Power Electronics And 3Phase Drives

3 Phase motor drives and DC drives dominate the industry in most applications from low to high power. (Single phase drives usually take care of the low power end.)

Basic 3Phase motors are:

• 3Phase induction cage rotor motor
• 3Phase induction wound rotor motor
• 3Phase synchronous motor

3Phase induction motors are used widely to serve general purpose applications, both adjustable speed and servo drives. 3Phase synchronous motor is found in special applications, mostly as servo drives. Some very large power adjustable speed drives also prefer synchronous motors because of the possibility of using low cost load-commutated-inverters (LCI) built from thyrestors.

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,

Single Phase Drives - Low Speed Control Mode

Power circuit for single phase drive - low speed control mode

At low speeds, motor voltage V should not have lower-order harmonics. An ideal would be a pure sinusoidal voltage but a compromise is acceptable.

The square wave voltage used in the high speed mode contains lower order harmonics of order 3,5,7,9...etc. So we con not use it for low speed operations. If attempted we will get some wobbling speed perturbations at low speeds.

We use switching strategy known as PWM (Pulse Width Modulation) to deliver near sinusoidal voltage for the motor. We have two operations of PWM.

(a). Bipolar PWM

(b). Unipolar PWM

Single Phase Drives - High Speed Control Mode

This mode of control is suitable when the speed is to be controlled at higher values; for example above base speed.

At higher speed the motor will not response to torque harmonics (because of filtering due to electromagnetic time constant) and hence we can apply simple square wave voltage input. The motor then responses to the fundamental component of voltage and (virtually) ignores harmonic voltages.

Switching pattern for squarewave voltage output

Ts - Switching cycle time.

Switching signals for squarewave voltage output