Variable Frequency Driver

A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor (AC motor) by controlling the frequency of the electrical power applied to the motor.

They are also known as Adjustable-Frequency drives (AFD), Variable-Speed Drives (VSD) or AC Drives. VFD's are widely used in ventilations systems for large buildings, variable-frequency (VF) motors save certain volume of air according to a previously established set point. VFD's are also use in others applications, pump, conveyors machines tools and many more.

In AC motors, the speed is determined by frequency of the AC and the number of poles in the stator, according to the relation:

RPM= (120xf)/p

RPM= Revolutions per minute

f= AC Power frequency (Hz)

p= Number of poles (an even number)

Note:

        The constant 120 is the 60 cycles per minute multiplied by 2 poles per pair. Thus sometimes 60 is used instead of 120, and the p is stated as pole pairs rather than poles.

For this example we will calculate the RPM for AC motor that has 6 poles (3 pairs of poles) operating at a 60 Hz frequency

RPM= (120xf)/p

RPM= (120x60)/6 = 1200

We can also use 60 instead of 120, in this case the equation would be:

RPM = (60xf)/p

RPM = (60x60)/3 = 1200

Note that we also change the value of p, since the motor has 6 poles and we use the constant 60 instead of 120, we had to state the p as pairs of poles rather than poles.