Strange dynamo problem

[iansoady]

I've just been trying to work out an odd problem with the dynamo on my 1961 A10 (positive earth, 6 volt system with solid state regulator and small gel battery).

Initially I had no charge showing on the bike's ammeter (this may not have been surprising as I'd crossed the field and dynamo wires - note to self, green fields). However, having sorted this out, I tried flashing the dynamo field from the live (-ve) battery terminal, checked that the dynamo was rotating, and measured the resistance to earth of field and dynamo connections (both a few ohms.- so the brushes and internal connections should be sound).

Then I tried the time-honoured test of connecting field and dynamo cables together (regulator out of circuit) and checking the voltage across the joined cables and earth. Nothing (well very little). In a fit of exasperation I tried connecting the field direct to the live side of the battery - and I had a healthy 12 volts and rising from the dynamo cable. But when I try the 2 wires connected together again, still nothing.

Surely my test connecting the battery direct indicates that the dynamo is working, but why nothing from dynamo and field connected? The only thing that comes to mind is that the battery is a low Ah model, but surely this is irrelevant when out of circuit? And it does seem to excite the field coil adequately.

All suggestions welcome.

[beezermacc]

Basically, the fundamental test is bridge F and D and connect to earth via a voltmeter or 12v bulb. If you are getting very little voltage there is something wrong. A few 'quickies' you might try..... 1) check the dynamo body is properly earthed onto the engine case. 2) Check F and D terminals are very clean and tidy  3) Clean the commutator with some spirit and finest emery you can find. 4) Check there is adequate pressure on the brushes. 4) Remove the brushes and check continuity across adjacent segments on the commutator - should be consitently low, not rising and falling as you work your way round.

[trevinoz]

Ian,
           Is your dynamo set to rotate in the correct direction? Was it working in your bike ever?
If you have it on the bench, try motoring it and ensure that it rotates anti-clockwise, viewed from drive end.
Trev.

[iansoady]

Thanks both. The reverse direction thing has been suggested by someone else - is there a way of telling which direction it's wired? When I bought the bike a couple of years ago I did rebuild the dynamo but it hasn't been used since then. I did however motor it when rebuilt and it was fine.

I could have connected the field backwards I suppose.

[KenF]

Ian,

It certainly sounds like your field connections have got swapped, or your brush connections have got swapped, but not both. What happens then is that the magnetic field produced by the field coil connected to the armature as the dynamo starts to rotate opposes the residual magnetism that enabled the armature to send some electricity to the field coil. As a result, the net magnetism hovers somewhere between the residual level and zilch.

It's usually easier to swap the field coil connections. Then motor the dynamo again, and it should rotate in the same direction that it is turned by the engine.

Ken.

[iansoady]

Many thanks all.

I'm away for a couple of days but will attack it with renewed vigour when I return!

[iansoady]

Hi all.

A few days interval turned into several weeks.....

I've removed the dynamo and stripped it, checking and cleaning everything. Field resistance is around 3 ohms. Resistance between dynamo brush and earth is 0.7 - 0.8 ohms for each pair of commutator segments. Dynamo motors in the correct direction when connected to a battery (+ve earth, 6 volts), taking 3 amps and rotating very smoothly.

I've tried swapping the field leads over, swapping the brush leads and swapping both at the same time. The dynamo works correctly with the leads in the original position (which conforms to the pictures in the Lucas service sheet) but only when I attach the battery to the field coil - giving a good 6 volts or so when turned with an electric drill. If I just connect the field & dynamo leads together as for the standard dynamo test I get about 0.5 volts again turning with the drill.

This has me totally baffled......

[warmshed]

How did you test the "motoring" of the dynamo? to determine correct rotation?  
With an output when the battery is feeding the field coil it should mean field coil is setting up a magnetic field OK. With the armature rotating then it is giving an output.  When you connect F and D together then the residual magnetism should generate an output on D which feeds the field so the cascade effect is started.  As you don,t get this, it does seem the field is reversed.

