The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: raindodger on 07.01. 2024 08:58
-
Hello, I've had my dynamo converted to 12v by Drags. When it arrived back I decided to test it before fitting to my 'A'. Connected in the usual way D linked to F, meter from D to frame. Had 7v max. Returned to Drags, they put it on their rig and got 29v. Got it back and a retest gave 7v. What am I doing wrong??
Regards.
Tim.
-
Could be that as it is a rebuild it requires "polarisation" in other words an application of power directly to the field coil to impart some magnetic field to the body and wake it from its slumber. All detailed in the electrical section. But to hear that Drags get 29v is strange in comparison. Best ask them how they did it! From that your test should give a good output, I use a headlamp bulb rather than a meter, in effect by joining F and D the D terminal is feeding ever increasing power directly back to the field coil, so the output should rise rapidly.
Swarfy.
-
Even a 6v should give over 12 on test at a decent RPM. Its should be giving 6v a bit above engine idle (I can't recall the chain ratio) so I'm guessing it should be doing a decent voltage by 2000 rpm at the dynamo.
-
G'day Tim.
D & F joined to +side of multimeter, - side of multimeter to body of dynamo.
Cheers
-
Hi Tim
How are you testing the dynamo?
Are you spinning it in the correct direction and what speed are you testing at?
A Swarfcut said use a 12v 60w headlamp bulb
John
-
OK. I'm spinning at 3000 anti clock. I've tried a couple more things. Fed the field coil from a 12v battery charger and got an output of 12v. Then tried a 12v 50w headlamp bulb - nothing - not even a slight glow and the meter showed 0v. One thing I have noticed and that is that the voltage creeps up slowly over about 3 seconds to a max of 7v and then steadies.
Regards. Tim.
-
Hi Tim,
Try swopping around the field wire connections, then flash the field coil again
John
-
Have you tried running it clockwise?
Why not try running it as a motor off a battery to see which way it actually spins? D&F bridged, battery one lead to the bridge and t'other to the dyn body - according to whatever earth you are set for- and see. (That'll incidentally confirm the polarity - which earth - for you.)
If it goes the wrong way, change the field wires over or swap the brush connections to reverse it as already stated. (Not both.)
If it's producing 7v in some circs, I kind of doubt the rotation is actually wrong (unless your 7v is actually 0.7v, which is about what I'd expect in those circs). Also, I'd have thought Drags would have got it right if you told them what engine it was going on.
When testing with a bulb, it's often best to get the dyn spinning and cut in before hooking 50W up. 50W/12v and a cold globe will take a fair bit of current to fire up (like maybe 8 amps+), and the thing won't necessarily be inclined to produce that much from a standing start.
The only other 'funny' would be if Drags had built the dyn for earth-side regulation, rather than the normal Lucas live side. That would be visible because the two field coil wires under the lid would be hooked between D and F, not D and Earth. Highly unlikely . . . but some regulators (JG for example) work that way and need it, and it was standard on Miller and most other non-Lucas equipment.
-
Hello, Gents. The unit motors happily on 12v and in the correct direction so, presumably, the field coil is connected correctly, the coil has a restance of 10 Ohms and is connected to D and ground. Spinning it clockwise gives no output. The 7v is correct, using the 50v scale. I'm seriously considering taking the dynamo and my test leads, meter and drill to Drags to see what they are doing that I'm not.
Regards. Tim.
-
Hello, Gents. The unit motors happily on 12v and in the correct direction so, presumably, the field coil is connected correctly, the coil has a restance of 10 Ohms and is connected to D and ground. Spinning it clockwise gives no output. The 7v is correct, using the 50v scale. I'm seriously considering taking the dynamo and my test leads, meter and drill to Drags to see what they are doing that I'm not.
Regards. Tim.
Assuming that's a typo Tim? F and ground we presume?
-
Correct, F and ground. bit of brain fade there!
-
chaterlea25, John. Questions for you, is there a correct way to connect a field coil? What I mean is should the inside/start lead be connected to D, or the outside/finish lead? Again, would that make a difference as the coil can be polarised eitherway? I hope that makes sense.
Regards. Tim.
-
Sorry I meant F.
-
10 ohm field coil sounds high, but it’s not impossible that could be right. A 6v coil has a resistance of around 3 ohms.
I would suspect the brushes are at fault….stuck or dirty, if they have a “high” resistance they prevent the self generation required for “start up” (eg the d+f joined voltage test). Putting 12v into the field coil should have got around that issue however, but didn’t which is odd.
-
If the dynamo rotates in the correct direction, then the field and brush connections are correct. Reversing polarity won't alter direction.
-
just a thought some carbons are no good and even only fit for the bin even before they have been fitted,
-
Checked the carbons, 0 ohms from pigtail loop to rubbing face.
I think I'll take it back to Drags and find out how they get 28 volts.
Regards.
Tim.
-
0 Ohms is good.
-
Hello, Groily and Swarfy.
Thanks for the comrehensive advise. I'm going to bite the bullet and take my test leads,
meter, drill and the dynamo to Drags next week. This has gone on long enough.
Watch this space.
Regards.
Tim.
-
Hi Tim,
No real way to identify the field winding ends when the coil is fitted, the way it works is correct *ex* *ex*
I think that 12 ohms is high resistance value for the field coil??
Does anyone have a 12v dynamo apart to check the value?
I'm not sure if I have one???? Must look
John
-
I think that 12 ohms is high resistance value for the field coil??
Does anyone have a 12v dynamo apart to check the value?
John
Yup.
Just measured three new unused 12v ones, UK-made and all are 12 ohms +/- . . .which is what I expect to see with them.
6v ones at a tad under 3 ohms typically, also as expected . . . and per Lucas at 2.8 /2.9 ohms (iirc).
-
My 12V field coils are about 11 Ohms. I used to wind them to 5.6 Ohms, and they were fine, but with the higher resistance the I squared R losses are much reduced resulting in less heat generated.
-
Hi. I'm happy to check it out f.o.c. Just send it to me and I'll tell you how it performs on my tester. 10 ohms is correct (or thereabouts) for a 12v field coil.
-
Dear all.
I'm an idiot!! I was not spinning it fast enough, that's all.
Increased the speed and all was well.
Thanks for all the advice, it's all sorted now. Hooray!!
Regards to all.
Tim.
-
Thought you told us you were spinning it 'at 3000rpm anti-clock'? Was there a 0 missing!?
Anyway, glad it works.
-
Yes, Idid say that but I was mistaken.
The drill is very old and makes the same noise no matter what gear is selected.
However, all is well!!
Regards.
Tim.
-
*smil* *smil* *beer* *beer*