Before starting this section, please ensure you read the warnings and suggestions in the general alternator voltage fault finding section and your alternator manual.

This section applies to machines OFF LOAD. If you have a different problem go back to the alternator voltage fault page. 

Alternator Low Voltage Between 10-20% of rated output 

On a 400V L-L Machine, this means you will have a voltage of between 40-80V. In my experience this is very often near the middle of this range at 60V. This indicates lack of excitation, which could be:

1. The Automatic Voltage Regulator (AVR) fuse (if fitted) has blown.
2.  The Automatic Voltage Regulator (AVR) has failed.
3.  A break in the excitation circuit between the AVR and the exciter stator.
4. Failure of the diodes.

Low Voltage Between 0-10V

In alternators that don't have a excitation system using a PMG, the alternator relies on its residual magnetism to initially build the voltage. If this residual has been lost, the alternator won't produce any voltage at all.

It is usually possible to regenerate this magnetism, you should consult someone qualified and your alternator manufacturer on the correct procedure for your model.