Split-phase Induction Motor

Split-phase Induction Motor

A split-phase induction motor is also known as a resistance-start motor. This type of motor is cheap, and it is suitable for easily started loads where the frequency of starting is limited.

The application of split-phase induction motor is;

  • washing machine
  • food mixture
  • grinders
  • air-conditioning fans
  • floor polishers
  • centrifugal pumps
  • small drills
  • dairy machinery, etc.

Generally, this type of motor is used in low starting torque applications. And it is used to drive load more than 1 kW.

Construction and Working

A split-phase induction motor has a single-cage rotor. And the stator consisting two windings; Main winding and auxiliary winding.

The auxiliary winding is also known as staring winding, with high resistance and low inductive reactance connected in this winding. This motor is also known as a resistance-start motor because of the high resistance to starting winding.

Main winding has high inductive reactance and low resistance. Therefore, the main winding and auxiliary winding are displaced at 90˚ in space, similar to a two-phase induction motor.

The connection diagram of the split-phase induction motor is shown in the figure below.

Split-phase Induction Motor
Split-phase Induction Motor

As shown in the connection diagram, a single-phase supply is given to the stator of the motor, and I current will draw from the supply. This current divides in both stator windings.

The current that passes through the main winding is IM, and the current that passes through the auxiliary winding is IA.

As the auxiliary winding has low inductive reactance, it is nearly in phase with the supply voltage. But the main winding has high inductive reactance. Hence, it lags behind the supply voltage, nearly about 90˚.

The phasor diagram of the split-phase induction motor is shown in the figure below.

phasor diagram of split-phase induction motor
phasor diagram of split-phase induction motor

The main winding lags supply voltage by 65-75 degrees. And the starting winding lags supply voltage by 35-45 degrees. Hence, the angle between starting winding and main winding is about 20-30 degrees.

Ideally, for a good starting condition, it is preferred to get an angle between IA and IM that is 90˚. But, in this type of arrangement, we can never get that angle.

But this phase difference is enough to produce a rotating magnetic field. The current that passes through both winding is not the same. Hence, the rotating magnetic field is not uniform either in time or space. But it is sufficient to start the motor.

At the time of starting, both windings are connected in parallel. When the motor reaches speed about 70-80 percent of synchronous speed, the starting winding is disconnected from the supply.

A centrifugal switch is used for higher-rated motors (100 kW or more), and for lower-rated motors, a relay is used to disconnect starting winding.

A relay is connected in series with starting winding. A heavy current will flow in starting winding at the time of starting that closes relay contacts.

As the motor reaches near the synchronous speed, the current passes through the starting winding decreases, and it decreases the current passes through the relay coil that opens relay contact. And it disconnects the starting winding from the circuit.

Torque-Speed Characteristics of Split-Phase Induction Motor

Typical torque-speed characteristics of split-phase induction motor are shown in the graph below.

Torque-Speed Characteristics of Split-Phase Induction Motor
Torque-Speed Characteristics of Split-Phase Induction Motor

The starting torque is about 1.5 times of full-load torque. The pull-out torque or maximum torque is about 2.5 times of full-load torque at 75% of synchronous speed. At the time of starting, the current flowing through the motor is 7 to 8 times higher than the full-load current.

The speed regulation of the split-phase induction motor is excellent. And in most fractional kilowatt motors, the percentage slip is about 4-6 percent.

Reversal of Direction

The resistance-start motor continues to run in the direction it is started. If we need to change the direction of this motor, we need to reverse the connection of either the main winding or starting winding.

The reversal of direction can be made when the motor is in standstill condition.

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