Diesel Generator voltage/ frequency failure 6

[mortimouse]

Any advice greatly appreciated.
Machine type cummins/onan admiral 400v genset. Started the machine today and only producing 200 v/phase as oposed to 400. Also no frequency registered. Have manually adjusted engine speed however there is no change. I am reasonably certain that the governing system is operating as it should.
Can anyone tell me what the likely cause is.
Many thanks in advance.

 

[dpc]

Any changes to the voltage regulator recently?

 

[mortimouse]

No changes to the voltage regulator. The casing of the machine recently got sprayed with sea water mist very briefly but the machine was not in operation and there is no sign of any intenal water damage.

 

[ScottyUK]

Excitation system fault. Difficult to say much more than that from the information given.


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If we learn from our mistakes I'm getting a great education!

 

[mortimouse]

Thanks scottyuk, could you please list any further info you may require for diagnosis, and any specific tests i should perform, test the excitation windings, rectifier, or i guess ultimately the avr etc. I have a specialist arriving next week as i only have basic test gear on board, but would be good to be armed with as much info as poss. Cheers.

 

[dpc]

If you can measure the dc voltage coming out of the regulator that would be helpful. With the machine shut down, the dc resistance of the pilot field would also be good to know.

The generator data should give dc voltage and amps required for no-load and full-load.

If this is a brushless exciter, you could have a failed SCR in the exciter or the VR could be bad.

You might visit the Basler website and look under Voltage Regulators for some troubleshooting tips.

 

[edison123]

May be rectifier diode(s) in one arm of the 3 phase bride rectifier failed leading to lower dc output. Though it is odd that frequency did not register at 200 V. If you have a multimeter with frequency function, you can check the frequency to see if the set is running at rated speed.

* I would go green if only I were not yellow *

 

[waross]

I am not surprised that the frequency meter does not register at 50% voltage. A lot of sets in that size range have frequency meters that need the voltage to be in the right ballpark.
See if the voltage responds to the adjustment on the AVR. Do't try yet to correct the voltage but no response points to an AVR problem and some response points to a rotating component problem.
Check the output of the Automatic Voltage Regulator. If it is low, 5 to 20 volts, the AVR is suspect. If the AVR is putting out 63 volts or more, check the brushless exciter and the rotating diodes. Then check the exciter field and rotor and the main field.
Also check that the power circuit is clear. If the power circuit has flashed over and tracked or shorted, you may have voltage collapse.
Don't worry about the frequency meter at this point.
BUT check the speed if you can. If for some reason the speed has dropped to about half speed, the Low Frequency Roll Off circuit in the AVR will have assumed that an overload has dragged the RPMs down and cut the voltage to about 50%.

respectfully

 

[ScottyUK]

Beaten to it by waross!

Definitely have a look at the currents and voltages on the exciter field winding and report back. If you can list the exciter nameplate ratings that would help too but I know they're not always apparent on small machines. We should be able to make a guess from the machine rating.

Does this unit have an electronic controller? There is usually a fairly comprehensive set of measurements of machine parameters. Often the problem is that what the controller thinks is happening and what is really happening are different. Look for mismatches between your independent measured values and whatever the controller is seeing.


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If we learn from our mistakes I'm getting a great education!

 

[mortimouse]

Thanks all for all your advice/help. Will start dismantling the casing and try to report back soon.

 

[mortimouse]

Having removed the outer casing and accessed the guts of it.
Firstly I tried adjusting the trimmer pot on the AVR. Instantly I could adjust the voltage to 400 and even to 420 volts. However it appears to be impossible to fix voltage at 400. I suspect that this pot has a dead spot and simply requires replacing. The only concern is that it appears to be possible to adjust as much as 15% voltage as oposed to the 3% stated in the manual, again maybe this is a function of the malfunctioning pot. It appears that there is no problem with excitation voltage and diode pack although I have not measured this. By deduction I guess if 400v is achievable at all they must be functioning correctly. Frequency is normal at 50hz when voltage is at 400. Have done no on load running yet as its difficult to fix the voltage. Anything else-Ill let you know. Again thanks for all the help. Cheers.

 

[waross]

From the information given, I suspect that the pot has been contaminated by the salt spray. Is it possible to clean the pot and the AVR in general with an aerosol cleaner?
respectfully

 

[mortimouse]

Hi Waross
The whole electrical end is effectively encased in first a sound proof casing system and inside this a second substatial metal cover. It is hard to imagine how salt contamination got in there, unless an aerosol mist found its way in via the fan openings. Regardless I am not a great believer in coincidence so probability points that way. I have liberally sprayed the pot with an electrical cleaner but have not yet accessed the AVR- this is mounted inside another casing system inside the affore mentined 2. I will access this and ensure its cleanliness. In any event there is absolutely no evidence of any salt contamination in the machine.
Again
Cheers

 

[waross]

There will be a voltage adjustment on the AVR board. The remote pot is generally a removable option. You can try disconnecting it and setting the voltage at the AVR. The majority of the sets that I install are stand alone and the remote pot is not recommended or installed. If you do not need operator voltage adjustment and removing the pot fixes your problem I would recommend running with out the pot.
If you must have it then by all means replace it.
respectfully

 

[mortimouse]

Hi Waross.
Just tried diconecting the pot, but voltage falls down to 204 the same as the problem described above-maybe some jumper resistor or something requires inserting in its place. Anyhow, removed the pot and examined it carefully-turns out the resistive track is worn slightly at the 400v position-vibration? meaning the pick up fails to contact correctly. Adjusted pick up spring tension, now all appears to operate correctly. Will put on load shortly. Will replace the pot when i source a new one.
Cheers

 

[ScottyUK]

If the pot sits permanently in one position, determine the resistance either side of the wiper then replace it with a pair of fixed resistors. No more track wear and one less thing for helpful fingers to play with. Pleased you got it sorted out.




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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

One more thing: Contact cleaners can dissolve the thin carbon resistive track/film on pots! When you turn the pot after you have sprayed it with aerosol contact cleaners - you just scraped the softened carbon resistive track!
My $0.02

 

[mortimouse]

Thanks again all those contributors. Machine has been on load for a couple of hours with no problems.

 

[waross]

Thanks for the feed back. We are glad to help.
respectfully

 

[slavag]

Sirs.
Great thread. I'm enjoy read it, it's high professional level.
Stars to Bill and Scotty.
Best Regards.
Slava