A Variable Speed Drive (VSD) is an electrical device used to change the speed and torque of an electric motor. This is done by controlling the frequency and voltage supplied to the motor. A Variable Frequency Drive (VFD) is a type of VSD that uses pulse-width modulation to convert AC power into DC power, and then manipulates the frequency and voltage of the output current to control the speed of the motor. This method of speed control provides superior performance compared to traditional VSDs, with increased efficiency and improved reliability.