T is directly proportional to square of the voltageâ€¦So Torque which can be delivered by the motor is proportional to square of the operating voltage. When the supply voltage increases by 10%, it becomes 1.1 times and therefore torque becomes 1.1^2 = 1.21 times i.e. 21% increase. When supply voltage reduces by 10%, it becomes 0.9 times, therefore torque becomes 0.9^2 times i.e. 0.81 times i.e. 19% less. Hence we can say that 10% change in supply voltage results in 20% change in torque.