If you are motoring it correctly then this will polarize the field coil as will putting the battery direct to the F tag.
 Go back to basics, check that it motors in the correct direction, its easy to get this muddled.
 Which direction does it motor and what end are you looking at.
 Are you happy that the other side of the field coil is going to a good earth along with the other brush??
Reassemble the dynamo and check on low ohms and ensure you get low earth resistance on both the F and the D, allowing for the field coil and armature resistance respectively
 If all well then it WILL work, laws of Physics.   Best of luck Dave.

Hey just noticed you live in Hall Green, shouldn't you have a Velocette?

[iansoady]

How did you test the "motoring" of the dynamo? to determine correct rotation?  

By connecting F&D together then connecting these to the -ve side of the battery and the dynamo body to +ve.

With an output when the battery is feeding the field coil it should mean field coil is setting up a magnetic field OK.

That was my reasoning but it doesn't seem to happen in practice.....

With the armature rotating then it is giving an output.  When you connect F and D together then the residual magnetism should generate an output on D which feeds the field so the cascade effect is started.  As you don,t get this, it does seem the field is reversed.

I've tried with the field connected both ways (polarising before retrying).....

If you are motoring it correctly then this will polarize the field coil as will putting the battery direct to the F tag.
 Go back to basics, check that it motors in the correct direction, its easy to get this muddled.
 Which direction does it motor and what end are you looking at.

The dynamo rotates anticlockwise looking from the drive end which is correct for the bike and consistent with the arrow on the body.

Are you happy that the other side of the field coil is going to a good earth along with the other brush??
Reassemble the dynamo and check on low ohms and ensure you get low earth resistance on both the F and the D, allowing for the field coil and armature resistance respectively

Yes, good earths and other connections all cleaned up. Degreased all connectors, commutator and brushes. Checked that brushes slide freely and that there is a constant 0.7 ohms between brushes when the armature is slowly rotated.

If all well then it WILL work, laws of Physics.   Best of luck Dave.

That's my reasoning but something doesn't add up..... I'll try it all again from scratch. I must be doing something wrong somewhere.

Hey just noticed you live in Hall Green, shouldn't you have a Velocette?

I did have a Venom for a couple of years (after 20-odd years of a Commando). Sold it to get the A10 (which actually I think is a much better bike).

[warmshed]

Have you checked on High ohms scale, between windings and earth on both the field and armature? Should be many Mohms.

[trevinoz]

Ian,
          I would guess that your armature is shot.
You may have one coil open circuit. The dynamo will still motor OK but will not generate enough to excite the field.
A good dynamo will motor as low as 2 volts applied whereas a crook one will not.
Get your armature either growler or voltage drop tested.

Trev.

[duTch]

Ian I had a similar situation last week with 2 Genny's, both motored ok but no generation so took Trevs' advice from another thread, and had them growled out, one has stuffed field, the other a stuffed armature. I put the good bits together and have power- magic!! Also tested the good armature and a field from a third unit with shredded armature and works well.
 Bottom line, track down a growler, at least you'll confirm something.
 Cheers ,duTch

[iansoady]

I eventually admitted defeat and took the dynamo over to Tony Cooper (who's been mentioned here before). He checked out armature and field, fitted new bearings and brushes and now it seems fine (at least when driven on the bench by a cordless drill).

Very nice chap, excellent work, very reasonable price and a shed full of lovely bikes. Highly recommended.

[lawnmowerman]

Get your armature either growler or voltage drop tested.

Trev.

OK i am going to show my ignorance here but what is a growler test?

Jim

[trevinoz]

Jim,
           A growler is a 'V' block with a wound coil fitted to it.
The armature is placed in the V and the A.C. mains voltage is applied to the coil.
A thin steel blade is passed over the steel sections of the armature progressively around the circumference.
If there are short circuits, the blade vibrates.
The test for open circuits is done with an A.C. voltmeter across each segment of the commutator, all should be about the same.
Alternatively, each pair of segments can be shorted with a pointed metal object across the mica and a healthy spark should be seen.
I have a growler but don't use it, I prefer to apply a D.C. voltage to the commutator and read the voltage drop across each pair of segments, a "drop test".
  Trev.