5. Principles of variable speed drives

To understand what a VSD does it is important to recall that the speed of an induction motor is directly proportional to the frequency of the supply. If we can vary the frequency of the power supplied to the motor, then we can vary the motor’s speed away from its rated synchronous speed.

A variable speed drive (VSD) controls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.

The VSD controller is a solid state device. A common method used for adjusting the motor voltage is pulse width modulation (PWM).

The PWM voltage control, inverter switches are used to divide the close-to sinusoidal output waveform into a series of narrow voltage pulses, and to modulate the width of the pulses.

PWM voltage control. Three phase power is rectified, smoothed and sampled:

This graph clearly shows the linear relationship between voltage, rpm, and frequency is directly related to each other.

You can see that at each frequency a new motor speed is selected.

Two valuable calculators (high efficiency motors and variable speed drives) are found at Sustainability Victoria’s web site.
(look under the Green Power Business Guide)