Abstract:
An electrical machine drive system is simulated with a circuit simulator to model normal operation, fault modes and control strategies for electric motor based vehicles. The electrical machine drive system includes a DC power source model ( 102 ), an inverter model ( 104 ) and an electrical machine model ( 106 ). The system is modified through substitution of components or removal of components to simulate faults. Post-fault control strategies are implemented in a similar manner and simulated. The model and methods associated therewith reduce modeling complexity and reduce simulation time, permitting thorough design and analysis of electric motor based vehicles.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electrical machine drive systems, and specifically, to a method and apparatus simulating fault modes and operational techniques for 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 fault affects 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. 
     Known methods for evaluating fault modes and operational techniques include modeling electrical drive system fault modes with a set of complex differential equations; simulating the system to determine the effects of the fault; selecting a post fault control strategy; modeling the post fault control strategy with a different set of complex differential equations; and simulating the system to determine the effects of the post fault control strategy. Welchko, Brian A. et al., “IPM Synchronous Machine Drive Response to a Single-Phase Open Circuit Fault,”IEEE Applied Power Electronics Conference (APEC), Mar 4-8, 2001, Paper Number 13A. 1, pp. 1-7, and Welchko, Brian A. et al., “IPM Synchronous Machine Drive Response to Symmetric and Asymmetric Short Circuit Faults,”EPE 2001—Graz, pp. 1-10, are exemplary of the conventional modeling and simulation techniques. Unfortunately, these known methods require complex differential equations, which must be revised for each model and post fault control strategy. In many fault modes the differential equations cannot even be derived. In addition, these methods require long simulation time. 
     Therefore, a need exists for a simple method and apparatus for simulating fault modes in electrical machine drive systems. 
     SUMMARY OF INVENTION 
     Accordingly, an object of the present invention is to provide an apparatus and method that reduces the complexity of modeling operational modes for electrical machine drive systems in electric or hybrid electric vehicles. 
     Another object of the present invention is to reduce the simulation time required to model electrical machine drive operation. 
     Yet another object of the present invention is to provide a method and apparatus to readily improve electric and hybrid electric vehicle design through simulation of electrical machine drive operation. 
     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, an electrical machine drive system is provided. The system includes a DC power source, an inverter model, and an electrical machine model. The DC power source is coupled to and drives the inverter model. The electrical machine model is coupled to the inverter to be driven by the inverter. The electrical machine model includes a first winding, a second winding, a third winding, a first mutual inductor, a second mutual inductor, and a third mutual inductor. The first, second and third winding are coupled together at a node. The first mutual inductor reflects a mutual coupling between the first winding and the second winding; the second mutual inductor reflects a mutual coupling between the second winding and the third winding; and the third mutual inductor reflects a mutual coupling between the third winding and first winding. Each of the first, second and third windings includes: (1) a voltage source coupled to the node intermediate the first, second and third windings; (2) a phase inductor coupled to the voltage source; and (3) a phase resistor coupled to the phase inductor. Preferably, the phase inductance of the phase inductor and the mutual inductance of the mutual inductor vary as a function of rotor position. Also, the voltage source preferably varies as a function of speed and electromagnetic field density. 
     In accordance with another aspect of the invention, a method is provided for simulating an electrical machine drive system simulation model. The method includes the step of simulating a response of the electrical machine drive simulation model. Preferably, the method further includes the step of selectively inserting faults in the electrical machine drive system simulation model. Faults are alternatively inserted by opening a connection in the simulation model or shorting a connection in the simulation model. The fault insertion may include adding or deleting components. 
     A further aspect of the present invention includes an alternative method for simulating an electrical machine drive system. In this method, the electrical machine drive system simulation model is stimulated with a plurality of ideal sinusoidal voltage sources. This simulates a steady state of the model in a relatively short simulation time. Then, if needed, the ideal sinusoidal voltage sources are disconnected from the model and further simulation continues with the electrical machine drive system simulation model being driven by the battery and/or inverter. And, the inverter is in turn controlled by a variety of pulse-width modulation signal generators. This simulates a transient behavior of the model and requires relatively longer simulation time due to the high frequency response of the inverter switches. 
    
    
     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 simulation model in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a schematic diagram illustrating an electrical machine drive system simulation model with faults selectively inserted in accordance with a preferred embodiment of the present invention. 
     FIG. 3 is a graph illustrating a simulation of the model of FIG.  2 . 
