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

Paul Boughton

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.

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 Uni: 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.


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:

  • Smooth ramp up and down causes less stress to the mechanics of fans and pumps and to air ducts and water piping.
  • Slowing down the speed rather than damping the output will result in lower noise levels.


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