Abstract:
A motor control circuit includes a processor configured to calculate a plurality of motor impedances from measurements of an excitation voltage on a power bus to a motor and measurements of a plurality of currents through the motor resulting from the excitation voltage, and the processor configured to calculate individual winding inductances in the motor, based on the measured motor impedances, and configured to determine whether there is an inter-turn winding fault based on the calculated individual winding inductances.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This continuation application claims priority to U.S. patent application Ser. No. 14/309,735, filed Jun. 19, 2014, which application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Inverter-driven motors are widely used in many industrial applications. The insulation on the stator windings of these motors is subject to deterioration due to high transient voltages, inverter cable reflections, mechanical stress, high temperatures, and aging. As a result, the insulation can fail, resulting in inter-turn winding faults (shorts from one part of a winding to another part of the winding). These inter-turn faults reduce efficiency and may cause excess heating, which may eventually cascade to irreversible damage such as melted conductors and motor failure. Given the cost of an interruption of critical systems being driven by these motors, it is important to detect inter-turn winding faults before irreversible damage occurs. Typically, detection of inter-turn winding faults involves monitoring of the motor currents and voltages and using a processor to analyze any asymmetries of currents through the motor, or to analyze differences in motor currents between a healthy motor and one with winding faults. In some embodiments, the inverter switches are used to inject voltages at a frequency higher than the normal switching frequency, and the resulting currents are analyzed by a processor. Analysis of motor currents requires significant processor resources, and using the inverter&#39;s switches to inject higher frequency voltages can result in audible noise. There is an ongoing need for improved detection of motor faults. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a block diagram schematic of an example embodiment of a motor driver circuit. 
           [0004]      FIG. 2  is a block diagram schematic of an alternative example embodiment of a motor driver circuit. 
           [0005]      FIG. 3  is a block diagram schematic illustrating additional detail for a motor impedance measurement circuit, which is part of the motor driver circuits of  FIGS. 1 and 2 . 
           [0006]      FIG. 4  is flow chart illustrating a method for detecting a motor winding fault. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]      FIG. 1  illustrates an example embodiment of a motor driver circuit  100  with details omitted to simplify illustration and discussion. In  FIG. 1 , a power supply  102  provides a DC voltage on a power bus (104,105). Electronic switches  106  function as an inverter (transforming DC to AC) to drive a motor  108 . The switches  106  sequentially drive stator windings  110  in the motor  108 . In the example of  FIG. 1 , a chopper circuit  112 , comprising a switch  114  and resistor  116 , generates an excitation voltage V E  on the power bus  105  to the motor  108 . A voltage divider  120  serves as a sensor for monitoring the amplitude of the excitation voltage V E  on the power bus  105 . A resistor  118  serves as a sensor for monitoring current through the motor  108  resulting from the excitation voltage V E . A motor impedance measurement circuit  122  measures the amplitude of the excitation voltage V E  and a plurality of currents resulting from the excitation voltage V E  at a plurality of switch states. A processor  124  computes a plurality of motor impedances from the amplitude of the excitation voltage V E  and the plurality of currents measured by the motor impedance measurement circuit  122 . The processor  124  then computes individual stator winding inductances from the motor impedance calculations. The processor  124  then determines whether one or more of the stator windings  110  has an inter-turn winding fault, based on the computed stator winding inductances. 
         [0008]    The chopper circuit  112  causes a bus capacitor C B  to linearly charge and discharge at the frequency of the chopper circuit  112 . Preferably, the chopper circuit  112  generates an excitation voltage with a frequency that is substantially higher than human audio perception and at least ten times higher than the switching rate of the switches  106 . Alternatively, if the switches are pulse width modulated (PWM), then the frequency of the excitation voltage is preferably at least ten times higher than the PWM carrier signal frequency, which may be on the order of 10 KHz. For example, the frequency of the excitation voltage may be on the order of 100 KHz. 
