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Circuit GlobeInduction MotorShaded Pole Induction
Motor
SHADED POLE INDUCTION MOTOR
Definition: The shaded pole induction motor is simply
a self-starting single-phase induction motor whose one of the pole is shaded by
the copper ring. The copper ring is also called the shaded ring. This copper
ring act as a secondary winding for the motor. The shaded pole motor rotates
only in one particular direction, and the reverse movement of the motor is not
possible.
WHY THE SHADED POLE INDUCTION MOTOR DESIGNS FOR LOW
POWER RATING?
The power losses are very high in the shaded pole
induction motor. And the power factor of the motor is low. The starting torque
induces in the induction motor are also very low. Because of the following
reasons the motor has poor efficiency. Thus, their designs are kept small, and
the motor has low power ratings.
CONSTRUCTION OF SHADED POLE INDUCTION MOTOR
The shaded pole motor may have two or four poles. Here
in this article, we use the two pole motor for the sake of simplicity. The
speed of the motor is inversely proportional to the number of poles used in the
motor.
Stator – The stator of the shaded pole motor has a
salient pole. The salient pole means the poles of the magnet are projected
towards the armature of the motor. Each pole of the motor is excited by its
exciting coil. The copper rings shade the loops. The loops are known as the
shading coil.
The poles of the motor are laminated. The lamination
means multiple layers of material are used for making the poles. So, that the
strength of the pole increases.
The slot is constructed at some distance apart from
the edge of the poles. The short-circuited copper coil is placed in this slot.
The part which is covered with the copper ring is called the shaded part and
which are not covered by the rings are called unshaded part.
Rotor – The shaded pole motor uses the squirrel cage
rotor. The bars of the rotor is skewed at an angle of 60º. The skew can be done
for obtaining the better starting torque.
The construction of the motor is very simple because
it does not contain any commutator, brushes, collector rings, etc. or any other
part. The shaded pole induction motor does not have any centrifugal switch.
Thus, the chances of failure of the motor are less.
The centrifugal switch is the type of electrical
switch that starts operating by using the centrifugal force, generated by the
rotating shaft. It is also used for controlling the speed of the shaft.
SHADED POLE INDUCTION MOTOR WORKING
When the supply is connected to the windings of the
rotor, the alternating flux induces in the core of the rotor. 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 circulating current develops the flux in the ring
which opposes the main flux of the motor. The flux induces in the shaded
portion of the motor, i.e., a and the unshaded portion of the motor, i.e., 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°.
The connection diagram of the Shaded Pole Motor is
shown below.
Shaded pole induction motor figAs 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 rotates from the unshaded portion to the shaded portion of the
motor.
APPLICATIONS OF THE SHADED POLE INDUCTION MOTOR
The various applications of the Shaded Poles Motor are
as follows:-
They are suitable for small devices like relays and
fans because of its low cost and easy starting.
Used in exhaust fans, hair dryers and also in table fans.
Used in air conditioning and refrigeration equipment
and cooling fans.
Record players, tape recorders, projectors,
photocopying machines.
Used for starting electronic clocks and single-phase
synchronous timing motors.
This type of motor is used to drive the devices which
require low starting torque.