Single Phase Induction Motors

We will study the different types of single-phase squirrel cage induction motors, working principle and their different parts. Single-phase motors also works on the same principle on which the polyphase (Two or three phase) induction motor works, i.e. “whenever a short circuited conductor or coil is placed in a rotating magnetic field, the conductor tends to move.” In construction, these motors are similar to polyphase induction motors with the exception that the stator has single-phase winding 

The magnetic field produced by the stator current is fixed in space instead of rotating, but its magnitude is changing sinusoidally. Such a field is equivalent to two fields of equal magnitude rotating in opposite directions at equal speed, each being half of the maximum value of the alternating field. So the single-phase motors are not self-start in the first instance. However, if the rotor of such motor is given a rotation in any direction by switching on the supply to the stator, a starting torque will be produced and motor pick up full speed in the direction in which the rotation was given.

As discussed above, these motors are not self-start. Splitting the stator field into two fields having an angle between them can solve this problem. The phase splitting up can be achieved by either providing an auxiliary winding in addition to main windings or dividing the stator field into two fields. Provision of an auxiliary or starting winding in addition to the main or running winding is easier and is spaced 90 0 electrical apart. Since the phase difference between the starting and running winding is large and hence behaves like a two-phase winding. These windings are connected in parallel and across the supply. So, according to the starting devices, the single-phase motors may be split phase motors, repulsion types motors and universal motors. And according to construction, these motors may be induction type motor, repulsion type motors and A.C. series type motors etc. Here we will discuss the different types of single-phase induction motors only, which are:

i. Split Phase Induction Motors

As the name indicates, the stator is temporarily converted into two-phase stator by providing an extra winding i.e. starting or auxiliary winding for the starting purpose only. The main winding has low resistance and high reactance whereas the starting winding has high resistance and low reactance, in order to produce phase difference between the magnetic field produced by these windings. Sometimes an extra resistance is added in series with the starting winding to increase the starting torque. These windings are connected in parallel and across the supply

When the supply is switched on, two magnetic fields will be produced having some phase difference and hence the combined effect of these magnetic fields will produce a starting torque in the rotor. When the motor attains 75 to 80 percent of full load speed, the centrifugal switch cut off starting winding from the main supply. This switch is mounted on the shaft of the motor and connected in series with the starting winding. In hermetically sealed units, the centrifugal switch is replaced by an electromagnetic relay. The relay is connected in series with the main winding and contact pair of the switch are connected in series with starting or auxiliary winding, which are normally open (NO type). When the supply is switched on, the main winding will draw excessively higher current and the relay contacts are closed. So, the motor starts working as split phase motor. The starting winding will be cut off from the supply when the motor attains more than 80 percent of the full load speed. Because the current drawn by the main winding will be reduced to too low enough to cause the electromagnetic relay’s contact open and the motor will continue to run on single winding as single phase induction motor. Generally these are available in fractional H.P. Reversing the direction of current either in starting or running winding, the direction of rotation can be reversed.

ii. Capacitor Start Induction-run Motors

Construction wise these motors are almost similar to split phase induction motors except the starting resistance is replaced by a static capacitor to produce starting torque

A centrifugal switch is connected in series with starting winding. As already explained,its function is to connect the starting winding across the supply and running winding,for producing starting torque and cut off the same when the motor picks up 75 % of the full load speed. If this winding remained in circuit it will damage due to heating. Because this winding is made of lesser number of turns of very fine copper or aluminium wire as compared to main winding. Starting and running procedure is same as split phase induction motor. In these motors also, reversing the direction of current either in starting or running winding the direction of rotation can be reversed. Generally all the small capacity motors up to 3 H.P. are widely used in domestic and industrial applications.

iii. Capacitor-start Capacitor-run Motor
This motor is also known as permanent capacitor motor. In these motors, the centrifugal switch is removed and the starting winding is so designed that it can work permanently without overheating .The resistance, inductive reactance and size of winding wires of both the windings are not exactly the same but it is almost same. So, the starting winding will remain in the circuit permanently without overheating and drawing excessive current from the supply. The overall starting and running torque is increased because the capacitor remains permanently in the circuit and hence the name.

iv. Shaded Pole Induction Motor

In such motors the rotor is same as in all the previously discussed induction motor except the stator has salient poles to accommodate the winding. The stator of the motor has four poles. The field coils connected in series for alternate polarity (North and South Pole). These poles are made of laminated silicon stampings. These stampings are clamped together to form stator poles. Approximately 1/3 rd portion of the pole is shaded to accommodate a thick copper ring, which acts as auxiliary or starting winding to produce starting torque.This thick copper ring is known as copper shading ring. When the supply is switched on, the alternating magnetic field is produced in each pole. This alternating flux will divide into two-portion i.e. shaded portion flux and un-shaded portion flux. The shaded portion flux will link with the shading ring and an electro-motive-force will be induced in the shading ring. This induced e.m.f. will circulate its eddy currents in the shading rings and opposes the shaded portion flux and hence the phase difference between the two fields will be produced. The combined effect of these fields will produce starting torque in the rotor and the motor will continue to run.These motors are available in very low capacity varying from 3 to 125 watts. The use of this type of motor is restricted due to poor starting as well as running torque, very less overload capacity and poor efficiency, etc. So, generally these motors are used for small fans, toys, ventilators, hair drier and small instruments, etc.