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.
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