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The Working Principle of Shaded Pole Fan Motor

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The Working Principle of Shaded Pole Fan Motor

Posted by Admin | 21 Apr
The working principle of Shaded Pole Fan Motor is as follows:
When AC current is passed through the power coil of stator winding, pulsating magnetic flux will be produced. As this pulsating magnetic flux changes, an induced EMF is produced in the shading ring which is made of high conductivity copper material.
The induced EMF of the shedding coil opposes the main flux by following Lenz’s law. As the flux reaches its maximum value, this induced EMF increases and a current is generated in the copper ring which flows in such a direction that it opposes the incoming flux.
This induced current in the ring produces an EMF in the circuit of the shaded portion which is short-circuited by this induced EMF. Since this part of the copper ring is short-circuited, a current is generated which flows in such a direction that it opposed the main flux.
As this induced current passes through the non-banded portion of the copper ring, it creates a lag in the change in magnetic field. This lag is such that it makes the magnetic center of attraction (MA) in the non-banded portion of the circuit slightly off-set from the MA in the banded portion.
It also causes a lag in the increase in magnetic field on the non-banded portion of the coil. This lag produces a rotating component in the magnetic field that helps the rotor to start turning.
A shift in the stator flux from the unshaded to the shaded portion of each pole occurs periodically during the negative half cycle of the alternating-current cycle. This shifting flux gives to some extent a rotating field effect which generates a low-starting torque and enables the motor to be self-starting.
In the first half cycle of the AC waveform, the alternating current produces an induced EMF in the copper ring which has a higher rate of rise than the other regions of the cycle. This increases the circulating current in the copper ring which again produces an induced EMF that opposes the main flux.
During the second half cycle of the AC cycle, the induced EMF in the copper ring becomes much lower and this decreases the circulating current in the copper ring. This again induces an induced EMF that opposes another portion of the main flux and this cycle continues.
As the induced EMF in the ring decreases, this reversing sequence occurs again and again. Eventually, the induced EMF is nearly non-existent and there are practically no opposing fluxes in the circuit of the shaded portion of the pole.
This reversal sequence is repeated in the complete cycle of the AC waveform and this is the basic working principle of shaded pole induction motors. As the alternating current continues, this sequence repeats again and again to produce a rotation in the magnetic field of the motor. The resulting rotating field is used to drive the rotor of squirrel-cage type motors.