Why the field windings are insulated? what is mean by hysteresis loss?

Question

Why the field windings are insulated? what is mean by hysteresis loss?

Answer 1

In order to produce a magnetic field, it is necessary to pass current through the field windings. If the wire was un-insulated, it would short so the current would bypass the coils and so there would be no field.

A piece of iron that is brought into a magnetic field retains some magnetization, even after the external magnetic field is removed. Once magnetized, the iron will stay magnetized indefinitely. In order to demagnetize the iron, it would be necessary to apply a magnetic field in the opposite direction. This effect is exploited commercially; for example, it provides the element of memory in a hard disk drive.

There are other manifestations of hysteresis but that should do since you are obviously interested in magnetics.