65 Kw alternator continued.

[George]

65Kw. Alternator Set Continued

Morning all.

After another two months I have moved one step forward and maybe another step back. The centrifugal clutch is working quite well. It does not drag at all when starting the engine and as soon as it gets going the centrifugal action comes in and the alternator starts turning, after a minute or so I put the throttle to running speed and away it goes with a slight shriek from the 3 belts. When the engine is stopped it freewheels immediately and comes to rest ages later.

Yesterday when I coupled it up to my lathe and ran it up to speed, all was OK until I connected the excitation, which is a 12 v dc supply from the starting battery. It only takes a few amps, but it immediately gets the volts up to about 400 ac between phases.

I noticed that the engine was labouring a bit and there was some smoke coming from the exhaust. I disconnected the supply to the lathe, which was not running anyway and it continued to labour. I am a bit worried about this, as there seems to be some current being used up somewhere. The full excitation to the excitation generator section is supposed to be about 17 vdc and the current specified, is a bit higher than I am seeing at 13 volts from the battery, but I wonder if I am missing something. If I up the excitation to what is specified I would imagine the output will reach the 440 vac between phases, but the 3 phase is OK at this voltage to run the lathe. There can really be no external shorts anywhere as the output from the alternator goes straight to a 16amp socket, which I have fixed to the side of the frame. I tried excitation from a battery charger and apart from the volts being a bit higher as the charger produced about 15 volts dc, the same drag from the alternator was apparent. Also, when I disconnect the excitation the engine power being used drops dramatically. Sounds horribly like an internal problem in the alternator to me, but what?

There are a couple more pictures of what I have done to include the centrifugal clutch, on my website

go to personal stuff and it is in old engines, but this has got nothing to do with the current problem, - no pun intended!

Does anyone have any ideas as to what might be happening, please?

Thanks and regards, George.

[Bob Minchin]

George,

I've only been an intermittent follower of your thread so not sure of the origins of the alternator. I wonder if it may have some shorted turns on a winding leading to power being absorbed internally?

Bob

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember George saying something like:

Internal short is a possibility, but make sure the star point is earthed, if the alt is wound that way. This happened to one I'd had re-wound and the original drawings missed that bit out. It would run up to speed and produce voltage ok, but when a load went on it 'cronk'd and tried to stall the engine. All it needed was an earth.

[George]

Do you mean earth the neutral? I have not connected the star point to anything but the neutral on the socket, because the lathe is only star connected.

If so this might be it! George.

[NT]

could check for carbon build up between commutator segments

NT

[Andy Wade]

Brushless machine, surely? Two machines on one shaft: a small exciter alternator with stator field to which the DC excitation is applied (usually from the AVR module); o/p from rotor of this is rectified by rotor-mounted semiconductor diodes (rotating rectifier) to supply the main machine's rotor. Main 3-ph o/p comes from the stator coils - no brushes, commutators or slip rings required.

Is the engine size adequate for a 65 kVA machine? Some power will be absorbed when excitation is applied since the exciter machine is generating and supplying current to the main machine's rotor windings.

Run it for a while (off load) - does anything, other than the engine, smell hot? Check waveform, each phase to neutral, using oscilloscope with suitable voltage divider. Check rectifier diodes.

[George]

Yes Andy, you are right about the type. There are some pictures of it and a lot of information on my website

you take a look you will see that is is a substantial alternator powered by a 3 cylinder Perkins P3 the idea being that the smaller engine will be quieter and cheaper to run and I only have one suitable alternator.

Come to think about it I have run a milling machine on the output some time ago and as far as I remember it ran it OK and this machine does have a neutral connected, I think. When there is no excitation it still generates 3 phase although at a much reduced voltage - less than half.

Grimly Curmudgeon suggested it might be a floating neutral fault and I am hoping this is correct - until it is proven wrong

Thanks George

[Harry Bloomfield]

NT wrote on 09/05/2010 :

This is an alternator, not a generator - no brushes, no comm.

[tony sayer]

In article , Harry Bloomfield scribeth thus

Slip rings for the moving magnet part?..

[George]

I have just looked at the picture of the alternator exciter on my own website and the notes about it and I see that the exciter has an output of 71 volts at 18 amps, if this is the power that the exciter is feeding into the alternator then it is less than 1400 watts, which is nothing compared with the power of the engine. Also on the picture of the exciter with the cover off I can see that the rotating rectifiers can be easily tested. This should not be too difficult - I thought I might have to take it all apart. George.

