Split Phase Induction Motor

Split Phase Induction Motor

A Split Phase Induction Motor is a particular type of single-phase induction motor where the stator is equipped with a starting or auxiliary winding (S) and a main or running winding (M). According to the figure, the main winding is 90 degrees away from the starting winding.

Only when the motor is starting up and for a short time after that does the starting winding turn on. The starting and main windings are constructed in such a way that the starting winding (S) has a high resistance and a low reactance while the main winding (M) has a relatively low resistance and a high reactance. This results in a phase difference (α) between the currents flowing in the two windings that is between 25° and 30°, as shown in the phasor diagram.

Operation of Split Phase Induction Motor

According to the connection diagram, the starting winding of the motor carries a current Is while the main winding carries a current Im when it is connected to a source of single-phase AC supply.

Because the main winding is made to be highly inductive and the starting winding is made to be highly resistive. As a result, there is a reasonable phase difference of between 25° and 30° between the currents Is and Im in the two windings. A weak rotating field is consequently created, which ignites the motor. The split-phase motor’s starting torque is determined by,

τst=kIsImsinα

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

The centrifugal switch cuts off power to the starting winding when the motor speed reaches about 80% of the synchronous speed. The motor now runs as a single-phase induction motor and keeps speeding up until it reaches its normal speed. The motor’s normal speed is lower than its synchronous speed, and it is influenced by the mechanical load it is under on its shaft.

Characteristics of Split-Phase Induction Motor

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

• The starting torque of a split-phase induction motor is about 1.5 times of the full-load torque.
• The split-phase motor has the starting current about 7 to 8 times of the full load current.
• Split-phase induction motors are less expensive, thus, they are very popular motors in the market.
• The maximum or pull out torque is about 2.5 times of the full-load torque at about 75% of synchronous speed.
• The power rating of split phase induction motors lies between 60 W and 250 W.
• Split phase induction motors work well in situations where there is a short starting period. The split-phase motor’s starting winding heats up quickly due to its high resistance. Unless the motor is protected by a thermal relay, the starting winding may burn out if the starting time is longer than 5 seconds.

Applications of Split-phase Induction Motor

Applications requiring a moderate starting torque and a brief starting period, such as, are suitable for split-phase induction motors.

• To drive fans
• Oil burners
• In washing machines
• Smalls machine tools
• Food mixers and grinders
• Blowers and centrifugal pumps, etc.