Different kinds of fan speed control switch
Voltage Control is the most popular method of speed control of induction motor driven fans. As the name suggests it controls the voltage appearing across the fan terminals. Theoretically fan torque is proportional to square of voltage applied. Below are three popular switch of voltage control for a fan.
1. Resistive/Rheostat control switch: These are the switch used back in old days. They are bulky and heats up at lower speeds. Here a variable resistor is connected in series with the fan. At full speed the resistance is zero. To decrease speed we increase the resistance by turning the knob. (Usually by steps). This will cause some voltage drop across the resistor. So the fan terminals receive a reduced voltage. The fan will rotate at a lesser speed. The drawback is the power dissipation in the resistor. ie to get reduced speed we are wasting some power into a resistor which causes heating and energy wastage. Due to this fan operating at full speed and lower speeds will be consuming almost same power. There will be heat sinks and openings on top to dissipate heat from resistors coils which makes the design bulky.
2. Capacitor Control switch: It is becoming popular nowadays. It is very similar to resistive control but less bulky and energy efficient. A variable capacitance in series (usually few capacitors connected together with some tappings corresponding to each step) is used in this regulator. As we turn the knob the capacitance increases and it reduces the voltage available to the fan. For example the fan rated 230V may get only 180V due to voltage division ( Rated voltage in India is 230V). This will reduce the fan speed. The interesting point to note is that there is nearly no power loss in this method. The losses incurred is due to resistive losses inside the capacitor which is negligible. So a fan with rating 230, 50W will consume only 25-30 Watts at low speeds. There is no heating problems and the capacitor improves the power factor of the circuit. They are less bulkier than resistive controllers.
3. Electronic regulator switch: Another popular controller. This utilizes power electronic devices such as Diac and Triac (they are essentially semiconductor switches). They chops the supply voltage waveform and reduces the effective voltage. Its like turning on/off the circuit at regular intervals (100/120 times in a second). These controllers are called AC full wave regulators. They are energy efficient as losses are negligible. The speed can be reduced by changing how much time the waveform is turned on and off. Fan at lower speeds consumes lesser energy.