Page 697 - Basic Electrical Engineering

To draw the torque–slip characteristics, we will take the value of T , T ,
m1
s1
and S in each case. For example, with R = 1 Ω, X = 8 Ω, T = 1.54. S m1
20
s1
m1
2
= 0.125 and T = 6.25 and with T = 0 at S = 0, we can draw the T–S
m1
characteristic with its slope on T α S for lower values of slip and for

higher values of slip.

We can now draw the four characteristics as shown in Fig. 8.13.

Figure 8.13 Torque–slip characteristics with increasing rotor-circuit resistance

By observing the characteristics, we can conclude that

i. increasing rotor-circuit resistance increases the starting torque;
ii. maximum torque remains the same for all values of rotor-circuit resistance;
iii. starting torque becomes equal to maximum torque when rotor-circuit resistance is made equal
to rotor-circuit reactance;
iv. slip at which maximum torque is developed changes with change of rotor-circuit resistance;
v. increasing rotor-circuit resistance beyond the value of rotor-circuit reactance will reduce the
starting torque (this can be verified by calculating T at R = 12 Ω (say) and X = 8 Ω).
s
20
2
8.14 STARTING OF INDUCTION MOTORS