Construction of Split Phase Induction Motor
The split phase induction motor is provided with an auxiliary stator winding called starting winding in addition to main or running winding. These windings are placed in the stator slots. Both the windings are put in parallel. The purpose is to get two different currents sufficiently displaced in phase from each other so that a revolving field approximating that of two phase machine is produced.
Working Principle of Split Phase Induction Motor
The main winding which is highly inductive is connected across the line in the usual manner. The auxiliary or starting winding has a greater resistance and lesser reactance as compared to main winding.
Current in the starting winding lags the supply voltage by lesser angle. The current in the main winding lags the supply voltage with greater angle due to its high inductance.
Now there are two currents which are not in phase with each other. As shown in the figure. IRun is the current in the main winding and IStart is the current in the starting winding.
The angle between the two currents is φ as shown in the figure. Now the torque produced is such that it gives circular movement to the rotor. The torque produced is proportional to the sine of angle φ i.e. best conditions are when the angle difference between currents is 90°.
A centrifugal switch which is normally closed is connected in series with the starting winding. When the motor comes up to speed about 75% of the synchronous speed it opens automatically due to centrifugal force and puts the starting winding out of the circuit. It is important that the centrifugal switch should open otherwise the auxiliary winding being made of thin wire will be overheated and thus burns.
Performance and Characteristics of Split Phase Motor
A typical torque speed characteristics of a split phase induction motor are shown in the figure.
- The starting torque is about twice the full load torque.
- The current at starting is about 6 to 8 times.
- Speed falls with the increase in load with only about 5% to 7% otherwise it is constant speed motor. Speed is governed by the relation
- Actual speed is less than synchronous speed Ns.
- For the same weight, its rating is about 60% that of the polyphase motor. The split phase induction motor has lower p.f. and lesser efficiency. Its P.f. is about 0.6 and efficiency is also about 60%. A 4 pole machine with speed 1440 r.p.m. is commonly manufactured.
Ns = 120f/P r.p.m.
Applications of Split Phase Induction Motor
As starting torque is not so high, so this machine is not used where large starting torque is required. It is used for smaller sizes. It is used in washing machines, fans, blowers, centrifugal pumps, wood working tools, grinders and various other low starting torque applications. At the present time, capacitor motors have replaced the split phase induction motors.
Rotation Direction Reversal
The direction of rotation of a split phase induction motor can be reversed by reversing (interchanging) the connections of either starting winding or running winding.