Patent Publication Number: US-2021194407-A1

Title: Large high reliability air-cooled motor with optimal ventilation system

Description:
This application is a continuation in part of U.S. patent application Ser. No. 16/258,608 filed on 27 Jan. 2019 that claims priority to Chinese Patent Application Ser. No. CN201810081395.0 filed on 29 Jan. 2018. 
    
    
     TECHNICAL FIELD 
     The present invention belongs to the technical field of reliability and durability of electromechanical power equipment, relates to a high reliability motor, and more particularly relates to a large high reliability motor with an optimal ventilation system. 
     BACKGROUND ART 
     Lots of heat will be generated due to existence of various energy losses in the running process of a motor, which results in that temperatures of various portions in the motor rise. If the heat cannot be discharged out in time, a motor temperature would continuously rise, if a motor corresponding to a certain insulation grade exceeds the allowable highest temperature, the motor insulation would accelerate ageing, even a direct breakdown is caused, an accident is caused, losses are caused, and safe and reliable running of the motor will be greatly influenced. Thus, an effective cooling measure must be taken for the motor to control a temperature rise of the motor. In the motor, temperature of a stator winding is highest, and when the motor is researched, commonly a temperature rise of the stator winding represents the temperature rise of the motor. Motors in different insulation grades are different in the allowable highest temperature. Currently, for a large direct-transmission water pump unit, due to the fact that a motor is large in volume, a heating density is not large, a ventilation duct is arranged in the motor, and a running temperature of the motor is commonly controlled by adopting a ventilation cooling method. However, on the one hand, the motor is designed according to determined factors, that is, under influences of various determined factors of designed running conditions, it is guaranteed that a running temperature of the motor winding is free of overtemperature, but due to the fact that factors influencing the motor temperature rise in actual conditions are complicated, uncertainty exists, which results in that the motor temperature rise deviates from a design value; on the other hand, the ventilation slot area of the motor, the ventilators selected and the number in operation are not reasonable; as a result, overtemperature occurs frequently, safety and reliability of the motor are affected, difficulty is brought to design, selection and use of the motor and selection of ventilators, and it is urgently needed to invent a motor with a high reliability degree of the temperature rise considering the influence factors of the motor temperature rise and their uncertainties. 
     SUMMARY OF THE INVENTION 
     A ventilation system of a large motor is shown in  FIG. 1 . The present invention is directed to a large motor with a high reliability degree of a running temperature rise with optimal ventilation system, the optimal system consists of 4 parts: a large number of radial ventilation slots in the motor stator iron core, an annular ventilation duct around the motor stator iron core, an direct main ventilation duct and 2 ventilators at the outlet of the main ventilation duct; a cross-sectional area the ventilation slot has been optimized, the two ventilators and the number in operation have been selected by optimization also, based on high running temperature rise reliability of the motor, in view of the above problem that uncertainty of the motor temperature rise is caused due to uncertainties of various influence factors of the temperature rise, and the unreasonable ventilation slot cross-sectional area, namely the motor could reach a required high reliability degree when its stator winding temperature does not exceed the allowable highest temperature. Main influence factors, including motor running power, power network voltage fluctuation, a winding insulation thickness, ventilation slot heat exchange areas and a ventilation flow rate, of a motor winding temperature rise are determined, then a temperature rise influence value range and a probability density of each factor are calculated, all the influence factors are composited, reliability degrees are calculated when an actual running temperature of the motor is lower than or equal to different temperatures, according to a given allowable highest running temperature of the motor, motor temperature rise reliability degrees under different environment temperatures are calculated and determined, safety of running of the motor may be guaranteed, and a more scientific basis is provided for improved design, reasonable selection and running management of the motor and a ventilation system thereof. 
     A technical scheme of the present invention is that: a large high reliability air-cooled motor with an optimal ventilation system, whose ventilation slot cross-sectional area optimized, ventilators and the number in operation selected by optimization based on high running temperature rise reliability, its flowchart shown in  FIG. 2 , includes following operation steps: 
     A. calculating a flow resistance of a ventilation system and selecting two ventilators of a motor, determining a ventilation flow rate; 
     B. calculating a heating quantity of a motor; 
     C. calculating temperature rises of a motor winding under different environment temperatures; 
     D. determining main influence factors of a temperature rise of a motor winding; 
     E. determining random numerical characteristics of influence factors of temperature rise of a motor winding; 
     F. generating possible minimum and maximum values of running temperatures of a motor winding under different environment temperatures; 
     G. calculating reliability degrees when running temperature of a motor winding is lower than a given temperature under different environment temperatures; 
     H. calculating a reliability degree of running temperature rise of a motor winding; 
     I. creating relations between a motor temperature rise reliability and a cross-sectional area of a ventilation slot at multiple different environment temperatures when one ventilator and two ventilators running respectively: and 
     J. providing a motor with high reliability of temperature rise by using optimal area of a ventilation slot and choosing ventilators and the number in operation. 
     In step A, a calculation of a flow resistance of a ventilation system and a selection of two ventilators of a motor are as follows. 
     Firstly, resistances of various portions of the ventilation duct and a ventilation system of the motor are calculated as follows. 
     A friction pressure loss is: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       p 
                       f 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         m 
                       
                        
                       
                         
                           λ 
                           i 
                         
                          
                         
                           
                             l 
                             i 
                           
                           
                             d 
                             i 
                           
                         
                          
                         
                           ρ 
                           2 
                         
                          
                         
                           v 
                           i 
                           2 
                         
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           m 
                         
                          
                         
                           
                             λ 
                             i 
                           
                            
                           
                             
                               l 
                               i 
                             
                             
                               d 
                               i 
                             
                           
                            
                           
                             ρ 
                             
                               2 
                                
                               
                                 A 
                                 i 
                                 2 
                               
                             
                           
                            
                           
                             Q 
                             2 
                           
                         
                       
                       = 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           m 
                         
                          
                         
                           
                             Z 
                             i 
                           
                            
                           
                             Q 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the formula, i—serial number of a friction loss of the ventilation loop; m—sum of the friction losses of the ventilation loop; A—friction resistance coefficient; l—flow channel length; d—flow channel equivalent diameter, and when a flow channel is a rectangular pipeline, 
     
       
         
           
             
               d 
               = 
               
                 
                   2 
                    
                   h 
                    
                   b 
                 
                 
                   h 
                   + 
                   b 
                 
               
             
             ; 
           
         
       
     
     h—height of a section of the rectangular pipeline; b—breadth of the section of the rectangular pipeline; ρ—density of air; v—air velocity; A—area of a cross-sectional of the flow channel; Q—flow rate of ventilation; and Z i —friction resistance of number i section. 
     A local pressure loss is: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                       p 
                       j 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           1 
                         
                         n 
                       
                        
                       
                         
                           ζ 
                           j 
                         
                          
                         
                           ρ 
                           2 
                         
                          
                         
                           v 
                           j 
                           2 
                         
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             1 
                           
                           n 
                         
                          
                         
                           
                             
                               
                                 ζ 
                                 j 
                               
                                
                               ρ 
                             
                             
                               2 
                                
                               
                                 A 
                                 j 
                                 2 
                               
                             
                           
                            
                           
                             Q 
                             2 
                           
                         
                       
                       = 
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             1 
                           
                           n 
                         
                          
                         
                           
                             Z 
                             j 
                           
                            
                           
                             Q 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In the formula: j—serial number of a local resistance; n—local resistance sum; ζ—local loss coefficient; and Z j —number j local resistance. 
     An equivalent air resistance of an air course formed by n air resistances connected in series is: 
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                      
                     
