VSD’s or VFD’s , also referred to as frequency converters or adjustable speed drives, are devices that convert fixed frequency supply voltage ( typically 50Hz or 60Hz) to a variable frequency voltage. The frequency of voltage supplied to a motor determines the speed at which that motor rotates.

By Cblambert (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

What are the benefits of a drive (VFD) over a motor starter?

• Drives protect motors from the in rush of current when starting.
• In applications where full speed operation is not required, drive saves energy by facilitating operation at lower speeds.
• Drives allow for speed regulation to maintain the set point of a process ( could be a pump motor speed for pressure and flow or fan speed for temperature)
• In applications where a high torque is required at a low speed, drives are able regulate both speed and torque at its output to allow for continuous operation a low speed. An example could be a hoist where the load is suspended ( at zero speed)in the air without the engagement of brakes.
• Drives are able to provide current and torque limiting functionality so as to prevent motor and other equipment damage.