What is a generator AVR or Automatic Voltage Regulator? What does an AVR do? How does it work?

What is a Generator Automatic Voltage Regulator?

An automatic voltage regulator (AVR) is a solid state electronic device for automatically maintaining generator output terminal voltage at a set value. It will try and do this as the generator load or operating temperature changes. The AVR is part of the alternators excitation system.

A typical looking AVR - a Stamford SX460

Who provides the automatic voltage regulators?

Normally in a generating set, the alternator manufacturer will supply an automatic voltage regulator with their AC alternator. The biggest manufacturers of alternators for diesel generators are Stamford AVK, Mecc Alte, Leroy Somer and more recently WEG. The model supplied will depend on the alternator and any accessories fitted to it, which may need a different AVR. An example of such accessory would be a PMG or auxiliary winding.

Where is an AVR located in a generator?

Normally the generator AVR is located in one of three places. It can be in the main control box of the generator, in the alternators terminal box or it could be (only on very small portable units usually) located under the alternators rear cover.

How does an AVR work?

It controls output by sensing the voltage from the generator terminals and comparing it to a stable reference. The error signal is then used to adjust the field current by increasing or decreasing the current flow to an exciter stator, which in turn will lead to a lower or higher voltage at the main stator terminals.

 Different AVR designs - what do they look like?

AVR's all look remarkable similar - they vary a bit in size and colour, but seemingly all have similar features.

You can find the AVR you need at the find the AVR support page.

What happens if a generator AVR fails?

If the AVR on your generator fails, then the generator will lose excitation. This loss of excitation will cause the voltage to fall suddenly at the generator and this loss of voltage should cause the generator to shut down on an under-voltage fault.

If your generator does not have under-voltage protection set, then the generator may continue to run, which could cause severe damage to your equipment.