                       Z 
                       i 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     An equivalent air resistance of an air course formed by n air resistances connected in parallel is: 
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     1 
                     
                       
                         ( 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             n 
                           
                            
                           
                             1 
                             
                               
                                 Z 
                                 i 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The required pressure curve of the ventilation system is as follows: 
         p=Z   1   Q   2   (5)
 
     In the formula: Z i —total resistance of the ventilation system; p—required total pressure of the ventilation system. 
     Two ventilators are selected according to the design ventilation flow rate of the motor. The flow rates of the one ventilator or two ventilators ventilation at an actual work condition is determined according to the intersection of a flow rate-full pressure performance curve of the one ventilator or two ventilators matched for use and a required pressure curve of the ventilation system as shown in formula (5); then the flow rate of the ventilation system of the motor is determined. 
     In step B, a calculation of a heating quantity of a motor is as follows. 
     Heat sources of the motor temperature rise include: an iron core loss, a winding copper loss and an excitation loss, and when motor cooling is carried out by adopting a manner of forced ventilation by draught fans, heat generated due to ventilation friction also needs to be considered. When a motor works, heat is generated due to the motor stator winding copper loss, a winding temperature is higher than an iron core temperature, the heat is transmitted to an iron core through winding insulation; and heat generated due to the iron core loss and the heat transmitted from the winding are subjected to convection heat exchange of cooling air in ventilation ducts, and the generated heat is brought out of the motor. 
     The heating quantity of the motor mainly comes from the iron core loss, the winding copper loss and the excitation loss. Heat generated by a mechanical loss of a thrust bearing and two guide bearings of the motor is brought away by cooling water in a cooler, and is not reckoned into a ventilation cooling load, wherein the iron core loss may be calculated with a following formula: 
     
       
         
           
             
               
                 
                   
                     P 
                     
                       F 
                        
                       e 
                     
                   
                   = 
                   
                     
                       K 
                       a 
                     
                      
                     
                       p 
                       0 
                     
                      
                     
                       B 
                       2 
                     
                      
                     
                       
                         
                           M 
                           
                             F 
                              
                             e 
                           
                         
                          
                         
                           ( 
                           
                             f 
                             
                               5 
                                
                               0 
                             
                           
                           ) 
                         
                       
                       1.3 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     In the formula: K a —experience coefficient; f—alternating frequency; p 0 —loss of per unit mass iron core when f is 50 Hz; B—magnetic flux density; M Fe —mass of the iron core. 
     A stator winding copper loss may be calculated with a following formula: 
         P   cu1   =mm   c   I   1   2   r   1   (7)
 
     In the formula: m—motor phase number; m c —insulation temperature rise coefficient, 1.4 is selected for Grade B insulation, and 1.48 is selected for Grade F insulation; I 1 —phase current; and r 1 —phase resistance: 
     A synchronous motor excitation winding copper loss may be calculated with a following formula: 
         P   Cu2   =I   2   2   r   2   (8)
 
     In the formula: I 2 —excitation current; and r 2 —excitation winding resistance. 
     The motor adopts the draught fan for ventilation, the draught fan sucks hot air from the motor, the hot air is discharged into atmosphere, negative pressure in the motor is caused, outside cold air is forced to enter the ventilation duct in the motor, and after heat is absorbed, the cold air is discharged into the atmosphere by the draught fan. A full air pressure generated due to ventilation of the draught fan is fully lost on ventilation system resistance, and is converted into heat, and the heat is also brought away by ventilation, that is ventilation friction resistance loss power is: 
         P   V   =Q   P   (9)
 
     In the formula: Q—ventilation flow rate; and p—full pressure loss generated in a process that air passes through the motor during motor ventilation. 
     In step C, a calculation of temperature rises of a motor winding under different environment temperatures is as follows. 
     According to air duct arrangement, an actual air velocity in each segment of the ventilation duct is also determined; and a heat exchange coefficient of a heat exchange surface is obtained from the air velocity, and then is substituted into a temperature rise calculation formula, and the motor temperature rise under a certain environment temperature is obtained. 
     A total area of stator ventilation slots is: 
         S   3 =π( D   1   +D   2 ) h   (10)
 
     In the formula, h n —slot height, b n —slot breadth; l 1 —stator iron core length; and z 1 —stator ventilation slot number. 
     A total area of ventilation openings of the stator iron core is: 
         S   2   =Z   i   b   n   h   n   (11)
 
     A total area of inner and outer cylindrical surfaces of the stator iron core is: 
         S   3 =π( D   1   +D   2 ) h   (12)
 
     In the formula: D 1 —outer circle diameter of the stator iron core; D 2 —inner circle diameter of the stator iron core; and h—height of the stator iron core; 
     A total heat dissipating area of the station iron core is: 
         S   Fe   =S   1   +S   3 −2 S   2   (13)
 
     A contact area of the stator winding and the iron core is: 
         S   4   =n   1   L   1   h   1   (14)
 
     In the formula: n 1 —winding branch number; L 1 —perimeter of a contact surface of the winding and the iron core; and h 1 —length of the contact surface of the winding and the iron core 
     An average air velocity in an air duct is: 
         v=Q/s   (15)
 
     In the formula: s—sectional area of the air duct. 
     A radial ventilation slot surface heat exchange coefficient: 
     
       
         
           
             
               
                 
                   α 
                   = 
                   
                     
                       1 
                       + 
                       
                         
                           0 
                           . 
                           2 
                         
                          
                         4 
                          
                         v 
                       
                     
                     
                       
                         0 
                         . 
                         0 
                       
                        
                       4 
                        
                       5 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     The winding temperature rise is: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           t 
                           m 
                         
                         = 
                           
                          
                         
                           
                             Δ 
                              
                             
                               t 
                               1 
                             
                           
                           + 
                           
                             Δ 
                              
                             
                               t 
                               
                                 Fe 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                           + 
                           
                             Δ 
                              
                             
                               t 
                               
                                 F 
                                  
                                 e 
                                  
                                 a 
                               
                             
                           
                           + 
                           
                             Δ 
                              
                             
                               t 
                               a 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 ϕ 
                                 
                                   C 
                                    
                                   F 
                                 
                               
                                
                               
                                 P 
                                 
                                   Cu 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                                
                               δ 
                             
                             
                               
                                 λ 
                                 1 
                               
                                
                               
                                 S 
                                 4 
                               
                             
                           
                           + 
                           
                             
                               q 
                                
                               
                                 L 
                                 
                                   Fe 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                                 2 
                               
                             
                             
                               1 
                                
                               2 
                                
                               
                                 k 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                             
                           
                           + 
                           
                             
                               P 
                               1 
                             
                             
                               α 
                                
                               
                                 S 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                             
                           
                           + 
                           
                             
                               ∑ 
                               P 
                             
                             
                               C 
                                
                               Q 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 ϕ 
                                 
                                   C 
                                    
                                   F 
                                 
                               
                                
                               
                                 P 
                                 
                                   cu 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                                
                               δ 
                             
                             
                               
                                 λ 
                                 1 
                               
                                
                               
                                 S 
                                 4 
                               
                             
                           
                           + 
                           
                             
                               
                                 ( 
                                 
                                   
                                     P 
                                     
                                       F 
                                        
                                       e 
                                     
                                   
                                   + 
                                   
                                     
                                       ϕ 
                                       CF 
                                     
                                      
                                     
                                       P 
                                       
                                         cu 
                                          
                                         
                                             
                                         
                                          
                                         1 
                                       
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 L 
                                 
                                   F 
                                    
                                   e 
                                    
                                   1 
                                 
                                 2 
                               
                             
                             
                               1 
                                
                               2 
                                
                               
                                 k 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                                
                               
                                 S 
                                 1 
                               
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                          
                         
                           
                             
