Rotor torque and Starting Torque of induction machines
The torque T developed by the rotor is directly proportional to:
(i) rotor current
(ii) rotor e.m.f.
(iii) power factor of the rotor circuit
where I2 = rotor current at standstill
E2 = rotor e.m.f. at standstill
cos Φ2 = rotor p.f. at standstill
Starting Torque (Ts)
Let E2 = rotor e.m.f. per phase at standstill
X2 = rotor reactance per phase at standstill
R2 = rotor resistance per phase
Generally, the stator supply voltage V is constant so that flux per pole Φ set up by the stator is also fixed. This in turn means that e.m.f. E2 induced in the rotor will be constant.
where K1 is another constant.
It is clear that the magnitude of starting torque would depend upon the relative values of R2 and X2 i.e., rotor resistance/phase and standstill rotor reactance/phase.