Chapter 16, Problem 16.1P

To determine

It is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value. Explain why this is so.

Answer to Problem 16.1P

To keep the ratio of voltage to frequency constant, it is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value.

Explanation of Solution

Given Information:

It is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value.

Consider the EMF equation for the induction motor:

  $E=4.44\varphi K.T.f$

where

  $\begin{array}{l}E=\text{Induced EMF in the induction motor}\\ \varphi =\text{Average airgap flux}\\ K=\text{Winding constant}\\ T=\text{Number of turns per phase}\\ f=\text{Frequency}\end{array}$

Therefore, the induced EMF is directly proportional to flux and frequency:

  $E\propto \varphi f$

Now, if the frequency is reduced, the magnitude of the induced EMF will also be reduced such that to maintain constant airgap flux.

Consider the applied voltage to the induction motor is V, such that, this applied voltage V is directly proportional to induced EMF E, i.e.,

  $E\propto V$

Therefore, if the frequency is reduced to maintain the constant flux in the air gap, there must be a proportional reduction in the magnitude of the applied voltage.

If the applied voltage is not reduced in proportion to the frequency, it results in excessive motor currents which may cause damage to the motor. Due to this, losses increase, and therefore, the efficiency of the motor is reduced.

So, it is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value.

Conclusion:

It can be concluded that it is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value to maintain constant flux in the airgap.