Joined: Aug 2017 Posts: 989 Threads: 6
Reputation:
12
Location: Scottish Borders
The battery limits it.
With the three brush dynamo, if the battery gets disconnected while the dynamo is charging the voltage can go very high. This can do damage, blow bulbs etc.
The 3 brush dynamo wants to generate a fixed current and the voltage rises until that is reached.
The same 12 or 6V
Jim
Joined: Aug 2017 Posts: 1,033 Threads: 54
Reputation:
5
Location: The delightful town of Knaresborough, North Yorkshire
Jamie, I think to answer your original question regarding what you need to do, to use the existing dynamo at 12 volts needs a 12 volt battery, then the best way seems to be to do away with the third brush and use an electronic regulator designed for the job, and field coils wound with more turns of thinner wire. This provides extra resistance in the field coils. Getting the armature re-wound as well might be the icing on the cake, but it seems not many do that.
Joined: Dec 2017 Posts: 189 Threads: 8
Reputation:
1
Location: East Yorkshire
Car type: Ruby sports trial special .
The thing not mentioned is to get 12v
From a 6v dynamo you have to spin it
Faster so 12v battery will only charge
At higher revs unless you rewind dynamo
For 12v .
My problem I ask questions that other people don't like?
Like have you got that for an investment or for fun?
Joined: Jun 2018 Posts: 519 Threads: 90
Reputation:
1
Thank you for the additional information. I recall the lack of charging at lower revs from my old Standard. I fitted an alternator to that instead. I shall see how I get on with the standard dynamo and see about rewinding it if charging becomes a problem.
Regards,
Jamie.
Joined: Aug 2017 Posts: 2,748 Threads: 31
Reputation:
95
Location: Auckland, NZ
To rewind an armature for 12v requires thinner wire and as overheating of the armature is the main problem this is self defeating. (It was common to rewind troublesome armatures with a thicker gauge but which delayed cut in )
Most dynamos I have seen have commutators too worn to warrant rewire. Do the reconditioners of Seven dynamos have a source of new comms?
I have to concede that modern sealed batteries are not suited to the original Seven charging. Do not like more than about 14.4/7.2 volts. The car system kept at a low rate except at night and maintained by a modern bench charger seems more ideal.
The idea of introducing electronics to a Seven troubles me, but I suppose if operating 2 wire, if the electronic regulators can limit max 2 wire field current mainly by switching, an earlier cut in is possible than otherwise. The switched field current has an exaggerated heating effect but the fields seem very tolerant.