         [0009]    High frequency harmonics in a square wave excitation voltage may result in significant current in the bus capacitor C B .  FIG. 2  illustrates an alternative example motor drive circuit  200 , which reduces the current in the bus capacitor C B  and improves the current measurement sensitivity. In  FIGS. 1 and 2 , elements having the same reference numbers are identical. In the example of  FIG. 2 , a square wave signal V S  is coupled to the power bus  105  through a series L-C circuit  202 . The series L-C circuit  202  functions as a band pass filter, resulting in a sinusoidal excitation voltage V E  on the power bus  105 . A sinusoidal excitation eliminates high frequency harmonics, which reduces the current through the bus capacitor C B . In  FIG. 2 , a resonant R-L-C circuit  204  serves as a sensor for monitoring current through the motor  108 . Circuit  204  has a resonant frequency that is the same as the frequency of the excitation voltage, which improves current measurement sensitivity while presenting a much lower impedance at the switching frequency or PWM carrier frequency of the switches  106 . 
         [0010]    At the frequency of the excitation voltage, the motor impedance is effectively an inductance. As the motor rotates, the motor impedance varies, and the variation of motor impedance modulates both the amplitude and phase of the current resulting from the excitation voltage. In  FIGS. 1 and 2 , the individual stator windings  110  have inductances labeled L A , L B , and L C . Switches  106  are also designated by A, B, and C, with the top switches having a subscript of “1” and the bottom switches having a subscript of “2”. During a first switch state, inductance L A  is switched to the bus voltage (switch A 1  is closed) and inductances L B  and L C  are switched to the bus return (switches B 2 , and C 2  are closed) (switches A 2 , B 1 , and C 1  are open). During the first switch state the motor impedance seen by the excitation voltage V E  is L A +(L B  in parallel with L C ). During a second switch state, inductance L B  is switched to the bus voltage (switch B 1  is closed) and inductances L A  and L C  are switched to the bus return (switches A 2  and C 2  are closed) (switches A 1 , B 2 , and C 1  are open). During the second switch state, the motor impedance seen by the excitation voltage V E  is L B +(L A  in parallel with L C ). During a third switch state, inductance L C  is switched to the bus voltage (switch C 1  is closed) and inductances L A  and L B  are switched to the bus return (switches A 2  and B 2  are closed) (switches A 1 , B 1 , and C 2  are open). During the third switch state the motor impedance seen by the excitation voltage V E  is L C +(L A  in parallel with L B ). 
         [0011]      FIG. 3  illustrates an example embodiment of the motor impedance measurement circuit  122 . The motor impedance measurement circuit  122  measures motor current at the frequency of the excitation voltage V E . As discussed above, the excitation current sensed by the motor impedance measurement circuit  122  is modulated in amplitude and phase. For current measurement, the motor impedance measurement circuit  122  comprises a synchronous demodulator comprising a differential amplifier  302  followed by two synchronous detectors ( 304 ,  306 ). Synchronous detector  304  comprises a multiplier  308  that multiplies the amplified current signal by a signal  310  having the same frequency and phase as the excitation voltage V E . The multiplier  308  is followed by a low pass filter (or integrator)  312 , optionally a logarithmic amplifier  314  (to optionally improve sensitivity), and an analog-to-digital converter (A/D)  316 . Synchronous detector  306  comprises a multiplier  318  that multiplies the amplified current signal by a signal  320  having the same frequency as the excitation voltage V E  but having a phase that is ninety degrees offset from the phase of the excitation voltage V E . The multiplier  318  is followed by a low pass filter (or integrator)  322 , optionally a logarithmic amplifier  324 , and an A/D  326 . The digital output of A/D  316  is the real part “a” of a complex number, and the digital output of A/D  326  is the imaginary part “b” of the complex number. During each switch state, the processor  122  receives the outputs of the A/D&#39;s  316  and  326  and computes the magnitude and phase of the motor current, as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     1 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           a 
                           1 
                           2 
                         
