AC drive is a device that is used to control the speed of an electric motor. The speed is controlled by changing the frequency of the electrical supply to the motor.

The three-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field.

Motor Speed Control

The speed is controlled by the ac drive converting the frequency of the network up to anything between 300Hz or even higher. The speed of the motor is thus controlled proportionally to the frequency.

The technology behind ac motor speed control consist of

Rectifier Unit: The ac drive is supplied by the electrical network via a rectifier. The rectifier unit can be unidirectional or bidirectional. When unidirectional, the ac drive can accelerate and run the motor by taking energy from the network. If bidirectional, the ac drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.

DC Circuit: The circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors.

Inverter Unit: The ac motor drive inverter unit takes the electrical energy from the dc circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed three-phase ac voltage output for the motor. The frequency can be adjusted to match the need of the process. The higher the frequency of the output voltage is, the higher the speed of the motor, and thus, the output of the process.

AC Motor Drive Benefits

The types of motors that ac drives control are normally operating at constant speed. Enabling the user to control the speed of motor potentially gives him various benefits in terms of process control, system stress and energy savings.

Process control: controlling the process output to match the need; synchronising different parts of the main process to secure smooth flow between subprocesses; easily changing the setup when the process requirements change.

In system stress: Reducing the start-up current, which allows the use of smaller fuses and supply connections and reduces peak loads on the electrical network; reducing the mechanical shock in start and stop situations.

Energy: Saving electrical energy compared to traditional methods of process control. For instance in pump and fan applications, energy savings are typically 20-50 per cent.