Abstract:
An electrical machine drive system ( 100 ) includes a DC power source ( 102 ), an inverter ( 104 ) and a three-phase electrical machine ( 106 ). In response to a fault condition, power switches ( 122, 124 ) in the inverter are opened. Any short circuits are then determined by examining the phase currents of the three-phase electrical machine. If short circuits are found in the inverter, then the inverter is made balanced by shorting all upper braches ( 122, 126 ) or all lower branches ( 124, 128 ), depending on the location of the short circuit. Torque ripples are avoided by balancing the circuit during the fault mode, thereby providing comfort to a user of an electric or hybrid electric vehicle employing the electrical machine drive system.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electrical machine drive controls, and specifically, to a method and system for mitigating fault modes in electrical machine drives for hybrid electric vehicles, electric vehicles and other systems. 
     2. Discussion of the Prior Art 
     The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Also, hybrid electric vehicles (HEV), which combine a smaller ICE with electric motors into one vehicle, attempt to address these needs. 
     Understanding electrical drive operation is critical in the design of electric vehicles and hybrid electric vehicles. In particular, where permanent magnet electric motors are employed, diagnosis and mitigation of fault modes are critical. This is due in part to the continuous existence of permanent magnet flux, which may produce pulsation torque, over voltage or over current conditions during certain fault modes. 
     An electrical machine drive system includes a DC power source, an inverter, and an electrical machine. The DC power source is coupled to, and provides power to, the inverter. The electrical machine is coupled to the inverter, and is driven by the inverter. The electrical machine typically includes a first phase, a second phase and a third phase. The inverter to drive this electric machine includes a first stage, a second stage, and a third stage. The first stage of the inverter is coupled to the first phase of the electrical machine; the second stage of the inverter is coupled to the second phase of the electrical machine; and the third stage of the inverter is coupled to the third phase of the electrical machine. Each stage of the inverter has an upper power switch and a lower power switch. An upper diode is coupled across the upper power switch and a lower diode is coupled across the lower power switch. 
     Certain faults in the phases or stages of the electrical machine or inverter may result in undesirable characteristics. The faults may include open power switches, open diodes, shorted power switches, or shorted diodes, which may degrade system performance. For example, asymmetric operation of the electrical machine drive system due to an open or short may cause torque ripples, which may result in an undesirable ride, in the case of an electric or hybrid electric vehicle. 
     Therefore, a need exists for a detection and mitigation strategy for certain fault modes in electrical machine drive systems. 
     SUMMARY OF INVENTION 
     Accordingly, an object of the present invention is to provide a method and system for mitigating effects of a fault in an electrical machine drive system of an electric or hybrid electric vehicle. 
     Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures. 
     In accordance with one aspect of the invention, a method is provided for fault mitigation in an electrical machine drive system. The electrical machine drive system includes a three-phase electrical machine, and an inverter coupled to the three-phase electrical machine. The inverter includes a first stage, a second stage, and a third stage. The first stage is coupled to a first phase of the three-phase electrical machine; the second stage is coupled to a second phase of the three-phase electrical machine; and the third stage is coupled to a third phase of the three-phase electrical machine. Each of the first stage, the second stage and the third stage includes an upper branch and a lower branch. The upper branch includes an upper power switch with an upper diode coupled across the upper power switch and the lower branch includes a lower power switch with a lower diode coupled across the lower power switch. The fault mitigation method includes the steps of: (1) opening the upper power switch and the lower power switch of each of the first stage, the second stage and the third stage of the inverter; (2) determining whether a diode or a power switch in the inverter is shorted; and (3) if a diode or a power switch in one of the first stage, the second stage, and the third stage is shorted, then commanding some power switches to close such that each phase current of the first phase, the second phase and the third phase is balanced. Ensuring balance requires insuring all lower power switches in the inverter are closed, if a lower diode or lower power switch has shorted, or ensuring all upper power switches in the inverter are closed, if an upper diode or upper power switch is shorted. 
     A further aspect of the present invention includes a system for fault mitigation. In addition to the electrical drive machine system described above, the system includes a control circuit and current sensors. The control circuit controls the power switches and receives inputs from the current sensors reflecting the phase currents. The control circuit implements the method steps given above. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing objects, advantages, and features, as well as other objects and advantages, will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which: 
     FIG. 1 is a schematic diagram illustrating an electrical machine drive system in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a flow diagram illustrating a method for fault mitigation in accordance with a preferred embodiment of the present invention. 
