EL008 ELECTRICAL

I stopped my 1957 Venom, which is fitted with a type D 6 volt dynamo, during a recent run out and when in for a cup of tea, only to be dragged out by someone who said there was a whirring noise coming from the dynamo! Sure enough the dynamo was running like an electric motor and it was hot! After disconnecting the leads I scratched my head - how could this be? A dynamo is supposed to make electricity not consume it! How could it run with a vee belt attached? I resumed my journey home, removed the vee belt cover only to find the belt in tatters. Are these two events related? It seems a big co-incidence. Could the dynamo spring into life and damage the belt due to some fault? What caused the dynamo to run like a motor? Is it a fault in the regulator or is it a fault in the dynamo? I do not think the belt was at the end of its life nor was it fitted too tight. (I have seen the advice given elsewhere on tensioning this belt - thank you). - John Shipman 9/5/2002

A dynamo is almost identical to a motor.
Almost all dynamos have a "cut-out" which keeps the battery disconnected from it until it is charging more volts than the battery has. If your cut-out doesn't, then the dynamo will motor - in fact this is the most basic test of a dynamo, wire out the cut-out and wire in the field coils and if it doesn't motor smartly it has a problem. You need to research your cut-out, which may be in the end cover of the dynamo or in a separate regulator and check it out - they are covered in the Velo service manuals. However, I suspect that it is more than just a stuck cut-out. Although closing the cut-out can cause the dynamo to rotate gently, it sounds as though yours has been turning faster than that. This means the field coils were fully energised, which means you either have a faulty regulator or a wire is shorting. If you have an electronic regulator I think it could fail such that it both 'closed the cut-out' and let full volts through to the field coils. -  TW 9/5/2002

I have been checking the wiring looking for a short, chafed wire, and comparing the bikes wiring to that in the manual (part no. VSL102) I got from the VOC. The wiring diagrams are not very clear but I am pretty sure the diagrams show the battery positive terminal is connected to the earth. On my bike, clearly the previous owner has replaced nearly all the wires as they are no longer the same colour as in the manual. This complicates matters a little! But more importantly (perhaps) the battery negative terminal is connected to earth and is also connected to the "E" terminal of the Regulator. This means that the battery positive terminal is connected to the ammeter and then on to the "A" terminal of the Regulator. The terminals "D" and "F" on the Regulator go to the dynamo.
Does it matter that the battery has been connected the wrong way around, or could this be the cause of my problem? - John Shipman 12/5/2002

I think you are looking at the wiring diagram for the later Lucas dynamo & regulator, which used a positive earth and had very different colour coding using 'stripes'.
I think you may have a Miller dynamo, though perhaps with a Lucas regulator.
Regardless of which dynamo you have, it doesn't matter whether you use +ve or -ve earth so long as the person who changes the polarity goes through the procedure to change it properly, and so long as there isn't an electronic regulator or any other electronic components. So if your dynamo was charging the battery while it ran, and keeping the lights lit, and the ammeter was showing charge and discharge as appropriate, the wiring was OK before something went wrong.
For some reason Velocette left the early Miller diagram out of the later manual but all the wiring diagrams are on our site, so you'll probably spot your combination in there somewhere. - TW 12/5/2002

DYNAMO MOTORING - From an article by John Hannis in FT326
The situation described by John Shipman in FT 325 is entirely normal if you have an electromechanical cut out and it has happened to me just as John described. The cut out requires a reasonable amount of reverse current to cause the contact points to open. This happens when the dynamo is connected to the engine and is brought to rest when you stop.
However, if you shed the dynamo belt with the engine running, the dynamo will continue to motor and the reverse current consumed when it is turning at speed can be insufficient to cause the cut out points to open so it just keeps going.
Disconnecting the battery or applying a gloved hand to slow down the spinning pulley would both cause the cut out points to open.
I don't think this could happen with an electronic regulator.

LETTER to FT326 from David Thomas
I would like to amplify TW's reply to the query in FT325.
My background is some 30 years working in Lucas Electrical design office (not admitted until I retired) so I do know a bit about the subject. It is quite possible that the only thing wrong with John Shipman's Venom was a broken belt! If this reached the point where the dynamo pulley was free to revolve with the engine running, then the unit could motor when the engine is stopped. This is because a mechanical cutout will not open until a certain value of reverse current passes from the battery through the dynamo. Normally this happens almost instantaneously, ( you may just see a 'flick' of the ammeter) but a free running dynamo may run without drawing enough current to operate the cutout. Applying a load to the pulley would cause the current to increase and open the cutout, but be careful in case there is another fault.
TW is wrong about dynamo motoring speeds - full field current gives a low speed, about 1000rpm, while a lower field current gives higher speed and current draw. The fault described above will result in motoring with full field, as the battery feeds the field through the regulator, as well as the armature.
Finally, for this to happen with a fully electronic regulator, there has to be a fault; recently I suffered a similar problem when the diode acting as a cutout became short circuited by a frayed wire.