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Three brush dynamo setup
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Glenn Crawford



Joined: 06 Dec 2007
Posts: 69
Location: Dorset, SW England

PostPosted: Thu Dec 10, 2009 10:33 am    Post subject: Three brush dynamo setup Reply with quote

I'm working on a car - it's a 1934 Riley Nine - which has a three-brush dynamo, which doesn't work. It's off the car, so I'm able to eliminate external factors to do with wiring, control boxes etc. There are no obvious wiring or commutator faults and I suspect it could just be that the brushes have been mis-aligned by some previous investigator, and that's easily done because the brushes are on circular carriers which can be rotated around to any orientation (limited only by the length of their wires) and then clamped in position.

The two main brushes are fixed at 90 degrees to one another and are on one carrier, and the third (field coil) brush is on its own carrier. Each carrier is independent to the other.

Does anyone know the correct method for getting everything in the optimum position to at least get the machine working? The adjustment of the third brush to give the optimum output should then be fairly simple.

Here's where I wish I had paid more attention during Electrical Machine lectures at college. Too late!

Finally, to make it clear, let me explain that this is being done for money as well as for love - I run a small business mending elderly vehicles. Many thanks for any help which you might be able to give.
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Thu Dec 10, 2009 12:44 pm    Post subject: Reply with quote

Hi Glenn,

I have no experience of them but how about these..

http://www.da7c.co.uk/technical_torque_articles/threebrush_dynamo.htm

http://www.austin7.org/dynamo_service.htm

Peter
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1939 SS Jaguar 2 1/2 litre saloon
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Glenn Crawford



Joined: 06 Dec 2007
Posts: 69
Location: Dorset, SW England

PostPosted: Thu Dec 10, 2009 7:46 pm    Post subject: Reply with quote

Hello Peter, I had found your previous articles on this topic but unfortunately excursions into the Austin Seven world don't help me. This is because the designers of that dynamo very sensibly fixed the positions of the brushes so that there could be no mistake!

When Mr Lucas turned his hand to the Riley dynamo design he must have decided that was too simple and made the brush holders fully mobile, which is why I am in the position I am in now. In fact not only can both the brush holders be rotated independently around the axis of the dynamo, but the end face of the dynamo itself can be screwed on in one of eight orientations 45 degrees apart - not helpful to the would-be repairer.

I have just found this potentially useful article from the world of tractors: http://www.ytmag.com/articles/artint4.htm: and although it doesn't give me exactly the answer I want, there are a few pointers contained in it which may help me to a solution. But a reply from someone who actually knows would make my day!
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Peter_L



Joined: 10 Apr 2008
Posts: 2680
Location: New Brunswick. Canada.

PostPosted: Thu Dec 10, 2009 8:53 pm    Post subject: Reply with quote

I haven't made a study of the 3 brush setup, but can remember recovering several dynamos in the past by "flashing" them.

http://www.da7c.co.uk/technical_torque_articles/flashing_the_dynamo.htm

I recall once having to flash the exciter on large diesel/methane gas generator after the fitters changed some bearings and the exciter output went to 0.
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Penman



Joined: 23 Nov 2007
Posts: 4750
Location: Swindon, Wilts.

PostPosted: Thu Dec 10, 2009 10:15 pm    Post subject: Reply with quote

Hi
From Odhams Motor Manual
Third Brush Regulation
Before the introduction of constant-voltage or current-voltage control, generator output was regulated by the constant-current or third brush control system.
In addition to the two main brushes, a third and smaller brush is situated between the main brushes.
The position of this brush can be altered and the effect of moving it in the direction of rotation is to increase the output from the generator; conversely, moving the brush in the opposite direction reduces the output.
The maximum output should be such that the total load, including the headlamps, is balanced, with one or two amperes indicated on the charging side of the ammeter.
The charge reading withall lamps and accessories switched off should not exceed10-15 amp.

Sometimes an additional control resistance was included in the generator field circuit to allow the generator ooutput to be regulated by a switch for "summer" (reduced charge) and "winter" (full charge) settings.
When the headlamps are switched on, this resistance is cut out to allow the generator to give it's full output irrespective of setting.
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Bristols should always come in pairs.

Any 2 from:-
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Glenn Crawford



Joined: 06 Dec 2007
Posts: 69
Location: Dorset, SW England

PostPosted: Thu Dec 10, 2009 10:35 pm    Post subject: Reply with quote

Hi Penman, thanks for taking the trouble; but part of my problem is that not only can the position of the third brush be altered, but on this dynamo so can the position of the other pair, although (thank goodness) at least they are fixed at right angles to one another. I need to know where they should be placed, for instance in relation to the field coil pole pieces. In line with the poles? Offset by some number of degrees?

That's (part) of my problem, the other is where to start with the third brush location, which can also be placed more or less anywhere, limited only by its wire length.
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Thu Dec 10, 2009 10:36 pm    Post subject: Reply with quote

Glenn,

It's been a while since I've worked on a 3 brush generator but I have never seen one with 45 degree brushes. Are you sure that is where they should be if each one is independant. The normal positions would be centred to the pole shoes. One main brush will connect to earth and one end of the field coil. The other main brush will go to the terminal that goes to the cut-out. The other end of the field coil connects to the 3rd brush which is position just ahead of the 'live' brush in terms of rotation. So if it's a 12volt dynamo the third brush will be in the 10 - 11 volt area. The voltage it collects will be applied to the field coil. From a very unrealiable memory, some circuits involved external resistors so the when the headlights were on the resistor was shorted out and the output was increased.

Art
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Penman



Joined: 23 Nov 2007
Posts: 4750
Location: Swindon, Wilts.

PostPosted: Thu Dec 10, 2009 11:06 pm    Post subject: Reply with quote

Hi
Found these
http://www.da7c.co.uk/technical_torque_articles/threebrush_dynamo.htm

http://www.austin7.org/dynamo_service.htm

http://www.smallfordspares.co.uk/en/plate.phtml?PlateID=17

They all appear to show the main brushes opposite each other while you seem to be saying yours are at right angles.
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Bristols should always come in pairs.

Any 2 from:-
Straight 6
V8 V10
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Glenn Crawford



Joined: 06 Dec 2007
Posts: 69
Location: Dorset, SW England

PostPosted: Fri Dec 11, 2009 7:50 pm    Post subject: Reply with quote

Hi Penman, thanks for the info, however the differences between the dynamo I'm working on and the ones described in the links mean that it hasn't helped me - at least not completely. BTW, the reason my brushes are at 90deg. to one another is that the dynamo is a 4-pole one (four field coils), whereas the ones illustrated are two-pole.

I am going to assume that the Muppets have been inside this dynamo since (as far as I can ascertain) the two main brushes should be aligned with the centres of the field coils - and these ones are not, they are exactly half-way between them. I will have a fiddle as soon as the temperature in the workshop starts with a '+' again.
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Glenn Crawford



Joined: 06 Dec 2007
Posts: 69
Location: Dorset, SW England

PostPosted: Wed Dec 16, 2009 8:59 pm    Post subject: Reply with quote

It didn't help that the field winding had broken and the loose end shorted to the casing part-way round the set of field coils, so the dynamo has gone off for a rewind. When it comes back I'll know how the brushes should have been set up!
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