How can the speed of a table fan be controlled?

• Voltage control is the most popular method of speed control of induction motor-driven fans such as table fans.
• This method controls the input voltage appearing across the fan terminals. Theoretically, fan torque is proportional to the square of the voltage applied. 

Popular three methods of voltage control are listed.

Resistive/Rheostat control:

• In this method, a variable resistor is connected in series with the fan. At full speed, the resistance is zero.
• The speed can be varied by varying the resistance by turning the knob (Usually by steps).
• This will cause some voltage variation across the resistor and the fan will rotate at a different speed.
• This method is not used now a days due to resistors are bulky & high-power wastage across resistors.

Capacitor Control:

• It is becoming popular now a days. 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 tapping corresponding to each step) is used in this regulator.
• As we turn the knob the capacitance changes and it varied the voltage available to the fan. This will change the fan speed.
• There are no heating problems and the capacitor improves the power factor of the circuit.

Electronic regulator:

• This utilizes power electronic devices such as Diac and Triac (they are essentially semiconductor switches).
• They chop the supply voltage waveform and reduces the effective voltage. It is like turning on/off the circuit at regular intervals (100/120 times in a second).
• These controllers are termed as 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. A fan at lower speeds consumes lesser energy.