The ac drives are electronic devices used to control speed and torque of threephase induction motors. An induction motor supplied by an ac drive can operate over a wide range of frequency, typically from 0 to 60Hz. This range of frequencies yields rotor speeds from 0 r/min to the rated value. Moreover, the ac drive can produce the rated torque at any frequency within this range from zero to the rated frequency.
This is a powerful characteristic for industrial processes that require torque-speed control. Although, the electrical installation of an ac motor-drive system is more expensive than an induction motor with a constant frequency sinusoidal power supply, the ac motordrive system can control not only the motor speed, but also can control and limit the starting torque and current, can adjust the acceleration and deceleration ramps, can maintain a constant torque for frequencies from zero to the rated frequency, and protect the motor against over voltages and over currents.