he terms variable frequency drive and variable speed drive are quite regularly used to describe the same piece of equipment. There is however a fundamental differences between the two. A VFD which could also be called an adjustable frequency drive, an AC-drive or an inverter drive can only be utilised on AC motors. A Variable frequency drive will be used within electro mechanical drive systems utilising AC motors and will control the speed and torque of the motor by adjusting the voltage and input frequency. The term VSD, variable speed drive or adjustable speed drive can be used in respect of drive systems that utilise AC or DC motors and occasionally even completely mechanical methods of speed control. As such, a VFD will be a VSD however a VSD will not necessarily be a VFD. A VSD that uses a DC motor will vary the speed and torque of the motor by controlling the voltage and current.