CONTROL CIRCUITS FOR SRM
For motoring operation the pulses of phase current must coincide with a period of accuracy inductance. The timing and dwell (i.e.) period of conductance of the current pulse determine the torque, the efficiency and other parameters. With fixed firing angles, there is a monotonic relationship exist between average torque and rms phase current but generally it is not linear. This may present some complications in feedback-controlled systems. Although it is possible to achieve ‗near servo-quality‘ dynamic performance, particularly in respects of speed range torque/inertia and reversing capability.
More complex controls are required for higher power drives, particularly where a wide speed range is required at constant power, and microprocessor controls are used. As high-speed operation, the peak current is limited by the self-emf of the phase winding. A smooth current waveform is obtained with a peak/rms ratio similar to that of a half sinewave.
At low speed, the self-emf of the winding is small and the current must be limited by chopping or PWM of the applied voltage.
Two types of control circuits used are:
1. Hysteresis type to maintain constant current
2. Voltage pulse width modulation control (or) duty cycle control.