What is Useful flux, leakage factor, leakage flux ?

Q. With the help of neat sketch, explain the following terms: Useful flux, leakage factor, leakage flux.

Ans.

The flux which is available at the air gap and can be utilized for some useful purpose is called as useful flux.

Useful flux (ϕ_u): The flux in an air gap is useful in many applications like motor, generator, moving coil, etc.

Leakage flux: The Total flux produced by the coil does not pass through the air gap as some of the flux leaks through the air surrounding the iron. This flux is called as leakage flux and phenomenon is called magnetic leakage.
It is impossible to confine all the flux to the iron path only, as there is no perfect insulator for magnetic path.

Leakage coefficient or leakage factor(λ) : The ratio of total flux (ϕt)to useful air gap flux(ϕu)is called leakage co-efficient.

The value of  λ  for modern electric machines varies between 1.1 & 1.25

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