Question

Is the flux really held constant by keeping V/f constant? If not, how do you ensure that flux will be held constant?

Answer 1

Let us try to understand the following equations. By Faraday’s law of induction

=o,n sin wt w.n. dt 2nf.n. = 4.44n. Ø - m

Now, from the above we can see V/f is constant as flux or flux density is fixed during the design of the motor.

Flux is proportional to the ratio v/f in an induction motor. So it has to be maintained constant to keep flux constant as all iron parts are designed for a particular flux density.

In case v/f becomes low there will be underflux situation and the rated torque will not be achieved.

In case it becomes high, overfluxing will occur and may saturate the iron partsMagnetic material of core of stator gets saturated if flux is increasing beyond limit. This is the reason to keep V/f ratio constant so that magnetisation of core remain same.