Patent Publication Number: US-RE42200-E

Title: Fault handling of inverter driven PM motor drives

Description:
BACKGROUND 
     The present invention relates to permanent magnetic motor drives. In particular, the invention relates to procedures in such drives for handling faults when detected. 
     Three-phase voltage inverters are commonly employed to the control the magnitude and frequency of the motor phase currents in hybrid vehicles (including electric and fuel cell powered). When the ac motor used is of the class of interior permanent magnet (IPM) type, the reaction of the system to various inverter based faults is of concern since the fault may cause an uncontrolled breaking torque in the motor. 
     Drive system faults can be classified as short-circuit type faults and open-circuit type faults. The behavior of various faults in permanent magnetic motor drives has been reported in the literature for many years. Modeling and system behavior of short-circuit type faults are described in B. A. Welchko, T. M. Jahns, W. L. Soong, and J. M. Nagashima, “IPM synchronous machine drive response to symmetrical and asymmetrical short circuit faults,” IEEE Trans. Energy Conversion, vol. 18, no. 2, pp. 291-298. June 2003. 
     For inverter driven IPM motors, an important class of open-circuit type faults results when the control gate signals to all of the six inverter switches are turned off, or disconnected. During this condition, the motor is connected to the dc source (e.g., battery, fuel cell, etc.) via the antiparallel diodes of the inverter switches. The antiparallel diodes create a potential path for current to flow which is dependant upon the motor operating condition and dc source voltage. The fault condition where the six gate signals have been turned off has been termed an uncontrolled generator mode (UCG mode) of operation since the motor would operate during the condition as a generator converting rotational power into electric currents. Modeling and system behavior during UCG mode operation is characterized in T. M. Jahns and V. Caliskan, “Uncontrolled Generator Operation of Interior PM Synchronous Machines Following High-Speed Inverter Shutdown,” IEEE Trans. Industry Applications, vol. 35, no. 6, pp. 1347-1357, Nov./Dec. 1999. 
     SUMMARY OF THE INVENTION 
     In a method example of the invention, a method for controlling a multi-phase inverter of a PM motor includes detecting a fault, sensing whether a speed signal indicates that a speed of the PM motor is greater than a transition speed, and applying either an open-circuit or a short circuit response. The open-circuit response is applied when the speed of the PM motor is greater  less than the transition speed when the fault is detected. The short-circuit is applied when the speed of the PM motor is less  greater than the transition speed when the fault is detected. The transition speed is either a fixed predetermined speed or an adjusted predetermined speed. 
     In an apparatus example of the invention, the apparatus includes a multi-phase inverter, a fault detector to indicate a detection of a fault, a sensor to provide a speed signal indicative of whether a speed of a PM motor is greater than a transition speed, and a controller. The controller is operable to apply either an open-circuit response or short-circuit response to the multi-phase inverter. The open-circuit response is applied when the speed of the PM motor is greater  less than the transition speed and a fault is detected. The short-circuit response is applied when the speed of the PM motor is less  greater than the transition speed and the fault is detected. The transition speed is either a fixed predetermined speed or an adjusted predetermined speed. 
     In a machine-readable medium example of the invention, the machine-readable medium includes sets of instructions operable in a controller to cause the controller to perform operations. The sets of instructions cause the controller to apply either an open-circuit response or a short-circuit response to a multi-phase inverter. An open-circuit response is applied when a speed signal from a sensor indicates that a speed of a PM motor is greater  less than a transition speed when a fault is detected. A short-circuit response is applied when the speed signal indicates that the speed of the PM motor is less  greater than the transition speed when the fault is detected. The transition speed is either a fixed predetermined speed or an adjusted predetermined speed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be described in detail in the following description of preferred embodiments with reference to the following figures. 
         FIG. 1  is a schematic diagram of a circuit exemplifying the invention. 
         FIG. 2  is a graph depicting the phase current of a PM motor as a function of RPM for both the short-circuit response and the uncontrolled generator response as produced by the circuit of FIG.  1 . 
         FIG. 3  is a graph depicting the torque of a PM motor as a function of RPM for both the short-circuit response and the uncontrolled generator response as produced by the circuit of FIG.  1 . 
         FIG. 4  is a flow chart of an exemplary method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention apply passive fault responses to inverters that differ depending on the rotational speed of the PM motor. After a fault, the residual control capacity of the faulted system is utilized to guide the system into a controlled fault response that globally minimizes the overall fault response characteristic of the system, for example, minimizing the uncontrolled breaking torque in the motor. As a result, the envelope of the faulted system response over the operating space is minimized, and the motor coasts to a stop at a controlled rate. 