     FIG. 4 is a schematic diagram illustrating an electrical machine drive system simulation model with faults selectively inserted in accordance with another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram showing an electrical machine drive system simulation model  100  in accordance with a preferred embodiment of the present invention. Electrical machine drive system simulation model  100  includes a DC (direct current) power source model  102 , an inverter model  104 , and an electrical machine model  106 . Electrical machine drive system simulation model  100  is a circuit component level model of an electrical drive machine system, such as, for example, a battery, inverter and electric motor of a hybrid electric or electric vehicle. As shown in FIG. 1, D.C. power source  102  is coupled to inverter model  104  to provide a source of D.C. power to inverter model  104 . The inverter model  104  is coupled to three-phase electrical machine model  106  to drive electrical machine model  106 . In a preferred embodiment, electrical machine drive system simulation model  100  is simulated in a circuit simulator, such as, the SABER circuit simulator, sold by Avanti of Freemont, Calif. The circuit simulator is executed on a computer using the system simulation model, which is typically stored on a computer readable medium. 
     In the preferred embodiment, D.C. power source model  102  includes a battery  108  connected in parallel to a capacitor  110 . Capacitor  110  is preferably a power capacitor. 
     In the preferred embodiment, inverter model  104  includes a plurality of power switches  112 . More specifically, a pair of power switches is provided in inverter model  104  for each of the three phases of electrical machine model  106 . Each pair of power switches that drive a phase is connected in a push-pull arrangement. Each power switch  112  preferably includes a transistor with a diode connected across the collector and emitter of the transistor. Each of the bases of the transistors in the pair is coupled together through an inverter such that one transistor base receives an inverted version of the signal received at the base of the other transistor of the pair. Most preferably, the bases of the transistors are driven by a pulse-width modulated source or signal generator  114 , which is selectively coupled to the bases via switches  116 . 
     Electrical machine model  106  preferably includes three windings: a first winding  120 , a second winding  122 , and a third winding  124 . The three windings model the three windings/phases of a three-phase electric motor. Electrical machine model  106  preferably models the characteristics of a permanent magnet induced electrical motor. As shown in FIG. 1, each of the three windings is separately driven by a pair of power switches  112  in inverter model  104 . More specifically, each winding is coupled at the junction of the emitter and collector of the transistors of the power switch pairs. 
     Each of the three windings  120 ,  122 ,  124  includes a phase resistor  126 , a phase inductor  128 , and a voltage source  130 . Preferably, phase resistor  126  is coupled to phase inductor  128 , which is coupled to voltage source  130 . Each of the voltage sources  130  of the three windings is coupled together at a node  132 . Mutual inductance that is created due to the mutual coupling between the three windings is modeled in electrical machine  106 . In particular, a first mutual inductor  134 , a second mutual inductor  136  and a third mutual inductor  138  are modeled in electrical machine model  106 . First mutual inductor  134  represents a mutual coupling between first winding  120  and second winding  122 . Similarly, second mutual inductor  136  represents a mutual coupling between second winding  122  and third winding  124 . Third mutual inductor  138  represents a mutual coupling between third winding  124  and first winding  120 . In accordance with the present invention, the phase inductances of the phase inductors  128  and the mutual inductances of the mutual inductors  134 ,  136 ,  138  vary as a function of a rotor position. Also, the voltage source  130  is a function of rotor speed and electromagnetic field density of the electric motor being modeled. 
     In accordance with an aspect of the present invention, electrical machine drive system simulation model  100  is a generic electric machine drive model composed of circuit component models known to a circuit simulator. This arrangement avoids a user generating complex differential equations to model and simulate operational and fault modes of the system. Rather, the circuit simulator handles this complexity automatically. As discussed further below, this permits ready simulation of operational, fault, and post-fault arrangements, circuits and strategies. 
     The phase inductor  128  and the first, second and third mutual inductors  134 ,  136 ,  138  are preferably improved over standard inductor models and standard mutual inductor models traditionally found in circuit simulators. In particular, the inductances of these inductors vary as a function of a rotor position in an associated motor. To accommodate this varying inductance the phase inductor models are modified. More specifically, the phase inductance and resultant voltage are calculated by, for each simulation step: (1) determining the rotor position θ; (2) calculating an inductance L as a function of the rotor position θ; (3) retrieving the current i in the winding from the simulator; (4) calculating a voltage drop due to the phase resistor  126  (Vr=i*r); and (5) calculating a total voltage Vt (Vt=Vr+L*di/dt+I*dL/dt). The. mutual inductances and resultant voltages are calculated by, for each simulation step: (1) determining the rotor position θ; (2) calculating the mutual inductance M as a function of θ; (3) retrieving the current it from the simulator, where i 1  is the current in the first winding that contributes to the mutual coupling; (4) retrieving the current i 2  from the simulator, where i 2  is the current in the second winding that contributes to the mutual coupling; (5) retrieving the current i 3  from the simulator, where i 3  is the current in the third winding that contributes to the mutual coupling; (6) calculating a new it with consideration of mutual inductance from the second and third winding; (7) calculating a new i 2  with consideration of mutual inductance from the first and third windings; and (8) calculating a new i 3  with consideration of mutual inductance from the first and second winding. 
