Abstract:
The motor power consumption and motor winding temperature are monitored and, responsive thereto, the flow of refrigerant to the motor is controlled so as to control the temperature of the motor. The motor power consumption in the primary control input because it anticipates cooling requirements whereas the motor temperature indicates current cooling requirements. The cooling flow may be provided in an on-off manner or there may be a constant flow portion with an on-off parallel cooling flow path.

Description:
BACKGROUND OF THE INVENTION 
     Electric motors are driven by supplying electricity to the motor windings. This results in the heating of the windings and associated motor structure due to losses in driving the motor. While motors are designed to operate at an elevated temperature, conventionally, motors are cooled responsive the motor windings reaching a predetermined temperature. For hermetic and semi-hermetic refrigerant compressors, cooling is achieved by causing refrigerant gas or liquid to flow through/over the motor structure before being supplied to the compressor with the suction or mid-stage pressure gas. Since the motor efficiency and equipment size requirements dictate limited flow path availability, the amount of cooling flow is somewhat limited. The motor structure, however, represents such a large thermal mass that the result is that there can be a significant time period before the motor cooling flow achieves the desired cooling effect to return the motor temperature to the desired level. During this time period the windings can experience a large deviation from the desired operating temperature. 
     SUMMARY OF THE INVENTION 
     The present invention uses the motor load to anticipate changes in motor cooling requirements and uses it as the primary process variable. Because of the time lag between changes in the motor load and a perceived temperature change, better motor temperature control is achieved than would be the case where cooling is responsive to motor temperature fluctuations from a set point. The motor winding temperature is used as a secondary variable to make minor corrections to the process output, i.e. the motor cooling flow. This mode of operation reduces operation at elevated temperatures above the design temperature and reduces the cooling requirement when the motor load has dropped but the motor is still at an elevated temperature due to the time lag in changes. 
     It is an object of this invention to reduce the time lag in a compressor motor cooling system. 
     It is another object of this invention to provide better motor temperature control by using motor load to anticipate changes in motor cooling requirements. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. 
     Basically, motor power consumption and motor winding temperature are monitored and, responsive thereto, the flow of refrigerant to the motor is controlled so as to control the temperature of the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic representation of a liquid cooled single-valve, non-economized system employing the present invention; 
     FIG. 2 is a schematic representation of a liquid cooled two-valve, non-economized system employing the present invention; 
     FIG. 3 is a schematic representation of a flashtank economizer system employing the present invention; 
     FIG. 4 is a schematic representation of a direct expansion economizer system employing the present invention; and 
     FIG. 5 is a schematic representation of liquid cooled two-valve economized system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the Figures, hermetic or semi-hermetic compressor  12  is driven by motor  14  and is fluidly connected to suction line  16  and discharge line  18  which are connected, respectively, to the evaporator  20  and condenser  22  of the refrigeration system. An expansion device  32  which may be a thermostatic or an electronic expansion valve is located between condenser  22  and evaporator  20 . Microprocessor  10  receives temperature (air or water) inputs and controls the refrigeration system responsive thereto. According to the teachings of the present invention, microprocessor  10  receives inputs representative of the power input to motor  14  and the temperature of the windings of motor  14 . Additionally, since microprocessor  10  controls motor  14  responsive to demand, control can be responsive to the approaching of demand satisfaction, for example. 
     The present invention controls the cooling of motor  14  responsive to the power draw of motor  14  and the temperature of the windings of motor  14  by supplying liquid or gaseous refrigerant to motor  14  by modifying the basic system described above as in one of the specific manners described below. 
     In FIG. 1, numeral  100  designates the basic refrigeration system described above and further includes branch refrigerant line  24  extending from liquid refrigerant line  23  from a point upstream of expansion device  32  and extending into motor  14 . Solenoid valve  34  is located in line  24  and is controlled by microprocessor  10  responsive to motor power consumption sensed by power transducer  14 - 1  and to motor winding temperature sensed by temperature sensor  14 - 2 . Solenoid valve  34  meters the flow of liquid refrigerant to motor  14  via line  24  in order to keep motor  14  in the designed operating temperature range. The liquid refrigerant is metered through an expansion orifice and solenoid valve combination to reduce its saturation temperature, thus changing the refrigerant from the liquid state to a two-phase mixture of liquid and gas in line  24 - 1 . There is no flow of refrigerant to motor  14  via line  24  when valve  34  is closed. The duty cycle of solenoid valve  34  is common to the embodiments of FIGS. 1 through 5 and is described below. 
