Help with Leroy Somer alternator

[Waqalevu]

We are in the process of trying to recommission our diesel generator after undergoing mechanical repairs. The recommissioning process requires that we run the generator at sliw speeds for short periods of time off load. Gradually increasing the speed untill we reach full speed 1500 RPM, then gradually apply load.
The alternator connected to the engine is a Leroy Somer TYPE LSA54-XL95-4P and generates at 11000 Volts.
Now we ere running the engine and alternator at low speed, with the alternator diconnected from busbars/load. I noticed that the alternator ammeter is trying to drive downscale below the zero. I also noticed that the alternator housing us getting a bit warm (62 deg C), so I have shut downn the engine.
Can anyone advise on why the alternator is getting warm? I have a feeling that I may need to dicconnect the excitation. If this is correct, can anyone advise how/where is best place to disconnect the excitation. Also, is there anything else I need to be aware of?

 

[iop95]

Air flow is a function of speed at cube power. So, at low speed alternator have not enough air flow to cool, even with no load. What is excitation parameters (voltage/current)?
For low speed tests, add external cooling devices.

 

[waross]

The ammeter driving downscale is strange. With a simple conventional AC ammeter that can't happen. However some generator panels use no-conventional schemes. I suspect that the zero setting may be poorly calibrated and the meter sits below zero when there is no current flowing. With no load there should be no current indication.
I hope that your AVR has Under Frequency Roll Off. UFRO has been standard for many years now, but there are still a few AVRs in service without UFRO.
The old AVRs would generally fail in less than a minute if the set was operated at slow speed. Failure may result in the maximum current being applied to the field.
Even with UFRO, the AVR will be driving the field.
I would disconnect the power in to the AVR rather than the output to the field.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

The alternator housing getting warm on an unloaded generator is a concern. Do yo have stator temperature RTD's installed and can you read them? As Bill pointed out above if you disconnect excitation, do it by removing the power input from the voltage regulator, not by opening the field output, as damage may occur to the regulator.

Do you have the manual for this generator? Here is the general manual for this range,

http://www.leroy-somer.com/documentation_pdf/2327_en.pdf

Where is the reading of 62C coming from? is that the surface temperature of the housing or a reading of a stator temp?

What voltage regulator is installed, normally these units shipped with the AVR449, but depending on its age it may also have a digital regulator, either should have the UFRO feature Bill described, but it may not be properly adjusted.

How long was the unit down and in what environment? It happens occasionally where a unit this size has been shut down for service with the generator space heaters not working and when restarted a failure occurs. Older units or units in coastal areas or in marine applications as those most affected.

Have done any electrical testing on the generator? I think some basic checks as described in the manual may be in order before you try to proceed further.

Hope that helps, MikeL.

 

[waross]

After hurricane Mitch, I had catastrophic failures of two generators (350-400 kW) that were started up without being tested and dried. Actually one of the sets I had tested and warned against starting, but what do I know. I was over ruled. The day it was started was the last day of its life!
Back to the set at hand. Due to core damage, neither set ever ran again.
Further to Mikes comments, The set may have developed turn to turn shorts. What may fail explosively at rated speed, may develop heat at slow speed.
The good news is if you do have failing or failed windings, the unit may not have yet suffered core damage and so be rewindable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Waqalevu]

The alternator is about 15 years old has an R449 AVR fitted. Just downloaded the R449 manual as impossible to do in poor phone reception area. Anyway, before I got the manual, I disconnected wires 5 and 6 from E+ and E- terminals and that allowed us to get the no-load speeds completed. We are now half way through the on-load tests and have came up against another unrelated problem, so will get back to this tomorrow. I have attached the R449 manual and page 4 shows what wires I disconnected.

 

[catserveng]

It would have been preferably to have left the wires at E+ and E- attached and removed the wires from the PMG or the AREP as shown in the manual, hopefully you get lucky the AVR field output is still ok, since you state you are partially thru on load tests it looks like you are.
Good luck,
MikeL

 

[Waqalevu]

Thanks guys, I have had the alternator on load to 1950 kW, then we had another unrelated problem. I will let you know how I get on tomorrow.

 

[ScottyUK]

As a general comment I'd bring up the field manually using an external DC source and with the main terminals open where possible before committing to the AVR - a 30V / 3A bench-top power supply is ample for most small/medium sets and one of the bigger lab supplies will handle a large machine. It's not really an option for a utility-sized machine with a slipring-fed static exciter, but it gives you a lot of control on pretty much anything with a rotating exciter.