Shaded-Pole AC Induction Motor
A shaded-pole AC induction motor is one of type of AC induction motor that does not have start winding but has only one main winding. This type usually called “disposable motors because it better replace than repair. Besides cheap, the other advantages of this type are the speed can be adjusted by through a multi-tap winding or varying voltage, simple and can be produced in high volume. Here is the typical circuit of shaded-pole AC induction motor :
This type does not have start winding, so to start this motor use the means of the circuit. a continuous copper loop around small portion of each of motor poles is rounded by the means of this circuit. The magnetic field in shaded area will lag behind the field in the unshaded area because of the shaded portion of the pole. So, the shaft rotating can be achieved due to the reaction of the two fields.
However, this circuit has a low starting torque typically 25% to 75% of rated torque. Beside that, the other disadvantages of this circuit are has very low efficiency(below 20%) and it is a high motor with running speed 7% to 10% below the synchronous speed. The applications that uses this type is the applications where the high torque, high efficiency and lot sturdy don’t be required like multi-speed fans for household use. [Schematic diagram source: Microchip Application Note]