The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: Rex on 03.05. 2019 16:30

Title: The perennial charging issues.
Post by: Rex on 03.05. 2019 16:30
My recently rebuilt 1951 A7 has not been charging since completing the restoration a couple of hundred miles ago, but now it's back in the workshop for the usual newly-rebuilt issues and adjustments, I thought I'd check out the dynamo.
It was rebuilt with new brushes and bearings, and using the existing armature and field coil. The usual test of bridging F & D and checking the output shows 0.1 to 0.2V on an AVO Mk8 (more forgiving than a DVM for rotating DC machines, I find) and the field coil measures at 3 Ohms while the armature measures around the 3-4 Ohms on one armature, and 0.5 - 0.75 Ohms on a substitute (fitted for testing purposes).
Referencing the excellent "Matchless Clueless" diagnostic article these resistance figures don't seem far from the expected values.
Mounting the dynamo in the lathe and rotating it at the top speed of the lathe (F-D bridged) gives about 0.1 - 0.2V when either armature is fitted. It also motors using a small Cyclon battery on the bench. There is no breakdown of the insulation to Earth.
The dynamo has been flashed countless times, the brushes, field coil connections and rotation have all been tried in all possible combinations but this machine stubbornly refuses to work.
Either I'm missing something obvious or there's something very wrong somewhere, so before I start throwing big money at a new field coil and/or a new armature has anyone any ideas?
Looking back through the many past replies on this subject it seems like flashing the dynamo or checking the internal/external connections usually sorts it, but sadly not in this case. *sad2*


Title: Re: The perennial charging issues.
Post by: Del 60 on 03.05. 2019 17:49
Has the dynamo body been painted. ?
Could it be a bad earth   + / -
on dynamo bracket holding it in place.
Title: Re: The perennial charging issues.
Post by: Swarfcut on 03.05. 2019 17:54
Rex.   Try a test with an earth connected to a nice clean part of the dynamo body, F & D bridged, 12 volt headlamp bulb between this bridge and the direct earth. Then check the internal wiring, making sure all connections are sound and correct, insulation where it should be and that the D terminal is connected directly to one brush. The other brush is earthed, usually on the same point as one field coil lead. The other field lead goes to the F terminal. Make sure the brushes are free to move.  The dynamo may be incorrectly polarised, but this does not matter for this test.

 Still no good, first check that the brushes actually conduct. Then disconnect both field leads, check the field coil is open circuit between each lead and the body, but the coil itself has continuity. In other words  the field current  is flowing through all the field winding, and the current is not taking a short cut. Also worth checking the field coil pole piece is securely tightened against the body, as anything which reduces the inherent magnetic field will result in low output.

 The armature should have more or less the same resistance across opposite  commutator segments. Test the resistance  between adjacent segments, here again, although the two values may be different, they should be consistent for both sets of readings. Finally check each segment to the shaft. It should be open circuit, showing the shaft to be electrically isolated from the armature windings.

 Swarfy.
Title: Re: The perennial charging issues.
Post by: Triton Thrasher on 03.05. 2019 18:15
I have sometimes had to briefly put 6V from a battery through the field while doing the spinning (at least 2,500 rpm) test above, to wake the dynamo up.

