FAQs - Variable Speed Drives

Variable Frequency Drives (VFDs) vary the speed of an AC motor by varying motor frequency. Variable Speed Drives (VSDs) vary the voltage to the motor.

Dynamic braking is a way to increase the braking capability of a variable speed drive by dissipating the excess energy into a high power resistor connected across the internal DC link. When a VSD is given a stop signal, the VSD output decelerates to zero but the motor can carry on running due to load inertia and mechanical energy with the motor acting as a generator. This energy flows back from the motor into the DC link capacitors in the variable speed drive and causes the DC link voltage to rise. If the DC voltage exceeds 750Vdc the drive will trip to protect the capacitors and connected devices.

A Variable Speed Drive is a motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor in order to save energy and optimize the performance of the motor. Other names include Variable Frequency Drive (VFD), inverter drive, AC drive, micro-drive, adjustable speed drive and variable speed drive.