In small generators, what type of magnets are often used for pole pieces?

In small generators, permanent magnets are commonly used for pole pieces because they provide a constant magnetic field essential for the operation of the generator. These magnets are made from materials that have a high magnetic retentivity, meaning they retain their magnetism over time without requiring an external power source. This characteristic is advantageous because it allows the generator to be simpler, lighter, and more efficient, as it eliminates the need for a power supply or additional circuit components to generate the magnetic field.

In contrast, while electromagnets can provide stronger and more easily adjustable magnetic fields, they typically require a continuous electrical current to maintain magnetism, making them less practical for small, portable generators. Other options like resistive and temporary magnets do not provide the stable magnetic field required for consistent generator performance.