     In FIG. 2, the numeral  200  designates the basic refrigeration system described above and further includes branch liquid line  224  extending from liquid refrigerant line  23  from a point upstream of expansion device  32 . Line  224  branches into parallel branch liquid lines  224 - 1  and  224 - 2  which recombine into two-phase, liquid and gas, line  224 - 3  which extends into motor  14 . Solenoid valve  234  is in line  224 - 2  and is opened by microprocessor  10  whenever compressor  12  is operating so that there is a constant flow of liquid refrigerant to motor  14  via valve  234 . This constant supply of refrigerant is intended to provide a minimum amount of cooling for all load conditions. Valve  34  is sized to provide additional cooling for higher load conditions. Accordingly, valve  234  is open whenever compressor  12  is operating and valve  34  is duty cycled as described below. 
     In FIG. 3, the numeral  300  designates a refrigeration system that is the same as refrigeration system  100  of FIG. 1 with the addition of flashtank economizer  350  downstream of expansion device  332  and upstream of expansion device  32 . In a flashtank economizer, a portion of the refrigerant is evaporated in passing through expansion device  332  and is supplied via line  324 - 1  and line  24  to motor  14  as saturated flash vapor from the flashtank of economizer  350 . The saturated flash vapor provides a constant supply of refrigerant flow to motor  14 . Additional two-phase refrigerant is supplied to the motor  14 , as required, via line  24  and valve  34  which is duty cycled as described below. 
     In FIG. 4, the numeral  400  designates a refrigeration system that differs from refrigeration system  300  of FIG. 3 in employing a direct expansion economizer rather than a flashtank economizer. In this system, liquid refrigerant line  23  branches into lines  23 - 1  and  23 - 2  which are supplied to economizer  450 . The flow in line  23 - 1  serially passes through economizer  450  where it is further cooled, expansion device  32  and evaporator  20 . The flow in line  23 - 2  serially passes through expansion device  432 , economizer  450  where it changes state and further cools the flow in line  23 - 1 , leaving economizer  450  with some degree of superheat and is supplied to motor  14  via line  24 . This constant supply of gaseous refrigerant from economizer  450  will be supplemented by liquid refrigerant supplied to motor  14 , as required, via valve  34  which is duty cycled as described below. 
     In FIG. 5, the numeral  500  designates a refrigeration system that differs from refrigeration system  400  of FIG. 4 in replacing expansion device  432  with solenoid  534  upstream of economizer  550 . Solenoid  534  is open whenever compressor  12  is operating and is sized to provide the correct amount of cooling flow at the nominal operating condition. Valve  34  is sized to provide the correct amount of flow at the maximum load condition and is duty cycled as described below. 
     In each of refrigeration systems  100  through  500  valve  34  is controlled by microprocessor  10  responsive to motor power consumption sensed by power transducer  14 - 1  and to motor winding temperature sensed by temperature sensor  14 - 2 . Because valve  34  is capable of supplying a sufficient amount of liquid refrigerant for cooling at maximum load, it is duty cycled to control the cooling flow when the cooling requirements are intermediate those of no/minimal and maximum cooling. 
     The duty cycle of the valve  34  is determined primarily by the operating load of the motor  14  sensed by power transducer  14 - 1  and is then corrected based on the motor winding temperature sensed by temperature sensor  14 - 2 . The efficiency of the motor  14  is a specified variable. The motor cooling load can be approximated for any operating condition based on the power draw of the motor  14  sensed by power transducer  14 - 1  and the motor efficiency as shown in (1) 
       Q   motor =(1−η motor )× kW   motor   (1) 
     where: 
     Q motor  is the estimated cooling requirement for the motor 
     η motor  is the motor efficiency 
     kW motor  is the motor power consumption 
     To determine the primary load factor of the duty cycle, the load is then compared to the maximum load condition, and the constant cooling that is provided by either the economizer gas from economizers  350 ,  450 , or  550  or valve  234 , as shown in (2).              LoadFactor   =       (       Q   motor     -     Q     constant                 flow         )       (       Q     max                 load       -     Q     constant                 flow         )               (   2   )                                
     where: 
     Q motor  is the estimated cooling requirement for the motor 
     Q constant flow  is the constant flow cooling that is available 
     Q max load  is the cooling requirement at maximum load 
     Since the embodiment of FIG. 1 has no constant cooling flow, Q constant flow , equation 2 reduces to        LoadFactor   =       (     Q   motor     )       (     Q     max                 load       )                              
     The size (capacity ) of valve  34  is then selected such that the Load Factor=1 at the maximum load condition. Accordingly, in the FIG. 1 embodiment, valve  34  is sized to provide the maximum required cooling flow. For the embodiments of FIGS. 2 to  5 , after the Load Factor is determined according to equation 2, the size (capacity) of valve  34  is then selected such that the Load Factor=1 at the maximum load condition. The Load Factor of the valve  34  then decreases at lower load conditions until the motor cooling requirement no longer exists in the FIG. 1 embodiment or falls below the cooling that is provided by the constant flow of either the economizer gas from economizers  350 ,  450 , or  550  or valve  234  liquid refrigerant. The Load Factor provides the coarse control of the motor winding temperature. 
     The winding temperature is more finely controlled by adjusting the Load Factor according to the actual winding temperature. This correction is intended to extend the duration of the overall duty cycle when the winding temperature is higher than set point and to decrease it when the winding temperature is under set point. The temperature set point is given by the following equation (3): 
     
