Single-Phase Motors (part 3)

Shaded-Pole Induction Motors

The shaded-pole induction motor is popular because of its simplicity and long life. This motor contains no start windings or centrifugal switch.

It contains a squirrel-cage rotor and operates on the principle of a rotating magnetic field. The rotating magnetic field is created by a shading coil wound on one side of each pole piece. Shaded-pole motors are generally fractional-horsepower motors and are used for low-torque applications such as operating fans and blowers.

++++ A shaded pole.

++++ The shading coil opposes a change of flux as current increases. Pole piece; Shading coil

++++ There is opposition to magnetic flux when the current is not changing.

The Shading Coil:

The shading coil is wound around one end of the pole piece. The shading coil is actually a large loop of copper wire or a copper band. The two ends are connected to form a complete circuit. The shading coil acts in the same manner as a transformer with a shorted secondary winding. When the current of the AC waveform increases from zero toward its positive peak, a magnetic field is created in the pole piece. As magnetic lines of flux cut through the shading coil, a voltage is induced in the coil. Because the coil is a low-resistance short circuit, a large amount of current flows in the loop. This current causes an opposition to the change of magnetic flux. As long as voltage is induced into the shading coil, there is an opposition to the change of magnetic flux.

When the AC reaches its peak value, it’s no longer changing and no volt age is being induced into the shading coil. Because there is no current flow in the shading coil, there is no opposition to the magnetic flux . The magnetic flux of the pole piece is now uniform across the pole face.

When the AC begins to decrease from its peak value back toward zero, the magnetic field of the pole piece begins to collapse. A voltage is again induced into the shading coil. This induced voltage creates a current that opposes the change of magnetic flux. This causes the magnetic flux to be concentrated in the shaded section of the pole piece.

When the AC passes through zero and begins to increase in the negative direction, the same set of events happens except that the polarity of the magnetic field is reversed. If these events were to be viewed in rapid order, the magnetic field would be seen to rotate across the face of the pole piece. A pole piece with a shading coil is shown.

++++The shading coil opposes a change of flux when the current decreases.

++++ The shading coil is a large copper conductor wound around one side of the pole piece.

++++ Four-pole shaded-pole induction motor.

++++ Stator winding and rotor of a shaded-pole induction motor.

Speed:

The speed of the shaded-pole induction motor is determined by the same factors that determine the synchronous speed of other induction motors: frequency and number of stator poles. Shaded-pole motors are commonly wound as four- or six-pole motors. ++++ a drawing of a four-pole shaded-pole induction motor.

General Operating Characteristics:

The shaded-pole motor contains a standard squirrel-cage rotor. The amount of torque produced is determined by the strength of the magnetic field of the stator, the strength of the magnetic field of the rotor, and the phase angle difference between rotor and stator flux. The shaded-pole induction motor has low starting and running torque.

The direction of rotation is determined by the direction in which the rotating magnetic field moves across the pole face. The rotor turns in the direction shown by the arrow. The direction can be changed by removing the stator winding and turning it around. This is not a common practice, however. As a general rule, the shaded-pole induction motor is considered to be nonreversible. +++ f the stator winding and rotor of a shaded-pole induction motor.

Multispeed Motors

There are two basic types of single-phase multispeed motors. One is the con sequent-pole type and the other is a specially wound capacitor-start capacitor-run motor or shaded-pole induction motor. The single-phase consequent-pole motor operates in the same basic way as the three-phase consequent pole discussed earlier. The speed is changed by reversing the current flow through alternate poles and increasing or decreasing the total number of stator poles. The consequent-pole motor is used where high running torque must be maintained at different speeds. A good example of where this type of motor is used is in two-speed compressors for central air-conditioning units.

++++ A three-speed motor. L2 (common); Run winding; Start winding

Multispeed Fan Motors:

Multispeed fan motors have been used for many years. These motors are generally wound for two to five steps of speed and operate fans and squirrel cage blowers. A schematic drawing of a three-speed motor. Notice that the run winding has been tapped to produce low, medium, and high speed. The start winding is connected in parallel with the run-winding section. The other end of the start-winding lead is connected to an external oil-filled capacitor. This motor obtains a change of speed by inserting inductance in series with the run winding. The actual run winding for this motor is between the terminals marked high and common. The winding shown between high and medium is connected in series with the main run winding. When the rotary switch is connected to the medium speed position, the inductive reactance of this coil limits the amount of current flow through the run winding. When the current of the run winding is reduced, the strength of the magnetic field of the run winding is reduced and the motor produces less torque. This causes a greater amount of slip and the motor speed to decrease.

