Author Topic: 5kW diesel Gen - Regular AVR failure  (Read 1109 times)

0 Members and 1 Guest are viewing this topic.

ForeignerinPhils

  • Newbie
  • *
  • Posts: 2
  • Country: ph
5kW diesel Gen - Regular AVR failure
« on: August 04, 2017, 05:15:41 PM »
I am an ex-pat Brit, living in Philippines. I have plans to start harnessing solar power for my domestic needs but in the meantime I use a 5kW, single phase, Sumo diesel generator for emergency use during times when the utility supply goes off (very frequent occurrence here).  This appears to be a 'generic' chinese device - many of them around, just with different name/casing.  Our power standard here is 230V/60Hz.

After a local technician ruined my previous generator (left heat-shielding out, resulting in fire), I am reluctant to let anyone touch the new generator.

A month, or so, after I bought the new unit, the AVR failed leaving the output at 0volts.  I purchased/fitted a replacement AVR and the generator was fine for another couple of months but then failed again - this time producing 370V (no load)!  I managed to get this AVR replaced, free of charge (obviously, from the same supplier!) and this lasted for more than a year before also failing  to 370V.   (I'm becoming quite adept at repairing 230V PSUs!)  I purchased yet another AVR, from a different supplier, and significantly more expensive.  This one also failed - after two or three uses - but, thankfully, to 0V.

Now, my questions:
Have I just been unfortunate in receiving a series of faulty AVRs?
Is there an inherent fault with the generator, causing these failures (two different modes of failure)?
Should the generator produce 370V off-load and unregulated?
What test/checks can I carry out on the generator in order to ascertain whether it does have a problem, and what can I do to fix?

I can provide more information and/or photos, if that would aid diagnosis.
I do have a BSc in Electrical/Electronic engineering, but that was a long time ago and I never worked on 'heavy' current systems - I worked in an electronics development lab for a couple of years before switching to IT.

All assistance greatfully received.
« Last Edit: August 04, 2017, 05:44:02 PM by ForeignerinPhils »

oztules

  • Hero Member
  • *****
  • Posts: 1477
  • Country: aq
  • Village idiot
Re: 5kW diesel Gen - Regular AVR failure
« Reply #1 on: August 05, 2017, 02:27:58 AM »
Have a look at this.
http://www.anotherpower.com/board/index.php/topic,781.0.html

 There are some circuits I drew as well, which will give you a sense of how it works.... and maybe why it does not.

No-one can be that unlucky, and if you look at the circuits, we can see that the things that may cause you to over extend the AVR will be .

1.. too ,much load for too long with rpm starved. In this case the AVR tries to wind up more amp turns in the rotor... the more amps is the bit that affetcs it.

2. Some shorted turns in the rotor will cause the same problem, If we need to develop X amount of flux, we need Y amps for a set number of turns..... if we loose turns for some reason, internal short for instance, then more current again..... short life in the AVR

3. RPM too low.... in which case the AVR will try to get the volts up, but the speed is a little low, so the average amp turns needs to be higher to achieve the same output volts under your load.... the frequency will suffer, but he emf may be ok in this case.

So I expect there is some operational foible that is causing the AVR to work too hard ( rpm and ;loads for long periods perhaps),... or a nasty little short in the rotor or the stator.... something not terminal, but enough to cause over current in the rotor while it tries to compensate.

Failure mode will depend on if the diodes went first, or the switches went. If the switches go, then probably fail short... so max voltage will hit the brushes, if the bridge fails, then we have no DC emf to drive the rotor...... so I don't know for certain, but thats my guess.

I am not an elec engineer, but am marooned on an island in the middle of nowhere....so we fend for ourselves a bit.

 I have rewound them, fiddled the AVR, built AVR's for temporary use... etc..... but too much coincidence for my money.

Here is another 7k5 unit that had problems with the AVR and stator sense winding I did some time ago. "http://www.fieldlines.com/index.php/topic,147246.msg1015700.html#msg1015700"
I has to be something causing it.. thats my take anyway.

..........oztules



Flinders Island Australia

ForeignerinPhils

  • Newbie
  • *
  • Posts: 2
  • Country: ph
Re: 5kW diesel Gen - Regular AVR failure
« Reply #2 on: August 12, 2017, 09:52:04 AM »
Thank you for taking the time to reply - it is much appreciated.

