Shaded Pole Induction Motor

Shaded-Pole induction Motors

• Like any other motors the shaded pole induction motor also consists of a stator and rotor. The stator is of salient pole type and the rotor is squirrel cage type.

• The poles of shaded pole induction motor consist of slots, which are cut across the lamination . The smaller part of the slotted pole is short-circuited with the help of a coil. The coils are made up of copper and it is highly inductive in nature. This coil is known as shading coil. The part of the pole which has the coil is called the shaded part and the other part of the pole is called un-shaded part.

 

Construction of Shaded Pole Induction Motor:

 

A shaded pole motor may be 2 pole or 4 pole. Here we are considering a 2 pole shaded pole motor. The pictures in this article also shows a 2 pole motor.

Stator:

• The stator has salient poles. Usually 2 to 4 poles are used. Each of the poles has its own exciting coil. A part of each pole is wrapped by a copper coil. The copper coil forms a closed loop across each pole. This loop is known as the shading coil.

• The poles are laminated. A slot is cut across the lamination of the pole. The slot is approximately one third distance from the edge of the pole. The short circuited copper coil described above is placed in this slot. So we can call this part as the shaded part and other part of the pole as unshaded part. 

• Selecting a 2 poled stator gives a synchronous speed of 3000 rpm while a 4 poled stator speed will be 1500rpm for 50Hz supply.

Rotor:

• The rotor of shaded pole induction motors is Squirrel Cage type rotor. The rotor bars are provided with a 60 degree skew. This is to obtain an optimum starting torque and for limiting the torque dip during run up.

WORKING:

The working operation of  shaded pole motor can be easily understood .Our main aim is to produce rotating magnetic field with single phase AC supply this is possible with shaded poles.

• Alternating flux is produced by input ac supply .We can observe waveform of the flux in below figure .The distribution of this flux in the pole area is greatly influenced by the role of copper shading band .Now assume our ac supply as sine wave as shown in 1. 

During the portion OA:

• In OA portion  the alternating-current cycle [1], the flux begins to increase and an e.m.f. is induced in the shading coil. The resulting current in the shading coil will be in such a direction (Lenz’s law) so as to oppose the change in flux. Thus the flux in the shaded portion of the pole is weakened while that in the un-shaded portion is strengthened as shown in figure 2.

During the portion AB:

• 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 [Fig 3] since no current is flowing in the shading coil. 

During the portion BC:

• As the flux decreases (portion BC of the alternating current cycle), current is induced in the shading coil so as to oppose the decrease in current. Thus the flux in the shaded portion of the pole is strengthened while that in the un-shaded portion is weakened as shown in Fig 4.

• The effect of the shading coil is to cause the field flux to shift across the pole face from the un-shaded to the shaded portion. This shifting flux is like a rotating weak field moving in the direction from un-shaded portion to the shaded portion of the pole .The rotor is of the squirrel-cage type and is under the influence of this moving field. Consequently, a small starting torque is developed. As soon as this torque starts to revolve the rotor, additional torque is produced by single-phase induction-motor action. The motor accelerates to a speed slightly below the synchronous speed and runs as a single-phase induction motor.

Advantages:

• ·         Very cheap and reliable

• ·         Easy to construct

• ·         Extremely rugged in nature

Disadvantages:

• ·         Low efficiency

• ·         Low starting torque

• ·         Since the shading coil is made of copper, the copper loss is high.

Uses:

• Due to their low starting torques they are mostly employed in small instruments, toys, small fans, electric clocks, hair dryers, ventilators, circulators etc.

• Thus we have discussed in detail the working, constructional features and applications of Shaded pole Single Phase motors.