A VFD (Variable Frequency Drive) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive (VSD), adjustable speed drive, adjustable frequency drive, AC drive, Microdrive, and inverter.
VFDs are used to control the speed or torque on a motor. They are often used in combinations on a production line or process flow, where different parts of the line are under different loads, but a constant speed must be maintained. By adjusting the power to the motor, the speed of the line can be maintained under varying loads.
Protection of VFD against Transients
According to the image above:
1. A parallel connected, frequency attenuation SPD is installed on the incoming power to the drive, usually on the output side of the main breaker.
2. A series connected, frequency responsive electronic grade SPD is installed on the output side of the step-down transformer (or on the output side of the power supply if there is no step-down transformer).
3. If the run from VFD to the motor is at least 61 meters, a parallel connected, voltage responsive SPD can be installed on the VFD output lugs to protect the back end of the VFD from reflected wave effect surges generated on the line between the VFD and the motor.
4. If the run from VFD to the motor is at least 30.5 meters, a parallel connected, voltage responsive SPD can be installed at the lugs of the motor junction box to protect the motor from reflected wave effect surges generated on the line between the VFD and the motor.