Question

For controlling the speed of an induction motor the frequency of supply is increased by 10%. For magnetizing current to remain the same, the supply voltage must

• A. be increased by 10%
• B. be reduced by 10%
• C. remain constant
• D. be reduced or increased by 20%

Answer 1

The correct option is A be increased by 10%

In order to avoid saturation in stator and rotor cores which would cause sharp increase in magnetizing current, the flux $\varphi$ must be kept constant as 'f' is varied. To achieve this when 'f' is varied, 'V' must also be varied such that $\left(\frac{V}{f}\right)$ remains constant.