Ceiling Fan

Capacitor-run motor induction motor

A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.

In a ceiling fan there is an single phase induction motor.
Single phase stator produces a non rotating, pulsating magnetic field.The single coil excited by a single phase current produces two counter rotating magnetic field phases, coinciding twice per revolution at 0 degrees and 180 degrees. When the phases rotate to 90 degree and -90 degree they cancel. At 45 deg and -45 deg they are partially additive along the +x axis and cancel along the y axis. The sum of these two phases is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed.

So, in order to generate the starting torque we have to split the phase in two. This configuration comprises two windings main winding W1 and auxiliary winding W2, a centrifugal switch SW1 and a capacitor.

The two windings are wound with a geometric offset, effectively making a second set of poles phase shifted within the stator. The capacitor provides a phase shift to the current flowing in W1 and we therefore have a "two phase" motor while the switch is closed. When the motor is almost up to speed, the switch opens disconnecting W1 and the capacitor. The motor can be reversed by reversing the connections of either W1 or W2 (but not both)

The start winding (W1) and the start capacitor provide for a rotating magnetic field in one direction enabling the motor to start.

Most ceiling fans use a type of motor known as a "permanent split capacitor motor". These motors have two coils, a "start" and a "run" winding. A capacitor is connected in series with the start winding to insert a phase shift, but once the motor is up to speed it becomes an auxiliary winding. In order to reverse the motor, the capacitor is applied to the "run" winding which then becomes the "start" winding. In some cases the windings may have taps to provide different speeds, in others the value of the start/run capacitor is lowered to provide lower speeds. Yet other designs will have additional capacitors wired in series with the motor as a whole to regulate speeds.

The motor doesn't have to be very strong to move the air once the blades are moving, but that requires some torque so the capacitor start motor gives higher torque in proportion to the running power.

The ceiling fan works to fan air downwards in the correct rotation. The ceiling fan motor consists of a starting and running winding. The capacitor is usually connected in series
with the starting winding to give the unit enough starting torque. If the capacitor gets busted, the fan will not rotate.

Single phase induction motors do not have any starting torque. Starting torque is provided by a second winding that is phase shifted. The capacitor provides this phase shift. Ce ling fans generally have motors with capacitor permanently installed which is considered the quietest motor.