The split-phase machine has two windings from a single phase arranged in the stator. One is the main winding and other is starting winding, which is used only for starting purpose. The main winding has characteristics of low resistance but high reactance. The starting winding has high resistance but low reactance.
In an a.c. circuit with pure inductance, the current lags voltage by some phase angle. With this in mind, it is easy to understand the split-phase starting method of single phase induction motors.
Let Is be the current passing through starter winding. Im be the current passing through the main winding. It is evident from the circuit diagram that the voltage applied across both the main and the starter windings are same. Thus the current Is drawn by the starter winding lags behind the applied voltage V by a small angle, while the current Im passing through the main winding lags behind the applied voltage V, by a very large angle. Phase angle between Is and Im is made as large as possible, because the starting torque of a split-phase motor is proportional to sin α.
After the starter and main winding has developed a rotating magnetic field, the rotor starts to rotate and for further rotation of the rotor, the starter winding is not required. So a switch is provided that can be opened to isolate the starter winding from the motor circuit. Usually the switch may be of centrifugal type and thus after the motor attaining 70 to 80% of the full load speed, the centrifugal switch opens up, isolating the starter winding. The centrifugal switch S, is connected in series with the starter winding and it is located inside the motor. In some motors which are used for refrigerators may be of hermetically sealed compressor-motors. In these motors, it is customary to use electromagnetic relays instead of centrifugal switches. In such motors, the relay coil is connected in series with the main winding and the pair of contacts which are normally open, is included in the starter winding. During the starting period, when Im is large, relay contacts close thereby allowing Is to flow and the motor starts as usual. After the motor speeds up to 75% full load speed, Im drops to a value that is low enough to cause the contacts open.
Typical torque/speed characteristics curve is shown in the figure. It can be inferred that the starting torque is approximately 200 to 300 times the full load torque and the starting current is 6 to 8 times the full load current. These motors are usually preferred when compared with the capacitor-start motors. These motors are generally used for fans, blowers, centrifugal pumps, small machine tools, domestic appliances etc.
The direction of rotation of such motors can be reversed by reversing or interchanging one of the two stator windings. It is only for this purpose, these motors have four leads brought out of the frame. Also these motors are similar to the characteristics of three phase induction motor in terms of speed control. These motors can be operated varying speed 2 to 5 % from no load to full load. Thus these motors can be called as constant speed motors.
It is necessary to point out here that these motors are also called as “resistance-start split-phase" induction motors. In my next article, let us discuss on the capacitor-start induction-run motors.