     Electrical machine drive system simulation model  100  preferably includes ideal sinusoidal voltage sources  140  coupled selectively by switches  142  to drive electrical machine model  106 . In accordance with one aspect of the present invention, sinusoidal voltage sources  140  are selectively used to stimulate electrical machine model  106  instead of inverter model  104 . As discussed further below, this permits fast steady state simulation of electrical machine drive system simulation model  100  without the overhead of high frequency simulation of pulse-width modulated sources  114 . 
     Advantageously, electrical machine drive simulation model  100  is suited for simulation of operational, fault, and post-fault mitigation and control strategies. To simulate operational aspects of the electrical machine drive simulation model, the components of the model and the associated parameters, such as values (resistance, inductance, transistor characteristics, etc.), and timing, that reflect the electrical machine drive system to be simulated are input into the circuit simulator and simulation cycles are run. In accordance with a preferred aspect of the invention, ideal sinusoidal voltage sources  140  are initially selected to drive electrical machine model  106  by closing switches  142  and leaving switches  116  open and placing switch  118  in position A. Then after a steady state is obtained, ideal sinusoidal voltage sources  140  are selectively removed from the circuit model by opening switches  142  and pulse-width modulated sources  114  are selectively inserted into the simulation by closing switches  116  and placing switch  118  in position B. Then a transient response of the system is simulated for a period of time. The high frequency of the pulse-width modulated sources causes a long simulation time. Therefore, the capability to switch out the pulse-width modulated sources eliminates the high frequency responses of the transistors, but still simulates circuit operation. Switch  118  is used to connect DC power source  102  to the inverter model  104 . In position A, switch  118  connects DC power source  102  to an equivalent resistor  119 . Resistor  119  is selected to represent an input resistance of inverter model  104  and electrical machine model  106 . Connecting DC power source  102  to equivalent resistor  119  permits the DC power source to simulate a connection to inverter model  104  even though it is not directly connected to inverter model  104 . This feature is used to insure the DC power source is in a proper state when switched to connect with inverter model  104 . In some fault mode simulations, the pulse-width modulated sources are not needed and therefore switches  116  are left open. 
     To simulate a fault mode, electrical machine drive system simulation model  100  is configured to simulate a particular fault. Preferably, a component is added or deleted from the model to reflect the fault to be simulated. For example, a wire is added across a component or in place of a component to reflect a short circuit or other fault of that component. Or, a circuit is removed or a connection disconnected to reflect an open circuit or other fault. Where required, switches are preferably added to selectively insert the fault or replicate an intermittent fault. The selective insertion of ideal voltage sources or pulse-width modulated sources may be used in simulating faults. 
     FIG. 2 is a schematic circuit diagram of the model  100  with selective modification to simulate a fault mode. More specifically, FIG. 2 simulates a fault due to an open in the first winding. The open is selectively inserted into the model by opening switch  202 . FIG. 3 is a graph showing a plot of the three winding/phase currents for a simulation of the circuit of FIG.  2 . The phase currents are plotted versus time. At time 0.5 seconds, switch  202  is opened to simulate the open fault in the first winding. 
     Curve  302  is the phase current of the first winding; curve  304  is the phase current for the second winding; and curve  306  is the phase current of the third winding. Notably, phase currents before and after the fault are captured. 
     FIG. 4 is a schematic diagram of electrical machine drive system simulation model  100  configured to simulate a fault mode where all transistors in inverter model  104  are open due to an over current condition and one diode is shorted. The open transistors are simply removed from the model and a wire  402  represents the short of the diode. 
     As discussed above, the present invention provides a generic electrical machine drive system simulation model. The generic model is readily simulated without development of complex differential equations to model system operation. Fault modes and post-fault control and operational strategies are readily simulated through modification of the generic model. In addition, simulation time is advantageously reduced by the use of ideal sinusoidal voltage sources selectively used to replace high frequency switching due to pulse-width modulated sources. 
     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.