Shaded pole induction motor is constructed with salient poles in the stator. Each pole has its own exciting coil. A 1/3rd portion of each pole core is surrounded by a strap of copper forming a closed loop called the shading coil. The rotor is a usual squirrel cage type.
Shaded Pole Induction Motor Working Principle
- During the portion OA of the alternating-current cycle, the flux begins to increase and an EMF is induced in the shading coil. This flux induces voltage and hence current in the copper ring, and by Lenz’s law, the direction of current is such that it opposes the flux entering the coil. Hence, in the beginning, the greater portion of flux passes through the unshaded side of each pole.
- During the portion AB of the alternating-current cycle, the flux has reached almost maximum value and is not changing. Consequently, the flux distribution across the pole is uniform since no current is flowing in shading coil.
- As the flux decreases (portion BC of the alternating-current cycle) the current induced in shading coil now tends to prevents the flux linking with shading coil from decreasing and hence the greater portion of flux passes through the shaded side of each pole.
- Thus the flux reaches its maximum value first at the shaded side of the pole and later reaches its maximum on the unshaded side of the pole. This gives a motion of flux across the pole face in the direction of arrow (in the direction of rotation of rotor) as shown in the figure above.
- Under the influence of this flux small starting torque is produced. As soon as this torque starts to revolve the rotor, additional torque is produced for running because of the single phase induction motor action. The shaded pole motor accelerates to a speed slightly below synchronous speed and runs as single phase motor.
Characteristics of Shaded Pole Induction Motor
- Shaded pole induction motor produces very small starting torque about 50% of full load torque.
- Efficiency of shaded pole motor is low because of continuous power loss in shading coil.
- Used for small fans and small appliances.
- The direction of rotation of shaded pole motor depends upon the position of shading coil i.e. which half of pole is wrapped with shading coil. Therefore, the direction of rotation cannot be reversed unless the machine is constructed so that the shading coil can be shifted to another half of the pole.
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