Abstract:
An electric motor control system including a stator for producing a magnetic field, a surface mount permanent magnet rotor rotated by the magnetic field, a motor shaft coupled to the rotor, power electronics for controlling the magnetic field in the stator, and where the power electronics controls the q-axis and d-axis current components for the electric motor.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to an electric drive system comprised of a surface-mounted permanent magnet electric machine powered by a voltage source inverter and a controller. More specifically, the present invention relates to a method and apparatus to increase the shaft torque output for a surface-mounted permanent magnet machine.  
         BACKGROUND OF THE INVENTION  
         [0002]    In today&#39;s automotive market, there exist a variety of electric propulsion or drive technologies used to power vehicles. The technologies include electric traction motors such as DC motors, AC induction motors, switched reluctance motors, synchronous reluctance motors, brushless DC motors and corresponding power electronics. An electric motor may be described as generally comprising a stator and a rotor. The stator is fixed in position and the rotor moves relative to the stator. Permanent magnet excited synchronous machines are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to DC motors and AC induction motors.  
           [0003]    In permanent magnet excited synchronous machines, the stator is typically the current carrying component of the motor, generating a magnetic field to interact with the rotor. The field generated by the stator will propel or rotate the rotor relative to the stator via the magnetic field. Permanent magnet excited synchronous machines operate with a permanent magnet rotor. A permanent magnet rotor may be configured as a surface mount or interior or buried permanent magnet rotor. In a permanent magnet excited synchronous machine equipped with a surface mount permanent magnet (SMPM) rotor, magnets are mounted on the surface of the rotor.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is a method and apparatus for increasing the torque of a surface-mounted permanent magnet machine or motor by using saturation-induced reluctance torque.  
           [0005]    The electromagnetic torque of a three-phase SMPM machine is represented by equation (1).  
             Te= 3/2 PΨ   m   i   q   (1)  
           [0006]    It can be derived using the general equation of the electromagnetic torque in a reference frame attached to the rotor as follows:  
             T   e =3/2 P (Ψ d   i   q −Ψ q   i   d )  (2)  
           Ψ d =Ψ m   +L   d   i   d   (3)  
           Ψ q   =L   q   i   q   (4)  
           [0007]    Equation 2 may be represented as:  
             T   e =3/2 P[Ψ   m   i   q +( L   d   −L   q ) i   q   i   d ]  (5)  
           [0008]    where  
           [0009]    T e  is the electromagnetic torque,  
           [0010]    Ψ m  is the permanent magnet flux linkage,  
           [0011]    Ψ d  and Ψ q  are the direct and the quadrature axis stator flux linkages in the rotor reference frame,  
           [0012]    i d  and i q  are stator current components, and  
           [0013]    P is the number of pole pairs of the machine.  
           [0014]    A simplified phasor diagram of an SMPM machine is shown in FIG. 1 including the variables illustrated by equations 1-5. The d-axis is defined as being aligned to the permanent magnet flux Ψ m  and the q-axis is 90 electrical degrees in advance. V s  corresponds to the stator voltage, and I s  corresponds to the stator current. In traditional SMPM machine control theory, L d  is considered equal to L q , rending the second term of equation 5 equal to zero and making equation 5 the same as equation 1.  
           [0015]    At high stator current levels, when the effects of magnetic saturation cannot be neglected, the two magnetizing inductances can have different values where L d  is not equal to L q . In these cases, the difference (L d −L q ) is not zero, and additional torque can be obtained from the motor if the d-axis current is controlled to an optimal, non-zero value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a phasor diagram of an SMPM machine;  
         [0017]    [0017]FIG. 2 is a diagrammatic cut-away drawing illustrating an electric motor of the present invention;  
         [0018]    [0018]FIG. 3 is a control block diagram for a SMPM machine;  
         [0019]    [0019]FIG. 4 is a plot illustrating the increase in torque generated by the present invention; and  
         [0020]    [0020]FIG. 5 is a plot illustrating the torque/amperes generated by the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    [0021]FIG. 2 is a cut-away view of an electric motor  10  of the present invention. The electric motor  10  includes a stator  12  and rotor  14  separated by an air gap  16 . The electric motor  10  in the preferred embodiment is configured as a three-phase surface mount permanent magnet machine. The rotor  14  is configured as a surface mount permanent magnet (SMPM) rotor with magnets  18  coupled to the surface of the rotor  14 . The magnets  18  are preferably rare earth magnets.  
         [0022]    [0022]FIG. 3 is a control block diagram for the motor  10  in the preferred embodiment of the invention. A controller  20  contains software to control the power electronics  22  that operates the motor  10 . The controller  20  may comprise any known controller in the electronic and computer arts. The power electronics  22  are preferably comprised of a voltage source inverter (VSI). A high voltage DC bus V dc  provides power to the power electronics  22 . T e * is the torque setpoint input to block  24 . Block  24  transforms T e * into current setpoints i q * and i d *. The transformation at block  24  is executed as a function of the angle β made by the stator current I s  with the q-axis as follows:  
           i   q   *=I   s  cos β 
           i   d   *=I   s  sin β 
         [0023]    The angle β is assigned an initial value of zero. The angle β may then be varied to produce the desired current setpoints.  
         [0024]    The current setpoint i q * is input to a summing junction  28  along with current feedback i q  provided by block  38 . The resultant error is processed by proportional integral (PI) control block  34  and space vector modulator  36  to switch or drive the power electronics  22  in response to the error. Similarly, at block  26  the current setpoint i d * is summed with current feedback id at summing junction  26  to generate an error that is processed by PI control block  36  and the space vector modulator  36  to switch or drive the power electronics  22  in response to the error. In past control systems, the current id was assumed to be zero. The present invention controls the current id to a non-zero value to increase the torque of the electric motor  10 . Torque is also controlled by controller  20  with reference to feedback  40  providing θ r  position and ω r  speed information for the motor  10 .  
         [0025]    The machine measured torque output vs. stator current is illustrated in FIG. 4 for the motoring mode of the electric motor  10 . The torque of the motor  10  can be increased if the stator current i q  is not aligned along the q-axis of the motor, but rather slightly in advance, corresponding to the presence of a small negative d-axis current component. Plot  44  in FIG. 4 corresponds to an off-line estimation of motor torque according to equation  1 , plot  42  corresponds to an on-line or operating optimization of motor torque using equation  5 , and plot  46  corresponds to motor torque using equation  1 . As can be seen by comparing plot  42  and plot  46 , by controlling id to a non-zero value, torque for the electric motor  10  can be augmented. Similarly, in FIG. 5, plot  50  corresponds to an on-line or operating optimization of motor torque/ampere using equation  5 , and plot  52  corresponds to motor torque using equation 1. As illustrated by the plots of FIGS. 4 and 5, by controlling the current id during magnetic saturation conditions, the torque of the electric motor  10  can be increased, as compared to past motor control algorithms that ignored control of the id component to generate torque.  
         [0026]    While this invention has been described in terms of some specific embodiments, it will be appreciated that other forms can readily be adapted by one skilled in the art. Accordingly, the scope of this invention is to be considered limited only by the following claims.