So, I guess that it's time to take a look at the windings.  I'm not sure that I have the time, or the patience, to rewind at home, but inspection with a DMM could yield some clues.  As you say, too much coincidence to have four failed AVRs, but the first did fail within a fairly short run time, so the problem must have been there from new.

1) Too much load? ...  My previous set had several hundred hours on it, before it was taken away for a repair to the diesel engine.  I don't think that the typical/maximum load has changed significantly.  Also, I would be aware if the revs drop markedly - yes, if the 2KW electric kettle gets turned on, then there is a slight change in engine note, but that is all.  Further, there is a 25A mcb on the output of the generator - I would hope that it is calculated to trip before the load rises to the point of damaging the set.  The mcb certainly trips when the AVR fails to high voltage!

2) Shorted turns - maybe, but they must have been there from very early in the life of the set (from manufacture?).

3) RPM too low?  I regularly check the frequency and output voltage (60Hz/230V), so I don't believe that it is ever run out of spec.

One thing I have noted with both the old and the new generator - the UPSs on my computer systems get upset when driven from the generator.  I'm not sure whether this is because the output from the generator is 'spikey' or otherwise unclean.  After a few minutes of the generator running, this appears to settle down.  Also, if the load on the generator is increased, then the UPSs are happier.  At all times, the supply voltage and frequency, as reported by the UPS, are well within spec (within 2Hz and 10 volts).  In fact, the measured voltage appears to increase as the load increases - unless I'm missing something, this suggest to me, that the electrical load is not excessive.

Edit to add:
Perhaps the problem is inadequate cooling of the AVR?  Day time temperatures here are typically in the mid 30s and the AVR is mounted at the rear of a sound-insulated box (lots of acoustic? foam), shared with a hot diesel engine and with no forced cooling (the fan-driven cooling air is ducted around the engine and alternator).  The AVR is the standard crescent shape, designed to mount inside the alternator where there is some airflow.  I guess that the components used in the encapsulated AVR will be of commercial specification (0 - 85C)
« Last Edit: August 12, 2017, 10:17:29 AM by ForeignerinPhils »

dnix71

  • SuperHero Member
  • ******
  • Posts: 2268
Re: 5kW diesel Gen - Regular AVR failure
« Reply #3 on: August 12, 2017, 01:09:51 PM »
If the AVR's have capacitors in them, my guess is the caps are failing. There are a lot of off-spec caps out there. Many computer makers have been stung by caps that couldn't handle the rated voltage.

Electroylitics have a definite lifespan because the electrolyte eventually goes dry.

https://www.allaboutcircuits.com/news/calculating-the-lifespan-of-electrolytic-capacitors-with-de-rating/

http://www.instructables.com/id/What-is-a-Bad-Capacitor/

There is also the issue of mechanical damage. Anyhing mounted solidly on a heavy shaking piece of equipment is going to break down from vibration unless it is well soldered and solidly potted. If you still have some of the failed AVRs, cut them open and see what they look like inside.

You could even have wiring failure. I fixed a few mid-1980's GM automobiles that had reluctor rings under the distributor cap with solid wire coupled to stranded wire. The stranded wire would fail near the crimp from engine vibration, but the insulation would hide the break. I had a co-worker who drove her car to work and when she went to go home, it wouldn't run. I popped the distrubutor cap, massaged the wires near the crimp and the car then started and ran. It was a temporary fix, but the reluctor ring could only be replaced by removing the distributor, so she had to drive it to a shop.

oztules

  • Hero Member
  • *****
  • Posts: 1477
  • Country: aq
  • Village idiot
Re: 5kW diesel Gen - Regular AVR failure
« Reply #4 on: August 12, 2017, 04:23:22 PM »
If the revs are good, and the loads are ok, then we only have the problem that the AVR is working too hard for some reason.

The too much current I referred to is not the stuff going through the MCB, but the current from the AVR into the rotor wires only....to make the current that will  go through the MCB .

If we some how make it harder to achieve the required output volts, the AVR will have to work harder..... so winding problem in the rotor?. or as you point out, maybe temp...... but gee, I would expect temp not to be the big worry here.

In short.... I don't know, but thats where i would start the investigation now.


...........oztules

Flinders Island Australia

Mary B

  • Hero Member
  • *****
  • Posts: 794
Re: 5kW diesel Gen - Regular AVR failure
« Reply #5 on: August 12, 2017, 05:07:21 PM »
If the AVR's are not potted I would be willing to take a look at one for you. I can test the capacitors etc and see if they are in spec. Lot of Chinese crap used caps rated for to low of a temp and they dry out fast.