how does a genny work

[Justme]

I know that the engine turns the alt to make the elec. I am after a bit more than that.

 

Without going in to to many details just yet. I have a genny (no not a cheapy ebay jobby one but a £2k plus one) that has stopped making power. Its been diagnosed as having a faulty capacitor on the alt. The alt is brushless. They are saying that the cap blew / failed due to the engine running at to low & variable speed.

(which they are blaming on a slightly dirty air filter & the fuel filters that still flow fuel freely so cant be blocked at all)

 

Now I would assume that the caps job is to smooth out any variances in the speed especially as the genny was fitted with the add on AVR module so that the power produced was suitable for sensitive equipment.

 

Also of note is that the power is fed to the appliances via an inverter charger that monitors the power for V & Hertz & will disconnect if the values are not up to spec. The inverter has not been doing that.

 

 

So what does the cap do (the genny has two, the second is wire to the AVR but I dont know which one failed) & why would it fail. The engine sounded lovely to me, never missing a beat, easy to start & stop

[WotEver]

Depends entirely on what cap and where. Brushless generators with AVRs have reservoir caps within the power module to power the exciter stator. A cap can also be used to block DC and pass AC. I believe your question is somewhat too vague at present for anyone to venture a guess at the problem.

 

Tony

[Justme]

I can only give the info provided by the dealer.

 

All he has said is that the cap failed & that it was due to the air filter & fuel filter causing the engine to run "rough".

 

In my opinion the engine was smooth.

 

There are two caps wired into the AVR.

I assume there is also the original cap wired to the end of the alt as per usual on that type of alt.

But one of the ones near the AVR could be that one.

[Bobbybass]

I'm with WotEver on this...its a bit vague as to how it is being used...

 

For general interest...as I understand it :

 

THe cap is essentially two sheets of metal ( usually silver foil) with an insulator in the middle....between the plates...in the metal types ( electrolytic) the insulator is fibreglass type...with a liquid eletrolyte impregnated.

The foil can be quite long but are rolled up into a sausage and do not touch...

THe smaller square typs are metal plates with an insulator...can be ceramic....mica..or other types...

 

WotEver has quoted the most common uses....

 

If its a big metal looking cap with ratings in the Farad range ( example 1 Farad, 2 Farad etc ) then it is probably a reservoir cap designed to absorb any sudden (DC) demand from you switching on an appliance. ...in this case it would be in a DC reservoir mode. ...connected directly across the 12 volt positive/ negative. It wouldn't hold much..just enough for a second or less. Kids often fit them into the DC of their hot-hatch so that if the music demands a sudden surge ie...thumping bass note....the amplifier is not left short of current/voltage ...while the batteries catch up.

I could see sudden demand from you while the genny 'catches up' could cause it to be damaged....they usually go with a nice bang and smoke when this happens and beware of any electrolyte that comes out of the cap.

They are also used in smaller form across petrol engine points...to act as a reservoir for the voltage at the points...and to prevent burning.

 

If its a metal cap ( eletrolytic again ) in the mictofarad range ( ie 10 uf....22 uf) it is possibley DC blocking...if it is 'inline' as only AC will jump across the capacitor....and the AC frequency will be determined by the value of the cap...some frequencies will not jump the gap.

 

 

If its a plasticy looking thing...maybe square....maybe in the 0.1....0.01 uf range...it is possibley removing spikes that are in the supply...maybe from alternator rotor....in this case it is normally connected live to earth....so that any high frequency sparks jump across the cap...and are lost to earth...and dont interfere with TV / audio etc....

 

This is just a rough guide...

Don't blame me if its not quite right....

 

Bob

[Justme]

Its a 35uf 450v cap

 

Its for voltage regulation & stator flashing.

 

Whay I need to know is can an engine thats running slow (even though its not) can cause it to fail.

[Bobbybass]

Its a 35uf 450v cap

 

Its for voltage regulation & stator flashing.

 

Whay I need to know is can an engine thats running slow (even though its not) can cause it to fail.

 

As I understand it :

 

35uf is 35 micro farads...

Its not going to act as a reservoir for large demand items...

it hasn't enough capacity...so its likely part of a larger system...

It won't actually regulate the volatge...it will just reservoir...whatever voltage you happen to have...and act as a buffer if there is a sudden surge/ blip..but only to a small degree as its value/capacity is low. Only a fraction of a second.....and probably polarises the system as well...it may be a 'polarised' ( usually indicated by a groove in the case at one end ...or the wire from the centre of the sausage shape is positive)

 

My guess...only guess...its not so much that the engine is running slow...more that the restricted filter ( you say it is not blocked) is stopping the engine from increasing its speed fast enough to match demand.

Its slower response..literally a part of a second... may be putting too much pressure on the capacitor...

