Abstract:
A system and method detects coil-to-coil faults in an electric motor having a plurality of coils. The method includes applying a square wave signal to a first coil of the motor, connecting a capacitor and a resistor between ground potential and a second coil of the motor, applying the coil output voltage to an amplifying and peak detecting circuit, applying an output of the amplifying and peak detecting circuit to the input of the control unit and, after a delay period, generating a motor fault signal if the coil output voltage is below a predetermined threshold. The square wave signal is amplified by a circuit which prevents transmission of voltage signals from the coils to the output of the control unit during normal operation of the motor.

Description:
BACKGROUND  
       [0001]     The present invention relates to a system and method for detecting faults in an electric motor.  
         [0002]     Small, low cost electric motors and controllers are used in certain product applications, such as electric reel drives on greens mowers which includes a 3-phase permanent magnet brushless DC electric motor. The motor controller typically includes high power FETs which can be damaged by overheating or overcurrents. It is difficult to detect when such a motor fails, and when a system with such a failed motor is not working, a repair technician may attempt to repair the system by replacing the controller instead of the faulty motor. But, if the motor was faulty, it may merely damage the new controller. Thus, it is desired to have a means for detecting motor faults before the faulty motor can damage a controller.  
         [0003]     Typically, systems for detecting faults in such motors have been complicated and have required expensive circuitry. U.S. Pat. No. 6,381,110, issued to Nagashima et al. in 2002, describes a system for detecting isolation or coil-to-case faults of a motor. However, this system cannot detect inductance degradation or phase coil-to-phase coil short circuit faults.  
       SUMMARY  
       [0004]     Accordingly, an object of this invention is to provide a simple and inexpensive system for detecting faults in an electric motor.  
         [0005]     Another object of this invention is to provide a system for detecting phase coil-to-phase coil faults in an electric motor.  
         [0006]     Another object of this invention is to provide a system for detecting faults in an electric motor before the faulty motor can damage a controller.  
         [0007]     These and other objects are achieved by the present invention, wherein a system and method, upon start-up, detects coil-to-coil faults in an electric motor having a plurality of coils. The method includes applying a square wave signal to a first coil of the motor, connecting a second coil to ground, connecting a capacitor and a resistor between ground potential and a third coil of the motor, applying the third coil output voltage to an amplifying and peak detecting circuit, applying an output of the amplifying and peak detecting circuit to the input of the control unit and, after a delay period, generating a motor fault signal if the output of the amplifying and peak detecting circuit is below a predetermined threshold. The square wave signal is amplified by a circuit which prevents transmission of voltage signals from the coils to the output of the control unit during normal operation of the motor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic diagram of a motor fault detection system according to the present invention;  
         [0009]      FIG. 2  is logic flow diagram illustrating an algorithm executed by the ECU of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0010]     Referring to  FIG. 1 , the motor fault detection system  10  is connected to a conventional 3-phase brushless DC motor  12  which has phase coils  14 ,  16  and  18 . The motor fault detection system  10  includes a microprocessor-based electronic control unit (ECU)  20  which supplies a square wave excitation signal to a driver/current amplifier  22 .  
         [0011]     Driver/amplifier  22  includes resistors R 6  and R 7 , each connected between the output of ECU  20  and a base of a transistor Q 1  and Q 2 , respectively. The emitter of transistor Q 1  is connected to a supply voltage, Vcc, such as +5 volts. The collector of Q 1  is connected to the base of transistor Q 3  and to the collector of transistor Q 2  via resistor R 5 . The collector of transistor Q 2  is also connected to the base of transistor Q 4 . The emitters of Q 2  and Q 4  are connected to ground. The emitter of transistor Q 3  is connected to a supply voltage, such as +5 volts. The collector of Q 3  is connected to the anode of diode D 2 . The cathode of diode D 2  is connected to the collector of transistor Q 4 . Finally, the collector of transistor Q 4  is connected to phase coil  14  of motor  12 . Driver/amplifier  22  prevents transmission of voltage signals from the motor  12  to the control unit  20  during normal operation of motor  12 .  
         [0012]     A novel feature of the design of the driver/amplifier  22  is the inherent prevention of the “shoot-through” condition, where Q 3  and Q 4  are both conducting causing large currents and damage to Q 3  and Q 4 . This “shoot-through” condition can typically occur during the transition between driving a 0 volt output and a 5 volt output. When the ECU  20  applies a 0 volt input, transistor Q 1  is designed to be in saturation and transistor Q 2  is off. As the ECU  20  increases the input voltage to the driver  22  from 0 volts to 5 volts, transistor Q 2  is designed to be in saturation while Q 1  remains in saturation. The current through the collectors of transistors Q 1  and Q 2  is limited by R 5  so no damage occurs. Note that when both transistors Q 1  and Q 2  are in saturation, transistors Q 3  and Q 4  will be turned off, thereby adding “dead-time” where neither transistor is on and eliminating the shoot-through condition. As the ECU  20  increases the input voltage to driver  22  to 5 volts, transistor Q 1  turns off, allowing transistor Q 2  to draw current out of the base of transistor Q 3 , putting it into saturation.  