If the rotary switch is changed to the low position, more inductance is inserted in series with the run winding. This causes less current to flow through the run winding and another reduction in torque. When the torque is reduced, the motor speed decreases again.

Common speeds for a four-pole motor of this type are 1625, 1500, and 1350 rpm. Notice that this motor does not have wide ranges between speeds as would be the case with a consequent-pole motor. Most induction motors would overheat and damage the motor winding if the speed were reduced to this extent. This type of motor, however, has much higher impedance windings than most other motors. The run windings of most split-phase motors have a wire resistance of 1 to 4 ohms. This motor generally has a resistance of 10 to 15 ohms in its run winding. It’s the high impedance of the windings that permits the motor to be operated in this manner without damage.

Because this motor is designed to slow down when load is added, it’s not used to operate high-torque loads. This type of motor is generally used to operate only low-torque loads such as fans and blowers.

Repulsion-Type Motors

There are three basic repulsion-type motors:

1. The repulsion motor

2. The repulsion-start induction-run motor

3. The repulsion-induction motor Each of these three types has different operating characteristics.

Construction of Repulsion Motors

A repulsion motor operates on the principle that like magnetic poles repel each other, not on the principle of a rotating magnetic field. The stator of a repulsion motor contains only a run winding very similar to that used in the split-phase motor. Start windings are not necessary. The rotor is actually called an armature because it contains a slotted metal core with windings placed in the slots. The windings are connected to a commutator.

A set of brushes makes contact with the surface of the commutator bars.

The entire assembly looks very much like a DC armature and brush assembly. One difference, however, is that the brushes of the repulsion motor are shorted together. Their function is to provide a current path through certain parts of the armature, not to provide power to the armature from an external source.

Operation:

Although the repulsion motor does not operate on the principle of a rotating magnetic field, it’s an induction motor. When AC power is connected to the stator winding, a magnetic field with alternating polarities is produced in the poles. This alternating field induces a voltage into the windings of the armature. When the brushes are placed in the proper position, current flows through the armature windings, producing a magnetic field of the same polarity in the armature. The armature magnetic field is repelled by the stator magnetic field, causing the armature to rotate. Repulsion motors contain the same number of brushes as there are stator poles. Repulsion motors are commonly wound for four, six, or eight poles.

Brush Position:

The position of the brushes is very important. Maximum torque is developed when the brushes are placed 15 degrees on either side of the pole pieces. ++++ the effect of having the brushes placed at a 90 dgr. angle to the pole pieces.

When the brushes are in this position, a circuit is completed between the coils located at a right angle to the poles. In this position, there is no induced voltage in the armature windings and no torque is produced by the motor.

++++ Brushes are placed at a 90 dgr. angle to the poles.

++++ The brushes have been shifted clockwise 15 dgr.

++++ The brushes have been shifted counterclockwise 15 dgr.

The brushes have been moved to a position so that they are in line with the pole pieces. In this position, a large amount of current flows through the coils directly under the pole pieces. This current produces a magnetic field of the same polarity as the pole piece. Because the magnetic field produced in the armature is at a 0 dgr. angle to the magnetic field of the pole piece, no twisting or turning force is developed and the armature does not turn.

The brushes have been shifted in a clockwise direction so that they are located 15 dgr. from the pole piece. The induced voltage in the armature winding produces a magnetic field of the same polarity as the pole piece.

The magnetic field of the armature is repelled by the magnetic field of the pole piece, and the armature turns in the clockwise direction.

The brushes have been shifted counterclockwise to a position 15 dgr. from the center of the pole piece. The magnetic field developed in the armature again repels the magnetic field of the pole piece, and the armature turns in the counterclockwise direction.

The direction of armature rotation is determined by the setting of the brushes. The direction of rotation for any type of repulsion motor is changed by setting the brushes 15 dgr on either side of the pole pieces. Repulsion-type motors have the highest starting torque of any single-phase motor. The speed of a repulsion motor, not to be confused with the repulsion-start induction-run motor or the repulsion-induction motor, can be varied by changing the AC voltage supplying power for the motor. The repulsion motor has excellent starting and running torque but can exhibit unstable speed characteristics. The repulsion motor can race to very high speed if operated with no mechanical load connected to the shaft.

++++ The brushes are set at a 0 dgr. angle to the pole pieces.