The invention relates generally to torque control of a permanent magnet motor. More particularly, the invention relates to a technique for torque control of a permanent magnet motor operating above base speed.
Three phase interior permanent magnet synchronous motors (IPMSM) receive three phases of electrical voltage, which enter three stator windings of the motor to produce a rotating magnetic stator field. The rotating magnetic stator field interacts with a magnetic field associated with the permanent magnets of the motor. The rotor rotates based on interactions between the magnetic stator field and the permanent magnetic field of the rotor.
To more precisely control the output torque and speed of the motor, a synchronous reference frame may be employed, which is represented by a quadrature axis (q) and a direct axis (d) defined by the relative location of the rotor to the stator windings. In the synchronous reference frame, voltages for obtaining a particular torque and speed may be more easily determined. Once direct-axis and quadrature-axis voltages are obtained for the motor in the synchronous reference frame, a mathematical transformation may be used to produce the equivalent three-phase voltage in a stationary reference frame, in which (a), (b), and (c) axes are defined by the location of the stator windings of the motor. The three-phase voltage in the stationary reference frame may subsequently be used to drive the motor.
When operating below base speed (generally considered to be a speed at which a voltage limit has been reached and additional speed may be achieved primarily by weakening the magnetic fields of permanent magnets in the rotor), the amount of torque output by the motor may generally be adjusted by changing the magnitude and frequency of the driving voltages in a relatively straightforward manner. To operate above base speed, the stator current in the direct axis of the permanent magnet motor operates to weaken the magnetic field of the permanent magnets, and thus a motor operating above base speed may be referred to be operating in a field-weakening region. However, as the magnetic field of the permanent magnets is reduced, control becomes much more complex. Though techniques for generating a maximum torque per amperes with a permanent magnet motor have been developed, the techniques remain limited to very specific applications and may vary considerably from one motor to another.