Wednesday, December 22, 2010

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,


I = feedback current
I* = desired current
^I = width of current - band (e.g. hysteresis) within which I should stay about I*.

Q1 and Q2 is ON while, I < (I* + ^I/2)
Q3 and Q4 is ON while, I > (I* - ^I/2)

Hysteresis comparator circuit

Width ^I is set by hardware of the hysteresis comparator, e.g. we can alter a resistor and set ^I.

Principle of generation switching signals for Hysteresis current control mode

Hysteresis current control mode is possibly the extremely fast method of current control mode. A sufficient voltage Vd must be provided at the invertor input to force current rapidly, otherwise possibility exist for partial saturation of output current.

Output current I with partial satuation

An example of output current I with partial satuation of current controller due to inadequate Vd.

Switching frequency is not a constant value, but varies over the cycle of I*, with higher frequency near zero-crossing regions and lower frequency near peak regions. This varying switching frequency can pick up audible noise of varying pitches, which may be a nuisance to the ear. In a situation like an office where calmness is necessary we can not tolerate this noise, which means we can not use hysteresis current control.

(b). Fixed frequency current control PWM

This option is not as fast as hysteresis current control but maintains a constant switching frequency. To fix up switching frequency it uses a triangular high frequency carrier.

Arrangement of fixed frequency current control PWM modulator
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