Thursday, April 10, 2014

Shaded Pole Induction Motor

Shaded-Pole Induction Motor
 Cutaway view of a totally enclosed fan-cooled shaded-pole induction motor
Figure 1: Cutaway view of a totally enclosed fan-cooled shaded-pole induction motor
A shaded-pole motor maybe defined as a single-phase induction motor provided with an auxiliary short-circuited winding or windings displaced in magnetic position from the main winding.

Shaded-pole induction motors are used in a wide variety of applications requiring an induction motor of 1/2 horsepower or less, even down to less than 1 mhp. It is simple in construction, low in cost, and extremely rugged and reliable - like a polyphase induction motor. Its torque characteristics and applications are similar to those of a permanent-split capacitor motor, except that it has a lower efficiency and a lower power factor. It is used in extremely wide variety of applications: home appliances, such as rotisseries, fans of all kinds such as those most commonly found in homes such as home electric fans, humidifiers, slide projectors; small business machines such as photocopy machines; vending machines, advertising displays, etc.



Essential parts of shaded-pole motor
The essential parts of a shaded pole motors are shown in Figure 1. They are also as follows:
1.) A stator usually of concentrated field type and has a laminated core consisting of salient field poles on which coils are placed. The shading coil are shown in Figure 2.

However, some shaded pole motors have a slotted core like that of split phase motors in which windings are placed. These are called the distributed field type shaded pole motor. The shaded winding consists of coils of wire that are wound into slots. A| typical layout of the windings is shown in Figure 3.

2.) A rotor of the squirrel cage type like that of split phase motor; and

3.) Two end plates which fit the bearings.

A four-pole, shaded-pole motor showing the field poles and shading windings.
Figure 2: A four-pole, shaded-pole motor showing the field poles and shading windings.
Stator winding diagram and coil layout of a typical distributed field-type shaded-pole motor.
Figure 3: Stator winding diagram and coil layout of a typical distributed field-type shaded-pole motor.
Principle of operation of a shaded pole induction motor
The shaded pole motor is started by means of shading coils placed on one side of each of the stator poles. Typical motor of this type is shown in Figure 4. The four poles are wound to give alternate polarity.

Stator winding connection of a four-pole shaded motor.
Figure 4: Stator winding connection of a four-pole shaded motor.

On starting, a current is induced into the shaded poles from the main poles by transformer action. The shading coils establish a magnetic field which is out of phase with that established by the main fields and a shifting field is produced sufficient to give the desired starting torque. When current is induced into the shading coils, a flux is built up which opposes the flux which produces it. This is shown in Figure 6-A.  Because of the nature of the sine cycle, the shading coil flux will tend to keep the main pole flux in the unshaded part of the pole during change from zero to near maximum. This is shown in Figure 5.

Sine wave of an alternating current.
Figure 5: Sine wave of an alternating current.

Shifting of air gap flux in shaded-pole motor
Figure 6: Shifting of air gap flux in shaded-pole motor.
When the stator current and flux have both reached their maximum values, there is a brief instant when there is no change in either current or flux. At this instant there will be no voltage or current in the shading coil. As a result, no magneto-motive force is set up by the shading coil to oppose the stator flux, and the stator field will be uniform across the pole face as illustrated in Figure 6-B.  When the stator current and flux decreases, induced electromotive force (emf) and current in the shading coil sets up a magneto-motive force which aids the stator field. Consequently, as shown in Figure 6-C, there is a concentration of flux in the shaded portion. Since the flux has shifted from the unshaded part of the pole to the shaded part of the pole, the rotation of the motor will also be from the unshaded to the shaded part of the pole.

Shaded pole motors electrical circuit for reversing and speed adjustments
Some shaded-pole motors are constructed so that they can be reversed merely by throwing a switch. This type of motor has one main winding and two separate shaded windings. The stator of this motor has slots into which the windings are placed. Each of the shaded-pole winding has as many pole as the main windings, but only one shading coil is used at a time. Figure 7 illustrates this method of reversing rotation. Figure 8 shows the coil connection for each pole. To reverse the motor, it is necessary to open the closed shaded wiring circuit and close the other shaded-winding circuit. Thus, the position of the shaded pole is changed with reference to the main poles.

A reversible shaded-pole motor with two shading coil
Figure 7: A reversible shaded-pole motor with two shading coil.
Coil connection for a reversible shaded-pole motor
Figure 8: Coil connection for a reversible shaded-pole motor.
Another method of reversing the rotation of shaded pole motor is by energizing one main pole at a time. This is true only to motors constructed with two sets of main winding as shown in Figure 9.  For clockwise rotation, one main winding is used while the other is opened. For counter-clockwise rotation, the other main winding is closed while the other is opened.

However, most shaded pole motors cannot be reversed unless they are taken apart. To reverse this type of motor, disassemble the motor, reverse the stator end for end and reassemble. Because the direction of rotation is from main pole to the shaded pole, it can be seen that in Figure 11, the rotation is clockwise and in Figure 12 the rotation is counter-clockwise.

Reversing rotation by using one main pole at a time
Figure 9: Reversing rotation by using one main pole at a time.
Connection diagram for a multi-speed shaded-pole motor
Figure 10: Connection diagram for a multi-speed shaded-pole motor.
Position of the poles and shading coils before the stator is reversed
Figure 11: Position of the poles and shading coils before the stator is reversed.
Position of the poles after the stator is reversed end for end
Figure 12: Position of the poles after the stator is reversed end for end. Compare with Figure 11.
Note: Direction of rotation of a shaded-pole induction motor is from the unshaded pole to the shaded pole. 

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