How a Solid State Fan Speed Control Work | 12 Ways it Works


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A solid-state fan speed control is described, which allows the fan to be controlled to run quietly, thereby eliminating the annoying buzzing that the winding would otherwise generate. Let’s learn how a solid state fan speed control work.

For single-phase motors in fans, these controllers allow step-less electronic speed control. The speed can be adjusted between a minimum and maximum setting.

What is a Fan speed controller?

A fan speed controller or fan controller at low ambient temperature is a controller that is used to control the fan speed of an air-cooled condenser in a direct expansion system of a heat pump. The heat pump is used in temperate climates and can operate in cooling or heating mode. When used as a heating mode in winter, the inner coil acts as a condenser, and the outer coil acts as an evaporator.

When the unit is set to summer cooling mode, the inner coil will act as an evaporator and the outer coil as a condenser. This is achieved by using a 4-way valve that reverses the direction of refrigerant flow in the system. In the United States, the check valve will usually operate in cooling mode when the check valve is on and will operate in heating mode when it is off.

Fan Speed Controller Function

How A Solid State Fan Speed Control Work

When the outside temperature is low, such as 50 ° F (10 ° C) or lower, we usually want to run the heat pump in thermal mode to heat the room and keep the human body comfortable.

However, there are times when we want the heat pump to run in cooling mode, even when the ambient temperature is low. These locations may include the following.

  • Data centers or server rooms where all information technology equipment must be kept cool and at a stable temperature.
  • Base radio stations.
  • Supermarkets.
  • Food storage areas.

If you continue to operate the heat pump in cooling mode, the head will drop. The expansion valve requires a certain head pressure to function properly, and if the head pressure continues to drop, it stops the air conditioning system. This freezes the evaporator and blocks the compressor.

Compressor clogging is caused by liquid refrigerant entering the compressor cylinder during operation. This will damage the compressor’s valves, bearings, and other mechanical parts.

The solution to this problem is to install a fan speed controller that can control the speed of the external condenser fan. To use the controller, check the fan specifications to ensure the fan can be controlled by changing the supply voltage.

The sensor will be installed on the condenser duct, where its temperature will control the fan speed. Adjusting the speed does not reduce the head pressure too low, so the heat pump can still run in cooling mode even when the outside temperature is low.

How to Use a Fan Speed Controller

How to Use a Fan Speed Controller

A hard start option will be available in cooling mode, which increases the fan output voltage for the first few seconds to ensure proper fan operation. The temperature is then changed to match the air temperature.

  • The output voltage is not regulated in heating mode. It will run on the highest possible mains voltage. 
  • The condensing temperature affects variable field speed.
  • The current at which the output is rated. Some are capable of delivering up to 10 amps of current.
  • A wide range of voltages can be used as inputs.
  • Isolated input for powering the control unit (e.g., 24 VAC) is a matter of safety.

How a Solid State Fan Speed Control Work?

How a Solid State Fan Speed Control Work
  • Make sure the device is turned off before opening the junction box.
  • Existing wiring harnesses connect the contactor to the motor wiring harness in the electrical box of the device. For motors that receive semiconductor ESC, these wiring harnesses will be replaced with the new wiring harnesses included in the kit.
  • DO NOT remove the wiring harnesses that directly connect the motor to the junction box.
  • Several electrical plugs are connecting a semiconductor line for speed control in the base of the electrical box. Only remove the required number of plugs.
  • Remove the existing wiring harness inside the junction box that connects the unit contactor to the motor harness adapter.
  • Replace the existing wiring harness with the supplied wiring harness. This wiring harness is adapted for connection to and from a semiconductor speed controller, which must be connected in series with a variable speed mains voltage motor. Discard the original wiring harness that was removed from the device.
  • Using the two-position wire connectors connected to the replacement wiring harness, connect the on-site wires that will lead from the unit to the solid-state speed controller.
  • Using the supplied cable ties, secure and clean the new wiring harness inside the unit’s junction box.
  • Properly route the locally supplied wires from the unit to the solid-state speed controller. Use the free connectors provided in the semiconductor ESC chaining kit with the device cabling.
  • Follow the manufacturer’s directions for grounding solid-state speed controllers. Use the supplied connector to connect the ground wire to the cruise control.
  • Verify that the semiconductor ESCs are properly installed in the standard electrical cabinet and secured by the controller manufacturer’s instructions.
  • Make sure that all unit electrical cabinets and doors are properly closed.

Frequently Asked Questions 

How does fan speed control work?

The idea of the capacitor controller remains the same. It is about regulating the voltage on the fan motor. As the capacity of the capacitor grows, the voltage on the capacitor falls while the voltage on the fan motor grows, which allows the fan speed to increase.

How does the fan change speed?

The fan speed is reduced by connecting the capacitor in series with the switched power supply to the fan, thus reducing power. The condenser acts as a dropper. The speed depends on the capacitance of the capacitor in the circuit. To obtain different speeds, the speed control switch selects a different capacitor.

Can you set the ceiling fan speed?

Check the draft switch on the fan. An incorrect switch or speed setting will cause the fan to run slowly. Turn off the fan and let it stop. Turn the screw, switch the fan to the lowest setting, pull the chain, and listen to the fan motor as you go through the progressive speed settings.

Why is my ceiling fan running?

The cause of a ceiling fan’s slow rotation is nearly usually dirty or damaged ball bearings. The fan blades move slowly or not at all at full speed, causing the motor to overheat.

Why is a triac used in the fan controller?

Triac is used as a dimmer in this circuit because it is easy to design and operate and economical due to high efficiency and low acquisition costs. This is a schematic diagram of the simplest dimmer for a lamp or fan. The circuit works by changing the angle of the triac.


A solid-state speed controller includes components like triac, semiconductor devices with no moving elements that control current flow. They are useful because they can be controlled relatively accurately and turn specific signal parts on and off.

One of the problems with starting the fan is the inductance of the fan or the capacity of the starting winding. It causes problems with the precise control of the tripping times and the start of the triac. Hope you have learned how a solid state fan speed control work.

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