3-brush dynamo

[ScottyUK]

Have any of you who have been around for a little while ever come across a 3-brush dynamo, with two brushes carrying the main armature current and the third brush being used to regulate the field? I met one of these unusual machines today, bolted to the engine of a 1934 Rolls Royce after somehow getting roped in to assisting a friend fault-finding the charging system.

The 3-brush dynamo appears to behave more-or-less like a current source, with the armature output controlled by the field current as you might expect. I've found a few notes on them from vintage car enthusiast sites but some of the technical explanations appear to be plain wrong and some are confusing. Do any of you have any references, at a guess probably from a machines text of a similar vintage to this car, or can offer some insight from an engineer's perspective? I think the 3-brush design fell out of favour for automotive applications after a short period of time so I've not come up with much.

I did find the fault on the vehicle, or at least I think I did: dirt and corrosion, as is so often the case. Now I just want to learn a bit more about the technology involved.

 

[waross]

I remember reading a story about Henry Ford going to see Steinmetz with a question about charging systems. As the story goes, Steinmetz quickly sketched out a simple charging system. I understood that that was the three brush generator. I understand that the third brush was set to match the average driving pattern of the vehicle. For instance a vehicle doing a lot of night driving may have a higher setting so as to keep the battery charged,
I believe that by the 50s they had been replaced by two brush machines with a combined voltage and current regulator controlling the field.

Bill
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