         [0013]     The other end of coil  14  is connected to an end of coils  16  and  18  in a Y connected motor. The other end of coil  18  is connected to ground via normally open switch  30  which includes transistor Q 6  which is preferably part of the inverter (not shown) which supplies electrical power to the coils  14 - 18 . Switch  30  is opened and closed by a signal provided from an output of ECU  20 .  
         [0014]     The other end of coil  16  is connected to the collector of normally off or open transistor switch Q 5  via parallel connected capacitor C 1  and resistor R 4 . The emitter of transistor Q 5  is connected to ground and the base of transistor Q 5  is connected to a control output of ECU  20  through a resister R 8 . This other end of coil  16  is also connected to sense peak detector/amplifier circuit  24 .  
         [0015]     Circuit  24  includes a operational amplifier  26  with a + input connected to coil  16  (and resistor R 4  and capacitor C 1 ) and a − input connected to ground via resistor R 2 . The output of op amp  26  is connected to the anode of diode D 2 . The cathode of diode D 2  is connected the − input of op amp  26  via resistor R 1 , to an input of ECU  20  via resistor R 3  and to ground via resistor R 3  and capacitor C 2 . Circuit  24  amplifies the signal from coil  16  and generates a slowly varying DC output signal, the voltage of which varies as the peak voltage of the signal from coil  16  varies. This output signal is applied to an input of the ECU  20 .  
         [0016]     A microprocessor (not shown) of ECU  20 , executes an algorithm  100  represented by  FIG. 2 . The conversion of the above flow chart into a standard language for implementing the algorithm described by the flow chart in a digital computer or microprocessor, will be evident to one with ordinary skill in the art.  
         [0017]     Step  102 , in response to a startup signal, turns on transistor switches Q 5  and Q 6 , thereby connecting the output end of coil  16  to ground via resistor R 4  and capacitor C 1 , and connecting an end of coil  18  to ground via Q 6 .  
         [0018]     In step  104  the ECU  20  applies a square wave signal to the input of driver/amplifier  22 . Preferably, the square wave has a 7 kHz frequency, a 50% duty cycle, a duration of about 10 milliseconds and voltage levels of zero and 5 volts. The optimum frequency of the square wave excitation signal may be determined based on an empirical and statistical analysis of many motors to characterize their inductance and resistance. These along with the resistor R 4  and capacitor C 1  determine the resonant frequency of an inductor-capacitor-resistor circuit. Preferably, the frequency of the square wave signal is chosen to be the same as the resonant frequency of this circuit so that a maximum output signal will be generated by a motor with good phase-to-phase inductance. It should be understood that the current amplitude from the driver circuit  22  also affects the perceived inductance in the motor  12  (because of all the magnets, ferrous materials and magnetic field interactions), so testing is preferably done with the circuit planned to be used in the final product.  
         [0019]     This square wave signal is amplified by amplifier  22  and is applied to coil  14  of motor  12 . Amplifier  22  preferably generates a drive signal with a maximum drive current of 50 milliamps RMS and a drive voltage of between a maximum of 4 volts and a minimum drive of 0 volts. These values are application specific and can vary based on the winding inductance and resistance of the motor.  
         [0020]     In response to this drive signal applied to coil  14 , coil  16  generates a substantially sinusoidal low voltage signal which is applied to the + input of op amp  26 . Meanwhile, step  106  causes the system to pause for a time period (such as about 1 millisecond) sufficient for the response to the drive signal to stabilize the voltage across capacitor C 2 .  
         [0021]     In step  108  the ECU  20  compares the output of amplifier  24  to a threshold voltage, such as 0.5 volts, and if the output of amplifier  24  is greater than the threshold voltage, step  108  directs the algorithm to step  110  which turns off transistors Q 5  and Q 6 , and then to step  112  which ends the test algorithm  100  and allows the motor  12  to be operated normally. If the output of amplifier  24  is not greater than the threshold voltage, then step  108  directs control to step  114  which turns off transistors Q 5  and Q 6 . Then step  116  generates a fault indication signal, such as an audible signal from a speaker (not shown) and/or visible signal from a display device (not shown).  
         [0022]     Finally, step  118  prevents further operation of the motor  12 . Preferably, steps  102 - 118  would be applied to each of the motor coils  14 - 18 , one after the other by re-arranging the connections between the driver  22 , the peak detector  24  including resister R 4  and capacitor C 1 , the switch  30  and the motor  12 .  
         [0023]     The result is a simple low cost system and method which detects phase inductance and phase-to-phase faults prior to normal operation of the motor.  
         [0024]     While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. For example, the same circuitry could be connected to additional motor coils and the same algorithm executed. Also, a number of square wave signals having different frequencies could be applied and the resulting frequency response could be analyzed. Or, the frequency of the square wave could be varied gradually and the output monitored for resonant peaks. The circuit configuration could easily be adapted to the delta connected motor. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.  
       Assignment  
       [0025]     The entire right, title and interest in and to this application and all subject matter disclosed and/or claimed therein, including any and all divisions, continuations, reissues, etc., thereof are, effective as of the date of execution of this application, assigned, transferred, sold and set over by the applicant(s) named herein to Deere &amp; Company, a Delaware corporation having offices at Moline, Ill. 61265, U.S.A., together with all rights to file, and to claim priorities in connection with, corresponding patent applications in any and all foreign countries in the name of Deere &amp; Company or otherwise.