The perennial charging issues.

[trevinoz]

Rex,
Your field coil seems to be OK.
What you are getting is typical of a faulty armature.
You really need to drop test your armature.
On motoring, a faulty geny will motor on 6V but if you have a variable supply, try it on 4V and less.
Good ones will motor as low as 2.5V.

[KiwiGF]

Wot Trevor said...and....

The F+D test is a “quick” test but as others have said putting current through the field (as opposed to relying on the residual magnetism/armature to do it) is a much better test of whether it will generate, and generally will pinpoint an armature or field coil problem. It’s ok to apply 6v to a field coil for quite a few secs, at 3 ohms that’s only 2 amps.

Have you checked there is no short between the field coil and case? This is a common fault.....the wrapping around the field coil wears and wires touch the core.

On my A10 I found the dynamo would stop charging every few hundred miles, the cure being to clean the commentator, and eventually to replace duff brushes. The brushes were “soft” and particles sticking to the commentator caused enough resistance to prevent the “start up” of the dynamo, as the initial voltage from just the “flashing” is very small even a small resistance will prevent “start up”.

Urban myth says there are bad brushes in the marketplace...maybe you’ve been unlucky and got some.

[Swarfcut]

Following from Trevs thoughts on the armature, plus KiWi's experience, by chance are the gaps between the commutator segments choked with carbon and in need of a good clearout and undercut? Could be that simple. Up to now all the results from suggested electrical checks for basic set up seem reasonable so a simple mechanical fault is a final possibility. The brushes need a good looky look for conduction, compare with the old ones if you still have them.

 Swarfy

[duTch]

 

.......the cure being to clean the commentator, and eventually to replace duff brushes. The brushes were “soft” and particles sticking to the commentator caused enough resistance to prevent the “start up” of the dynamo,.......

 Is this a race or a game ?  ...........ahem, back to the topic, I had a similar thing and tracked down a growler to be told the armature is no good- mixeda nd amtched some bits  << better thaan my typing<<  and  had a working dynamo-humming...

[trevinoz]

High resistance between the brushes and the comm will certainly prevent the generator starting. A quick continuity test will confirm whether there is a good circuit.
Usually a good clean of the comm and a rub on the brush ends with an abrasive will sort this.

[Rex]

A good clean of the comm, a clean of the comm segment gaps and checking the continuity through the brushes was done as first steps in this saga.
Here's the latest-
Powering the field coil independently was a good idea, and on test the AVO showed 6-7V which given the speed of rotation is probably more than acceptable. This to me points to a problem with the field, so a good 12V was splashed across the field coil in an attempt to wake it up..but sadly it doesn't.
The same results also applied when  the alternative armature was fitted.
So, being cynical that anything would have changed with the pole piece which prevents it's retaining  any residual magnetism, I can only think that the field coil has been compromised in some way (although the resistance figures don't indicate that) and will need replacing.
Any thoughts?

[KiwiGF]

A good clean of the comm, a clean of the comm segment gaps and checking the continuity through the brushes was done as first steps in this saga.
Here's the latest-
Powering the field coil independently was a good idea, and on test the AVO showed 6-7V which given the speed of rotation is probably more than acceptable. This to me points to a problem with the field, so a good 12V was splashed across the field coil in an attempt to wake it up..but sadly it doesn't.
The same results also applied when  the alternative armature was fitted.
So, being cynical that anything would have changed with the pole piece which prevents it's retaining  any residual magnetism, I can only think that the field coil has been compromised in some way (although the resistance figures don't indicate that) and will need replacing.
Any thoughts?

Can the armature supply a decent current when the field is powered independently? Eg light a headlight bulb?

Did you check if the field is shorting to the core? 3 ohms resistance of the field indicates its good (they can get down to 2 ohms or less as insulation degrades), but it can be 3 ohms and still be shorting to the core.

[Bsareg]

It's certainly a weird one. It would seem it may be poor magnetic conduction between the field pole and the body. Did you use an expander to fit the field accurately ? I've  never tried it, but what about temporarily  placing a welders magnet adjacent to the field to boost it  (might need to spin it  to line up field ).

[Rex]

I've never had the field out, so it's still in the position that Mr Lucas put in in all those decades back. Magnetic conduction?

