The common method of AC fan speed control
1.AC fan speed control by using TOROIDAL TRANSFORMER.
You can use TOROIDAL TRANSFORMER to supply lower voltages to the fan in order to reduce speed of the fan.
For example, if you are using 230Vac fan, you can supply 110v to the fan for low speed or supply 160v for medium speed.
Some of our customers use TOROIDAL TRANSFORMER to change 230v to 2 lower voltages.
Then, You only need to design an electrical circuit for switching 3 different voltages to connect to the fan.
For instance, when you switch to low speed, the electrical circuit supplly 110V to the fan.
When switching to medium speed, the electrical circuit connect 160V to the fan.
It does not cause damage to fans by providing a lower voltage to fans.
The advance to of this control method is that the fan can be running very smooth when running at low speed.
However, the drawback is the materials cost is higher than second and third methods because of high-cost TOROIDAL TRANSFORMER.
Some of our customers using this control method in the medical equipment which has a strict requirement for noise.
2.Using “dimmer” to control AC fan speed.
You can use a diac and triac to control supply voltages.
This method is to control supplying time of voltage.
This technic is widely used as a “dimmer” for controlling light bulb.
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The advance of this AC fan speed control method is that the cost for producing the control unit is much cheaper than using TOROIDAL TRANSFORMER.
On the other hand, the fan will cause vibration and ticked sound by using this AC fan speed control method.
Please check if your application is able to accept small ticked sound.
Moreover, the control range is limited if this method is used for controlling shaded pole motor.
3.《ONLY for Capacitor-Start Motor》Simply changing the capacitor of fans to carry out AC fan speed control.
Reducing Capacitance value of the capacitor is also one of AC fan speed control method. The advance to use this method is that this is a low-budget method and fans are running smoothly at lower speed.
On the other hand, the speed range that is controlled by this method is not wide. Also, the capacitors are degrading when start using and it is suggested to change capacitors which are used for controlling speed every two years.