Split-Phase Induction Motor – Operation and Characteristics

A split-phase induction motor is a type of single-phase induction motor in which the stator is provided with a starting or auxiliary winding (S) and a main or running winding (M). The starting winding is displaced by 90° from the main winding as shown in the figure.

The starting winding operates only during the brief period when the motor starts up. The starting and the main windings are so designed that the starting winding (S) has a high resistance and relatively low reactance while the main winding (M) has relatively low resistance and high reactance so that the currents flowing in the two windings have reasonable phase difference (α) of about 25° to 30° as shown in the phasor diagram.

Operation of Split-Phase Induction Motor

When the starting winding of the motor is connected to the source of single-phase AC supply, the starting winding carries a current I_s while the main winding carries a current I_m as shown in the connection diagram.

As the starting winding is made highly resistive whereas the main winding highly inductive. Therefore, the currents I_s and I_m in the two windings have a reasonable phase difference of about 25° to 30° between them. As a result, a weak revolving field is produced which starts the motor. The starting torque of the split-phase motor is given by,

$$\mathrm{τ_{𝑠𝑡} = 𝑘𝐼_{𝑠}𝐼_{m}\:sin\:𝛼}$$

Where, k is a constant of proportionality whose value depends upon the design of the machine.

When the motor speed reaches about 80% of the synchronous speed, then the centrifugal switch isolates the starting winding from the circuit. Now, the motor operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is less than the synchronous speed and it depends upon the mechanical load on the shaft of the motor.

Characteristics of Split-Phase Induction Motor

The characteristics of split-phase induction motors are given as follows −

• The split-phase motor has the starting current about 7 to 8 times of the full load current.

• The starting torque of a split-phase induction motor is about 1.5 times of the full-load torque.

• The maximum or pull out torque is about 2.5 times of the full-load torque at about 75% of synchronous speed.

• Split-phase induction motors are less expensive, thus, they are very popular motors in the market.

• Split-phase induction motors are suitable for the applications where the starting period is small. Since the starting winding of the split-phase motor has high resistance, it heats up quickly. If the starting period exceeds 5 seconds, the starting winding may burn out unless the motor is protected by a thermal relay.

• The power rating of split-phase induction motors lies between 60 W and 250 W.

Applications of Split-phase Induction Motor

The split-phase induction motors are suitable for the applications where a moderate starting torque is required and the starting period is small such as −

• To drive fans

• In washing machines

• Oil burners

• Smalls machine tools

• Blowers and centrifugal pumps

• Food mixers and grinders, etc.