What is an AC Alternator on a diesel generator?

An AC Alternator or generator is a electrical generator that converts rotary mechanical energy into electrical energy in the form of alternating current. This electricity is used to power schools, homes, hospitals, factories, telecommunications base stations and many other applications. The diesel generator can be either used as the main power source, Prime Power (PRP) or Continuous Power (COP), or as a backup power source, Standby Power (ESP) or Limited Time Power (LTP).

What is the AC alternator made from?

The AC alternator is predominantly made internally from electrical steel and copper windings, while the external frame is usually made from a low carbon steel. Due to the force on the alternator as the diesel engine rotates, the frame of the alternator must be stiff enough to resist the forces for its lifetime, which could be several decades. While the copper windings and electrical steel need to be insulated to prevent short-circuits to earth or other windings.

How does the AC alternator work?

Typically an AC alternator on a diesel generator is brushless and self excited. This description applies to this type of alternator only.

The rotary mechanical energy, supplied by the diesel engine, is used to spin the alternators main rotor. This rotor has a magnetic field, which generates the AC current in the main stator. The excitation system on the alternator provides the current to the main rotor, which it alters to control the magnetic field and hence the voltage from the main stator. The main stator usually has cables connecting it to the generator terminals.

A basic excitation system on a self excited generator is made up of the AVR, the exciter stator, the exciter rotor and the main rotor. The AVR monitors the voltage at the generator terminals and sends a current through the exciter stator. The exciter stator induces a current in the exciter rotor as it spins. The current from the exciter rotor is then rectified by the diodes. It then passes through the main rotor to generate the magnetic field which induces the AC current in the main stator.

The AVR increases the voltage to the exciter stator when the main stator voltage needs to be increased and decreases the voltage to the exciter stator when the main stator voltage needs to be decreased. You can read about what happens when load is applied to an AC alternator.

The engine speed also has an effect on the voltage, read what happens when you apply load to a fixed speed diesel engine or when you apply load to a diesel generator.