In an induction motor, if the supply voltage is halved, the starting torque becomes 

Concept:

The torque equation of a three-phase induction motor is given by,

At starting slip is equal to one.

By the above expression, we can say that starting torque of an induction motor is directly proportional to the square of applied voltage, when all other factors are constant.

∴ T_st ∝ E²

Where,

R₂ & X₂ are rotor side resistance & reactance respectively

N_s is synchronous speed is rps

Application:

If the supply voltage is halved, then the starting torque becomes one fourth of its previous value.