What is the reason for the presence of residual magnetism in the field poles?

Answer 1

I'm not quite sure what you're asking, but the reason that there is magnetism at the poles has to do with the fact that magnetic field vector lines have no beginning or end, which is described mathematically through Maxwell's equations; specifically through Gauss' law for magnetism which states that the divergence of a magnetic field is 0, or ∇ ● B = 0. Divergence is a term meaning how much of something is exiting an enclosed surface. Since the divergence of a magnetic field is zero, there must be, always, the exact same amount of magnetic field exiting a surface as entering it, leaving the net divergence as 0.

Thus, a magnetic field vector line has to "exit" from somewhere and loop around to "enter" somewhere else, and these two "somewheres" have to be connected (like a circuit). We call these two "somewheres" the magnetic poles.

Answer 2

Residual magnetism is present in the field poles because the magnetic material used to make the poles retains some magnetization even after the magnetic field is removed. This residual magnetism helps the generator to self-excite and start generating power when it is first put into operation.