AC synchronous generators are generally used as stand-by electric power supply for domestic purposes and also as isolated power supply in remote and hilly areas where grid supply is not available. It is coupled to a prime mover like internal combustion engine, or wind turbine for using as generator set. Brushes and slip-rings are used in the machine to tap out the power.

AC variable frequency drive is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable speed drive. Variable frequency drives are also known as adjustable frequency drives (AFD), variable speed drives (VSD), AC drives or inverter drives. A variable frequency drive system generally consists of an AC motor, a controller and an operator interface. The motor used in a VFD system is usually a three-phase induction motor. Variable frequency drive controllers are solid-state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier bridge. The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed.