Looking at an old 2 brush dynamo, there are two terminals at the top. Which is “F” and which is “D”,there are no marks on them ? Are these dynamos self earthing or must there be an Earth cable ? Thanks
Can anyone help with this query,please ?
I think that they must be self earthing, as they have a metal body which is firmly clamped to the saddle on top of the magneto, which itself earths via its mounting bolts and the magneto platform. I’m no ‘Sparks’ myself as I am partly colour blind, which is a real problem when playing around with wiring harnesses ! My late father was even more paranoid about electrics… “You can’t see it, you can’t smell it, and when you feel it ,it kills you.”
Yep, they earth through the dynamo frame. F stands for field which is the windings around the (fixed) yokes. The regulator adjusts the current through the field coils which alters the magnetic strength and thus the amout of juice generated by the spinning commutator. The commutator output is connected to D via the brushes. If my recoolection is correct, one brush is earthed to the dynamo frame and the other goes to ‘D’. The connection to the field winding should go to ‘F’.
Hope that helps,
One of the terminals should have about 3 ohms resistance between its self and the generator body. This will be the F.
The other should have not much between it and the generator body and it may vary a bit as the armature is rotated because there is two brushes between it and completing the circuit to earth. The F terminal resistance should not vary as it does not go through brushes.
This assumes it is a Lucas and has not been modified for say a Bosch regulator.
If you can see inside the geni the wire from the D terminal will go to one of the brushes and the F will go to the field coil. The other brush and other end of the field coil should go to earth.
The generators earthed through the body so a really good paint job will mean no earth. Rare that paint insulates that well with all the bolts and clamps and stuff but worth checking back to battery earth.
Thanks for the replies. I do not fully understand electrics, so brushes and field coils are a mystery to me. I have attached a photo to this posting and hope that someone can tell me which terminal is F and which is D. Thankyou,
Sorry, but hard to see from the pics and I’m not familiar with that type. One brush will be connected to the dynamo frame. The D connection should be to the other brush. There should be a connection (not to earth) that goes to a wire vanishing inside the outer coil – that is F. As suggested, a multimeter would be my first check. A good test is to connect D and F together and connect to 6V (try to get polarity correct to the intended machine) with the other connection to the dynamo frame) then it should run as a motor.
It does not look like a 2 brush Lucas dynamo. I do not say I know all of them but it is not like any Lucas one I have seen. That means D and F may not be valid.
Bosch use D and F but their regulator is the earth side of the field coil and Lucas is on the power side. The internal wiring in the dynamo can normally be changed to suit either regulator.
Miller have something similar to Lucas but the cut out is inside the dynamo and the connections are D and S. I have no practical experiance with Miller but the dynamo internal wiring can be changed to suit the Lucas regulator. Apparently this is an improvement.
If it is a BTH it will work excellently by some obscure theory that no one understands any more but a few will know how to make it work.
The few PAL and CAV dynamos I have seen are similar to the Bosch. To motor them the F is connected to earth and D to power.
I would suggest you take the dynamo to some one who knows about old dynamos as they can all have their internal wiring changed to suit the various regulators.
Didn’t the Miller dynamo have the regulator in a little pressed tin box on the outside ? Funnily enough I’ve just been asked to look out for one at the next Newark Autojumble, by someone who needs one for his MOV Velocette. It’s nearly thirty years since I had a Velo with a Miller dynamo, and I can’t remember what it was like, except that it couldn’t keep the battery charged when I was riding in the dark, (literally !) We curse Lucas stuff, but Miller was worse. I don’t really blame them, because it was the bike manufacturers who were screwing them down on price all the time, and quality suffered as a result. BTH and Bosch stuff was a lot better.
It is definitely a Lucas Magdyno and I think it is a type MDB-1. The base is stamped with the Lucas emblem and then 336 LUCAS.
UPDATE…..Following a Google link for Lucas Magdyno MDB 1, I was taken to a Brough Superior site with comprehensive diagrams and service instructions relating to a Lucas Magdyno MDB,MDBR 1929,clearly showing the “F” and “D” terminals. Whether a previous owner fitted this item to my 1936 FS or whether it was original,I don’t know,but at least I can find the details I was looking for. If it was good enough for Scott then it should be good enough for a lesser bike !!!!
One of the very good things about Lucas mags and magdynos is that they are date stamped. I think 336 = March 1936, so it probably is the original unit. I am no expert, but I think it might have been a three brush job in 1936.