Author Topic: The perennial charging issues.  (Read 780 times)

Offline Rex

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The perennial charging issues.
« 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*



Offline Del 60

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Re: The perennial charging issues.
« Reply #1 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.

Offline Swarfcut

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Re: The perennial charging issues.
« Reply #2 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.

Offline Triton Thrasher

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Re: The perennial charging issues.
« Reply #3 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.

Online bsa-bill

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Re: The perennial charging issues.
« Reply #4 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
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline Rex

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Re: The perennial charging issues.
« Reply #5 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..

Offline chaterlea25

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Re: The perennial charging issues.
« Reply #6 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
1961 Super Rocket
1963 RGS (ongoing)

Offline trevinoz

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Re: The perennial charging issues.
« Reply #7 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.

Offline Triton Thrasher

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Re: The perennial charging issues.
« Reply #8 on: 04.05. 2019 07:29 »
On most lathes, it is at least simple to spin it in both directions.

Offline Rex

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Re: The perennial charging issues.
« Reply #9 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*


Online Bsareg

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Re: The perennial charging issues.
« Reply #10 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 ?
C11,B40,B44 Victor,A10,RGS,M21,Rocket3,REBSA

Offline Swarfcut

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Re: The perennial charging issues.
« Reply #11 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.

Offline Rex

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Re: The perennial charging issues.
« Reply #12 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.

Online Bsareg

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Re: The perennial charging issues.
« Reply #13 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.
C11,B40,B44 Victor,A10,RGS,M21,Rocket3,REBSA

Offline Rex

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Re: The perennial charging issues.
« Reply #14 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.