What is a PMG system on a generator? How does it work and why use one?

What is a generator PMG system?

An AC alternator with PMG has an AVR which is independently powered from a separate permanent magnet generator (PMG). Made of two parts, the PMG rotor and the PMG stator. The PMG rotor is mounted on the main alternator rotor shaft and voltage is induced in the PMG stator as the PMG rotor moves the permanent magnets. The power from the PMG stator is then used to power the AVR.

A Diagram from Stamford showing a PMG excited alternator.

Why use a PMG system rather than a standard self excited generator?

In a standard AC generator, the AVR is fed from the generator terminals. In systems which have high inductive loads that cause a large transient voltage dip in the stator voltage before the AVR can compensate, the AVR suffers, because its input voltage is reduced. This can lead to situations where the AVR is not able to bring the voltage back to its preset value, because the terminal voltage has fallen too far and hence its power source isn't available.

The advantage from the PMG is that the power to the AVR is kept near constant, allowing the AVR to quickly recover the voltage of the AC generator back to its nominal preset value. The AVR reacts quicker reducing the transient voltage dip associated with applying a load to an AC alternator.

The PMG system also provides the advantage that no residual magnetism is required in order to start the excitation process at start up, whereas for shunt or auxiliary wound machines, if the residual magnetism is lost, the generator voltage will fail to build up.

What are the disadvantages of a PMG?

A PMG system is more expensive than a shunt or auxiliary system, it also increases the alternators weight by several KG, depending on the size. PMG systems also sit on the back of the alternator shaft and therefore increase the length.

Can you retro-fit a PMG? What does a PMG look like?

Sure! On most alternators where PMG is a factory option, PMG can be retrofitted as well. Fitting a PMG will increase the length of the alternator.

So we'll show you the difference on a Stamford HCI544D1 alternator and what the PMG looks like. So this is the non-drive end of the alternator as it arrives, without the PMG fitted. These are abbreviated fitting instructions that omit key steps to show the difference between the machines. Always refer to the full fitting instructions before fitting a PMG and ensure you have the correct tools to avoid damaging the alternator.

Here is the alternator without the PMG fitted.	With the the rear cover removed, you see the alternators shaft in the centre and the four mounting points for the PMG rotor.
Now we fit the locating in in the shaft:	We can now fit the PMG rotor to the shaft and fix with the centre bolt.
Now we add the PMG stator to the mounting brackets. as you can see the PMG adds to the length of the alternator.	Here is another view end on. You can see the flying lead which will later be used to upgrade the AVR to either an MX321 or MX341 depending on the customers requirements.
Finally, after taking the lead up and fitting the new AVR, we fit the rear cover. As you can see the rear cover needs to be changed because of the additional length.