                               
                                 P 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                               + 
                               
                                 
                                   ϕ 
                                   
                                     C 
                                      
                                     F 
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       P 
                                       
                                         cu 
                                          
                                         
                                             
                                         
                                          
                                         1 
                                       
                                     
                                     + 
                                     
                                       P 
                                       
                                         C 
                                          
                                         u 
                                          
                                         2 
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               α 
                                
                               
                                 S 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                             
                           
                           + 
                           
                             
                               ∑ 
                               P 
                             
                             
                               
                                 C 
                                 a 
                               
                                
                               Q 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     In the formula: Δt 1 —winding insulation layer temperature drop; Δt Fe1 —iron core interior average temperature difference; Δt Fea —temperature difference between a surface of an iron core segment and air; Δt a —air temperature rise; φ CF —loss component transmitted to the iron core from copper; q—unit volume heat flowing in axis direction of the iron core; L Fe1 —iron core length; P 1 —loss dissipated through the iron core; λ 1 —winding insulation heat conduction coefficient, the insulation heat conduction coefficient is relevant to temperature, and insulation heat conduction coefficients under different environment temperatures are obtained through an iterative approximation method; k Fe —coefficient; α—surface heat exchange coefficient of ventilation slot; ΣP—total heating quantity of the motor; C a —air volume specific heat capacity; and Q—ventilation flow rate. 
     The temperature rises of the motor winding under effects of the determined influence factors and under different environment temperatures are calculated with the formula (17), and by adding an environment temperature, motor running temperatures are obtained, as shown by a curve  1  in  FIG. 3 . 
     In step D, main influence factors of temperature rise of a motor winding are determined as follows. 
     According to calculation and comparison, the main factors influencing the motor temperature rise include a motor running power, a power network voltage, a winding insulation layer thickness, a ventilation slot heat exchange area and a ventilation flow rate. 
     In step E, a determination of random numerical characteristics of influence factors of temperature rise of a motor winding is as follows: 
     Influences of a random error of the motor running power on the motor temperature rise are considered. A ratio of the running power in random change and originally determined running power is relative power δ P  of the motor, and a random value range of δ P  is [δ Pmin , δ Pmax ]. 
     Assuming that motor running efficiency is unchanged, the motor stator and rotor winding copper losses, the iron core loss, the ventilation friction resistance loss and the like are all converted into heat, and according to the temperature rise calculation formula and a relationship between the various types of motor losses and the motor running power, influences of motor power change on the motor temperature rise are calculated as follows: 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                        
                       
                         t 
                         P 
                       
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 L 
                                 
                                   F 
                                    
                                   e 
                                    
                                   1 
                                 
                                 2 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     ϕ 
                                     
                                       C 
                                        
                                       F 
                                     
                                   
                                 
                                 ) 
                               
                             
                             
                               1 
                                
                               2 
                                
                               
                                 k 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                             
                           
                           + 
                           
                             
                               1 
                               + 
                               
                                 ϕ 
                                 CF 
                               
                             
                             
                               α 
                                
                               
                                 S 
                                 
                                   F 
                                    
                                   e 
                                 
                               
                             
                           
                           + 
                           
                             
                               
                                 ϕ 
                                 
                                   C 
                                    
                                   F 
                                 
                               
                                
                               δ 
                             
                             
                               
                                 λ 
                                 1 
                               
                                
                               
                                 S 
                                 4 
                               
                             
                           
                           + 
                           
                             1 
                             
                               
                                 C 
                                 a 
                               
                                
                               Q 
                             
                           
                         
                         ] 
                       
                        
                       Δ 
                        
                       
                         P 
                         F 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     = 
                     
                       
                         
                           K 
                           P 
                         
                          
                         
                           ( 
                           
                             
                               δ 
                               P 
                             
                             - 
                             1 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           g 
                           1 
                         
                          
                         
                           ( 
                           
                             δ 
                             P 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
     In the formula, ΔP F —motor heating quantity change caused by the motor running power change; and K P —power change influence coefficient. 
     Influences of the power network voltage fluctuation on the motor winding temperature rise are considered. A ratio of a power network voltage in random change and an originally determined power network voltage is a relative voltage δ V , a random value range of δ V  is [δ Vmin , δ Vmax ], according to the motor temperature rise calculation formula and a relationship between the voltage change and the motor power, an influence value of the relative voltage fluctuation on the motor temperature rise is calculated, and its calculation formula is: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                             t 
                             V 
                           
                         
                         = 
                           
                          
                         
                           2 
                           [ 
                           
                             
                               
                                 ( 
                                 
                                   
                                     
                                       
                                         ϕ 
                                         
                                           C 
                                            
                                           F 
                                         
                                       
                                        
                                       
                                         L 
                                         Fe 
                                         2 
                                       
                                     
                                     
                                       1 
                                        
                                       2 
                                        
                                       
                                         K 
                                         
                                           F 
                                            
                                           e 
                                         
                                       
                                        
                                       
                                         S 
                                         1 
                                       
                                     
                                   
                                   + 
                                   
                                     
                                       ϕ 
                                       
                                         C 
                                          
                                         F 
                                       
                                     
                                     
                                       α 
                                        
                                       
                                           
                                       
                                        
                                       
                                         S 
                                         
                                           F 
                                            
                                           e 
                                         
                                       
                                     
                                   
                                   + 
                                   
                                     
                                       
                                         ϕ 
                                         
                                           C 
                                            
                                           F 
                                         
                                       
                                        
                                       δ 
                                     
                                     
                                       
                                         λ 
                                         1 
                                       
                                        
                                       
                                         S 
                                         4 
                                       
                                     
                                   
                                   + 
                                   
                                     1 
                                     
                                       
                                         C 
                                         a 
                                       
                                        
                                       Q 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 P 
                                 
                                   C 
                                    
                                   u 
                                    
                                   1 
                                 
                               
                             
                             + 
                           
                         
                       
                     
                   
                   
                     
                       
                         
                             
                            
                           
                             
                               ( 
                               
                                 
                                   
                                     L 
                                     Fe 
                                     2 
                                   
                                   
                                     1 
                                      
                                     2 
                                      
                                     
                                       K 
                                       
                                         F 
                                          
                                         e 
                                       
                                     
                                      
                                     
                                       S 
                                       1 
                                     
                                   
                                 
                                 + 
                                 
                                   1 
                                   
                                     
                                       a 
                                       _ 
                                     
                                      
                                     
                                       S 
                                       
                                         F 
                                          
                                         e 
                                       
                                     
                                   
                                 
                                 + 
                                 
                                   1 
                                   
                                     
                                       C 
                                       a 
                                     
                                      
                                     Q 
                                   
                                 
                               
                               ) 
                             
                              
                             
                               P 
                               
                                 F 
                                  
                                 e 
                               
                             
                           
                           ] 
                         
                          
                         
                           δ 
                           V 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               K 
                               V 
                             
                              
                             
                               ( 
                               
                                 
                                   δ 
                                   V 
                                 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                           = 
                           
                             
                               g 
                               2 
                             
                              
                             
                               ( 
                               
                                 δ 
                                 V 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     In the formula, K V —voltage fluctuation influence coefficient. 
     Influences of the winding insulation layer thickness on the motor winding temperature rise are considered. A ratio of the winding insulation layer thickness in random change and an originally determined winding insulation layer thickness is a winding insulation layer relative thickness δ D , and a random value range of δ D  is [δ Dmin , δ Dmax ]. It can be known that according to the motor temperature rise calculation formula, the winding insulation layer thickness and the motor winding temperature rise are in a linear relationship, and a calculation formula of an influence value of the winding insulation layer relative thickness on the motor temperature rise is: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                       t 
                       D 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             ϕ 
                             