     In  FIG. 1 , a circuit for controlling a multi-phase machine that has a stator with plural stator windings is depicted. The circuit includes a multi-phase inverter made from a connection of six commutation switches (Q 1 , D 1 ), (Q 2 , D 2 ), (Q 3 , D 3 ), (Q 4 , D 4 ), (Q 5 , D 5 ) and (Q 6 , D 6 ). The commutation switches are organized in pairs with each pair connected to a respective phase. Phase A is connected to both the first switch (Q 1 , D 1 ) and the fourth switch (Q 4 , D 4 ). Phase B is connected to both the second switch (Q 2 , D 2 ) and the fifth switch (Q 5 , D 5 ). Phase C is connected to both the third switch (Q 3 , D 3 ) and the sixth switch (Q 6 , D 6 ). The ends of the first, second and third switches that are not connected to any phase are connected to the PLUS bus. The ends of the fourth, fifth and sixth switches that are not connected to any phase are connected to the MINUS bus. The six switches are controlled by respective control inputs C 1 , C 2 , C 3 , C 4 , C 5  and C 6 . Phases A, B and C connect to a permanent magnet motor PM. 
     A controller provides electronic signals C 1 , C 2 , C 3 , C 4 , C 5  and C 6  to control respective commutation switches (Q 1 , D 1 ), (Q 2 , D 2 ), (Q 3 , D 3 ), (Q 4 , D 4 ), (Q 5 , D 5 ) and (Q 6 , D 6 ). A speed sensor senses the rotational speed of permanent magnet motor PM and provides a signal characteristic of the rotational speed to the controller. 
     Controllers of the type used to control am multi-phase permanent magnet motor are frequently constructed around a micro-processor or equivalent. Other technologies might be used to mechanize a controller ranging from discrete components to application specific integrated circuits (ASICs) and everything between. Such a controller often has built in fault detection circuitry. Fault detection circuitry detects faults such as “out of limit” conditions, open and short circuit faults or even software faults. Faults may also be detected with separate circuits and reported to the controller as a fault signal. In any case, the fault is detected by a fault detector, whether the detector is external or internal to the controller. 
     For any fault requiring that the motor be shut down in a controlled manner, one of two fault responses are applied: an open-circuit fault response and a short-circuit fault response. The selection of response to be used depends on the rotational speed of the motor. For purposes of example, a transition rotational speed of about 7,000 rpm will be assumed for examples and embodiments discussed herein. 
     When the motor rotates at rotational speeds below the transition speed, the controller mechanizes the open-circuit fault response by forcing all commutation switches (Q 1 , D 1 ), (Q 2 , D 2 ), (Q 3 , D 3 ), (Q 4 , D 4 ), (Q 5 , D 5 ) and (Q 6 , D 6 ) to be off (i.e., open circuit). With such a response, the motor is put into an uncontrolled generator mode (UCG mode); however, the motor generates no current (as depicted in  FIG. 2 ) and produces no breaking torque (as depicted in  FIG. 3 ) when the motor rotates at rotational speeds below the transition speed. On the other hand, if the motor were rotating at rotational speeds above the transition speed, the UCG mode generates increasingly more current (as depicted in  FIG. 2 ) with increasing speed and produces increasingly more breaking torque (as depicted in  FIG. 3 ) with increasing speed. Therefore, the controller avoids mechanizing the open-circuit fault response at motor rotation speeds above the transition speed. Instead, a short-circuit response is mechanized. 
     A short-circuit response is mechanized by forcing all commutation switches on the PLUS bus (Q 1 , D 1 ), (Q 2 , D 2 )), and (Q 3 , D 3 ) to be on (short circuit) while all commutation switches on the MINUS bus (Q 4 , D 4 ), (Q 5 , D 5 ) and (Q 6 , D 6 ) are forced to be off (open circuit). Alternatively, the short-circuit response may be mechanized by forcing all commutation switches on the PLUS bus (Q 1 , D 1 ), (Q 2 , D 2 ) and (Q 3 , D 3 ) to be off (open circuit) while all commutation switches on the MINUS bus (Q 4 , D 4 ), (Q 5 , D 5 ) and (Q 6 , D 6 ) are forced to be on (short circuit). In either case, at motor rotation speeds below the transition speed, the short-circuit response generates increasing more current from the motor (as depicted in  FIG. 2 ) as speed is increased from zero rpm and initially produces increasingly more breaking torque at low rpm but diminishing torque as the rpm approaches the transition speed (as depicted in FIG.  3 ). 
