Types of Single Phase Induction Motor

Types of Single Phase Induction Motor

The single phase induction motors can be classified according to the phase difference produced between the currents in the main and auxiliary windings.

The types are:

(1) Split-phase motors.

(2) Capacitor-start motors.

(3) Capacitor-start capacitor run motors.

(4) Shaded-pole motors.

Split-Phase Motors:

It consists of two stator windings. One is the main winding or running winding and another is the auxiliary or starting winding. These two windings axis are displaced by 90 electrical degrees. The auxiliary winding has high resistance and low reactance and main winding has low resistance and high reactance. I_r is the current flowing through the starting winding. The auxiliary winding is only used for starting period. When the motor speed is about 75% of synchronous speed, the auxiliary winding is disconnected from the circuit. This is done by connected a centrifugal switch in the auxiliary circuiting. After this, motor runs only because of main winding.

The starting torque of the motor can be increased by connecting a resistance in series with the auxiliary winding. Split-phase induction motor is also called as resistance start induction motor. Applications are fans, blowers, washing machines.

Capacitor Start Induction Motor:

Here, a capacitor is connected in series with the auxiliary (or) starting winding. It is also used to get higher starting torque.

Single phase supply is applied to the two windings. The starting current I_S leads the line voltage because of the capacitor present in the auxiliary winding. The running current I_r lags the line voltage.

The phase displacement between the two currents are approximately equal to 90° during starting.

Again the auxiliary winding is disconnected from the circuit by centrifugal switch at 75% of the synchronous speed, i.e., the capacitor is used only during starting period. The direction of rotation of the motor can be changed by changing the corrections of one of the windings.

Applications:

1. Compressors,

2. Pumps,

3. Conveyors,

4. Washing machine,

5. Refrigerators.

Capacitor-Start Capacitor-Run Motors:

Here two capacitors are used one capacitor is used for starting purpose and another capacitor is used for running purpose. In this motor, we can get high starting torque, because of two capacitors.

The value of the starting capacitor is large and the value of running capacitor is less. The running capacitor is permanently connected in series with auxiliary winding. When the motor speed picks upto 75% of synchronous speed, the centrifugal switch is opened and the starting capacitor is disconnected from the circuit.

The starting capacitor is used for developing high starting torque and capacitor Cis used to improve the power factor.

Applications:

It is mainly used for low noise and high starting torque applications, such as:

1. Compressors,

2. Pumps,

3. Conveyors,

4. Refrigerators.

Shaded-Pole Single Phase Motor

In these type of motors, the splitting of phases is done by Induction. These type of motors will have salient (or) projecting poles stator and a squirrel cage rotor. A 4-pole stator with the stator poles connected in series. Those poles will have a slot cut across the laminations approximately one-third distance from one edge. A short is called as shaded circuited copper coil called as shading coil is inserted into the slotted portion. This part is called unshaded part.

Now, when a alternating current is passed through the stator, the axis of the pole shifts from unshaded part to the shaded part for the positive half cycle of the exciting current. This seems to be equivalent to a physical movement of the pole. Hence this magnetic field will crosses the air-gap and cuts the short circuited rotor. Hence the rotor starts rotating in the directions of this shift i.e., from unshaded part to the shaded part.

The motor starts due to the production of starting torque. This starting torque is low which is about 40 to 50% of the full load torque shaded motors are commercially available in vary small sizes from 3 W to 125 W. Due to the absence of centrifugal switch and capacitors its construction is simple and small.

Advantages:

(i) Simple in construction

(ii) Reliable and cheap

(iii) Compact size.

Disadvantages:

(i) Low starting torque

(ii) Carry little overload capacity

(iii) Low efficiency (5% to 35%).

Applications:

Used in small toys, hair dryers, ventilators, clock, etc.