One battery terminal connected to your joined D and F.  Other battery terminal to dynamo case, other field lead and other brush pigtail.
Title: Re: The perennial charging issues.
Post by: bsa-bill on 03.05. 2019 19:00
just going to confirm what has been said as I found this on my Flash after I unthinkingly mounted the dynamo on rubber strips and the body had been painted, a solution was to scrape some of the paint off from below the retaining strap and slip a bit of copper braid in there before tightening it down.
Hope you get  yours fixed as easily
Title: Re: The perennial charging issues.
Post by: Rex on 03.05. 2019 21:48
To try and answer those replies as a group....the body is painted but the dynamo is mounted on the lathe without the band being fitted, so the aluminium rear bearing support gives a perfect earth. The internal circuitry has been checked numerous times for continuity and that (and the armature and field coil) have resistance values as stated. The brushes are new and slide easily in the brush boxes, and the field coil has no continuity to earth. When the dynamo was spun up on the lathe I tried a known working 6V 10W test lamp from the F-D link to earth....not even a glimmer of life, and that was immediately  after yet another flashing.
I'm fast running out of possible solutions..
Title: Re: The perennial charging issues.
Post by: chaterlea25 on 03.05. 2019 23:00
Hi Rex,
When you motor the dynamo from a battery (using the chosen polarity of the bike) the dynamo should turn in the correct direction when fitted to the bike
It must be spun in the same direction for testing
As mentioned by TT connect a 6v battery across earth and the Field connection
Connect the headlamp bulb from D to earth
When spun the bulb should light ,  *bright idea*
a 12v 60watt bulb should light full brightness if the armature is good

John
Title: Re: The perennial charging issues.
Post by: trevinoz on 04.05. 2019 00:27
When testing an armature it is not really much use testing between segments unless there are a lot of open circuits in the windings.
Sure, test for earth but it only needs to be done from one segment, they are all connected.
A growler test will find the faults, if you have one.
I have one but rarely use it, I prefer to "drop test" the armature.
This is done by setting the armature between two contacts on the commutator and applying a voltage, it doesn't need to be very high, just enough to be able to get a reading.
Test between each segment and rotate until each pair has been tested. I use the 2000 millivolt scale and adjust the input voltage so that I get about 15 mV across each segment, the good ones.
A high reading indicates an open circuit and a low a short circuit. Most commonly they have open circuits.
Low voltage output is caused by wrong rotation, high resistance between the brush and commutator or a buggered armature.
Motoring the generator does not prove that it is any good, they will motor quite happily with faulty armatures.
Title: Re: The perennial charging issues.
Post by: Triton Thrasher on 04.05. 2019 07:29
On most lathes, it is at least simple to spin it in both directions.
Title: Re: The perennial charging issues.
Post by: Rex on 04.05. 2019 09:33
Thanks for the replies.
On a Myford it certainly is possible to run it in reverse, and it has been tried. It does motor in the correct direction when connected across a battery. I don't have a growler and indeed I haven't even seen one since my apprenticeship days X decades ago, but a quick-and-dirty check of continuity across the commutator should at least give enough indication that the armature isn't full of dead-spots, and it isn't.
That said, I have tried another armature and it performs the same. While it's far from impossible that both armatures are somehow U/S, it's unlikely, given the visual and simple-test condition of each.
Lost.... *conf*

Title: Re: The perennial charging issues.
Post by: Bsareg on 04.05. 2019 10:30
If the dynamo motors smoothly , once it is forcibly spun above its natural motoring revs the current drawn will reduce until it becomes a charge. The construction of a brushed motor and a dynamo is very similar. The fact that yours motors should indicate all is well. Have you tried feeding 6v into the field only, then checking dynamo output ?
Title: Re: The perennial charging issues.
Post by: Swarfcut on 04.05. 2019 11:15
Rex, This is becoming even more of a mystery. The fact it motors at least means the field coils are producing some magnetic field, so it could be that for some reason the body is not retaining enough residual magnetism to allow any major output to be produced, yet it is not completely dead. Powering the field coil independently is a good idea, see if that works. A rotating coil in a magnetic field should produce a current, according to Faraday.

 "Ye cann'ae break the Laws of Physics",  Scotty used to say, so it must work.

 Just an observation on the FD bridge test. With the bridge connected, dynamo spinning, any small output is fed directly back into the field coil, strengthening the field, increasing the output. Then connect the test lamp from  the bridge to earth. Connecting the lamp at the start means any output is drained through the lower resistance of the lamp, never adding to the field coil strength.

