AC fan speed control


Type of controllers for AC fans

AC fans


These type of fans are powered by an AC motor. These motors require an AC supply voltage. Most AC fans have voltage controllable motors. This means that the fan speed is reduced by reducing the motor voltage. Voltage controllable motors can be controlled via electronic fan speed controllers, transformer fan speed controllers or frequency inverters. When the nominal motor voltage is applied, the fan runs at nominal speed.

In some cases, the AC motor is not completely integrated in the enclosure of the fan. That means the energy efficiency of the AC motor can be tested separately. The energy efficiency of AC motors is defined in IEC 60034 30-1. It is marked on the motor name plate. The following international classifications are defined:

  • IE1 Standard Efficiency
  • IE2 High Efficiency
  • IE3 Premium Efficiency
  • IE4 Super premium Efficiency

AC fan speed can be controlled by:

In order to achieve the IE4 efficiency level, most motor manufacturers are forced to use permanent magnets to minimise energy losses in the rotor. This makes most IE4 motors synchronous AC motors. In human language: most IE4 motors can only be controlled by frequency inverters.

In many cases, the requested fan speed can be adjusted on the device itself via the integrated potentiometer or key button interface. Another possibility is to adjust the fan speed remotely via a control signal.

This control signal can be digital (Modbus RTU) or analogue (typical 0-10V). More information about these control signals can be found on the next page. First, we will provide more details about the different technologies to control AC fan speed.

Type of controllers


Transformer fan speed controllers

Transformer fan speed controllers


Transformer fan speed controllers are intended for stepped speed control of the electric motors. They use autotransformer technology to reduce the motor voltage – and fan speed - in steps. Thanks to this technology, they generate a motor voltage with a perfect sinusoidal shape. This results in exceptionally quiet motor operation and extended service life.

Autotransformers are electrical transformers with a single coil. Via their different voltage taps, reduced voltages are available. A special impregnated coating reduces the electrical noise from the autotransformers. However, in quiet environments the typical humming noise caused by the transformer technology might be noticeable.

Transformer fan speed controllers are cost efficient and have proven to be very reliable and robust. They can also be used in circumstances with unstable power supply.

These type of fan speed controllers are easy to install. They don’t require any configuration. Some transformer fan speed controllers have an integrated rotary switch to manually adjust the fan speed. Other variants can be controlled remotely via Modbus RTU or via an analogue control signal.

Electronic fan speed controllers


Electronic or variable fan speed controllers offer infinitely variable speed control for AC fans. Typically, these controllers are used to optimise the speed of AC fans or pumps in HVAC applications. They use phase angle control - TRIAC technology - to reduce the motor voltage and to control fan speed. Triac fan speed controllers are typically available for motor currents up to 10 A. Thanks to this technology, these fan speed controllers are completely silent. They reduce the mains voltage by eliminating parts of it. The remaining motor voltage will not have a perfect sinusoidal shape. Microprocessor control makes it possible to optimise the zero-cross detection. This means that the TRIACs can be controlled more accurately. This results in a quiet motor operation.

Nevertheless, depending on the motor type some additional motor noise at low speed might occur due to the non-sinusoidal shape of the motor voltage. Increasing the minimum motor voltage will reduce the noise level.

Some electronic fan speed controllers have an integrated potentiometer to manually adjust the fan speed. Other variants can be controlled remotely via Modbus RTU or via an analogue control signal.

Electronic fan speed controllers do not require configuration in most circumstances. The low speed level can be adjusted via a trimmer or via Modbus RTU.

Electronic fan speed controllers
Frequency inverters

Frequency inverters


Frequency inverters or Variable Speed Drives provide infinitely variable speed control for AC fans. Just like electronic fan speed controllers… So what is the difference? A frequency inverter is using Pulse Width Modulation – IGBT technology – to adjust the motor voltage and frequency. This offers an almost perfect sinusoidal motor voltage and exceptionally quiet motor control in all circumstances. Depending on the settings, the frequency inverter itself can also operate very silently.

Due to the high switching frequency of the IGBT’s, special filters are required to reduce the EMC pollution in the power grid. This extra hardware has a cost. Precise motor control also means: more complex to configure. The conclusion is that frequency inverters are more expensive than electronic fan speed controllers, more complicated to configure and more critical to install, but they offer very precise motor control. These type of speed controllers are very energy efficient and capable of controlling high motor currents.

The requested motor speed can be adjusted via the integrated controls (potentiometer or push buttons). Next to that, it is also possible to adjust the motor speed remotely via Modbus RTU or an analogue control signal.

Thermal protection for AC fans


An AC motor is a robust device with an extended service life. However, operating an AC motor during a longer period at low speed is not without risks. At low speed, the motor is cooled less. This can cause overheating of the motor windings. Motor overheating can cause degradation of the insulation of the motor windings. This can cause electric leakages, short circuits, and eventually motor failure. To prevent motor failure it is important to prevent the motor from being overheated.

For this purpose, many AC motors are equipped with thermal contacts – often called TK contacts. These thermal contacts measure the temperature in the motor windings. In case of motor overheating, the TK contacts are opened. Some fan speed controllers provide extra protection against overheating via their TK monitoring function. This functionality deactivates the motor in case of overheating to prevent motor damage. At the same time, the alarm output will be enabled to indicate a motor problem. In case your motor is not equipped with TK contacts, don’t forget to bridge the TK terminals of the fan speed controller or to deactivate the TK monitoring function.

Thermal protection for AC fans

How to operate a motor in a safe way?


Switch disconnectors

Switch disconnectors


In case of maintenance or when a motor needs to be replaced, it is important to make sure that the motor power is - and remains - disabled during the intervention. The best guarantee is to install a switch disconnector with padlockable OFF position. These switches can be blocked in OFF position with a padlock to guarantee the safety of the maintenance technician. Typically, switch disconnectors are installed nearby the motor or at the entrance of a room or zone.

Motor circuit breakers


Motor circuit breakers are Low voltage circuit breakers with thermal overload relay functionality. They protect motor branch circuits against overload, phase-loss, and short-circuit. Typically, motor circuit breakers are installed in or nearby the electrical cabinet.

Motor circuit breakers