What distinguishes a permanent magnetic voltage regulator from a self-exciting voltage regulator?

A permanent magnetic voltage regulator is specifically characterized by its use of a permanent magnet to generate the necessary magnetic field for voltage regulation. This type of regulator operates independently of the generator's output voltage and utilizes the constant magnetic field provided by the permanent magnet to control voltage levels effectively.

In contrast, a self-exciting voltage regulator relies on the generator's output to supply the excitation current needed for its magnetic field, typically using an inductor or winding connected to the generator. The distinction lies in the fundamental design of how the magnetic field is created, emphasizing the use of a permanent magnet in the former rather than relying on generated electrical output for excitation.

Regarding the other options, while it’s true that some designs of voltage regulators may not have moving parts or may incorporate soft iron cores, these characteristics are not exclusive to the permanent magnetic voltage regulator and do not clearly differentiate it from self-exciting types. Efficiency can vary across different designs, but it is not a defining characteristic that distinguishes between these two types of regulators. Thus, the correct answer highlights the unique feature of using a permanent magnet over a soft iron core in voltage regulation.