Charging woes

[Peter Arnold]

Just had the dynamo checked out as the ammeter has only ever shown a discharge when operating the car. We have one of those rare breeds who knows all about old vehicles and he has not yet retired. He found one of the field coil leads was shorting intermittently and fixed that, Checked the operation of the cut-out, cleaned up the comm and the brushes and set the charge rate at about 7.5 amps. After reinstating the dynamo the ammeter still showed a discharge. Reading up notes from various clubs (Cornwall etc) one of the things that could affect charging is the small current from the ignition warning light which I had previously found to be faulty. Installed a new light and fired the old girl up. The warning light stayed on and then when I revved the engine it went brighter then blew. I then installed a spare ammeter in the feed from the battery and the generator is certainly charging. I have checked the wiring and thus far have found nothing amiss. I am running the electrics on 6V negative earth (originally positive earth). Suggestions would be appreciated.

Cheers, PeterA.

[David Stepney]

If the warning lamp bulb blew when the engine was revved, it sounds as if the cutout is not operating and the dynamo is trying to charge through the lamp. Make sure that the cutout is wired correctly. Take off the top cover and check that it contacts are closing when the engine is revved. If not, try pushing them together briefly to see whether, when they are closed, the ammeter shows a charge. This will establish whether the cutout is faulty. If you can, try another known good cutout.

[Bob Culver]

When you push the cutout closed it will or should lock closed. Must pull open or release by disconnecting the battery briefly.
Contacts can be cleaned as above. The ignition bulb should vaguely approximate the originall bulb or combination.
Not clear which lead you put the ammetr in. But assuming the summer resistor is not on the dynamo, if ammeter in the lead from dyn it supplies some current for its own field independant of the cutout operation.

[Peter Arnold]

Thank you gents, I have checked the cut out and I cannot get a good reading across the shunt winding, it does not appear to be open circuit so will arrange for a more accurate meter from one of my sons-in-law who is an electrician. I have not been game enough to push the cut out closed as it happened once by accident and scared the heck out of me. I suppose I could do it with the battery lead clamp only pushed on so that a quick disconnect could be effected. Cheers, PeterA.

[David Stepney]

Peter,
Just for clarification ( and I hope I am not teaching granny to suck eggs here), the cutout works as follows:-

The bobbin has two windings. A very fine wire winding that is connected from the dynamo output to earth which is the voltage winding and a much thicker winding that is the current winding that is connected between the dynamo output to the battery via the cutout points. The voltage winding will have a high resistance. I can't give you a figure off the top of my head. The warning light is also connected between the dynamo output and the battery, effectively bridging the cutout points. The cutout operates as follows:

With the ignition on, and dynamo not charging, current flows from the battery through the warning light and earths through the dynamo windings. As the dynamo begins to produce current, the voltage winding is energised and pulls the cutout armature to the bobbin, closing the cutout points. Current now flows through the current winding, increasing the magnetic pull on the cutout armature. as the current path through the current winding is now at less resistance to the path though the warning lamp, the lamp goes out, and the dynamo begins to charge the battery.

Conversely, as the dynamo stops charging, say at tickover, the dynamo voltage drops and current starts to flow back to the dynamo through the current winding. This reverses the magnetic pull in that winding, counteracting the pull from the voltage winding ( which is falling anyway) and the contacts open with the help of a small spring. This disconnects the dynamo from the battery.

It is important that the contacts in the cutout are clean. Just rub them through with a little fine rubbing down paper to ensure that there is a good contact between them.

Hope this helps and I wish you all success in sorting out the problem.

[Bob Culver]

My comment about closing the cutout rferred to engine stopped conditon. Unless there is a dead short somewhere poking the cutout will not cause harm as long as it is not left locked down. The current flow heats the dynamo and flattens battery. They do close with a vicious snap and quite tight to pull apart. If it does not snap down either the points are not making contact or there is a poor link somewhere in the circuit to or inside the dynamo. 
Even an electrician would probably benefit from John Cornforths circuit diagram . See also the parallel current post.
Dunno how you set the charge off the car?
If the car has much more than original 18w lamps the dynamo will not charge with lights on, but should give about 8 amps on ammeter Winter setting, lights off.

[AustinWood]

Check that the cutout is earthed.