[Bsareg]

Magnetic conduction: The magnetism doesn't just come out on the armature side but spreads into the body and around to meet the opposite pole of the armature. The reason armatures and bodies are made from varnished lamination is to limit losses through generating eddy currents within the metal. All testicle stuff this.

[chaterlea25]

Hi All
Reading volts on a meter is one thing
But lighting a 60w 12v bulb is another
Repeat the test feeding the field coil with 6v and the 12v 60 w headlamp bulb from D to dyno body
On one reluctant dynamo I got results by pushing the brushes against the comm with a small screwdriver as the dynamo was spinning when all else seemed good
John

[Rex]

Magnetic conduction: The magnetism doesn't just come out on the armature side but spreads into the body and around to meet the opposite pole of the armature. The reason armatures and bodies are made from varnished lamination is to limit losses through generating eddy currents within the metal. All testicle stuff this.

Ahhh, you confused me there. Did you mean "magnetic induction" , and what could possibly have happened to a simple Lucas yoke and pole piece to cause problems in this regard?
So, on the premise that the EMF generated can swing the AVO needle but not necessarily light a test lamp, the thinking now is that it's either a duff pair of armatures, ie unable to generate when rotated in a known good magnetic field, or a duff field coil in that it cannot create sufficient field strength to create enough residual magnetism in the pole?

[Bsareg]

  Magnetic induction is when within the field, conduction requires contact, but would seem not to be your dynamos problem. Do you know the current draw at 6v of the field coil without the armature fitted ?

[KiwiGF]

Magnetic conduction: The magnetism doesn't just come out on the armature side but spreads into the body and around to meet the opposite pole of the armature. The reason armatures and bodies are made from varnished lamination is to limit losses through generating eddy currents within the metal. All testicle stuff this.

Ahhh, you confused me there. Did you mean "magnetic induction" , and what could possibly have happened to a simple Lucas yoke and pole piece to cause problems in this regard?
So, on the premise that the EMF generated can swing the AVO needle but not necessarily light a test lamp, the thinking now is that it's either a duff pair of armatures, ie unable to generate when rotated in a known good magnetic field, or a duff field coil in that it cannot create sufficient field strength to create enough residual magnetism in the pole?

I think re- doing the test with “independent” current going through the field with a real load on the armature will reveal the probable cause of the problem. Whether or not the field core retains residual magnetism is not relevant for that test, what IS relevant is whether current you THINK is going through the field coil is actually going to earth via a short in the field coil to “earth”, and we don’t know if you have checked for that short yet (unless I missed that). Even a near end of life field coil with 2 ohms resistant will create enough of a magnetic field to make a dynamo light a high watt bulb at 1000 rpm.

I personally think it’s extremely unlikely that the problems you are having have anything to do with residual magnetism in the field core. Given the tests you’ve done it’s more likely in the brush area, I recently found a brush with high enough resistance in the internal wire to carbon joint to prevent “start up” and that was only revealed when pushed I on it when the dynamo was rotating, and charging level increased.

It also might be worth checking the rpm you are testing the dynamo at, they run faster than you think, I’ve never worked it out but you need a relatively fast drill (1500rpm plus?) to spin them fast enough to get to “start up” rpm (eg only using residual magnetism) especially if the brushes etc aren’t 100%.

Someone no doubts knows but I guess the dynamos run well above engine rpm?

[groily]

It also might be worth checking the rpm you are testing the dynamo at, they run faster than you think, I’ve never worked it out but you need a relatively fast drill (1500rpm plus?) to spin them fast enough to get to “start up” rpm (eg only using residual magnetism) especially if the brushes etc aren’t 100%.

Someone no doubts knows but I guess the dynamos run well above engine rpm?

Around 1.25 x engine speed ideally.
It may often take somewhere around 1800rpm to get a dyn to kick in. Sometimes, they need a really good blast to encourage them to behave - after which they will often stay behaving. When it's got into the groove, it might play from a little over 1000. Cordless drills and slow lathes aren't much good for doing some of this.
A bit of light extra pressure on the brushes can often get 'em going. (But if there's anything wrong with the armature, it's all in vain.)
A cold bulb hooked up won't light in a hurry, especially a 6v one - it will need pre-warming using a battery, or to be hooked up after the dynamo has started to deliver.