                               C 
                                
                               F 
                             
                           
                            
                           
                             P 
                             
                               C 
                                
                               u 
                                
                               1 
                             
                           
                         
                         
                           
                             λ 
                             1 
                           
                            
                           
                             S 
                             4 
                           
                         
                       
                        
                       
                         
                           δ 
                           m 
                         
                          
                         
                           ( 
                           
                             
                               δ 
                               D 
                             
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         
                           K 
                           D 
                         
                          
                         
                           ( 
                           
                             
                               δ 
                               D 
                             
                             - 
                             1 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           g 
                           3 
                         
                          
                         
                           ( 
                           
                             δ 
                             D 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
           
         
       
     
     In the formula, δ m —originally determined winding insulation layer thickness; and K D —insulation layer thickness influence coefficient. 
     It is considered that the ventilation slot heat exchange area has influences on the motor winding temperature rise. A ratio of the ventilation slot heat exchange area in random change and a determined ventilation slot heat exchange area is a ventilation slot relative heat exchange area δ A , and a random value range of δ A  is [δ Amin , δ Amax ]. According to a maximum value and a minimum value of the ventilation slot relative heat exchange area, a plurality of points are taken between the maximum value and the minimum value, different ventilation slot relative heat exchange areas are substituted into the temperature rise calculation formula, a calculation result is subtracted from a calculation result of the originally determined ventilation slot heat exchange area, and motor temperature rise changes under different δ A  are obtained. A curve is fitted according to splattering values, and a calculation formula of the motor temperature rise change Δt a  under any δ A  is obtained 
       Δ t   A   ==g   4 (δ A )  (21)
 
     It is considered that the ventilation flow rate has influences on the motor temperature rise. A ratio of the ventilation flow rate in random change and an originally determined ventilation flow rate is a relative ventilation flow rate δ Q , and a random value range of δ Q  is [δ Qmin , δ Qmax ]A plurality of points are taken between a largest ventilation flow rate and a smallest ventilation flow rate, different ventilation flow rates are substituted into the temperature rise calculation formula, a calculation result is subtracted from a result of the originally determined ventilation flow rate, and motor temperature rise changes under different δ Q  are obtained. A fitting curve is made according to splattering values, and a calculation formula of the motor temperature rise change Δt Q  under δ Q  is obtained 
       Δ t   Q   ==g   5 (δ Q )  (22)
 
     A probability density function determination method of random changes of relative values of the influence factors is as follows. 
     According to the random change range [x min , x max ] of the influence factors of the motor winding temperature rise, a probability density function ƒ(x) is determined, a probability density distribution type is parabolic distribution, an opening faces downwards, and a calculation formula is: 
       ƒ( x )= ax   2   +bx+c ( a≠ 0)  (23)
 
     According to non-negativity of the quadratic polynomial probability density function, an upper limit and a lower limit of the random change range of the influence factor are substituted in, a probability density value of 0 is obtained, and probability density values of other values in the domain of definition are all larger than 0; and according to normativity of the probability density function, an area surrounded by a probability density function curve and x axis is 1. Specific formulae are as follows: 
         ax   min   2   +bx   min   +c= 0  (24)
 
         ax   max   2   +bx   max   +c= 0  (25)
 
       ∫ x     min     x     max   ƒ( x ) dx= 1  (26)
 
     The coefficients a, b and c of the probability density function are solved from the three equations (24), (25) and (26). Corresponding probability density functions are respectively solved for the several types of influence factors of the temperature rise of the motor winding by adopting the method. 
     In step F, a generation of possible minimum and maximum values of running temperatures of a motor winding under different environment temperatures is as follows. 
     A motor running basic temperature under a certain environment temperature and extreme values of decrease or increase, caused by the various random factors, of the temperature rise are accumulated to obtain possible minimum and maximum values of the running temperature of the motor winding under the environment temperature, and a calculation formula is as follows: 
         t   Cu1min   =t   a   +t   m   +Δt   Pmin   +Δt   Vmin   +Δt   Dmin   +Δt   Amin   +Δt   Qmin   (27)
 
         t   Cu1max   =t   a   +t   m   +Δt   Pmax   +Δt   Vmax   +Δt   Dmax   +Δt   Amax   +Δt   Qmax   (28)
 
     In the formula: t a  is an environment temperature. Under different environment temperatures, schematic views of the possible lowest and highest running temperatures of the motor winding are respectively as shown by curve  2  and curve  3  in  FIG. 3 . 
     In step G, a calculation of reliability degrees when running temperature of a motor winding is lower than a given temperature under different environment temperatures is as follows. 
     The random value ranges and probability density functions of the influence factors of the motor power δ P , the power network voltage δ V , the ventilation flow rate δ Q , the winding insulation thickness δ D  and the ventilation slot heat exchange area δ A  are known and the probability density functions of the influence factors are respectively ƒ P (δ P ), ƒ V (δ V ), ƒ Q (δ Q ), ƒ D (δ D ) and ƒ A (δ A ), the reliability degree is calculated when the running temperature of the motor winding is lower than the certain temperature, that is, the running temperature of the motor winding, t=t a +t m +Δt P +Δt V +Δt Q +Δt D +Δt A , for a set motor winding temperature t 5  (a subscript 5 shows that five factors are considered), a reliability degree P 5  is calculated when the running temperature of the motor winding t≤t 5 . Firstly, two influence factors are composited, a probability P 2  is calculated, and analysis is as follows. 
     The random value range of a relative value of the first factor motor power is δ 1 =[δ Pmin , δ Pmax ], and the probability density function of the first factor motor power is ƒ P (δ P ) as shown in  FIG. 4 . At any point δ P  in a range [δ Pmin , δ Pmax ] of an abscissa, a micro-component area ƒ P (δ P )dδ P  with a micro width being dδ P  and a height being ƒ P (δ P ) is taken, and the micro-component area is a probability when δ P  is valued therein. 
     A probability P 2  when t a +t m +Δt P +Δt V ≤t 2  is solved, namely a sum of products of all micro area probabilities ƒ P (δ P )dδ P  and a probability P 1  when t a +t m +Δt V ≤t 2 −Δt P =t 1 , namely, P 2 =∫ δ     Pmin     δ     Pmax   P 1 ·ƒ P (δ P )dδ P , wherein a probability P 1  when Δt V ≤t 1 −t a −t m , namely, δ V ≤(t 1 −t a −t m )/K V +1 is an area Q V  of a figure on left side of the line δ V =(t 1 −t a −t m )/K V +1 in  FIG. 5 , then P 2 =∫ δ     Pmin     δ     Pmax   Q V ·ƒ P (δ P )dδ P , wherein Q V =∫ δ     Vmin     (t     1     −t     a     −t     m     )/K     V     +1 ƒ V (δ V )dδ V ; a Q V  expression is substituted into the P 2  calculation formula, and a probability when the running temperature of the motor winding is lower than or equal to t 2  may be obtained. Then the third factor, the fourth factor and the fifth factor are considered, recursive integrals continue to be deduced with the same method, and a probability P 5  when the running temperature of the motor winding is lower than or equal to t 5  is finally obtained: 
         P   5 =∫ δ     Amin     δ     Amax   ∫ δ     Dmin     δ     Dmax   ∫ δ     Qmin     δ     Qmax   ∫ δ     Pmin     δ     Pmax   ∫ δ     Vmin     (t     1     −t     a     −t     m     )K     V     +1 ƒ V (δ V ) dδ   V ƒ P (δ P ) dδ   P ƒ Q (δ Q ) dδ   Q ƒ D (δ D ) dδ   D ƒ A (δ A ) dδ   A   (29).
 