The capacitor would have been calculated to respond in a certain timeframe....which you may be exceeding...

 

Guess others will have more ideas....

 

Hope this helps.

 

Bob

[WotEver]

Its a 35uf 450v cap

 

Its for voltage regulation & stator flashing.

 

Whay I need to know is can an engine thats running slow (even though its not) can cause it to fail.

Okay, that's what we needed to know, The answer is no.

 

Two things will kill an electrolytic in double-quick time with the satisfying accompaniment of a bang and release of the magic smoke - over-voltage or reverse-voltage. I can't possibly see how a slow-running engine could cause either of those scenarios. I stated in my first reply that a common use for a cap in this context was to power the exciter stator and you've confirmed that use above.

 

You suspect you're being conned - I would tend to agree.

 

I could be wrong.

 

Tony

 

Quick little edit to explain that you can't 'stress' a capacitor. In a power circuit the cap is used to hold a small charge which it releases when required (not a great explanation but it suffices for these purposes). If you 'call' for current from a capacitor then it'll deliver it until the (very small) charge is exhausted. This doesn't in any way hurt the cap, it's just sat there 'empty' until it gets 'refilled'.

 

I'll also point out that the boy racer types as mentioned by Bobbybass are also being conned by fitting these big caps to feed their amps. A battery is a far bigger capacitor than anything they could buy.

Edited December 3, 2009 by WotEver

[Justme]

Thanks.

 

Thats what I thought.

[Sir Nibble]

An awful lt of almost true above.

A capacitor has a property called reactance, which gives it a kind of ersatz resistance called impedance. Capacitive reactance is calculated by the formula one over 2 pi fc, where f is the frequency in hertz and c is the capacitance in farads. This means that the impedance of the capacitor falls with increased frequency. Frequency rises and falls with speed, so if the engine runs slow the capacitor will present a greater "resistance" and therefore a larger voltage will be dropped across it as the regulator tries to maintain output (presuming it is a series capacitor employed to cause a phase shift for excitation purposes). So I suppose theoretically a slow running engine could over volt the cap. Having said that, in practice I think it most unlikely that a 450V cap would have any bother. All this really means is that quite frankly it is most unlikely you will get a definative answer except from someone whe knows these machines (not me for instance ) actually looking at the thing. Also, you state that the engine is not running slow, how can you tell if the generator isn't working to load it?

[Justme]

Also, you state that the engine is not running slow, how can you tell if the generator isn't working to load it?

 

 

Thanks for the all that info.

 

Can you rephrase that question.

 

Ah I get it you mean how can I tell that its not running slow now its not producing any power.

 

All I can say is that when it was used 4 days before it was working fine all the way up to the point it shut down. On starting it produced no power. From that I deduce that it failed after it was started last time but once started is not needed as its just to "flash" the windings to get it going. Also the engine sounded just fine when started (which was done a few times to run basic tests before I returned it) & sounded just like it always did.

 

Also if one incident of slow running is enough to cause the cap to fail what about starting & stopping? It must spend more time & at a lower rpm then than it can every whilst under load when running normally. Even adding a big load could not slow the engine enough to get to starting & stopping speeds.

[Sir Nibble]

Right, I teach electrical engineering but I am not au fait with this kind of machine, however if as you say the only role this capacitor plays is to start the excitation then like a starting capacitor on a motor it will need to be disconnected at some point, by either a relay or centrifugal switch. In which case, yes, underspeed will kill it.

[Justme]

As far as I can see physically & from the description there is no relay or speed switch & even if there was I would think it would cut in at much under 3000rpm.

 

As 3000rpm makes 50hertz each 60 rpm would mean a 1 hertz drop in the power. As the inverter would drop out at 48 hertz thats only 120rpm drop in speed from the 3000rpm. I cant see that being the trigger for any switch or relay. Also as the previous time it was running at 53/52 hertz (as it did from new so it its over running thats the issue its still their fault as they were notified of the fast running) so about 3120-3180rpm.

 

I still cant see why a "big name brand" would quibble over a £5-8 part on a machine in warranty.

[Sir Nibble]

I would think it would cut in at much under 3000rpm.

 

I cant see that being the trigger for any switch or relay.

I still cant see why a "big name brand" would quibble over a £5-8 part on a machine in warranty.

Fine, are those assumptions based on experience and training? or an overwhelming urge to win the argument with your supplier? It would be very easy for the VRG electronics to incorporate a switch sensitive enough to operate within 1 hz either way if that's what the designer wanted.

Your dispute with your supplier does not involve me, you asked a question, I have given you an opinion. I could be right, I am more likely wrong. The main thing is that yes there is a theoretical basis for the claim that a slow running engine could blow a capacitor, I know it's not what you wanted to hear but I would not go around accusing people of ripping you off on the basis of a claim that a capacitor has been demolished by slow running.

