Dynamo Polarisation

[Mosin]

Perhaps I am being particularly thick today, but I've read and re-read the section of the Lucas Workshop Instructions concerned with generator polarity and I'm not finding it particularly clear which side of the battery I should be using to "flash" my dynamo - depending on the way you read the instructions they could be interpreted either way.

Anyway, I am running a 12v positive earth system - the battery positive terminal being connected to the bike's frame. So, my question is: which side of the battery do I need to flash to the dynamo "F" terminal?

Cheers,

Simon

[iansoady]

Negative. It's always the one that isn't earth. If you think about it, connecting the positive (earth) side to the field windings will do precisely nothing as both ends of the field will be at the same potential.

The earth (positive) side of the battery should of course be connected to the frame when you do this.......

[Mosin]

That's great. Thanks Ian!

[wilko]

To avoid confusion, just close the cutout points on your regulator a few times. With mechanical regulators anyway!

[iansoady]

True, but if you have the RB108 regulator with the mickey mouse alloy cover, the bent over tabs rapidly decide they're going to depart after being removed a couple of times. The clip on the earlier type was much better.

[A10Boy]

If they go wrong, just throw them in a ditch and fit a solid state jobbie.

[wilko]

I have one of each on my bikes ,and surprisingly my mechanical one has worked perfectly for 6 years!

[Mosin]

Hmmm. I have just flashed the dynamo - I didn't really think that there was anything wrong with it, but I figured it was something which could be easily eliminated - and it hasn't made any difference at all. The bike still isn't charging.

The dynamo has been very recently rebuilt by Sean Hawker so I would be surprised if it proves to be faulty, but I have run the test connecting the two terminals together and then using a voltmeter from them to an earth point and I am not getting any charge reading at all. I've also removed the timing case cover to check that the drive chain is intact and turning correctly and it is.

Is there anything else I might have overlooked?

Simon

[iansoady]

If it's just been rebuilt it's possible the brushes aren't making good contact with the commutator. If you connect an ohmmeter between the D terminal on the dynamo and earth you should see a resistance of around 1 ohm - IIRC. Obviously disconnect everything else from the dynamo first. If you gently turn the engine over this resistance should slightly fluctuate.

I once had a problem whereby the dynamo body wasn't making good electrical contact with the engine earth (it had been painted).

[Dynamo Regulators Mike]

Simon
Could the dynamo be connected internally for opposite direction?
If so reversing field (or aramture, one OR the other) connections will sort it.

[bsa-bill]

sorry if this is obvious Simon but do remember the bike is positive earth when using your meter ( I and I bet many others occasional get caught even after years of A ing).
Using a bulb is a polarity proof way of testing

good luck with it

[Mosin]

Sorry guys, I should have made it clear that since being rebuilt, the dynamo has been running fine on the bike for a couple of months and charging fine through a DVR2. It just stopped working all of a sudden last time I was out on it.

As regards positive earth Bill, I am using a digital multimeter so even if I had got the probes the wrong way around it would have just given me a negative reading. As it is, I am not getting any reading at all coming off the dynamo, either positive or negative!

I've also just tried the resistance test suggested by Ian and found I have 1.5 ohms of resistance between terminal D and earth which did fluctuate when I rotated the engine.

Back to the drawing board then...

[wilko]

Unfortunately if it was working before it may have been overcharging and thrown the solder off your commutator segments.Before you pull it apart though check your brushes for free movement in their holders and field wires are all intact.I've saved an armature by soaking each segment with a heavy duty soldering iron only last week.

[a101960]

This what BSA service sheet 809 says:

Field coil

"Measure the resistance of the field winding by means of an ohm meter. If this not available connect a 6v DC supply with an ammeter in series across the coil The ammeter reading should be approximately 1.9 amperes". This is for the E3H dynamo. For the E3L which yours probably is, the value is 2.1 amperes. No reading on the ammeter indicates an open circuit in the field winding.

Running the engine dynamo test BSA service sheet 809 "It is sufficient to run the generator up to a speed of 1000 r.p.m. If there is a low reading of approximately 1/2 volt the field winding may be at fault. If there is a reading of approximately 1 1/2 to 2 volts the armature winding may be at fault".

Switch your multimeter to amps and swap the meter leads to the amp position (10 amp). to test, or alternatively disconnect your ammeter leads on the bike and put that in series.

For the running test common F and D to the multimeter red wire and run black lead to earth. Use crocodile clips to ensure a good connection.

John

Check also that the insulation on the wires to the terminals have not been damaged. This can happen if the wires are pinched by the end cover. Also check the continuity of the conductor between the terminals and the brushes in case the conductors have broken.