Starting Torque of 3-Phase Induction Motors

Starting Torque of 3-Phase Induction Motors

The rotor circuit of an induction motor has low resistance and high inductance. At starting, the rotor frequency is equal to the stator frequency (i.e., 50 Hz) so that rotor reactance is large compared with rotor resistance. Therefore, rotor current lags the rotor e.m.f. by a large angle, the power factor is low and consequently the starting torque is small. When resistance is added to the rotor circuit, the rotor power factor is improved which results in improved starting

torque. This, of course, increases the rotor impedance and, therefore, decreases the value of rotor current but the effect of improved power factor predominates

and the starting torque is increased.

(i) Squirrel-cage motors. Since the rotor bars are permanently short circuited, it is not possible to add any external resistance in the rotor circuit at starting. Consequently, the stalling torque of such motors is low. Squirrel  cage motors have starting torque of 1.5 to 2 times the full-load value with starting current of 5 to 9 times the full-load current.

(ii) Wound rotor motors. The resistance of the rotor circuit of such motors can be increased through the addition of external resistance. By inserting the proper value of external resistance (so that R2 = X2), maximum starting torque can be obtained. As the motor accelerates, the external resistance is gradually cut out until the rotor circuit is short-circuited on itself for running conditions.

Torque-Slip Characteristics

the motor torque under running conditions is given by;

T =(K₂* s R₂)\(R₂2+S²X₂²)

If a curve is drawn between the torque and slip for a particular value of rotor

resistance R2, the graph thus obtained is called torque-slip characteristic. Fig.1

(8.19) shows a family of torque-slip characteristics for a slip-range from s = 0 to

s = 1 for various values of rotor resistance.

Fig.1

The following points may be noted carefully:

(i) At s = 0, T = 0 so that torque-slip curve starts from the origin.

(ii) At normal speed, slip is small so that s X2 is negligible as compared to R2.

T ᾳ s /R2

                                     ᾳ s     ... as R2 is constant

Hence torque slip curve is a straight line from zero slip to a slip that

corresponds to full-load.

(iii) As slip increases beyond full-load slip, the torque increases and becomes

maximum at s = R2/X2. This maximum torque in an induction motor is

called pull-out torque or break-down torque. Its value is at least twice the

full-load value when the motor is operated at rated voltage and

frequency.

(iv) t o maximum torque, the term

s2X22  increases very rapidly so that

R22 may be neglected as compared

T ᾳ s / s2 X22

                                  ᾳ 1/ s         ... as X2 is constant

Thus the torque is now inversely proportional to slip. Hence torque-slip

curve is a rectangular hyperbola.

(v) The maximum torque remains the same and is independent of the value

of rotor resistance. Therefore, the addition of resistance to the rotor

circuit does not change the value of maximum torque but it only changes

the value of slip at which maximum torque occurs..