Single Phase Drives - Servo Control Mode

Servo control use current control for rapid adjustment of motor torque. Voltage control will not be good for servo applications due to inherent delays before the control passes to adjust current. In PWM it is a delay in the motors electrical time constant L/R; in square wave control it is a sequence of delays at the capacitor of DC-link, electric time constant L/R of motor etc.

To obtain current control we use, so called, "current controlled PWM". There too, we have two options;

(a). Hysteresis current control mode

(b). Fixed frequency current control mode

(a). Hysteresis current control mode

This PWM acts to constrain the motor current I to a specified shape and amplitude, as suggested by the outer loops (e.g. Speed loop) of the closed loop control system. This requires motor current feedback as an input to the PWM modulator. Desired current is the other input.Switching principle is,

Power Converter Technology

Apart from power switching devices, the power-converter technology has also gone through significant developments in the recent past. Resonant converters specially the multi resonant converters are critical examples. High-phase-number converters is another area of development. (where the motors are no longer restricted to 3 phases).

Motor technology also went through a similar phase of developments, mainly after 1980. In 1985, the development of high energy permanent magnet, known as Nd-Fe-B (Nyoaliminium Iron Boron) resulted in a set of new motors such as brussless DC motors, switch reluctant motors, hybrid stepper motor, permanent magnet linear motor ...etc. The brussless DC motor is a good alternative to conventional (brushed) DC motors, it delivers the same performance without problems associated with a mechanical commutator.

Note: High energy permanent magnet materials especially Sm-Co (Samarian Cobolt) was in use prior to developing Nd-Fe-B but the high cost of Sm-Co and not allow its usage in industrial grade motors. It was used expensively in aerospace applications.