     A calculation formula of the running temperature of the motor winding is obtained by accumulating the motor running basic temperature under a certain environment temperature and values of decrease or increase, caused by the various factors, of the temperature rise, according to the method of step F, for different environment temperatures, progressive increasing is performed at a 0.2° C. winding running temperature step size for iterative calculation, and the reliability degrees are obtained when the running temperature of the motor winding is lower than or equal to given different temperatures; as shown in  FIG. 3 , the reliability degrees are calculated when the running temperature of the motor winding is lower than or equal to the given different temperatures, equal reliability degree points are connected with a curve, the reliability degree of the curve  2  is P=0, the reliability degree of the curve  3  is P=100%, and the reliability degree of the curve  4  is P=95%. 
     Relationship curves of the reliability degrees of the motor temperature rise and the given motor winding temperature under different environment temperatures are made, as shown in  FIG. 6 , and curves with serial numbers being 1-8 represent different environment temperatures. 
     In step H, a calculation of a reliability degree of running temperature rise of a motor winding is as follows: 
     Corresponding to the allowable highest temperature of the motor winding for the motor insulation grade, a horizontal line is drawn on  FIG. 3 , intersection points of the horizontal line and curves of different equal reliability degrees are reliability degrees of the motor temperature rise under corresponding environment temperatures, and change of the temperature rise reliability degrees along with the environment temperatures is made, as shown in  FIG. 7 , and may be used for motor design, selection and running. 
     In step I, a creation of relations between a motor temperature rise reliability and a cross-sectional area of a ventilation slot at multiple different environment temperatures when one ventilator and two ventilators running respectively is as follows: 
     In  FIG. 7 , for a certain motor, the length-to-width ratio of the section of the ventilation slot is set to be constant. Given a number of different cross-sectional areas of the ventilation slot, a number of relationship curves between the motor temperature rise reliability and the environment temperature for different cross-sectional areas of the ventilation slot are calculated using the method in step G when running one ventilator. For a certain environment temperature, a vertical line perpendicular to the horizontal axis is drawn and intersect multiple reliability curves. Taking the area of a single ventilation slot at the intersection point as the abscissa and the temperature rise reliability of the intersection point as the ordinate, a number of the intersection points are fitted, the relationship curve between the reliability and the ventilation slot area at a certain environment temperature is obtained shown in  FIG. 8 . Choosing two ventilators reasonably, using the same method, the relationship curves are obtained between the motor temperature rise reliability and the cross-sectional area of the ventilation slot at multiple different environment temperatures when one ventilator and two ventilators running respectively. 
     In step J, a motor provided with high reliability of temperature rise by using optimal area of a ventilation slot and choosing ventilators and the number in operation are as follows: 
     Observing and analyzing the curves in  FIG. 8 , at a certain environment temperature, when the environment temperature is high and one ventilator is running, as the cross-sectional area of the ventilation slot increases, the heat exchange area between the motor iron core and the air increases, and the ventilation system loop resistance decreases, the ventilation rate increases slightly, therefore, the motor temperature rise decreases and the temperature rise reliability increases. But when the cross-sectional area of the ventilation slot continues to increase, the air velocity in the ventilation slot decreases, and the convection heat exchange coefficient between the air and the iron core decreases, when its influence exceeds influence of the increase of the heat exchange area, the reliability of the motor temperature rise decreases instead, therefore, corresponding to the highest temperature rise reliability, there is an optimal ventilation slot cross-sectional area for design use shown as a vertical dashed line in  FIG. 8 . 
     When the environment temperature is higher, even if the motor ventilation slot adopts the optimal cross-sectional area, the reliability of the motor temperature rise still cannot meet the requirements when one ventilator running, then two ventilators should be put into operation to ensure the reliability of the motor temperature rise; if the reliability still cannot meet the requirements when two ventilators running, the two bigger air volume ventilators should be selected, step A to step J above should be repeated until the reliability is satisfied; herein, the motor, the ventilation system and its operation scheme with high reliability of the temperature rise are provided. 
     The present invention has the beneficial effects that a large motor of high reliability degree of the running temperature rise is provided by determining the main influence factors of the temperature rise of the motor winding, calculating the heating quantity and the temperature rise of the motor under influences of certain factors, determining the random numerical characteristics of the main influence factors of the temperature rise of the motor winding, calculating possible minimum values and possible maximum values of running temperatures of the motor winding under different environment temperatures, calculating the reliability degrees when the running temperature of the motor winding is lower than the given temperature under different environment temperatures, and calculating the reliability degree of the running temperature rise of the motor winding, providing a large motor with high reliability of the temperature rise by optimizing area of a ventilation slot of the motor, choosing the ventilator and its number of operation. The present invention can improve a probability when the running temperature of the motor is lower than the allowable highest temperature under influences of the plurality of uncertain factors, the improvement is more scientific, the results are more reasonable, a scientific basis is provided for design, selection and application of the motor and the ventilation cooling system of the motor, a high reliability motor is provided, and important theory academic value and engineering application significance are achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a ventilation system of a large motor. 
         FIG. 2  is a design flowchart of a large motor with high reliability degree of running temperature rise. 
         FIG. 3  is a schematic view of a running temperature of a motor stator winding under different environment temperatures in the present invention. 
         FIG. 4  is a schematic view of a probability density function ƒ P (δ P ) and composite calculation of a reliability degree of motor power in the present invention. 
         FIG. 5  is a schematic view of a probability density function ƒ V (δ V ) and composite calculation of a reliability degree of power network voltage in the present invention. 
         FIG. 6  is a schematic view of changes of a motor temperature rise reliability degree along with a given motor winding temperature in the present invention. 
         FIG. 7  is a schematic view of changes of the motor temperature rise reliability degree along with environment temperatures in the present invention. 
         FIG. 8  is a schematic view of changes of a motor temperature rise reliability degree along with a ventilation slot area at different environment temperatures and number of fans in operation in the present invention. 
         FIG. 9  is a running temperature chart of a motor stator winding corresponding to different reliability under different environment temperatures when one draught fan or two draught fans run according to an embodiment of the present invention. 
         FIG. 10  is a change diagram of the motor temperature rise reliability degree along with a given motor winding temperature under different environment temperatures when two draught fans run according to the embodiment of the present invention. 
         FIG. 11  is a change diagram of a motor temperature rise reliability degree along with environment temperatures when one draught fan or two draught fans run and allowable highest temperature of a motor winding is 100° C. according to the embodiment of the present invention. 
         FIG. 12  is a change diagram of a motor temperature rise reliability degree along with a ventilation slot area at different environment temperatures when one ventilator and two ventilators running according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is further illustrated below in conjunction with the accompanying drawings and embodiments. 
     A motor matched with a main water pump of a certain pump station for use is a synchronous motor, a rated voltage is 6000 V, a rated current is 180 A, a phase number is 3, an insulation grade is Grade F, an iron core mass is 3.693 t, an iron core height is 370 mm, an iron core inner diameter is 2290 mm, an iron core outer diameter is 2600 mm, the number of the ventilation slots of the motor is 216×6=1296, a height of a ventilation slot is 10 mm, a slot width is 18 mm, an iron core length is 155 mm, a cross-sectional area of a ventilation slot is 180 mm 2 , phase resistance is 0.2416Ω when a stator is at 75° C., an excitation current under a rated load is 177 A, and winding resistance is 0.6398Ω. 
     In step A, a calculation of flow resistance of ventilation system and a selection of two ventilators: 
     A ventilation duct resistance coefficient of the motor may be calculated with formulae (1) to (5), The flow resistance of the original ventilation system is 8.15 Ns 2 /m 6 . Two ventilators of TS35 mode were selected for a motor, the rated parameters of one ventilator: flow rate is 9.89 m 3 /s, full pressure is 450 Pa, power is 5.5 kW, which can meet the design requirements of flow rate 8.7 m 3 /s and full pressure 200 Pa of the motor. 
     In step B, the heating quantity of the motor is calculated: 
     A motor iron core loss, a stator winding copper loss, an excitation winding copper loss and a ventilation friction resistance loss were calculated with formulae (6) to (9). For example, when the environment temperature is 20° C., the iron core loss is 7.001 kW, a stator winding copper loss is 34.756 kW, an excitation winding copper loss is 20.044 kW, and the ventilation friction resistance loss is relevant to the ventilation flow rate and resistance. 
     In step C, temperature rises of the motor winding under different environment temperatures are calculated: 
     It is known that an area of the ventilation slots of a stator is 11.249 m 2 ; a total area of the ventilation openings of a stator iron core is 0.233 m 2 ; a total area of inner and outer cylindrical surfaces of the stator iron core is 5.603 m 2 ; a total heat dissipating area of the stator iron core is 16.386 m 2 ; a contact area of the stator winding and the iron core is 11.872 m 2 , and the temperature rise of the winding may be calculated and determined with formulae (10) to (17). For example, when the environment temperature is 20° C., the flow rate provided by two draught fans selected for use is 6.32 m 3 /s, and the temperature rise of the winding is 52.644° C. Running temperature rises of the motor stator winding under other environment temperatures and under effects of determined influence factors are obtained by calculation in the similar way, and by adding the environment temperature, the running temperatures of the motor are obtained, as shown by curves  1  and  1 ′ in  FIG. 9 . 
     In step D, the main influence factors of the temperature rise of the motor winding are determined. 
     When an environment temperature is 20° C., one ventilator operates and the flow rate is 6.32 m 3 /s, based on calculation, temperature rise error ranges caused by various factors having influences on the temperature rise of the motor winding are as shown in Table 1: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 The motor temperature rise error ranges caused in case that the  
               
