What Is the Role of the Capacitor in Starting and Running a Ceiling Fan?

 A ceiling fan uses a single-phase induction motor, which cannot start on its own because it produces no initial rotating magnetic field. This is where the capacitor becomes essential. It performs two major functions:



1. To Start the Fan: Creating a Rotating Magnetic Field

Why a capacitor is needed for starting

In a single-phase AC supply, the motor produces only an alternating magnetic field—not a rotating one.
A rotating magnetic field is needed to start the rotor spinning.

How the capacitor solves this

  • The ceiling fan has two windings:
    Main winding and auxiliary (starting) winding

  • The capacitor is connected in series with the auxiliary winding.

  • The capacitor shifts the phase of the current in the auxiliary winding.

Result

The phase shift creates a second magnetic field, which is out of phase with the main one.
These two fields combine to produce a rotating magnetic field that gives the fan the initial push to start.

Without the capacitor, the fan will hum but not start.

2. To Run the Fan Smoothly: Improving Torque and Efficiency

Even after the fan starts, the capacitor continues to work.

Running role of the capacitor

  • It maintains the phase difference between the two windings.

  • This keeps the magnetic field rotating steadily.

  • Ensures:

    • Efficient airflow

    • Higher torque

    • Stable speed

    • Lower power consumption

Fans that use two capacitors (one for starting, one for running) are called capacitor start–run motors and offer smoother performance.

3. Why Speed Changes When You Change the Capacitor

The capacitor value (usually 1.5–2.5 µF) affects the:

  • Phase shift

  • Motor torque

  • Speed

If the capacitor weakens:

  • The fan runs slowly

  • Takes time to start

  • May not start at all

Replacing the capacitor restores normal speed.



4. Summary

FunctionWhat the Capacitor Does
StartingCreates a rotating magnetic field by shifting current phase
RunningMaintains phase difference for smooth torque and airflow
SpeedDetermines efficiency and rotational speed

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