Fixing up the 7500w genny for a friend

[oztules]

One of the fellows on one of the outer islands had a problem with his 7500w petrol genny....  It's broke he said.

He tossed it in a boat and bought it to this island, and I had a look at it......took a bit to tussel it out, but here it is.



After I encouraged it to fall to bits, I found the wiring to be suspect.



It's not supposed to look like this, so it was time to pull it to bits, for a look see.



Below is the rotor out of one of this unit. It is simply two coils of wire (one each side) connected to the slip rings.

This effectively means any emf driven into the rotor windings as a  DC current makes a spinning magnet in the center of the stator coils. It has slip rings to energise these coils, and by changing the current in the coils we change the flux in the rotor core. This will be what we use to induce current in the stator for driving the 240v output, the 12v battery charger for external batteries, and the third coil group which power the Automatic Voltage Regulator that in turn powers the rotor via the slip rings.

It relies on remnant magnetism from the previous use,  to set up a very small voltage (.7v or more) to start putting power into the AVR, which in turn drives this into the rotor, which makes more field, so more flux into the drive coil whose EMF goes back into the AVR and around it goes. I quickly gets up to a point where there is enough voltage in the drive coil to power the AVR to drive the rotor to drive the stator to full voltage..... whew...

Looks like this:



More gory shots of the stator.





Here we are part way through stripping out the old:



The sick coil is the drive coil for the AVR. The AVR has a few wires going to it, two from the drive coil for it's own power supply, two go to a sense coil (really a tapped segment of the output coil) and two wires for the brush outputs.

This coil set used 8 slots in the stator, made up of 4 coils in a 180 degree orientation.

Sadly, I got so interested in winding and rewiring and sealing, I forgot to take any more pics until near the end

Here it is rewound and almost back together:



nearer the end



And the final result:



So a new AVR from China (50 bucks) and a few hours work and it's back in business.


At this point it all turned nasty.

I got this idea to check if the original AVR was really sick..... so I replaced the new one with the old one to test it.
It sprang into life, and showed 240v..... so I mused I'd wasted the blokes money buying the AVR..... oh well, we have a spare.....


But no,.......... smoke started billowing out of the alternator and so I realised too late that it was all in vain.

Now I know what cooked the drive winding... the AVR shorted internally, and a single diode stayed alive. The fet had gone where fets go when they are unhappy, but not short.
The end result was that the AVR drew max current from the drive coil, drove the stator, and shorted the drive coil at the same time......and cooked it...... again.... grrrr.

So I pulled it apart again, stripped out the new burnt coils, rewound another set, put it back in, put in the new AVR and stood back.

It fired it up nicely , the voltage stood at 240v... and I waited..... no smoke...... still no smoke.

So I decided to drive a load.... the worst load I had for it was the big 6kw inverter charger... so I hooked it onto the shore power  input and equalised the batteries for the next 4 hours.... no problems so I'm calling this fixed.

Alls well that ends well.... just another day.


...... but no....... news travels fast, and another burnt out wreck arrived only a few hours ago...... maybe I'll remember the camera, as it is interesting how we wind the coils on a jig, and place them in the stator......... next time.




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


Edit,
He'll be charged the $50 for the coil, and must bring a few cans of beer so we can drink to it's good health.... I'm sure he'll be fine with that too.

[ghurd]

Wow.
Nice work.

"I got so interested in winding and rewiring and sealing..."
Only you would say that.  LOL
G-

[DamonHD]

Hey, it's called "flow" and what I aspire to!  Not that anything requiring that amount of manual skill is likely to get me more that some colourful language and a sweaty brow...

Rgds

Damon

[phil b]

Your postings are interesting and full of information Oztules. The generator seems fairly straight forward.

The AVR on the other hand has always a mysterious black box full of secrets. I got to look at one when I repaired a 3 phase generator with a 4 cylinder Wisconsin engine. It had about 50 wires going into the AVR and 30 coming out going to various relays and finally to the brushes on a four pole armature.
 I posted about it several years back on this forum. I never could figure out how that thing actually worked. I replaced it and it ran fine until the owner attempted starting it about 2 years later. He got low volts out of the electrical plugs. I checked it out later and it had blown the new $1000 AVR then too. All the windings were fine.

What gave you the idea of that peticular type of AVR was found in that generator? Are they all basically the same? I'd like to hear more about what you can tell us about these black boxes and how they work, If you please?

Good luck on the second one. I'm sure you will have it repaired in short order. And please post about it also.

[oztules]

There are probably lots and lots of ways to do it...... but essentially it it the same as a car alternator if you look at it .

A rotor, slip rings, a stator..... yep a big car alternator... nothing spectacular at all.

And yes in theory you could use a car regulator circuit like I have published elsewhere here and the backshed.com

In it's simplest form, you simply need to measure the output volts (they use a low voltage tap on the output field to keep it simple), so thats our input to a comparator. The other side of the comparator only needs a reference voltage... from the aux or the same  place but regulated... ie zener and voltage bridge will do.
They use an aux winding of about 100v in a lot of cases (rectified to DC),  to power the rotor via a darlington or fet...... this allows a rotor resistance of about 50R.

They usually put a filter cap on the output to smooth the power to the rotor, buts thats essentially it.... perhaps just use a transistor and a fet/darlington and zener etc, not even a comparator required


As you can imagine, it is easy to make that little setup as complex as you wish, but it must do just the same thing.

For three phase, we assume a balanced load, and so still need to only read one leg.

Fancier forms probably use dividers over the three legs I guess, and that may help balance the output nearer to the desired voltage.

So we use the aux to make a DC source of chopped voltage to the rotor (with a electro on the output and freewheel diode).
We use a reference against the sample to chose the pulse width.
and thats all we need to run.

I've never bothered to try and open one up as they are always epoxy embedded, but that's how I will/would build one... and it would work fine.

The thing likely to cuase problems with them originally, will be overload. If the voltage sags in the stator, the aux will try to drive excessive current into the rotor. If the Fet/darlington/scr  fails from over current in the silicon...... it will short out, with differing results. Now twice in a row it seems the coils copped the result.... we'll see when I try to get the wire out

"Good luck on the second one. I'm sure you will have it repaired in short order. And please post about it also."...... eeech... I'll need it.
It looks like this::


 
Its a little version of the 7500. The AVR for this one is only about $10.00 from china, so not worth building  really.... may just for fun



I'ts just a little bit burnt...... and then some.

The wires hanging out the front will have to be researched before I can start it too...... why do folks do that to me?....


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

[SparWeb]

One of the fellows on one of the outer islands had a problem with his 7500w petrol genny....  It's broke he said.
He tossed it in a boat and bought it to this island, and I had a look at it......

Have you noticed that the country icon (under your username) shows Antarctica?  That's a pretty long boat ride, just to fix a generator!

Anyway, thanks for the walk-through.  Do you have to re-wind all the coils in that second one?
And: did peeling out those AVR wires do any damage to the primary coils' insulation?

[oztules]

Hmm yes .... it felt like Antarctica too this year.

Yes, it looks pretty bad for the main coils, but there's only 10 of them from the looks. The AVR coil looks fair at this stage.
It may take a full rewind, won't know until I pull it to bits.... retirement seems awfully busy somehow.

The main coils on the big one were ok. The charring was the separator plastic, not the enamel.... thankfully.


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

[phil b]

People think that since you are retired, you have nothing to do. So, when any project comes along....

Thanks for the explanation Oztules. AVR's are fascinating.