               
                 various influence factors change randomly when the ventilation  
               
               
                 flow rate is 6.32 m 3 /s 
               
            
           
           
               
               
            
               
                 Influence factor of temperature rise  
                 Temperature rise error  
               
               
                 of motor winding 
                 range (° C.) 
               
               
                   
               
               
                 Running power 
                 [−10.509, 12.449] 
               
               
                 Power network voltage fluctuation 
                 [−4.414, 4.414] 
               
               
                 Ventilation flow rate 
                 [−2.959, 2.624] 
               
               
                 Ventilation slot heat exchange area 
                 [−1.987, 1.895] 
               
               
                 Winding insulation thickness 
                 [−1.440, 1.440] 
               
               
                 Stator winding resistance 
                 [−0.344, 0.344] 
               
               
                 Power network frequency fluctuation 
                 [−0.0206, 0.0196] 
               
               
                 Iron core length 
                 [0, 0.076] 
               
               
                   
               
            
           
         
       
     
     Through comparison, it may be known that, the main factors influencing the temperature rise of the motor include five factors of the motor running power, the power network voltage fluctuation, the winding insulation layer thickness, the ventilation slot heat exchange area and the ventilation flow rate. 
     In step E, the random numerical characteristics of the influence factors of the temperature rise of the motor winding are determined: 
     Factors such as design, installation and running may cause the running power of a pump station unit to change. Prototype and model conversion error, a water pump characteristic error, a pipeline characteristic error, a vortex of entering flow and a pump station head change all may generate a random error on the running power of the motor. Through analysis, a random change range of relative running power δ P  of the motor is [0.9025, 1.1155]; 
     As stipulated by a national power supply standard, an allowed range of the power network voltage fluctuation is ±5%, and therefore a range of a random change rate δ V  of a power network voltage is [0.95, 1.05]; 
     As stipulated by a manufacturing standard, an error of insulation of the motor winding does not exceed ±7%, and therefore a random change range of a winding insulation relative thickness δ D  is [0.93, 1.07]; 
     As stipulated by the standard, a machining error of a size of the motor stator ventilation slot does not exceed 10%, and therefore a random change rate of a size of a cross-sectional of the ventilation slot is [0.9, 1.1], and correspondingly, a random change rate change of a ventilation slot relative heat exchange area δ A  is [0.81, 1.21]; 
     It is hard to avoid errors during calculation of the resistance coefficient of the ventilation duct of the motor, and draught fan performance may also bring an error for determination of the ventilation flow rate. After analysis, when an original ventilation flow rate is 6.32 m 3 /s, a random change range of a relative ventilation flow rate δ Q  is [0.9, 1.14], and when the original ventilation flow rate is 5.339 m 3 /s, a range of a random change rate δ Q  of the ventilation flow rate is [0.932, 1.075]. Temperature influence coefficients of the various influence factors under different random change rates may be calculated with formulae (18) to (22), as shown in Table 2 and Table 3. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Temperature influence coefficients of the various influence 
               
               
                 factors when the ventilation flow rate is 6.32 m 3 /s 
               
            
           
           
               
               
               
               
               
               
            
               
                 t a   
                 K P   
                 K V   
                 K D   
                 Δt A   
                 Δt Q   
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 5 
                 102.46 
                 84.37 
                 20.16 
                 −4.29δ A   2 + 26.73δ A  − 22.44 
                 −17.40δ Q   3  + 72.49δ Q   2  − 14.77δ Q  + 59.69 
               
               
                 10 
                 104.28 
                 85.71 
                 20.30 
                 −4.39δ A   2  + 27.36δ A  − 22.97 
                 −17.93δ Q   3  + 74.68δ Q   2  − 18.21δ Q  + 61.46 
               
               
                 15 
                 106.00 
                 86.97 
                 20.44 
                 −4.49δ A   2  + 27.98δ A  − 23.49 
                 −18.40δ Q   3  + 76.62δ Q   2  − 21.28δ Q  + 63.05 
               
               
                 20 
                 107.78 
                 88.27 
                 20.57 
                 −4.59δ A   2  + 28.60δ A  − 24.01 
                 −18.91δ Q   3  + 78.75δ Q   2  − 24.62δ Q  + 64.78 
               
               
                 25 
                 109.54 
                 89.57 
                 20.70 
                 −4.69δ A   2  + 29.20δ A  − 24.51 
                 −19.42δ Q   3  + 80.88δ Q   2  − 27.96δ Q  + 66.51 
               
               
                 30 
                 111.38 
                 90.92 
                 20.82 
                 −4.78δ A   2  + 29.80δ A  − 25.02 
                 −19.99δ Q   3  + 83.22δ Q   2  − 31.62δ Q  + 68.39 
               
               
                 35 
                 113.08 
                 92.15 
                 20.95 
                 −4.88δ A   2  + 30.38δ A  − 25.50 
                 −20.47δ Q   3  + 85.24δ Q   2  − 34.80δ Q  + 70.03 
               
               
                 40 
                 114.83 
                 93.42 
                 21.07 
                 −4.97δ A   2  + 30.99δ A  − 26.02 
                 −20.98δ Q   3  + 87.34δ Q   2  − 38.11δ Q  + 71.75 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Temperature influence coefficients of the various influence factors when  
               
               
                 the ventilation flow rate is 5.339 m 3 /s 
               
            
           
           
               
               
               
               
               
               
            
               
                 t a   
                 K P   
                 K V   
                 K D   
                 Δt A   
                 Δt Q   
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 5 
                 109.50 
                 89.40 
                 19.98 
                 −5.98δ A   2  + 32.14δ A  −  
                 −20.49δ Q   3  + 84.32δ Q   2  −  
               