 

Swarfy.
Title: Re: The perennial charging issues.
Post by: Rex on 04.05. 2019 14:17
Thing is Swarfy, the test lamp was tried more in desperation than expectation, but then the AVO I've been using ( a real blast from the past!)  current draw is next to SFA anyway so shouldn't affect the output.
I'll try and give it the 6V injection to the field coil and see what the output is then.
Title: Re: The perennial charging issues.
Post by: Bsareg on 04.05. 2019 15:19
While you're there, have you tried ohms between commutator and shaft with the brushes isolated ? Current might just be shorting to earth. Where do you buy Avo mk8 batteries from, haven't been able to get them for some years. Great meters though. I still rewind our mags on a Avo douglas electric winder, brilliant piece of kit.
Title: Re: The perennial charging issues.
Post by: Rex on 04.05. 2019 16:52
Yep, done the insulation test of armature to earth and it's next to infinity.
AVO batteries..one's the common large 1.5V cell which used to be termed a U2 but has now changed it's designation, and smaller one came from Ebay (where else?) and is a little stack of small round batteries.
This one sat unused for years in at work ever since the times when cheap DVMs came available, and I only dished it up to use as a matter of interest, but it's analogue needle stability has been very useful when trying to measure small and unsteady voltages.
Title: Re: The perennial charging issues.
Post by: trevinoz on 04.05. 2019 22:31
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.
Title: Re: The perennial charging issues.
Post by: KiwiGF on 05.05. 2019 06:22
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.
Title: Re: The perennial charging issues.
Post by: Swarfcut on 05.05. 2019 07:53
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
Title: Re: The perennial charging issues.
Post by: duTch on 05.05. 2019 09:33

 
Quote
.......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 ?  *smile*...........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  *eek* << better thaan my typing<<  and  had a working dynamo-humming...
Title: Re: The perennial charging issues.
Post by: trevinoz on 05.05. 2019 22:09
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.
Title: Re: The perennial charging issues.
Post by: Rex on 06.05. 2019 11:13
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?
Title: Re: The perennial charging issues.
Post by: KiwiGF on 06.05. 2019 11:53
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.
Title: Re: The perennial charging issues.
Post by: Bsareg on 06.05. 2019 11:59
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 ).
Title: Re: The perennial charging issues.
Post by: Rex on 06.05. 2019 15:39
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?
Title: Re: The perennial charging issues.
Post by: Bsareg on 06.05. 2019 19:16
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.
Title: Re: The perennial charging issues.
Post by: chaterlea25 on 06.05. 2019 19:39
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
Title: Re: The perennial charging issues.
Post by: Rex on 06.05. 2019 21:28
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?
Title: Re: The perennial charging issues.
Post by: Bsareg on 06.05. 2019 22:09
  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 ?
Title: Re: The perennial charging issues.
Post by: KiwiGF on 06.05. 2019 22:17
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?

Title: Re: The perennial charging issues.
Post by: groily on 07.05. 2019 07:01

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.
Title: Re: The perennial charging issues.
Post by: Rex on 07.05. 2019 10:47
The latest-  it motors slowly at 6V, nothing at all at 4V.
Pressing the brushes to ensure good contact was done as a first step..it made no difference.
I'm doubtful the brushes are at fault in any way as they came from a reputable and well-known dynamo/mag rebuilder.
So, what's the consensus, new armature first or new field coil?
Title: Re: The perennial charging issues.
Post by: Swarfcut on 07.05. 2019 11:59
Rex It hinges on your ultimate aim. Cheapest option, halfway house or top quality replacement.  It would be good to know what is wrong, but in the grand scheme of things another serviceable unit may not be cheapest, but is far easier.

 Replacing the field coil is tricky to get right, making sure of good physical all over contact of the pole piece and body, hence the screwjack Lucas Tool. Maybe you can pick up a body with a good coil, failed armature.

 If you decide to change both armature and field coil, the cost of an upgrade to 12V is not much more, but needs the 12v regulator cost to be factored. Assessment by a specialist is your other alternative.

