LEAKAGE FLUX: Leakage flux is that flux which does not follow the specified path for it, that means it follows the path which is not intended for it.In this figure, when a current is passed through a solenoid magnetic flux is produced by it. The flux in the iron ring air gap is known as useful flux and the flux through the air surrounding the ring known as leakage flux. Useful fluxes utilize for useful purposes and leakage fluxes are wastage. For best designed dynamos 15 to20% flux leaks without use.
Leakage flux is denoted by øL.

FRINGING: The useful flux passing across the gap tends to bulge outwards because the lines of force repel each other when passing through a non magnetic material, and thereby reducing the effective area of the gap and reducing the flux density. This effect is known as fringing.
Fringing increases with increasing air gap length.

HOPKINSON’S LEAKAGE CO-EFFICIENT: Leakage co-efficient is the ratio of total flux to the useful flux.
Leakage co-efficient = λ= total flux/useful flux.
If we denote øt= total flux and øu=useful flux,
Then λ= øt/ øu
The value of λ of modern electric machine is between 1.1 and 1.25.

Because of leakage magnetic flux, weight and cost of a machine increase. So needs to minimize the magnetic leakage. It can be minimized by placing the winding at the closest position on to the air gap or magnetic circuit for maximum utilization of flux.


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