Electronic VFDs are speed control devices which vary the voltage and frequency to an induction motor using a technique called Pulse Width Modulation (PWM). VFDs have become the preferred way to achieve variable speed operation as they are relatively inexpensive and very reliable.
VFDs use power semiconductor devices called insulated-gate bipolar transistors (IGBT). Using PWM, the speed of the motor and torque characteristics can be adjusted to match the load requirements. They convert the fixed frequency AC supply voltage to a variable frequency, variable voltage AC supply to the motor and can regulate the speed of an induction motor from about 10% to 200% with wider ranges possible depending on the model and options selected.
The speed accuracy is affected by the slip of the motor, resulting in slightly slower operation than the synchronous speed for a given frequency. The accuracy can be increased greatly by using tachometer feedback. Extremely precise speed and position control of the motor shaft can be achieved by using a VFD with Vector Control.
The VFD can provide many solutions depending on the required application. For example, a VFD can provide the following:
- Energy savings on fan and pump applications
- Better process control and regulation
- Speeding up or slowing down a machine or process
- Inherent power-factor correction
- Bypass capability in the event of an emergency
- Protection from overload currents
- Safe acceleration.