What exactly is an ac drive and just how does it work?

Olli Tevä outlines ac drive technology and, in particular, its uses in HVAC applications

An 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.

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:

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.

In heating, ventilation and air conditioning (HVAC) applications the main processes are related to heating, cooling, drying and circulating air. Supporting processes are mostly related to taking the extra heat out of the building or providing additional heat energy to the building.

The majority of HVAC applications where ac drives are used are fans, pumps and compressors.

Fans and pumps: Using an ac drive to control the fan or pump output rather than using dampers, vanes, valves or on/off control brings substantial energy savings, if the required output is less than nominal most of the time.

The ac drive controls the speed of the pump and fan by changing the electrical energy supplied rather than damping the air- or water flow. It is like reducing the speed of a car by pressing less on the accelerator instead of using the brake to slow down the speed. The payback time of an ac drive is typically one year or less.

Other benefits of using ac drive to control the speed of fan or pump are: