Split Phase Induction Motor

SPLIT PHASE INDUCTION MOTOR

The stator of a split‒phase motor provided with an auxiliary or starting windings in addition to the main or running winding M. The starting winding is located 90° electrical from the main winding [See Fig. 5.28] and operates only during the brief period when the motor starts up. The two windings are so designed that the starting winding s has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance as shown in the schematic connections in Fig. 5.28 (b) Consequently, the currents flowing in the two windings have reasonable phase difference α (25° to 30°) as shown in the phasor diagram in Fig. 5.28 (c).

This can be done by having main winding of large diameter and number of turns while starting winding of a fine wire of a small number of turns.

When the two stator windings are energised from a single‒phase supply, the main winding carries current I_m while the starting winding carries current I_S.

Since main winding is made highly inductive and the starting winding highly resistive, the currents I_m and I_S have a reasonable phase angle α (25° to 30°) between them as shown in Fig. 5.28 (c) Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. The starting torque is given by:

T_S = kI_mI_S sin α                     …………(5.32)

Where, k is a constant whose magnitude depends upon the design of the motor.

When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single‒phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.

Characteristics of Split phase Induction motors

The important characteristics of split‒phase induction motors are

1. The starting torque is 1.5 to 2 times the full‒load torque and the starting current is 6 to 8 times the full‒load current.

2. Due to their low cost, split‒phase induction motors are the most popular single‒phase motors in the market.

3. Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built‒in‒thermal relay. This motor is, therefore, suitable where starting periods are not frequent.

Applications of Split phase Induction motors

These motors are suitable where a moderate starting torque is required and where starting periods are not frequent e.g., to drive:

• Fans

• Washing machines

• Oil burners

• Small machine tools etc.

The power rating of such motors generally lies between 60 W and 250 W.