Fixing Portable Generator

[Barx]

Can somebody just confirm that the rotor on a brushed portable genny should have continuity when tested at the slip rings? It's just a winding isn't it?

Head is a little fried and no power coming out this bloody thing. It's testing open circuit so thinking it's knackered.

With the avr disconnected, getting 18v on brushes which is apparently too high and also indicates knackered rotor.

Cheers

 

[Geoff1946]

Assuming we are talking about one of the small (~2kVa) cheap and cheerful type, I think there may be a diode in the rotor winding as well.

Bit hazy now but I did fix one a couple of years ago and I had a generic circuit diagram. I'll see if I can find it later.

 

[Geoff1946]

I've found my diagram, but sadly it isn't going to help you.

Mine was actually only a 600 - 700 watt job, but had no external connection to the rotor. i.e. no slip rings.

The rotor just comprised a winding short circuited with a diode. 

As yours has slip rings then I would expect continuity. Presumably its fairly heavy wire and I would be looking carefully at the terminations if it's O/C.

 

[Barx]

Hi @Geoff1946

Yeah, that's the type of unit I'm talking of. Thinks it's  2.5kW.

Got another one turning up soon (hopefully tomorrow) so will check that one out to answer my own question

Thought it would be an easy one for people with brains (not me), but posted here and 2 x facebook groups. So far, your the only person that has replied, lol.

 

[Doc Hudson]

Hi @Geoff1946

Yeah, that's the type of unit I'm talking of. Thinks it's  2.5kW.

Got another one turning up soon (hopefully tomorrow) so will check that one out to answer my own question

Thought it would be an easy one for people with brains (not me), but posted here and 2 x facebook groups. So far, your the only person that has replied, lol.

Facebook!?  I didn't think we mentioned that here? It is rumoured you could be banned for a month for quoting any Facebook references. Lets hope no one else noticed! Do let us now what you find out as or when this 'other' generator arrives.

Doc H.

 

[Barx]

Facebook!?  I didn't think we mentioned that here? It is rumoured you could be banned for a month for quoting any Facebook references. Lets hope no one else noticed! Do let us now what you find out as or when this 'other' generator arrives.

Doc H.

Ok, new genny turned up today. Looks pretty much same except a few bits of metal are different shape etc. I whipped the cover off the alternator (after checking that it works fine first), removed connections to brushes and tested between the brushes (which are obviously connected to the slip rings. 60 Ohms!!!

Tried it both ways too to see if there are any diodes in place and got same reading both ways.

So, looks like the rotor is indeed kaput on the old one.

 

[binky]

makes sense, any motor winding will have  a resistance albeit not very high.

 

[Barx]

makes sense, any motor winding will have  a resistance albeit not very high.

Thats what I originally thought @binky but it was O/C on the non-working unit and then I doubted myself. Not a master of motors or alternators. I started to question how a coil withing a coil generates electricity. Where the magnetic field comes from etc. As mentioned head was a little fried and had little info with me. Turns out my theory was correct, which is nice. I also now wish to learn / revise a bit about the whole subject.

 

[binky]

know what you mean, I've forgotten so much basic physics and priciples of electricity / motors coils etc etc due to lack of use...

 

[Geoff1946]

Here is the diagram I mentioned earlier. This was a very small (~600w) cheap & cheerful, I think from Aldi'

As you see there are no sliprings and no regulator. Somewhat surprisingly it worked OK once I "fixed" it.

It hadn't worked from new, and I dismantled tested and rebuilt it more times than I should admit. I even replaced the capacitor, as I couldn't test that.

Eventually, picking up on basic principles,  a penny finally dropped!

it relies on residual magnetism in the stator to initially energise the rotor. There clearly wasn't sufficient residual so nothing happened.

I connected the stator output winding to a car battery and counted ten. Put it all back together for the final time and it worked fine. 

View attachment Generator.pdf

 

[Tony S]

Here is the diagram I mentioned earlier. This was a very small (~600w) cheap & cheerful, I think from Aldi'

As you see there are no sliprings and no regulator. Somewhat surprisingly it worked OK once I "fixed" it.

It hadn't worked from new, and I dismantled tested and rebuilt it more times than I should admit. I even replaced the capacitor, as I couldn't test that.

Eventually, picking up on basic principles,  a penny finally dropped!

it relies on residual magnetism in the stator to initially energise the rotor. There clearly wasn't sufficient residual so nothing happened.

I connected the stator output winding to a car battery and counted ten. Put it all back together for the final time and it worked fine. 

View attachment 9660

Ahh, that old trick, flashing the stator to reintroduce the residual magnetism. I’ve only ever done it a couple of times to get the exciters working on our MG sets.

Another way is to give it a good clout with a lump hammer, it jolts the magnetic flux back in to line.

 

[Geoff1946]

Ahh, that old trick, flashing the stator to reintroduce the residual magnetism. I’ve only ever done it a couple of times to get the exciters working on our MG sets.

Another way is to give it a good clout with a lump hammer, it jolts the magnetic flux back in to line.

I think if I hit that Aldi one with a lump hammer it would never work again! A tack hammer maybe.