Variable Frequency Drive (VFD)
A variable frequency drive (VFD) controls the speed and torque of an AC motor by varying the frequency and voltage of its supply. A rectifier converts the incoming AC to DC, a DC link smooths it, and an IGBT inverter synthesises a variable-frequency output using pulse-width modulation (PWM). VFDs provide soft starting, speed control and energy savings on variable-torque loads such as pumps and fans, as well as process control on conveyors and compressors. Selection parameters include motor power and current, supply voltage, overload duty (normal or heavy), control mode (V/f, vector or direct torque control), enclosure IP rating, harmonic mitigation, EMC filtering and communication protocols. Relevant standards include the IEC 61800 series for adjustable-speed electrical power drive systems and IEEE 519 for harmonic limits.
Examples
- Pump speed control to maintain constant pressure
- Fan drive in an air-handling unit or cooling tower
- Conveyor drive with controlled acceleration