Answer Posted / pavan

Mechanical degrees: the degree of physical or mechanical
rotation a single conductor in an alternator.

Electrical degrees: the degree or the cycle of emf induced
in a single conductor in an alternator

Consider a Two pole Alternator, when a conductor rotates in
the feild, the emf induced in the conductor will be maximum
when the conductor is at the center of the pole and the emf
induced will be minimum when the conductor is in the middle
of the gap between the two poles. so when we consider the
graph of the induced emf versus the position or degree of
rotation. For one complete rotation of the condutor in the
feild - One complete cycle of emf is generated in the
conductor (Positive half cycle at south pole and negative
halfcycle at north pole).

The same way when we consider a 4 pole alternator a single
conductor when completing one mechanical rotation will
cross the four poles there by giving the induced emf graph
with two positive half cycles and two negative halfcycles
which gives us two complete cycles of induced emf.

So for One Mechanical rotation (360 degrees) the induced
emf completes two cycles (360 * 2 = 720 degrees in terms of
the sine wave)

So the relation between Electrical and Mechanical degrees
can be written as

Elect. Degrees = (N0. of Poles / 2) * Mech. Degrees

Source: Elecrical Machinary by P.S.Bimbhra

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