     FIGS. 3A-C are curves illustrating phase currents for a certain fault mode. 
     FIGS. 4A-C are curves illustrating phase currents for another fault mode. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram showing an electrical machine drive system  100  in accordance with a preferred embodiment of the present invention. Electrical machine drive system  100  includes a DC (direct current) power source  102 , a control circuit  103 , an inverter  104 , and an electrical machine  106 . In a preferred embodiment, electrical machine drive system  100  is a drive component for a hybrid electric or electric vehicle. Most preferably, electrical machine drive system  100  is mechanically coupled through gears to drive the wheels of a hybrid electric or electric vehicle in any manner known in the art. As shown in FIG. 1, DC power source  102  is coupled to inverter  104  to provide a source of DC power to inverter  104 . The inverter  104  is coupled to three-phase electrical machine  106  to drive electrical machine  106 . 
     In the preferred embodiment, DC power source  102  includes a battery. In alternate embodiments the DC power source is a fuel cell or another electric machine, etc. Electrical machine  106  includes a first phase  108 , a second phase  110 , and a third phase  112 . The three phases are coupled together at a node  113 . As an alternative to coupling the three phases at a node, the phases may be coupled by a Delta connection or any other suitable connection. 
     In the preferred embodiment, inverter  104  includes three stages that drive the three phases of electrical machine  106 . More specifically, first stage  116  drives first phase  108 ; second stage  118  drives second phase  110 ; and third stage  120  drives third phase  112 . Current sensors  114   a ,  114   b , and  114   c  are coupled to sense the currents in first phase  108 , second phase  110 , and third phase  112 , respectively. 
     Each of the stages  116 ,  118 ,  120  of inverter  104  includes a pair of power switches arranged in a push-pull configuration. More specifically, each stage has an upper power switch  122  and a lower power switch  124 . Upper power switch  122  has its emitter coupled to the collector of lower power switch  124 . The power switches shown in FIG. 1 are bipolar transistors. However, the invention is not so limited, and the power switches may be any suitable power switches, including MOSFET, or other semiconductor power switches. The substitution of bipolar power switches with other switches is known to those of skill in the art. 
     The power switches in FIG. 1 are labeled with an alphabet in addition to the reference numeral to reflect whether the switch is associated with the first stage (a), second stage (b) or third stage (c). For example, upper power switch  122   a  is associated with the first stage and lower power switch  124   b  is associated with the second stage. When the power switches are referred to generally, that is, without specificity to a particular stage, the alphabet is not used. Such convention shall be followed throughout this specification, including with respect to other components. 
     Each upper power switch  122  has an upper diode  126  coupled across it to form an upper branch. More specifically, each upper diode  126  is coupled at one end to the emitter of an upper power switch  122  and at the other end to the collector of an upper power switch  122 . Similarly, each lower power switch  124  has a lower diode  128  coupled across it to form a lower branch. More specifically, each lower diode  128  is coupled at one end to the emitter of a lower power switch  124  and at the other end to the collector of a lower power switch  124 . 
     Control circuit  103  controls the upper power switches  122  and the lower power switches  124 . More specifically, control circuit  103  drives the bases of upper power switches  122  and lower power switches  124  to turn the switches on and off. Control circuit  103  also receives the outputs of current sensors  114 . Control circuit  103  preferably includes a microprocessor, micro-controller, digital signal processor or the like, for performing the functions specified herein. Alternatively, control circuit  103  is hardwired logic and other circuitry for performing the functions specified herein. As discussed further below, in accordance with the invention, fault conditions are sensed and control circuit  103  operates to mitigate the fault by configuring inverter  104 . 
     FIG. 2 is a flow chart illustrating a method for fault mitigation in accordance with the present invention. Also, the operational aspects of the preferred embodiment are described below with respect to FIG.  2 . 
     The fault mitigation strategy begins with the detection of a particular fault condition ( 200 ). According to the invention, the fault condition is typically a condition of over voltage or over current. The fault condition is directly sensed by current sensors  114  or through other sensing circuits in the system or associated with the vehicle. 
