AC fan speed control

AC fans

Since many years, AC motors are widely represented in industrial applications and in the HVAC industry. Thanks to the wide range of variable speed drives and the increasing intelligence of control solutions, the number of possible applications seems endless.

AC motors usually have a squirrel-cage rotor. Electric alternating current running through the stator windings generates a rotating magnetic field. This magnetic field induces currents in the rotor windings (Faraday’s law of induction). The induced currents generate the rotor magnetic field. The magnetic rotor field lags behind the magnetic stator field. This is called the motor slip. The rotor is not synchronised with the rotating magnetic field. The higher the load, the higher the slip and the higher the motor torque. For this reason, these motors are called asynchronous motors or induction motors.

Type of controllers for AC fans

Some recent, high-efficient AC motors are equipped with a permanent magnetic rotor to minimise energy losses. Due to the permanent magnets, the rotor runs synchronously with the rotating magnetic field. For this reason, these motors are called synchronous motors.

We distinguish types of AC motors:

AC motors are extremely reliable and very robust. They hardly require any maintenance and if they break down, they are easy to repair. AC motors are the industry standard and are therefore widely available in a very wide power range.

Type of motors

Induction motors

Induction motors are asynchronous motors with a squirrel-cage rotor. Their magnetic field is created by induced currents in the rotor windings (Faraday’s law of induction). Due to this technology, the rotor is not synchronised with the rotating magnetic stator field. The higher the load, the higher the slip and the higher the motor torque.

For HVAC applications like fans and centrifugal pumps, V/f frequency inverters can be used to regulate motor speed. The frequency will keep the ratio between voltage and frequency constant. As a result, the motor flux is kept constant to prevent the magnetic field from weakening and to assure sufficient motor torque.

Induction motors are the standard for most industrial applications. In the HVAC industry, induction motors are typically used in larger installations. Worldwide, electric motors consume approximately 60 % of the total energy required. That is why a lot of time and energy has been invested during recent years on making electric motors more energy efficient. Under the influence of the Energy efficiency guidelines, significant energy savings have been achieved. The energy efficiency of induction motors is defined in the standard IEC 60034 30-1. This information is usually 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 (PMSM)
  • IE5 - Ultra-premium Efficiency (PMSM)

The major difference between these variants is the energy consumption. In many cases, more copper is used to minimise losses, in some IE3 motors the complete rotor is made in copper. This has an important impact on the price of the motor.

Voltage controllable motors

Voltage controllable motors

Voltage controllable motors are asynchronous motors whose speed can be controlled by reducing the voltage. When the nominal voltage is applied, the motor runs at high speed. When the motor voltage is reduced, the motor will slow down.

When the motor voltage decreases, the maximum motor torque also decreases. As long as the motor remains powerful enough to drive the load, the motor speed can be controlled by reducing the voltage. Note that not all motors are voltage controllable. Commonly used voltage controllable motor types are single-phase permanent split-capacitor motors or single-phase shaded pole motors.

Voltage controllable motors

Voltage controllable motors are often single-phase motors.

Whether voltage control is possible also depends on the load attached to the motor. If this load is a fan, it is more likely that the motor can be voltage controlled. A fan has a quadratic torque curve. This means that the relation between motor speed and motor torque is not linear. E.g. if the fan speed is reduced by 10 %, the required torque to maintain that speed will be 20 to 30 % lower than the required torque to maintain high speed. That’s why many fans with AC motor are voltage controllable. In case of constant torque applications, this is not always possible.

If the motor is voltage controllable, this is always indicated in the technical data sheet of the motor. If the technical data sheet does not state that the motor is voltage controllable, consult the manufacturer before connecting a speed controller that reduces the motor voltage.

The main advantage of voltage controllable motors is their simplicity. They are easy to control and easy to connect. On top of that they are very robust and reliable. If they break down, they are easy to repair.

Following techniques can be used to regulate the speed of voltage controllable AC motors:

  • 5-step speed controllers – Autotransformer technology
  • Electronic variable speed controllers - Triac based phase-angle technology
  • Frequency inverter – IGBT based Pulse Width Modulating (PWM) technology

PMSM or Permanent Magnet Synchronous Motors

Electric motors: 4 pole, 50 Hz

Permanent Magnet Synchronous Motors diagram

Output power [kW]

Efficiency [%]

To achieve the highest energy levels, manufacturers are forced to use permanent magnet rotors. This is the case in most IE4 motors and certainly for IE5 motors. Motors with permanent magnet rotors must be controlled by a frequency inverter, they cannot work without a frequency inverter. Obviously, permanent magnet motors have their own, permanent magnetic field.

Thermal protection for AC motors

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. During the maintenance activities, they prevent the motor from being activated.

Switch disconnectors can also be used to disconnect the motor in case of an emergency. The electrical contacts are oversized so that they can also interrupt inductive electrical circuits. The forced open contacts fulfil load break requirements up to 690 Volts.

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 electric motors against overload, fluctuations in input current or unscheduled interruptions to the main circuit. This includes line faults and phase loss or imbalance in three-phase motors.

When an AC motor is energised, a high inrush current occurs. Typically, the inrush current is 4 to 8 times the normal current during several seconds. As the motor reaches the nominal speed, the current subsides to its normal running level. Because of this high inrush current, AC motors require special overload protective devices that can withstand the temporary overloads associated with starting currents and yet protect the motor from sustained overloads. Motor circuit breakers are designed for motor overload protection.

Typically, motor circuit breakers are installed in or nearby the electrical cabinet.

Motor circuit breakers