[Justme]

Fine, are those assumptions based on experience and training? or an overwhelming urge to win the argument with your supplier? It would be very easy for the VRG electronics to incorporate a switch sensitive enough to operate within 1 hz either way if that's what the designer wanted.

Your dispute with your supplier does not involve me, you asked a question, I have given you an opinion. I could be right, I am more likely wrong. The main thing is that yes there is a theoretical basis for the claim that a slow running engine could blow a capacitor, I know it's not what you wanted to hear but I would not go around accusing people of ripping you off on the basis of a claim that a capacitor has been demolished by slow running.

 

Thank you for your opinion.

 

Your right what you say is not what I want to hear but if its the truth / facts then I accept them.

 

From my side what they were saying it just seemed odd.

 

So far over about 5 forums I have had lots of advice.

Some good some bad but so far no one that agrees with you (not saying your wrong).

More than one has been from other people in the generator supply & repair field.

 

 

 

If its true that a slightly dirty air filter can cause this to fail then

 

a, I would happily pay for the repair

b, question the design of the item as it would seem to be not fit for purpose if it can not make it from one service to another without self destructing.

 

 

I am just waiting for the repair quote now.

I expected to be paying for the courier each way (as per the warranty conditions) which will make up the largest part of the bill any way.

I would have paid for a dealer service whilst it was there any way.

So the cost of the actual repair to them would have been the cost of the capacitor & 10 mins extra on the service time to fit it.

 

Just seems like bad business practice to me.

 

Edit to add

 

The selling dealer is being good.

 

(Just Generators)

Edited December 4, 2009 by Justme

[WotEver]

(presuming it is a series capacitor employed to cause a phase shift for excitation purposes).

Why do you assume that? If the generator in question is an SEIG then the capacitor will be permanently connected across the stator terminals of the generator to create the self excitation. A slow running engine would mean that the excitation voltage never reaches the designed steady state, so the cap would be operating well within its design parameters.

 

Of course this is all theoretical, I have never designed nor built a generator, brushless or otherwise, so what have I misunderstood?

 

Tony

[Justme]

I think its a 4 wire cap as well.

[Sir Nibble]

Why do you assume that? If the generator in question is an SEIG then the capacitor will be permanently connected across the stator terminals of the generator to create the self excitation. A slow running engine would mean that the excitation voltage never reaches the designed steady state, so the cap would be operating well within its design parameters.

 

Of course this is all theoretical, I have never designed nor built a generator, brushless or otherwise, so what have I misunderstood?

 

Tony

I don't presume it, I guess. The original question was could a slow running engine blow a capacitor? I have been trying to think of ways in which this MAY be possible. The point is that the person who originally diagnosed this knows far, far more about this machine than I do so I am inclined to believe it. I am just seeking to rationalise that by considering possibilities.

[Justme]

My thought is how slow would it have to be to cause the fault?

 

(I should ask them to explain how slow running causes the fault once I get it back)

 

It should run at 3000rpm

at 2840rpm it would be unloaded by the inverter / charger (very quickly as it would not even trip a computer or tv just like a UPS).

So the speed would increase. So can a reduction of 160rpm (5.33%) cause the fault?

 

If it can that seems a very tight rpm working (well not working but breaking so working range would be even smaller) range.

[WotEver]

I don't presume it, I guess. The original question was could a slow running engine blow a capacitor? I have been trying to think of ways in which this MAY be possible. The point is that the person who originally diagnosed this knows far, far more about this machine than I do so I am inclined to believe it. I am just seeking to rationalise that by considering possibilities.

Fair enough

[Justme]

Well its arrived back.

 

So I refilled it with oil & started her up. Was running smooth (remember this is with no load & before the service).

 

I then connected it up to my system to see what it was doing with a very small load & a larger load.

I then fitted the service parts & repeated the test.

 

I was hoping to be able to show a difference in the before & the after shots but they were the SAME.

 

This is small load

 

 

This is a larger load

 

 

 

UMains = incoming mains voltage

IMain = incoming current

UOut = voltage entering the domestic meter

IOut = current being used via the domestic meter

Udc = battery bank voltage

Udc = ripple is battery bank voltage variation

Idc = charging current going into the battery bank

Freq Out = Hertz at domestic meter

Freq In = hertz from genny

 

The first pic shows a 0.3 amp load on mains & no charger (the other 0.1 amp is the system load) so 0.4amps from genny.

The second pic shows a 9 amp total load on genny with the same 0.3 amps to the mains meter & the charger using the rest to charge at 106 amps.

 

As you can see the herts (so engine speed) is high at 54 hertz with virtually no load & still slightly high at 52 hertz with the largest load we can apply.

 

It is also interesting to see that the £200 AVR module seems to not be working very well as the genny voltage alters from 226v no load to 219v under half load.