     In response to the fault condition, in particular an over voltage or over current condition, a first mitigation approach is to open, i.e., turn off, the upper power switches  122  and lower power switches  124  of inverter  104  ( 202 ). When the upper power switches  122  and lower power switches  124  are opened, the power switches approximate open circuits. 
     Unfortunately, opening the upper power switches  122  and lower power switches  124  alone, may not be sufficient to mitigate the fault condition. In particular, certain simulations and studies have shown that a short may persist in the inverter after all the power switches are open. Therefore, after the power switches are opened ( 202 ), a determination is made as to whether there are short-circuited switches or diodes in the inverter  104 , that is, whether an upper branch or lower branch is shorted ( 204 ). In the preferred embodiment, this determination is generally made through the examination of the average phase currents of each phase of the electrical machine. Most preferably, control circuit  103  determines an approximate location of a short circuit through examination of the average phase currents provided to the control circuit  103  by current sensors  114 . More specifically, if the average phase current for a particular phase is greater than zero, and the average phase currents of the other two phases are each less than or equal to zero, then the upper power switch  122  or upper diode  126  (upper branch) of that particular phase is likely shorted. For example, if the second phase  110  has an average phase current that is greater than zero and the first phase  108  and third phase  112  each have average phase currents that are less than or equal to zero, then the second phase likely has a shorted upper diode  126   b . Or, the upper power switch  122   b  is actually shorted. On the other hand, if the average phase current for a particular phase is less than zero and the average phase currents of the other two phases are each greater than or equal to zero, then the lower diode  128  or lower power switch  124  (lower branch) of that particular phase is likely shorted. For example, if the third phase  112  has an average phase current that is less than zero and the first and second phase each have average phase currents that are greater than or equal to zero, then lower diode  128 c is likely shorted. Or, lower power switch  124   c  is shorted. 
     In any event, if a short is found ( 204 ), then a mitigation strategy is implemented in accordance with the present invention ( 206 ). If no short is detected ( 204 ), then the strategy of opening all power switches is employed. The mitigation strategy employed according to the invention where a short is detected requires an attempt to balance the inverter in light of the location of the short. In particular, the inverter is balanced if all lower diodes or lower power switches (lower branches) of all stages are shorted or if all upper diodes or upper power switches (upper branches) of all stages are shorted. Therefore, if a determination is made that an upper diode or upper power switch (upper branch) is shorted in a particular stage, as discussed above, then the upper diodes are shorted in the remaining stages or in all stages. That is, if the first stage has an upper diode or upper power switch short, then the control circuit shorts the remaining stages or all stages by turning on the upper power switches of the remaining stages or all stages. Similarly, if a determination is made that a lower diode or lower power switch (lower branch) is shorted in a particular stage, as discussed above, then the lower switches of the remaining stages or all stages are shorted. For example, if the second stage has a lower diode short, the control circuit shorts, i.e., closes, the lower switches of the first and third stages or shorts the lower switches of the first, second and third stages. 
     FIGS. 3A-C illustrate phase currents for a fault condition where all power switches are open and an upper diode or upper power switch (upper branch) is shorted. In particular, FIGS. 3A-C show that the curve of the first phase current  302  is on average greater than zero; and the curve of the second phase current  304  and the curve of the third phase current  306  are on average less than or equal to zero. This corresponds to a fault condition where all power switches are open and the upper branch of the third stage of the inverter is shorted. 
     FIGS. 4A-C illustrate phase currents for a fault condition where all power switches are open and a lower diode or a lower power switch (lower branch) is shorted. In particular, FIGS. 4A-C show that the curve of first phase current  402  is on average less than zero; and the curve of the second phase current  404  and the curve of the third phase current  406  are on average greater than or equal to zero. This corresponds to a fault condition where all power switches are open and the lower branch of the third stage of the inverter is shorted. 
     As described above, a fault mitigation strategy includes detecting a fault condition and opening power switches in an inverter in response to the fault condition. Then the presence of a short is determined and the circuit is made balanced based on the location of the short. Advantageously, a fault condition is mitigated to provide more comfort to a user of a vehicle experiencing the fault. 
     The above-described embodiments of the invention are provided purely for purposes of example. Many other variations, modifications, and applications of the invention may be made.