       
         TemperatureFactor=( T   winding   −T   control point )×Gain  (3) 
       
     
     where: 
     T winding  is the actual motor winding temperature 
     T control point  is the desired winding operating temperature 
     Gain is a factor to modify the sensitivity of this correction 
     The Duty Cycle of the valve is then determined by adding the Load and Temperature Factors, as shown below. The Duty Cycle is limited to the range from zero to one, zero meaning the valve does not open, and one meaning the valve remains open all the time. 
     
       
         Duty cycle=LoadFactor+TemperatureFactor 
       
     
     
       
         
           
             
               
                 
                   DutyCycle 
                   = 
                   
                       
                   
                    
                   
                     
                       [ 
                       
                         
                           ( 
                           
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       η 
                                       motor 
                                     
                                   
                                   ) 
                                 
                                 × 
                                 k 
                                  
                                 
                                     
                                 
                                  
                                 
                                   W 
                                   motor 
                                 
                               
                               ) 
                             
                             - 
                             
                               Q 
                               
                                 constant 
                                  
                                 
                                     
                                 
                                  
                                 flow 
                               
                             
                           
                           ) 
                         
                         
                           ( 
                           
                             
                               Q 
                               
                                 max 
                                  
                                 
                                     
                                 
                                  
                                 load 
                               
                             
                             - 
                             
                               Q 
                               
                                 constant 
                                  
                                 
                                     
                                 
                                  
                                 flow 
                               
                             
                           
                           ) 
                         
                       
                       ] 
                     
                     + 
                   
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     [ 
                     
                       
                         ( 
                         
                           
                             T 
                             winding 
                           
                           - 
                           
                             T 
                             
                               control 
                                
                               
                                   
                               
                                
                               point 
                             
                           
                         
                         ) 
                       
                       × 
                       Gain 
                     
                     ] 
                   
                 
               
             
           
         
                 
         
             
         
      
     
     Although preferred embodiments of the present invention have been illustrated and described, other changes will occur to those skilled in the art. It is therefore intended that the present invention is to be limited only by the scope of the appended claims.