                         + 
                         
                           b 
                           1 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
             
               
                 
                   
                     I 
                     2 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           a 
                           2 
                           2 
                         
                         + 
                         
                           b 
                           2 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
             
               
                 
                   
                     I 
                     3 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           a 
                           3 
                           2 
                         
                         + 
                         
                           b 
                           3 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
             
               
                 
                   
                     θ 
                     1 
                   
                   = 
                   
                     tan 
                      
                     
                       ( 
                       
                         
                           b 
                           1 
                         
                         
                           a 
                           1 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   4 
                 
               
             
             
               
                 
                   
                     θ 
                     2 
                   
                   = 
                   
                     tan 
                      
                     
                       ( 
                       
                         
                           b 
                           2 
                         
                         
                           a 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   5 
                 
               
             
             
               
                 
                   
                     θ 
                     3 
                   
                   = 
                   
                     tan 
                      
                     
                       ( 
                       
                         
                           b 
                           3 
                         
                         
                           a 
                           3 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   6 
                 
               
             
           
         
       
     
         [0012]    In the above equations, for example, a 1  is the output of A/D  316  during the first switch state and represents the real part of a complex number, and b 1  is the output of A/D  326  during the first switch state and represents the imaginary part of a complex number. I 1  is the computed magnitude of the motor current resulting from the excitation voltage V E  during the first switch state. Θ 1  is the computed phase of the motor current resulting from the excitation voltage V E  during the first switch state. 
         [0013]    As discussed above, the motor impedance circuit  122  also measures the excitation voltage V E . In  FIG. 3 , the output of the voltage divider  120  is amplified by an amplifier  328  and the resulting signal is processed by a synchronous detector  330 . The synchronous detector  330  enables rejection of square wave harmonics and enables measurement of the synchronous frequency component of the excitation voltage V E . The synchronous detector  330  comprises a multiplier  332  that multiplies the amplified voltage signal by a signal  334  having the same frequency and phase as the excitation voltage V E . The multiplier  332  is followed by a low pass filter (or integrator)  336 , and an analog-to-digital converter (A/D)  338 . The processor  124  monitors the digital output of the A/D  338 . The processor  124  then computes motor impedance as the measured excitation voltage V E  divided by the motor current, as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     1 
                   
                   = 
                   
                     
                       V 
                       E 
                     
                     
                       I 
                       1 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   7 
                 
               
             
             
               
                 
                   
                     Z 
                     2 
                   
                   = 
                   
                     
                       V 
                       E 
                     
                     
                       I 
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   8 
                 
               
             
             
               
                 
                   
                     Z 
                     3 
                   
                   = 
                   
                     
                       V 
                       E 
                     
                     
                       I 
                       3 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   9 
                 
               
             
           
         
       
     
         [0014]    The processor  124  computes the motor impedance (Z 1 , Z 2 , Z 3 ) for each switch state. The processor then generates three equations with three unknowns (L A , L B , L C ) as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     1 
                   
                   = 
                   
                     
                       L 
                       A 
                     
                     + 
                     
                       
                         
                           L 
                           B 
                         
                          
                         
                           L 
                           C 
                         
                       
                       
                         
                           L 
                           B 
                         
                         + 
                         
                           L 
                           C 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   10 
                 
               
             
             
               
                 
                   
                     Z 
                     2 
                   
                   = 
                   
                     
                       L 
                       B 
                     
                     + 
                     
                       
                         
                           L 
                           A 
                         
                          
                         
                           L 
                           C 
                         
                       
                       
                         
                           L 
                           A 
                         
                         + 
                         
                           L 
                           C 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   11 
                 
               
             
             
               
                 
                   
                     Z 
                     3 
                   
                   = 
                   
                     
                       L 
                       C 
                     
                     + 
                     
                       
                         
                           L 
                           A 
                         
                          
                         
                           L 
                           B 
                         
                       
                       
                         
                           L 
                           A 
                         
                         + 
                         
                           L 
                           B 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   12 
                 
               
             
           
         
       
     
         [0015]    The processor  124  then solves for the three individual winding inductances (L A , L B , L C ) and the computed winding inductances are analyzed for an inter-turn winding fault. If one of the individual winding inductances (L A , L B , or L C ) is different than the others, or different than an expected value, then that motor winding may have an inter-turn fault. 
         [0016]    The processor  124  can also detect a failure of the bus capacitance C B . An increase in the amplitude of the excitation voltage V E  may indicate a failed bus capacitance C B . That is, if the bus capacitor&#39;s equivalent resistance is increased, then the impedance of the bus capacitor C B  is increased, and the attenuation of the excitation voltage V E  is decreased, and the magnitude of the excitation voltage increases. 
         [0017]      FIG. 4  illustrates a method  400  for detecting a motor-winding fault. At step  402 , a processor computes a plurality of impedances of a motor using a plurality of voltages on a power bus to the motor and a plurality of currents through the motor. At step  404 , the processor computes individual motor winding inductances based on the computed motor impedances. At step  406 , the processor determines whether the motor has an inter-turn winding fault, based on the computed motor winding inductances. 
         [0018]    While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.