Question

With reference to squirrel cage, induction, electric motors:

(a) Describe the construction of such a motor;

(b)  Sketch the torque against speed curve of such a motor.

(c) Describe a method employed by a retro-fitted device used to improve the part load performance of an induction motor.

The stator of an  induction  motor is,  in principle, the same as that of a synchronous motor or generator. it is  made up of a number of stampings which are slotted to receive the windings.

The stator carries a 3-phase winding and is fed from a 3-phase supply, it is wound for a definite number of poles, the exact number of poles being determined by the requirements of speed.

Greater the number of poles, lesser the speed and vice versa. 

The stator windings, when supplied with 3-phase curr­ents, produce a magnetic flux which is of constant value but which revolves (or rotates) at synchronous speed (given by Ns = 120fp). This revolving magnetic flux induces an e.m.f. in the rotor by mutual induction. Stator windings of an induction motor can be arranged in various ways so that a supply of three-phase alternating current will produce a rotating magnetic field. The method shown in Figure  has three sets of coils at different pitch circles with a 30% overlap.

  Method of connection of the coils

The outer set are connected as in Figure so that current flow between A, and A2 will produce magnetic fields simultaneously in the four coils but that the polarity of the top and bottom will be the same and opposite to that of the other two. Current flow follows in coil sets B and C, and then the sequence is repeated with the direction of current reversed. The effect is to cause a rotating field.

 

Almost 90 per cent of induction motors are squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core  with   parallel  slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consist of heavy bars of copper, alumi­nium or alloys. One bar is placed in each slot, rather (he bars are inserted from the end when semi-closed slots are used. The rotor bars are  brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us, what is so picturesquely called a squirrel-cage construction.

It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes.

The rotor slots are usually not quite parallel to the shaft but arc purposely given a slight skew (Fig. 29-4). This is useful in two ways :

(1) It helps to make the motor run quietly by reducing the magnetic hum and

(2) It helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.

 

T= Rated troque.

Starting troque = 1.5T

Maxi,um troque = 2.5t

T = KφsE2R2 / (R2)2 + (sX2)2

At 0 slip T = 0

Under normal condition [running] slip is minimum

Factor sX2 is negligible  with respect to R2

 

 

 

Therefore T α s / R2

Hence for low values of slip, the torque / slip curve is approximately a straight line.

As slip increases [for increasing load on the motor] the torque also increase and becomes maximum where s = R2 / X2

This torque is known as pull out or break down torque or stalling torque.

As slip further increases [i.e. motor speed falls] with further increase in load, then R2 becomes negligible as compared to sX2 therefore for larger values of slip T α s / sX2  α 1/s

Hence, the torque/slip curve is a rectangular hyperbola. So, we see that beyond the point of maximum torque, any further increase in motor load results in decrease of torque developed by the motor. The result is that the motor slows down and eventually stops. The circuit-breakers will be tripped open if the circuit has been so protected. In fact, the stable operation of motor lies between the values of s =0 and that corresponding to maximum torque. The oper­ating range is shown shaded in Fig.

It is seen that although maximum torque does not depend on R2, yet the exact location of Tmax is dependent on it. Greater the R2, greater is the value of slip at which the maximum torque occurs.