GEC Alsthom Alternator

PMG in any Generator is for Excitation Voltage if this voltage is less than required voltage than you must ensure the rated rpm. You have written 11kva,500rpm is it right? or it is 11kv, 500rpm.

The PMG is for input power to the voltage regulator so that power is not taken from the generator itself via a transformer. It produces rated voltage simply by spinning at rated speed. There is no other magic components involved. Check the integrity of the windings of the PMG.

When any magnetic field crosses a conductor such as a copper wire, current flow occurs in a complete circuit. There is only two pieces. The magnets and the windings.

The idea of a PMG is to have a clean uninterrupted source of power to the regulator for the regulator to function.

If the power for the regulator was taken from the generator itself, if there was a short circuit of the main leads the potential would drop to zero, the regulator input would drop to zero, and it would shut off and it becomes difficult to trouble shoot the fault.

The PMG provide 300% current output capacity for field forcing during a short circuit in order to burn free the short circuit and help trip breakers, fuses, or simply burn the short free so you can find it.

Field flashing is a term utilised to build residual magnetism in the conventional alternator and will not help this issue and may damage things if not done properly.

Good luck and post the solution for learning, please.

G

Hi thanks for the help

One of the document says 're-magnetize' the PMG. Any idea how to go about re-magnetizing?

I'd be interested in hearing what others say, but I think that if a PMG has been demagnetized, then replacing the magnets with new magnets would be cheaper than having the existing magnets removed, remagnetized, and reinstalled.

I wonder if a short in the field coils could be pulling down the voltage output of the PMG? If so, it would get quite hot I suppose -- and that could lead to demagnitization. Permanent magnets are permanent... until they get hot.

Dear it looks that short turns in the stator Coils of the PGM causing the low voltage output.

Please check the resistance of the each coil in the PMG stator and you can find the

problem there.

Best Regards,

Younis Sabir

.It looks me that there might be short turns in the PMG Stator Coils that is why it is not producing the rated voltage. The shorted turn coils resistance must be less than the other one. You can check with milliohm meter and might be able to locate those coils.

In my opinion you must have to get rewind the PMG Stator otherswise there might be a big problem ahed having a big bang and output will be 000000000.

Younis Sabir

Hi freiend

Here's the events of my exoerience with the PMG

1. This is a GEC Alsthon Generator 10MW, 11kV

2. The PMG is rated as 220V ac at terminals

3. On running the machine at rated rpm (500) I measured the voltage at PMG terminals to be 140V ac.

4. I took the chance of switching on the AVR and was able to build up the generator terminals to 11kV.

5. At 11kV on the generator terminals, the voltage at the PMG terminals dropped to 134 - the current measured was 5.5A. I could not monitor the temperature of the PMG.

6. The machine was operated for about 10 minutes at 11kV.

Comments please

Domnic

These are some thoughts from someone who is by no means an expert in generators of this size:

1. The best way to know for sure whether the PMG is operating correctly is from operating history. If you have records showing the the voltage under load was originally 220, then the idea of repairing the PMG makes sense. But perhaps the 220 V is the peak rating of the PMG, that is never seen in the application?? History or a consultation with the manufacturer or someone with a similar unit would tell you if there is a problem.

2. If the current is only 5.5 A, there is certainly not so serious a short in the field coils fed by the PMG that the output of the PMG is being pulled down. 5.5 A seems, if anything, low (although understand I am speaking from complete and total ignorance) which supports the idea that the PMG is indeed putting out less voltage than intended.

3. I think you cannot know unless you get the actual design output current for the PMG, in this application. If you have a manual that says that, in this application, the PMG voltage, under load should be 220V, then either replacing the PMG (and repairing it in the shop to serve as a spare, or repairing it with the generator down, would make sense) Once the generator is off the machine, then testing the armature and magnets would be very easy. The problem (if there is one) could be either weak magnets or partly shorted coils.

4. Is there a symptom with the output of the main alternator that indicates that the PMG is malfunctioning? I gather that at least the voltage is correct.

Good luck.

Hello friends

The issue regarding the PMG is resolved.

The Permanant magnet were of magnetizing type. I arranged welding machines to feed 70 volts 600 Amps to the magnets and now the PMG is generating 260 volts at the output terminals.

Now I need to de-magnetize a bit

regards

Domnic

I looked up this model, it seems it is not 11kVA but more likely 11MVA! It may also be 11kV.

Hello there Dennis.

We aren’t ignoring you, you have posted the query on a thread that has been dormant for nearly twenty years. It is not likely to get much attention.

If you wish to attract more attention to your search for replacement equipment that appears to be about thirty years discontinued, starting a new discussion will get you on more member radar screens.