Motor Design

Ceiling fan capacitor help

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.

Split-phase induction motor

Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch.

In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the stationary centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding.

The phase of the magnetic field in this startup winding is shifted from the phase of the mains power,...