What type of generator is specifically described as having its pole pieces excited by residual magnetism?

A self-excited generator is designed to have its pole pieces energized by the residual magnetism present in the iron of the rotor or stator when the machine is started. This residual magnetism allows the machine to generate a small amount of electrical output even without external excitation. As the generator begins to produce electricity, this output flows to the field windings, reinforcing the magnetic field and allowing the generator to increase its output.

In the context of the other types of generators, a separately excited generator relies on an independent external source of power for its field winding, rather than relying on residual magnetism. A shunt generator has a configuration that allows a portion of its output to be fed back into the field windings, but it typically does not depend solely on residual magnetism for its operation. A series generator has field windings that are connected in series with the load, but it also does not primarily rely on residual magnetism to excite its poles at the start.

Thus, the defining characteristic of a self-excited generator—its dependence on residual magnetism to initiate operation—distinguishes it from the others, making it the correct choice for this question.