A motor drive system is a combination of hardware and software that is used to control the speed, torque and direction of an electric motor. The main component of a motor drive system is the drive, also known as an inverter or a variable frequency drive (VFD). The drive receives input from a control system, such as a programmable logic controller (PLC) or a human-machine interface (HMI) and uses this input to adjust the voltage and frequency of the power supplied to the motor. This, in turn, controls the speed and torque of the motor.
The drive may be a separate unit or it can be built into the motor. The drive converts the incoming AC power to DC and then inverts back to AC with variable frequency and voltage to control the speed of the motor. In construction, motor drive systems are primarily in HVAC systems for controlling the speed of fans and pumps.