[George]

I have just read it again and the output of the exciter is, - of course 3 phase, so the excitation power to the alternator is 3 times

1400 which is a lot more. How does the external DC excitation get to the exciter if there are no slip rings - and I am pretty sure there aren't any, at least with all the ventilation covers, etc. off I can't see any? of does this just get fed into a field winding?

George.

[Andy Wade]

Surely not? The output of the exciter is 2-wire DC (rectified from polyphase AC, as in a car alternator). There's only one lot of 71 volts times 18 amps. The required mechanical input is higher of course as the exciter m/c may not be too efficient. Also the cooling fan power may be significant.

Yes, it's all built into the combined rotor assembly, including the rectifier diodes. (Well that's how the 100 kVA set that I used to look after at a former place of work was built).

Your engine does look quite small in relation to the generator - although I guess you don't intend to connect anything approaching 85 kW load. The aforementioned work m/c was powered by an 11 litre 6-cyl diesel, directly driven. You appear to have a 1.5:1 step-up arrangement, which of course will increase the torque load on the engine. Can you not run the engine faster and step-down, to reduce the required torque? (Just thinking aloud...)

[George]

Hi Andy'

The specification plate clearly states that the output of the excitation generator is 3 phase. There is a transcript of it on the website. I copied it from the alternator. I think there is dc in to the excitor then this is rectified and goes into the main alternator as DC again from the rotating rectifiers. If you look at the picture of the end of the exciter you can see two wires from the diodes going into the shaft which must go to the alternator proper from there.

If you look at the history, i.e. all the earlier photos, it shows the plan A as directly coupled, but I did not like the vibration and noise that I got from the engine running at 1500 rpm. I lowered it to make room for a belt drive to let the engine run at about 1000 , not be perched on a structure that seemed to encourage vibration and geared it up, then I had to move the alternator back a bit to install a centrifugal clutch to allow the engine to start off load. I only need about 10Kw really and I think this will be available from the Perkins engine - even at this speed. It is a lot quieter and happier now. I have learned a lot.

Regards George.

[Andy Wade]

I'm not so sure. It clearly states that it's a 3-ph machine, but I suspect the voltage and current figures refer to the rectified DC o/p, especially as they exactly match the excitation requirement stated for the main machine.

Yes, it does look that way - a simple 2-wire DC output, not a 3-ph AC output at that point :-) It also looks as if it should be fairly straightforward to check the diodes.

Engine coupling comments noted. Fairynuff. We're not really any nearer to knowing why the engine seems to be labouring on what should be a relatively light load though. I can't see why earthing the neutral point of the main alternator should make any difference but I'm sure you'll be back soon to say whether or not it does...

[Tim Lamb]

In message , Andy Wade writes

Tractor diesel engines are likely to reach peak torque at 1500r/m or so. Standard pto speed coincides with about 1800 on most of my old machines. Somewhere there will be performance figures published as the farming press were fond of comparisons.

regards

>

[NT]

Quite right, I should have been asleep. Carbon dust can go astray on any machine with carbon brushes, and waste power.

NT

[<me9]

That will be the output for full load. I'd expect considerably less for no load.

[George]

Hi again,

Today I had to buy a new laptop and this is the first thing I have set up on it!

This morning I checked all the 6 diodes, they were all OK.I connected the neutral star point to earth, but it had no effect. The motor still sounded as if it had a big load. I measured the various windings for shorts but it was so low a resistance anyway that it was inconclusive.

I popped across to our industrial estate and described the symptoms to the guy at a motor rewind shop, where I have bought things and he said it sounded like shorted turns in the main alternator, He said it might have got damp inside and said that the best way to check was to lower the excitation to a level where the load did not slow down the engine and measure the 3 phase voltages and if one is low then that is what it sounds like. I shall do this tomorrow as I still have a lot of work to do on my laptop. I cannot afford to have it rewound and so if it has got shorted windings, then I shall scrap it. It should fetch enough to buy a smaller alternator of 15-20 Kw. It will be a shame though as it is a real quality piece of kit.

That's all for now. George.

[Andy Wade]

No, for any given values of exciter input current and shaft speed the exciter output power will be constant. Well it will vary a bit depending on the rotor winding temperatures, but won't vary with the load on the main machine.

[Harry Bloomfield]

George submitted this idea :

Have you got/ could you borrow a 1,000v meggar and perhaps try checking the resistance phase to ground?

If this unit has not been run for some time and stored in less than perfect conditions, then moisture may well have got into it - especially if it is an air vented unit. Could perhaps blow some dry warm recirculated air through it (fan heater), with some silica gell in the circuit to help absorb the moisture?