Question

The supply voltage to an induction motor is reduced by 10%. By what percentage approximately will the maximum torque decrease?

• A. 5%
• B. 10%
• C. 20%
• D. 40%

Hint:

The maximum torque in an induction motor is proportional to the square of the supply voltage. A reduction in voltage results in a decrease in torque by the square of the voltage ratio.

Answer 1

The Correct Option is C

The maximum torque of an induction motor is proportional to the square of the supply voltage. If the supply voltage is reduced by a certain percentage, the torque will decrease by the square of that percentage.
Given that the supply voltage is reduced by 10%, the reduction in maximum torque can be calculated as:
\[ \text{New Torque} = \text{Old Torque} \times \left( \frac{V_{\text{new}}}{V_{\text{old}}} \right)^2 \] \[ \text{New Torque} = \text{Old Torque} \times (0.9)^2 = \text{Old Torque} \times 0.81 \] Thus, the torque decreases by approximately: \[ 1 - 0.81 = 0.19 = 20% \] Therefore, the maximum torque decreases by approximately 20%.