 The armatures you have are an enigma, both failed the same, one good, one bad, both good? Substitution in a good working body would be a way forward if you can lay your hands on one.

 Swarfy.

Title: Re: The perennial charging issues.
Post by: KiwiGF on 07.05. 2019 12:25

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.

This thread IS confusing but I think we were talking about a test that involves independently powering the field coil with 6v, then spinning the dynamo up and powering a high wattage bulb. In this scenario the dynamo should easily light the bulb.
Title: Re: The perennial charging issues.
Post by: KiwiGF on 07.05. 2019 12:38
This thread is long but has quite a bit of info including test results.

https://www.a7a10.net/forum/index.php?topic=13999.0

I found fitting a new field coil pretty easy, I just used a piece of 3/4 inch pipe through the middle of the case and a bench vice one end and vice grips the other to clamp the core against the case, then an impact driver on the screws (with  Loctite). It was pretty obvious if the core was held hard against the case or not as the retaining screws would tighten within half a turn once the core was hard against the case.

Having said that, I doubt a coil with 3 ohms resistance could be faulty unless it is shorting to the core.

I’ve not experienced an armature failure, do they short out between coils? Or short to the armature “core”?



Title: Re: The perennial charging issues.
Post by: groily on 07.05. 2019 14:31


This thread IS confusing but I think we were talking about a test that involves independently powering the field coil with 6v, then spinning the dynamo up and powering a high wattage bulb. In this scenario the dynamo should easily light the bulb.
[/quote]

You're right Kiwi - wasn't following properly, sorry.  So yes, it should light.


Title: Re: The perennial charging issues.
Post by: trevinoz on 07.05. 2019 22:19
As I have said all along, armature.
Can you borrow a known good one?
It's fairly simple to set up a drop test rig. I screwed a couple of pieces of aluminium channel onto a piece of timber and drilled a hole through each at comm centre height and fitted a round head brass screw through each with a spring for tension and a nut on the outside.
Apply a low voltage to same and read voltage drop across each pair of segments.
They all should be fairly close in value to each other, they will vary slightly due to the different resistance in each coil.
Title: Re: The perennial charging issues.
Post by: trevinoz on 08.05. 2019 01:08
Rex,
I set up a generator with a known open circuited coil armature.
I excited the field from another source and drove it up to around 2500 rpm.
The output was in the order of 12V, however when I reverted back to the bridged terminals test, I could only get 0.7V.
Title: Re: The perennial charging issues.
Post by: Rex on 23.05. 2019 15:44
Righto, here's an update.
As the tests etc didn't definitively show a faulty armature or field coil, and as 12V items are the same price as 6V ones, I decided to bite the bullet and buy both, on the basis that I didn't want to be doing the job twice.
An impact driver made short work of removing the field coil, and the new one installed by using two deep-throat G clamps on the pole piece and again the impact driver.
The dynamo was then assembled and tested by motoring...nothing. Getting the trusty AVO out showed that the field coil was O/C straight from the pack. Unbelievable, so this was returned to the seller (another week wasted) and a replacement sent.
The new one was assembled into the dynamo, and during testing (on the lathe) all output ceased. Bizarrely the AVO had chosen that moment to give up the ghost..
Now the dynamo looked to be a goer, but due to the man-handling it needed a respray..easy enough, but the spray gun just happened to choose this moment to block one of the side air bleeds and the paint spattered badly, necessitating a clean and a re-respray.
Now it was looking good so reassembled into the bike, the terminals bridged and the meter showed a healthy output.
Reconnected it to the DVR2 ( a known worker from another bike fitted at the start of this saga), and...nothing. Regulator appears to have given up the ghost, and yes I did reconfigure it for 12V operation.
 I swear this charging system is cursed in some way, but sod it, today was a lovely day for a ride, so I did just that. Replacement can wait for another day as I've already lost enough riding days this year.