Fundamental of single-phase AC motors
Types of single-phase AC motors
Single-phase AC motors are also very widely used in industrial, commercial, as well as residential usage.
The main types of AC single-phase motors are:
• Induction motors
• Universal motors
• Synchronous motors.
Most of the single-phase AC motors are induction motors with different arrangements for starting.
In the previous section, we have seen how the three-phase supply sets up a rotating magnetic field that can start the motor. The induction motor, however, is a single-phase motor. Problems are avoided by introducing a second phase artificially to produce a rotating magnetic field for starting.
A schematic diagram of a two-pole single-phase induction motor is shown in Figure 4.7. The alternating current is supplied to the stator winding, the stator field axis remains fixed along with the main axis, joined at the centers of the two poles, alternating in polarity and varying sinusoidally.
It can be shown mathematically that a pulsating magnetic field can be resolved into two equal rotating magnetic fields revolving in opposite directions, each trying to drag the rotor with it. Therefore the rotor remains stationary. However once the rotor starts rotating in any direction, the torque developed in that direction becomes higher and the motor continues to pick up speed till it reaches its rated speed. Thus a single phase induction motor is not self starting but requires some external means of initiating rotation.
This is done by having two stator windings. In addition, to produce a phase difference between two windings currents, a capacitor or an inductance is introduced in the starter- winding circuit. Once the motor reaches a normal speed, the starting winding is switched shut.
Once the motor starts, it will continue to run in the direction it has started. To reverse the direction of the motor any one of the winding leads has to be reversed.
Characteristics of single-phase induction motor:
• Works on a single-phase supply
• Auxiliary means of starting is required
• Are larger for the same horsepower compared with a three-phase motor
• Torque produced by motor is pulsating and irregular
• Mostly used in domestic and residential application.
Types of single-phase induction motors
Split-phase induction motor
This is a single-phase split in two-phase currents for creating a two-phase condition.
Thus, phase splitting creates a rotating field. In the split-phase motor, an auxiliary winding in the stator is used for starting, with either a resistance connected in series with the auxiliary winding, known as the resistance start, or a reactor in series with the main winding, known as the reactor start.
The start winding of a split motor is made up of a few turns of a small diameter wire giving a high resistance and a low reactance.
They are used in small machines that require low torque.
Capacitor-start induction motor
A single-phase induction motor with only a squirrel-cage rotor has no starting torque.
In this motor, a capacitor of suitable size is introduced in the starting winding circuit and a phase difference is created between the two windings.
In the capacitor-start single-phase motor, an auxiliary winding in the stator is connected in series with a capacitor and a centrifugal switch. During the starting and the accelerating period, the motor operates as a two-phase induction motor.
At about 67% of the full load speed, the switch disconnects the auxiliary circuit and the motor thereafter runs as a single-phase induction motor.
Single-value capacitor or capacitor split-phase motor
In a single-value capacitor, or a capacitor split-phase motor, a relatively small, continuously rated capacitor is permanently connected in one of the two-stator windings. This ensures that the motor starts and runs like a two-phase motor. The capacitor and the winding must be continuously rated. These motors are generally used for small pumps or such similar applications.
Capacitor-start / capacitor-run motor
In the capacitor-start, capacitor-run motor, the starting or the auxiliary winding remains connected in the circuit during running.
In order to have a high-starting torque, the capacitor rating should be high, but during running, a small value capacitor is suitable.
To solve this issue, two capacitors of different ratings are used. One is known as the starting capacitor and the other, as the running capacitor.
The starting capacitor is switched out of circuit once the motor reaches full speed. These motors have good starting torque and a quiet running.
Shaded-pole induction motor
This is another method to create two phases, with phase differences using pole shading. The shaded pole has salient poles, each pole split into two sections. A small part of each pole face is wound with a short-circuited copper ring.
When supply is given, the flux under the shaded portion lags behind that of the un- shaded portion. Thus a phase difference is created. This causes the creation of a rotating magnetic field.
The shaded pole motor is inexpensive, since it does not consist of an auxiliary winding or other mechanisms. These are low-torque motors, generally used for small fans and blowers.
Repulsion-start single-phase motor
The principle of operation of the repulsion-type single-phase motor is an interesting contrast with other motors.
In a repulsion-start single-phase motor, a drum-wound rotor is used, which is similar to a squirrel-cage rotor. The circuit is connected to a commutator with a pair of short-circuited brushes. These are set such that, the magnetic axis of the rotor winding is inclined to the magnetic axis of the stator winding. The current flowing in the rotor circuit reacts with the field, to produce a starting and an accelerating torque. At about 67% of the full load speed, the brushes are lifted, the commutator is short-circuited and the motor runs as a single-phase squirrel-cage motor.
The repulsion induction motor employs a repulsion winding on the rotor during the start and running. The repulsion induction motor has an inner squirrel-cage winding and an outer winding on the rotor, which acts as a repulsion winding. As the motor speeds up, the induced rotor current partially shifts, from the repulsion winding to the squirrel-cage winding and the motor runs partly as an induction motor.
Repulsion-start motors have a high-starting torque. Changing the position of the brushes can vary their speed.
Series-wound single-phase or universal motor
The series-wound single-phase motor, has a rotor winding in series with the stator winding, similar to the series-wound DC motor.
This is similar to a DC series motor with a similar high-starting torque. This motor is also called the universal motor because it is designed to operate on either AC or DC supply.
When an AC is applied, there is an instantaneous change in the field and armature polarities. Since the field winding has a low inductance, it creates reversals in the current direction at the proper timings.
As the motor can also be operated on direct current (DC), it is also known as the universal motor.
Single-phase synchronous motor
This is similar to a three-phase synchronous motor in terms of speed, as this also runs at a constant speed. The stator winding is connected to a single-phase supply, while the rotor is made up of a permanent magnetic material.
The resistor and the capacitor are connected in series with one winding. This results in a 90° phase shift between the two windings.
For the first 90°, one set of the windings produces a force that attracts the electromagnet rotor. The second set of phase windings again attracts a new position. This provides the rotor, the force required to start and continue running.
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