Why does a speed of ceiling fan decreases?

There are 2 Answers to this Question.(Simple and Long)

But before you read the answer and decide what to do, please clean your fan blades regularly because it will make a lot of difference in fan speed. Please try it.220V DC Brushless External Rotor Motor Fan

Let's get to the Answers.

Simple Answer is Capacitor which connected to the fan is lost its capacitance value. If you replace the capacitor it will work as new fan.

And now for long answer

Ceiling fan is a single phase induction motor. This motor is NOT self-starting i.e. it requires external force to run it at the time of starting. For starting any type of motor there must be resulting magnetic field to create torque to rotate the rotor.

In single phase motor without capacitor the field created by stator winding is positive in half cycle and negative in other half so resulting net field is zero so torque is also zero. To overcome this drawback, a capacitor is used. It is connected across starting winding and running winding in the fan motor. This creates a phase difference between the windings. Thus, a field is produced which is resultant of the two fields. The rotor rotates according to the direction of the resultant field.

Capacitor is not only used to start your fan but also to run. see below diagram how it is connected.

Where C is capacitor and L & N are Live, Neutral which is our input power supply. You can see that capacitor is connected across two windings, L2 is starting winding and L1 is running winding in fan. once power was given, capacitor will create a RMF(Rotating Magnetic Field) in motor. if capacitor is not connected or damaged fan just produce humming noise and start heating and damage the windings also.

As mentioned above, Capacitor creates RMF and thus torque produces in motor and motor will rotate. As long as capacitor produces good torque fan will rotate at good speed. once capacitor losses its capacitance value torque will be reduced resulting in slowing the fan speed.

Below mentioned the capacitor which uses in regular ceiling fans in India.

Polyester/Film Capacitors are used in ceiling fans. There are no defined polarities for these type of capacitors.

They are of rating approx ~2.2uF/440V for a 50-60 Watt fan.