Understanding fan speed control

Get Started

Why to control fan speed?


Indoor Air Quality

Fresh air supply is essential to create a healthy and comfortable environment. Since most people spend the majority of their time indoors, indoor air quality has a direct impact on their health. Together with the increased thermal insulation of our buildings, the importance of ventilation systems is rising in order to maintain a good indoor air quality.

Temperature, relative humidity, CO2 and TVOC are the most important parameters that determine an occupant's comfort and productivity. For more information about indoor air quality, it is advisable to visit www.air-quality-monitor.eu

When many people share a confined space, fresh air supply will be maximised in a well-dimensioned ventilation system. The fan speed and thus the air volume flow will reach their maximum level. This is necessary to guarantee a good indoor air quality and to keep the CO2 and TVOC concentrations at an acceptable level. When fewer people are present in the same room, a lower supply rate of fresh air will be sufficient. This variable regulation of the ventilation system will increase the comfort of the residents and save energy.

COVID-19 contamination via aerosols

The risk of becoming infected with the COVID-19 virus is higher in crowded and poorly ventilated spaces. Transmission of COVID-19 via aerosols rarely occurs outdoor or in enclosed spaces with a large volume. Some ventilation systems partially recover indoor air to minimise heat losses. This internal air circulation can spread viruses through the building. Therefore, the general recommendation is to deactivate indoor air recirculation, to increase the supply rate of fresh air and the extraction rate of stale air. The ventilation system should remain active on a continuous basis. Most ventilation systems are dimensioned to provide crowded spaces with sufficient fresh air. When the room is not occupied, the air volume flow rate can be reduced while still supplying sufficient fresh air.

COVID-19 contamination via aerosols

Energy savings

What is the advantage of an apparently complex fan speed controller? Why not run the fans continuously at maximum speed? Then there is certainly sufficient fresh air supply… A slight reduction in fan speed has a major impact on the electrical energy consumption of the fan. A typical HVAC fan follows a quadratic torque curve. Depending on the motor type, a reduction of 25 % air volume flow corresponds with 50 % less energy consumption. In addition, a lower air volume flow rate also results in a quieter operation.

Input power (%)

Energy savings

Flow rate (%)

Minimise heat losses

In colder or moderate climates, extracted warm indoor air is replaced by fresh, colder air. The less warm indoor air has to be replaced by cold outdoor air, the lower the heat loss. Modern ventilation systems are nowadays equipped with a heat exchanger to minimize such heat losses. Nevertheless, additional energy can be saved by reducing the fan speed when possible. By measuring the air quality of the extracted indoor air, the fan speed can be continuously optimised while the indoor air quality is guaranteed.

Minimise heat losses

Extended service life

Air filter service intervals are significantly extended by reducing the air volume flow rate. The more air that passes through the filters, the higher the risk of contamination of the filters. A reduced air volume flow rate also has a positive effect on the service life of the mechanical parts of the fan. These prolonged service intervals reduce the operational costs and the total cost of ownership.

Extended service life

Increased comfort

Reduced risk for Covid-19 contaminations

Energy savings

Minimise heat losses

Extended service life

Optimal indoor air quality

EC fan or AC fan?


If indoor air quality remains guaranteed with reduced supply of fresh outdoor air, it is interesting to reduce the fan speed. The comfort of the residents will increase due to the quieter operation of the ventilation system. Energy consumption and operational costs are reduced.

By decreasing fan speed, the rotational speed of the fan blades is reduced. This results in a lower air volume flow rate.

Before we can answer the question ‘How to control fan speed?’ or before installing a fan speed controller, sufficient information about the fan and its motor must be known. Not every fan is speed adjustable!

Control signals diagram

A fan is driven by an electric motor. The technical data of this electric motor determines the possible choice of fan speed controllers. We distinguish between following motor types:

To simplify things, consider an EC motor as an AC motor with a built-in fan speed controller. An AC motor is a motor that requires a separate fan speed controller (different technologies are available). For now, we will give it the general name AC fan speed controller.

Both an EC motor and an AC fan speed controller need to know the 'desired fan speed'. They can receive this ‘desired fan speed information’ via a control signal. Typical control signals are a 0-10 Volt or 0-20 mA signal (analogue signals). Although these analogue signals have been popular for many years, the HVAC market starts to discover the advantages of digital control signals. Communication networks like e.g. Modbus RTU are increasingly used to distribute information. Also here, different technologies are available.

In the HVAC industry, Modbus RTU is a commonly used communication network. Modbus RTU is a serial communication protocol, based on RS485 technology. It guarantees a reliable communication between different devices over longer distances.

