What’s the Purpose of VFD’s?
A VFD (Variable Frequency Drive), or rather a VVFD (Variable Voltage and Frequency Drive) is a precision electronic device specifically designed and used to control the speed of AC induction motors (single as well as three phase) without affecting the electric consumption, torque, impedance, magnetic flux, etc. of the motor. It is integrated to an operator interface for receiving the required speed control commands (using keypads). Why can’t VFDs be replaced by other straightforward means? The following discussion will provide the exact purpose of using VFDs to control AC motor speed.
The fundamental speed of any AC motor is inversely proportional to its number of stator poles and directly proportional to the supply voltage's frequency. Therefore, to alter the speed of an AC motor, we need to either change the frequency or the number of stator poles. Since the number of stator poles for every motor is fixed, obviously we cannot change them. By varying the frequency of the supply voltage through some simpler means, the speed of the motor can be changed.
However, changing only the frequency at a constant voltage (120 or 230) causes the equivalent impedance of the motor to decrease, resulting in greater magnetic flux and causing the motor to start drawing dangerously huge currents. Therefore it becomes imperative that the supply voltage is also proportionately reduced along with the frequency at a particular fixed ratio. Failing to do this would cause the magnetic flux of the motor to saturate and the motor to become damaged. Varying the frequency and voltage proportionately also ensures a constant torque since the magnetic field in the air gaps is constant.
The purpose of a VFD is specifically intended to control the speed of an AC motor by strictly observing the above parameters. Here, the speed of the motor is varied by changing the magnitude of the input voltage as well the frequency at a constant ratio and thus the motor is able to maintain a constant torque even at lower speeds.