What is a main rotor in an AC alternator?

The main rotor is a component part an AC generator (or alternator) and forms part of the alternators excitation system. It is manufactured primarily from wound copper and electrical steel.

What does the main rotor do?

The main rotor has DC current fed through it from the exciter rotor, after being rectified to DC current by the diodes. Its function is to create a magnetic field and induce a current in the main stator to power an electrical load.

How does a main rotor work?

The main rotor spins because it is mounted on a shaft with a bearing at either one end (single bearing alternator) or both ends (two bearing alternator) which is connected to something that rotates, normally a diesel engine. As power from the rest of the excitation system is passed through the rotor it generates a magnetic field. The magnetic field induces a current in the main stator as it spins, which is then fed to the generator terminals.

The power to the main rotor varies as the machine runs as the excitation system responds to the changes in voltage at the main generator terminals. Changing the power in the circuit changes the strength of the magnetic field and therefore the voltage induced in the main stator.