What is Shaded-Pole Motor ?
Introduction :-
The Shaded-Pole Motor is the AC single phase induction motor. A Shaded-Pole motor is a small squirrel cage motor.
In which the auxiliary winding is constructed of a copper ring or bar surrounding a portion of each pole. When single phase AC supply is given to the stator winding and then rotating magnetic field is generated. This auxiliary single-turn winding is called a shading coil.
Construction:-
Shaded-Pole motor have a two main part stator and rotor.
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| Shaded-Pole Motor inner construction |
Stator :
The stator has salient poles. Usually 2 to 4 poles are used. This poles has it's own exciting coil. The exciting coil are wrapped between the pole. This exciting coil is known as the shading coil. The laminated poles are made by a thin high-grade alloy steel to reduce eddy current losses. A slot is cut across the lamination of the pole. The slot is placed approximately one-third distance from the edge of the pole. So we can call this part as the shaded part and other parts of the pole as unshaded part.
Rotor:
The rotor of shaded Poe induction motors is the squirrel cage type rotor. The rotor bars are provided with a 60° skew. This is to obtain an optimum Starting torque and for limiting the torque dip during the run up. Air-gap length between stator and rotor is of the order 0.25 to 0.5 mm. Too short air gap may result in starting torque variation due to rotor slotting.
The shaded pole induction motor has no commutator, brushes, collector and ring. So, it is relatively cheaper simpler and extremely rugged in construction and reliable.
Shaded-Pole Motor working principle:-
When the AC supply is connected to the main winding of the rotor. The sine wave current passes through the winding.
Let, us consider the positive half cycle of the AC current as shown in fig below.
When the current raises from "Zero" value of point "0" to a point "a" the current change it's polarity rapidly. Hence, it reduces an EMF in the shaded coil on the basic of Faraday law of electromagnetic induction.
The small portion of the flux link with the shaded coil of the motor as because it is short-circuited. The variation in the flux induces the voltage inside the ring because of which the circulating current induces in it.
The induced circulating current in the shaded coil produces a flux in turn. Produces a flux in according with Lenz's Law. This induced flux opposes the main flux of the motor. The flux induces in the shaded portion of the motor (a) and the unshaded portion of the motor (b) have a phase difference. The main motor flux and the shaded ring flux are also having a space displacement by an angle of 90° .
As there is time and space displacement between the two fluxes, the rotating magnetic field induces in the coil. The rotating magnetic field develops the starting torque in the motor. The field rotating from the unshaded portion to the shaded portion of the motor.
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