83 Pm Synchronous Motor Drives

The advantage of a PM synchronous machine is that it can be driven in the vector-controlled mode, delivering high performance, unlike the PM trapezoidal machine, which has to be driven more in the six-step mode. Of course, high-precision position information is needed to implement vector control in PM synchronous machines. A typical PM synchronous motor drive consists of a PM synchronous motor, a three-phase bridge inverter, gate drivers, position sensor, current or voltage sensors, a microprocessor, and its interfacing circuits, as shown in Figure 8.19.

Vector control of a PM synchronous motor is simpler than that of an induction motor, because the motor always rotates at synchronous speed. In vector calculations, only the synchronously rotating reference frame is necessary. The system controller sets the reference or command signal, which can be position, speed, current, or torque. The variables needed for the controller are the feedback signals from the sensing circuits (position, speed, current, or voltage) or estimated values in the signal processor. The error signals between the reference and actual variable signals are transformed to gate control signals for the inverter switches. The switches follow the gate commands to decrease the error signals by injecting desired stator currents into the three-phase stator windings.

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