               
                   
                   
                   
                   
                 26.16 
                 32.41δ Q  + 68.58 
               
               
                 10 
                 111.52 
                 90.88 
                 20.12 
                 −6.12δ A   2  + 32.89δ A  −  
                 −20.49δ Q   3  + 84.32δ Q   2  −  
               
               
                   
                   
                   
                   
                 26.78 
                 32.41δ Q  + 68.58 
               
               
                 15 
                 113.43 
                 92.27 
                 20.25 
                 −6.25δ A   2  + 33.63δ A  −  
                 −21.68δ Q   3  + 89.19δ Q   2  −  
               
               
                   
                   
                   
                   
                 27.38 
                 39.99δ Q  + 72.48 
               
               
                 20 
                 115.41 
                 93.71 
                 20.38 
                 −6.39δ A   2  + 34.38δ A  − 
                 −22.29δ Q   3  + 91.69δ Q   2  −  
               
               
                   
                   
                   
                   
                 27.99 
                 43.88δ Q  + 74.48 
               
               
                 25 
                 117.36 
                 95.13 
                 20.51 
                 −6.53δ A   2  + 35.10δ A  −  
                 −22.90δ Q   3  + 94.20δ Q   2  −  
               
               
                   
                   
                   
                   
                 28.57 
                 47.78δ Q  + 76.48 
               
               
                 30 
                 119.40 
                 96.63 
                 20.63 
                 −6.66δ A   2  + 35.83δ A  −  
                 −23.58δ Q   3  + 96.97δ Q   2  −  
               
               
                   
                   
                   
                   
                 29.17 
                 52.07δ Q  + 78.68 
               
               
                 35 
                 121.28 
                 98.00 
                 20.75 
                 −6.79δ A   2  + 36.52δ A  −  
                 −24.16δ Q   3  + 99.35δ Q   2  −  
               
               
                   
                   
                   
                   
                 29.73 
                 55.77δ Q  + 80.58 
               
               
                 40 
                 123.23 
                 99.40 
                 20.87 
                 −6.93δ A   2  + 37.26δ A  −  
                 −24.77δ Q   3  +  
               
               
                   
                   
                   
                   
                 30.33 
                 101.83δ Q   2  − 59.63δ Q  +  
               
               
                   
                   
                   
                   
                   
                 82.6 
               
               
                   
               
            
           
         
       
     
     With the winding insulation thickness of the influence factors of the temperature rise of the motor winding when two draught fans run as an example, a corresponding probability density function ƒ D (δ D ) is calculated, a change rate range of δ D  is [0.93, 1.07] and is substituted into formulae (24) to (26), and calculation is as follows: 
       0.93 2   ×a+ 0.93× b+c= 0
 
       1.07 2   ×a+ 1.07× b+c= 0
 
       ∫ x     min     x     max   ( ax   2   bx+c ) dx= 1
 
     Simultaneous solving is performed, and the probability density function of the influence factor of the motor winding insulation thickness is obtained: 
       ƒ( x )=2186.5889 x   2 +4373.1778 x− 2175.8746
 
     Corresponding probability density functions are respectively solved for the other several types of influence factors of the temperature rise of the motor winding by adopting the method, as shown in Table 4 and Table 5. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 The probability density functions of the various influence factors  
               
               
                 when the ventilation flow rate is 6.32 m 3 /s 
               
            
           
           
               
               
               
            
               
                   
                 Motor winding influence  
                   
               
               
                   
                 factor 
                 Probability density function 
               
               
                   
                   
               
               
                   
                 Motor power δ P   
                 ƒ (x) = −620.8868x 2  + 1252.9496x −  
               
               
                   
                   
                 625.0708 
               
               
                   
                 Power network voltage δ V   
                 ƒ (x) = −6000x 2  + 12000 x − 5985 
               
               
                   
                 Ventilation flow rate δ Q   
                 ƒ (x) = −434.0278x 2  + 885.4167 x −  
               
               
                   
                   
                 445.3125 
               
               
                   
                 Winding insulation  
                 ƒ (x) = −2186.5889x 2  + 4373.1778x −  
               
               
                   
                 thickness δ D   
                 2175.874636 
               
               
                   
                 Ventilation slot heat  
                 ƒ (x) = −93.75x 2  + 189.375x − 91.8844 
               
               
                   
                 exchange area δ A   
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 The probability density functions of the various influence factors  
               
               
                 when the ventilation flow rate is 5.339 m 3 /s 
               
            
           
           
               
               
            
               
                 Motor winding influence  
                   
               
               
                 factor 
                 Probability density function 
               
               
                   
               
               
                 Motor power δ P   
                 ƒ (x) = −620.8868x 2  + 1252.9496x −  
               
               
                   
                 625.0708 
               
               
                 Power network voltage δ V   
                 ƒ (x) = −6000x 2  + 12000x − 5985 
               
               
                 Ventilation flow rate δ Q   
                 ƒ (x) = −2051.8383x 2  + 4118.0395x −  
               
               
                   
                 2055.7368 
               
               
                 Winding insulation  
                 ƒ (x) = −2186.5889x 2  + 4373.1778x −  
               
               
                 thickness δ D   
                 2175.8746 
               
               
                 Ventilation slot heat  
                 ƒ (x) = − 93.75x 2  + 189.375x − 91.8844 
               
               
                 exchange area δ A   
               
               
                   
               
            
           
         
       
     
     F. Possible minimum and maximum values of the running temperatures of the motor under different environment temperatures are calculated: 
     A motor running basic temperature under a certain environment temperature of an embodiment and extreme values of decrease or increase of the temperature rise caused by the various factors are accumulated to obtain the possible minimum and maximum values of the running temperature of the motor winding under the environment temperature, and the calculation results are as shown by curves  2  and  2 ′ and curves  3  and  3 ′ in  FIG. 9 . 
     G. The reliability degrees when the running temperature of the motor winding is lower than a certain given temperature are calculated: 
     With the environment temperature being 20° C. and the given motor winding temperature being 80° C. as an example, the reliability degree P 5  when the running temperature of the motor winding is lower than 80° C. is calculated, δ P =[0.9025, 1.1155], δ V =[0.95, 1.05], δ Q =[0.9, 1.14], δ D =[0.93, 1.07] and δ A =[0.81, 1.21] and the probability density functions ƒ P ((δ P ), ƒ V (δ V ), ƒ Q (δ Q ), ƒ D (δ D ) and ƒ A (δ A ) are known, and at the moment, the given motor winding temperature is: 
     
       
         
           
             
               
                 
                   
                     t 
                     5 
                   
                   = 
                     
                    
                   
                     
                       80 
                        
                       ° 
                        
                       
                           
                       
                        
                       
                         C 
                         . 
                       
