Making Single Phase Induction Motor Self Starting

MAKING SINGLE PHASE INDUCTION MOTOR SELF‒STARTING

A single phase motor is very similar to a 3‒phase squirrel cage induction motor. It has (i) a squirrel‒cage rotor identical to a 3‒phase motor and (ii) a single‒phase winding on the stator.

Unlike a 3‒phase induction motor, a single‒phase induction motor is not self‒starting but requires some starting means. The single‒phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel‒cage rotor. However, if the rotor of a single‒phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single‒phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed for a motor located at some inaccessible spot.

Making self‒starting

To make a single‒phase induction motor self‒starting, we should somehow produce a revolving stator magnetic field. This may be achieved by converting a single‒phase supply into two phase supply through the use of an additional winding. When the motor attains sufficient speed, the starting winding (i.e., additional winding) may be removed depending upon the type of the motor. As a matter of fact, single‒phase induction motors are classified and named according to the means employed to make them self‒starting.

(i) Split‒phase motors - started by two‒phase motor action through the use of an auxiliary or starting winding.

(ii) Capacitor motors - started by the motion of the magnetic field through the use of an auxiliary winding and a capacitor.

(iii) Shaded‒pole motors - started by the motion of the magnetic field produced by means of a shading coil around a portion of the pole structure.

1. Double‒field Revolving Theory

This theory makes use of the idea that an alternating uni‒axial quantity can be represented by 2 oppositely ‒ rotating vectors of half magnitude. Accordingly, an alternating sinusoidal flux can be represented by 2 revolving fluxes, each equal to half the value of the alternating flux and each rotating synchronously in opposite direction.

As shown in Fig. 5.26 (a) let the alternating flux have a maximum value of ϕ_m. Its component fluxes A and B will each be equal to ϕ_m/2, revolving in anticlockwise and clockwise directions respectively. After some time, when A and B would have rotated through angle +θ and –θ in Fig. 5.26 (b), the resultant flux would be = 2 θ_m/2 cos(2θ /2) = θ_m cosθ.

After a quarter cycle of rotation, fluxes A and B will be oppositely directed as shown in Fig. 5.26 (c) so that the resultant flux will be zero [Fig. 5.26 (d)]. After half a cycle, fluxes A and B will have a resultant of ‒2 × ϕ_m/2 =  ‒ϕ_m. After 3 quarters of a cycle, again the resultant is zero, as shown in Fig. 5.26 (e). and so on.

If we plot the values of resultant flux against θ between limits θ  = 0° to θ = 360°, then a curve similiar to the one shown in Fig. 5.26 (f) is obtained. That is why an alternating flux can be looked upon as composed of 2 revolving fluxes, each of half the value revolving synchronously in opposite directions.

2. Torque ‒ Slip Characteristics

It may be noted that if the slip of the rotor is s with respect to the forward rotating flux [i.e., one which rotates in the same direction as rotor], then its slip with respect to the backward rotating flux is (2 ‒ s). It may be proved thus

If N is the r.p.m of the rotor, then its slip with respect to forward rotating flux is

s = (N_S‒N) / N_S = 1 ‒ N/N_S

 N/N_S = 1 ‒ s

Keeping in mind the fact that the backward rotating flux rotates opposite to the rotor, the rotor slip with respect to this flux is

s_b= (N_S‒(‒N)) / N_S = 1 ‒ N/N_S

 N/N_S = 1 + (1 ‒ s)

 s_b = 2 ‒ s

Each of the 2 component fluxes, while revolving round the stator, cuts the rotor, induces an emf and this produces its own torque. Obviously the 2 torques are oppositely directed, so that the net / resultant torques is equal to their difference as shown in Fig. 5.27.

Now, power developed by a rotor, is

If N is the rotor r.p.s. then torque is given by

Hence the forward and backward torques are given by

 Total torque T = T_ƒ + T_b

Fig. 5.27 shows both torques and the resultant torque for slips between 0 to 2. At standstill s = 1 and 2‒s=1. Hence. T_f and T_b are numerically equal, but, being oppositely directed, produce no resultant torque. That explains why there is no starting torque in a single phase motor.

However if the rotor is started somehow, say in the clockwise direction, the clockwise torque starts increasing and, at the same time, the anticlockwise torque starts decreasing. Hence there is a certain amount of net torque in clockwise direction accelerating the motor to full speed.