     In an embodiment of a method, the method for controlling a multi-phase inverter of a PM motor being depicted in  FIG. 4 , the method includes detecting a fault and sensing whether a speed signal indicates that a speed of the PM motor is greater than a transition speed. The method further includes applying an open-circuit response when the speed signal indicates that the speed of the PM motor is greater  less than the transition speed when the fault is detected, and applying a short-circuit response is applied when the speed signal indicates that the speed of the PM motor is less  greater than the transition speed when the fault is detected. The transition speed is either a fixed predetermined speed or an adjusted predetermined speed. 
     In a first variant of the method embodiment, the applying the open-circuit response includes controlling all switches in the multi-phase inverter drive to be open. 
     In a second variant of the method embodiment, the applying the short-circuit response includes controlling selected switches in the multi-phase inverter drive to connect all phases of the multi-phase inverter to a single bus and controlling all other switches in the multi-phase inverter drive to be open. 
     In a third variant of the method embodiment, the transition speed is the fixed predetermined speed and is defined based on parameters characteristic of the PM motor. 
     In a fourth variant of the method embodiment, the transition speed is the adjusted predetermined speed and is defined based on parameters characteristic of the PM motor adjusted according to either a temperature of the PM motor, or a voltage of a voltage source, or both. 
     In an embodiment of an apparatus, the apparatus includes am  a multi-phase inverter, a fault detector to indicate a detection of a fault, a sensor to provide a speed signal indicative of whether a speed of a PM motor is greater than a transition speed, and a controller. The controller is operable to apply an open-circuit response to the multi-phase inverter when the speed signal indicates that the speed of the PM motor is greater  less than the transition speed and a fault is detected. The controller is additionally operable to apply a short-circuit response to the multi-phase inverter when the speed signal indicates that the speed of the PM motor is less  greater than the transition speed and the fault is detected. The transition speed is either a fixed predetermined speed or an adjusted predetermined speed. 
     In a first variant of the apparatus embodiment, the application of the open-circuit response by the controller controls all switches in the multi-phase inverter to be open. 
     In a second variant of the apparatus embodiment, the application of the short-circuit response by the controller controls selected switches in the multi-phase inverter to connect all phases of the multi-phase inverter to a single bus and controls all other switches in the multi-phase inverter to be open. 
     In a third variant of the apparatus embodiment, the transition speed is the fixed predetermined speed and is defined based on parameters characteristic of the PM motor. 
     In a fourth variant of the apparatus embodiment, the transition speed is the adjusted predetermined speed and is defined based on parameters characteristic of the PM motor adjusted according to at least one of a temperature of the PM motor and a voltage of a voltage source. 
     In an embodiment of a machine-readable medium, the machine-readable medium includes sets of instructions operable in a controller to cause the controller to perform operations. The sets of instructions are operable to cause the controller to apply an open-circuit response to a multi-phase inverter when a speed signal from a sensor indicates that a speed of a PM motor is greater  less than a transition speed when the fault is detected. The sets of instructions are further operable to cause the controller to apply a short-circuit response to the multi-phase inverter when the speed signal indicates that the speed of the PM motor is less  greater than the transition speed when the fault is detected. The transition speed is either a fixed predetermined speed or an adjusted predetermined speed. 
     In a first variant of the machine-readable medium, the operator of applying the open-circuit response includes controlling all switches in the multi-phase inverter drive to be open. 
     In a second variant of the machine-readable medium, the operation of applying the short-circuit response includes controlling selected switches in the multi-phase inverter drive to connect all phases of the multi-phase inverter to a single bus and controlling all other switches in the multi-phase inverter drive to be open. 
     In a third variant of the machine-readable medium, the transition speed is the fixed predetermined speed and is defined based on parameters characteristic of the PM motor. 
     In a fourth variant of the machine-readable medium, the transition speed is the adjusted predetermined speed and is defined based on parameters characteristic of the PM motor adjusted according to at least one of a temperature of the PM motor and a voltage of a voltage source. 
     Having described preferred embodiments of a novel method, apparatus and media for handling faults of inverter driven PM motor drives (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope of the invention as defined by the appended claims. 
     Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.