                     
                     = 
                     
                       
                         t 
                         a 
                       
                       + 
                       
                         t 
                         m 
                       
                       + 
                       
                         Δ 
                          
                         
                           t 
                           P 
                         
                       
                       + 
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           t 
                           V 
                         
                       
                       + 
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           t 
                           Q 
                         
                       
                       + 
                       
                         Δ 
                          
                         
                           t 
                           D 
                         
                       
                       + 
                       
                         Δ 
                          
                         
                           t 
                           A 
                         
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       t 
                       a 
                     
                     + 
                     
                       t 
                       m 
                     
                     + 
                     
                       10 
                        
                       
                         7 
                         . 
                         7 
                       
                        
                       8 
                        
                       1 
                        
                       6 
                        
                       
                         ( 
                         
                           
                             δ 
                             P 
                           
                           - 
                           1 
                         
                         ) 
                       
                     
                     + 
                     
                       88.2742 
                        
                       
                         ( 
                         
                           
                             δ 
                             V 
                           
                           - 
                           1 
                         
                         ) 
                       
                     
                     + 
                   
                 
               
             
             
               
                 
                     
                    
                   
                     
                       20.5710 
                        
                       
                         ( 
                         
                           
                             δ 
                             D 
                           
                           - 
                           1 
                         
                         ) 
                       
                        
                       
                         - 
                       
                        
                       4.5905 
                        
                       
                         δ 
                         A 
                         2 
                       
                     
                     + 
                     
                       28.5996 
                        
                       
                         δ 
                         A 
                       
                     
                   
                 
               
             
           
         
       
       
         
           
             
               
                 - 
               
                
               2 
                
               
                 4 
                 . 
                 0 
               
                
               0 
                
               9 
                
               1 
             
             - 
             
               1 
                
               
                 8 
                 . 
                 9 
               
                
               090 
                
               
                 δ 
                 Q 
                 3 
               
             
             + 
             
               7 
                
               
                 8 
                 . 
                 7 
               
                
               495 
                
               
                 δ 
                 Q 
                 2 
               
             
             - 
             
               12 
                
               
                 4 
                 . 
                 6 
               
                
               2 
                
               1 
                
               4 
                
               
                 δ 
                 Q 
               
             
             + 
             
               6 
                
               
                 4 
                 . 
                 7 
               
                
               805 
             
           
         
       
     
     Programming calculation is performed by utilizing MATLAB software, in the random value range of each influence factor, a reasonable iteration step size is set, micro widths being dδ P , dδ V , dδ Q , dδ D  and dδ A  are sequentially taken from small, a constraint condition that the running temperature of the motor winding t≤t 5  is met, that is, t a +t m +Δt P +Δt V +Δt Q +Δt D ≤t 5 −Δt A =t 4 , t a +t m +Δt P +Δt V +Δt Q ≤t 4 −Δt D =t 3 , t a +t m +Δt P +Δt V ≤t 3 −Δt Q =t 2 , t a +t m +Δt V ≤t 2 −Δt P =t 1  and δ V ≤(t 1 −t a −t m )/K V +1 are sequentially met, a probability P 1 =∫ δ     Vmin     (t     1     −t     a     −t     m     )/K     V     +1 ƒ V (δ V )dδ V  is obtained, and is substituted into a P 2 =∫ δ     Pmin     δ     Pmax   P 1 ·ƒ P (δ P )dδ P  calculation formula, then P 2  is substituted into P 3 =∫ δ     Qmin     δ     Qmax   P 2 ·ƒ Q (δ Q )dδ Q , the rest can be done in the same way, that is, P 5  may be obtained due to calculation with a formula (29). 
     For the environment temperature from 5° C. to 40° C., valuing is performed every other 5° C., the reliability degrees under eight different environment temperatures when a step size of the running temperature of the winding is given, and iteration is performed at 0.2° C. progressive increase, linear interpolation is performed on data, given motor winding temperatures are taken when the reliability degrees P are respectively 0, 30%, 50%, 80%, 95%, 98%, 100% and the like, and corresponding equal reliability degree lines are made. For conciseness and clearness, equal reliability degree lines when the reliability degrees P are respectively 0, 95% and 100% are given in  FIG. 9 . 
     Curves  1 ,  2 ,  3  and  4  respectively represent equal reliability degree lines when two draught fans run, the influence factors of the stator winding temperature rise are determined with a random factor P=0, 100% and 95%, and the curves are shown with solid lines; curves  1 ′,  2 ′,  3 ′ and  4 ′ respectively represent equal reliability degree lines when one draught fan runs, the influence factors of the stator winding temperature rise are determined with a random factors P=0, 100% and 95%, and the curves are shown with imaginary lines. Along with rising of the environment temperature, the curves are all in a tendency of monotone increasing. Under the same environment temperature, for the same given motor winding temperature, the reliability degree of the motor temperature rise when one draught fan runs is lower than the reliability degree of the motor temperature rise when two draught fans run; or under the same allowable highest motor winding temperature, the environment temperature at which two draught fans can run is higher than the environment temperature at which one draught fan can run. 
       FIG. 10  is a change diagram of the temperature rise reliability degree along with the given motor winding temperature, each curve represents one environment temperature, the environment temperature is sequentially increased from left to right, it can be known from the  FIG. 10  that, under the various environment temperatures, corresponding to one given motor winding temperature of an abscissa-namely the allowable highest temperature, the higher the environment temperature is, the more rightwards the reliability degree line moves, and the lower the temperature rise reliability degree is. 
     H. The reliability degree of the running temperature rise of the motor winding is calculated: 
     In  FIG. 9 , more equal reliability degree lines of changes of the motor winding temperature along with the environment temperature are calculated and determined, the allowable highest temperature corresponding to the motor insulation grade is 100° C., a horizontal line with a temperature being 100° C. is drawn on  FIG. 9 , the horizontal line is intersected with different equal reliability degree curves of one draught fan and two draught fans, intersection points are the reliability degrees of the motor temperature rise under corresponding environment temperatures, and a change diagram of the reliability degree of the temperature rise of one draught fan and two draught fans along with the environment temperature is made, as shown in  FIG. 11 . 
     It is known from  FIG. 11  that, along with rising of the environment temperature, the reliability degree of the motor temperature rise is lowered, and if it is stipulated that the reliability degree of the temperature rise is required not to be lower than 95%, a horizontal line of a 95% reliability degree is intersected with one draught fan and two draught fans at points A and B respectively. When the environment temperature is 30° C. or below, one draught fan may be selected to run, so that ventilation cost is saved; when the environment temperature is 30° C. to 34.3° C., the two draught fans should run, which is able to guarantee that the reliability degree of the motor temperature rise is larger than or equal to 95%; and when the environment temperature is larger than 34.3° C., even though two draught fans run, the reliability degree of the motor temperature rise is still lower than 95%, and especially when the environment temperature reaches 40° C., the reliability degree of the motor temperature rise is only 68.5%, but the situation hardly occurs. 
     I. The relations between the motor temperature rise reliability and the cross-sectional area of the ventilation slot are calculated at multiple different environment temperatures when one ventilator and two ventilators running respectively: 
     A change diagram of the motor temperature rise reliability degree along with the ventilation slot area at different environment temperatures when one ventilator and two ventilators running according to an embodiment of the present invention is shown in  FIG. 12 . 
     J. A motor is provided with high reliability of the temperature rise by using optimal area of a ventilation slot and choosing the ventilators and the number in operation: 
       FIG. 12 , when only one ventilator running, there is optimal ventilation slot cross-sectional area 21.6×12=259.2 mm 2  corresponding to the highest temperature rise reliabilities 97.21%, 90.23% and 78.33% at environment temperatures of 30° C., 32.5° C. and 35° C. respectively, compared with the temperature rise reliabilities 94.97%, 85.95% and 72.56% respectively for original ventilation slot cross-sectional area 18×10=180 mm 2 , therefore, the area of a ventilation slot of the motor should adopt 21.6×12=259.2 mm 2 . At 32.5° C. and 35° C. environment temperatures, two ventilators should be put into operation to meet 95% reliability degree of the motor temperature rise. 
     The results of calculation and optimal design in the embodiment explains that the motor, provided by the present invention, has the highest reliability degrees of temperature rise under different environment temperatures by using the optimal area of a ventilation slot, and combining selection of ventilators and the number in operation, the high reliability of temperature rise of a motor can be guaranteed. Therefore, the present invention has important theory academic value and engineering application significance in design, selection and operation of a large and motor.