Why are capacitors used in fans?

 

An ordinary ceiling fan (a split-phase induction motor) essentially consists of a Running Winding and a Starting Winding. It doesn’t have the property of self-starting. The capacitor is connected in series with the starting winding. Here’s a circuit diagram:

I’ll first talk about what will happen in the absence of the capacitor…

Imagine there was no capacitor in the circuit. That would mean the Starting winding and the Running winding are connected in parallel with the 220V AC supply. Both windings will be having the same phase. When we have same phase for 2 windings there will be no magnetic rotation! And that means the Rotor will NOT rotate in the absence of the capacitor.

Here’s the main problem : To create magnetic rotation, we need to supply 2 different phases for each windings. But in our homes, we have single phase connection.

This problem is resolved by simply adding that capacitor in series with the Starting winding.

Now when the capacitor is present, same phase enters the Running winding and the capacitor. The phase which enters the capacitor will get shifted in phase inside the capacitor (leading by about 90°) and comes out. And now this shifted phase passes through the Starting winding while the original applied phase is in the Running winding. This provides a starting torque.

Since now the voltages across the starting winding and the running winding have different phases, there will be magnetic rotation and the Rotor starts rotating!

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