How to control fan speed?


Thus, a control signal is required to regulate fan speed. That control signal instructs the EC motor or the AC fan speed controller (and AC motor) to maintain a certain speed. The control signal (specifying the requested fan speed) can be generated manually via a knob or automatically via an HVAC sensors or via an HVAC controller. Automatic fan speed control is often referred to as ‘demand based fan speed control’.

If you want to control fan speed manually, the control signal can be generated via:

Control signals diagram
  • Control switch - A device thtat generates a 0-10 Volt signal in steps. The advantage of control switches is that a limited number of predefined fan speeds can be selected easily.
  • Potentiometer - A device that generates a continuously variable 0-10 V control signal. Potentiometers are available with or without OFF position. The control signal can be generated from high to low (10 – 0 Volt) or from low to high (0 – 10 Volt).

If you want to control fan speed automatically or demand based, the control signal can be generated via:

  • Intelligent HVAC sensors - Intelligent HVAC sensors are typically used to control extraction fans or ceiling fans. A standard HVAC sensor or HVAC transmitter measures certain values like temperature, humidity, CO2, air quality, etc. The measured values are transmitted via analogue outputs or via Modbus RTU communication. Intelligent HVAC sensors have one output. This output is controlled by an intelligent algorithm. The output controls fan speed to keep one or more parameters within range. E.g. if fan speed needs to increase when CO2 or humidity levels are too high, an extraction fan can be controlled via an intelligent CO2 – humidity sensor. The intelligent sensor’s output controls the EC fan or AC fan speed controller. Fan speed will increase when CO2 or humidity level are increasing. More information about HVAC sensors can be found on www.air-quality-monitor.eu.
Intelligent HVAC sensors
HVAC controllers
  • HVAC controllers - To control a complete ventilation system, Heat Recuperation Unit or Air Handling Unit, the market offers universal controllers which have to be programmed. Depending on the connected devices, this can be reasonably to very complex. However, full-range manufacturers also offer complete, turn-key solutions that are often more efficient as the complex universal HVAC controllers. Since these HVAC control solutions are made for a specific application, they are usually very user-friendly, intelligent and easy to install.

How does an electric motor work?


So we distinguish EC motor and AC motor with voltage controllable motor or AC fans with induction motor. In the next chapters, we will explain the difference between these motor types in detail.

Let’s start with the basics: An electric motor is a machine that converts electrical energy into mechanical energy. The interaction of a magnetic field and electric current in a coil (motor winding) generates a force (torque) on the motor shaft.

It is not possible to explain the working principle of an electric motor without some basic laws of physics. The most relevant physical laws are Faraday’ law of induction, the law of Lenz and the Lorentz force. Without going too much into details, we will try to explain what these laws are about.

How does an electric motor work - diagram 1

At the end of the 19th century, more and more experiments were done with electromagnetism. Mr. Lorentz discovered that when a current-carrying wire (or coil) moves through a magnetic field, it experiences a force. This force is called the Lorentz Force. This is the fundamental operating principle of a magnetron, radar, and… an electric motor. The current-carrying conductor moves through the magnetic motor field and is pushed away by the Lorentz Force. This theory explains why an electric motor rotates and how motor torque is created. It explains how electrical energy is converted into kinetic energy.

How does an electric motor work - diagram 3

So for an electric motor to work, a current-carrying conductor and a magnetic field are required. Some new, expensive motors use permanent magnets to create this electric field, but the majority of the electric motors use the principle of electromagnetic induction to create this magnetic field. Faraday’s law of induction explains how electromagnetic induction works. It is one of the basic laws of electromagnetism and it tells us how a changing magnetic flux induces electric current in a coil. It is the fundamental operating principle of electric transformers, motors, generators, etc. Based on this law, Mr. Emil Lenz described the induced EMF and the direction of the induced current in Lenz law.

Enough theoretical knowledge. Now some background information about the mechanical construction of a motor. A motor has a moving part and a stationary part. We call the moving part the rotor, the stationary part is the stator.

In most classic AC motors, the motor windings (coils) are integrated in the stator. The rotor holds the motor shaft in position. The rotor is mounted in the stator with ball bearings. Some motor types have an external rotor. In that case the rotor is mounted around the stator.

So we distinguish EC fans and AC fans with voltage controllable motor or AC fans with induction motor. In the next chapters, we will explain the difference between these motor types in detail. Remember that an EC fan can be seen as an AC fan with integrated fan speed controller.

An AC fan requires a separate speed controller to regulate fan speed. The different technologies with their advantages and disadvantages will be discussed in the next chapters.

How does an electric motor work - diagram 4