The shaded-pole motor is the original type of AC single-phase motor, dating back to at least as early as 1890.[1] A shaded-pole motor is a motor, in which the auxiliary winding is composed of a copper ring or bar surrounding a portion of each pole to produce a weakly rotating magnetic field.[2] When single phase AC supply is applied to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated. This auxiliary single-turn winding is called a shading coil. Currents induced in this coil by the magnetic field create a second electrical phase by delaying the phase of magnetic flux change for that pole (a shaded pole) enough to provide a 2-phase rotating magnetic field. The direction of rotation is from the unshaded side to the shaded (ring) side of the pole.[2] Since the phase angle between the shaded and unshaded sections is small, shaded-pole motors produce only a small starting torque relative to torque at full speed. Shaded-pole motors of the asymmetrical type shown are only reversible by disassembly and flipping over the stator, though some similar looking motors have small, switch-shortable auxiliary windings of thin wire instead of thick copper bars and can reverse electrically. Another method of electrical reversing involves four coils (two pairs of identical coils).[3]
The common, asymmetrical form of these motors (pictured) has only one winding, with no capacitor or starting windings/starting switch, making them economical and reliable. Larger and more modern types may have multiple physical windings, though electrically only one, and a capacitor may be used. Because their starting torque is low, they are best suited to driving fans or other loads that are easily started. They may have multiple taps near one electrical end of the winding, which provides variable speed and power by selection of one tap at a time, as in ceiling fans. Moreover, they are compatible with TRIAC-based variable-speed controls, which often are used with fans. They are built in power sizes up to about NaNhp output. Above NaNhp, they are not common, and for larger motors, other designs offer better characteristics. A main disadvantage is their low efficiency of around 26%.[4] A major advantage is that the motor's stall current is only slightly higher than the running current, so there is low risk of severe over-heating or tripping the circuit protection if the motor is stalled for some reason.
Even by the standards of shaded pole motors, the power output of these motors is usually very low. Because there is often no explicit starting mechanism, the rotor of a motor operating from a constant frequency mains supply must be very light so that it is capable of reaching running speed within one cycle of the mains frequency.A further development dispenses with the shading rings altogether. The application of power gives the magnetised rotor enough of a 'flick' to move it fast enough to establish synchronism. A mechanical means prevents the rotor from starting in the wrong direction. This design will only work satisfactorily if the standstill load is near to zero and has very little inertia. This is similar to the motor used in quartz-timed mechanical clocks.In more recent times, the use of variable frequency controls permits synchronous motors to start slowly and deliver more torque.