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BTH generator - wiring diagram

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Joe Hurley25/06/2022 20:36:55
4 forum posts
2 photos


I'm totally new here....

I have a British Thomson-Houston (BTH) Dynamo/DC generator ... 40 Amps at 27 to 40 Volts. I believe it may have been used to drive a fairground ?

I need a wiring diagram. It has 2 light wires coming out of it, but I do not know where they "went" .... I suspect to a rheostat... for perehaps voltage control ?

Can anybody point me to anywhere I could get the diagrams (I suspect it is 1930 to 1950 vintage.



john fletcher 126/06/2022 11:42:13
805 forum posts

Can you take a picture of the machine and its terminal box and are you certain that it is a generator and not a motor, circuit wise they are the same. Those thin wires are the shunt field pair, one of the thin pair would have been be connected to one of the main terminals and the other to some form of voltage regulator then to the other main terminal, a parallel group. I have seen at a traction engine rally where the operator used a rheostat to control the voltage to the the bulbs around the engine. John.

bernard towers26/06/2022 14:37:11
691 forum posts
141 photos

Yes if it was a dynamo I would have expected only one output wire and usually an insulated stud in the body, the second connection being the frame.

SillyOldDuffer26/06/2022 15:51:56
8903 forum posts
1999 photos

Simplest possible circuit is like this:


The wires allow amps to flow through a field winding inside the dynamo, much increasing the magnetic field and the machine's ability to generate power whilst also providing a way of controlling the output voltage to suit the load. The rheostat is a high-power resistor, usually wire-wound, and cranking the handle controls the strength of the magnetic field by adding more or less resistance to the circuit.

The circuit is very old-fashioned in that it requires an operator to manage the output. Typically only used on constant loads such as a set fair-ground lights. Automatic control circuits were soon added; these cope with varying loads and don't need an operator. Various ways of doing it: motorising the rheostat, using contactors to disconnect the winding when the volts went too high and reconnect it when they went too low. Most common I think was an arrangement of contactors that provided fine control by adding or removing fixed resistors from a selection. These days it's done electronically. Stabilising circuits are more complicated.

Various problems with rheostats:

  • what value in ohms, and how many watts does it need to dissipate without going up in smoke?
  • avoid burns and getting rid of the heat
  • finding one - being grossly inefficient, rheostats are rarely used these days, and hard to buy

Guestimating wildly - I hope someone knows better - I'd start by assuming the field current to be about 10% of the dynamo's output, so:

  • rheostat ohms = 40V/4A or 10ohms, and
  • heat I²R = 4A * 4A * 10ohms = 160W.

Suggests a rheostat of, say, 0 to 40ohms and rated for 300 or 400 watts. Probably necessary to experiment - I don't know what the field current of a BTH Dynamo should be - when new, it would have been in the manual!


Joe Hurley26/06/2022 17:20:21
4 forum posts
2 photos

Thanks v much Dave and all for informative and technical replies.

I have photos of the brass plate and the wiring. I plan to post them when I figure out how ?


SillyOldDuffer26/06/2022 17:28:23
8903 forum posts
1999 photos

Welcome to the forum Joe. The forum's photo publishing is a little peculiar but easy enough when you know how! Explained here.

Another quirk: the obvious search box top right of this page is pretty naff. There's a better one hiding halfway down the Home Page. Keeps us on our toes...


John Olsen27/06/2022 07:02:35
1256 forum posts
94 photos
1 articles

The older style of single bar electric radiator element should be about 60 Ohms and is rated for 1kW for the whole element. About 4 Amps maximum current. So you would probably want to arrange two pieces in parallel so it does not have to run at red heat. A Carbon brush scavenged from a motor would make a good slider.

That is if Dave's estimates are about right, they seem reasonable.

Sometimes older generators used a vibrating contact style of regulator, these were quite usual on cars into the sixties, until alternators took over. The on/off time of the contacts reduced the current through the field to regulate the output. These were of course used with a battery which would help greatly in reducing any output voltage variations.

These days it might be easier to come up with a circuit using power MOSFETs to control the field current. The circuit could easily be small enough to conceal somewhere if you want to look authentic.


noel shelley27/06/2022 11:00:11
1445 forum posts
23 photos

Johns idea is good ! I needed a power resistor and happened to have a stock of new/old electric fire elements ! for variable resistsnce a starter motor brush would work as the slider.DO NOT try to run this machine without a load ! A 28V(24v) CAV regulator as used on the AC7 alternator might be able to be adapted to suit. Noel.

SillyOldDuffer27/06/2022 11:58:06
8903 forum posts
1999 photos

Found a book describing a 500W dynamo that quoted 2A for the field winding, so I'm a shade more confident the 4A guesstimate is in the right ball-park.

Electric fire elements are a good idea, and for a test, you could just substitute an electric fire for the rheostat, wiring a domestic socket so it can be plugged straight in. Doesn't matter if L is connected to + and N to - or the other way round. Earth not needed on a 40V circuit, but as always treat electricity with due respect, shorting out 40A could start a fire, and DC arcs aren't self-extinguishing.

Don't use a fan heater - the element might overheat because 40V won't run the fan needed to keep it cool.


Joe Hurley28/06/2022 21:56:56
4 forum posts
2 photos

bth dc generator plate.jpgbth dc generator wiring.jpg

Joe Hurley28/06/2022 21:58:10
4 forum posts
2 photos

It will be a week or two before I get back to it, but I will update.


Howard Lewis29/06/2022 17:36:16
6314 forum posts
15 photos

My first car was a Singer 9. The charge rate from the 12 volt dynamo was controlled by bthe lighting switch.

It had three positions

"Off" fed the field coils through two resistors in series to give minimal charge rate (About enough to equate tom the ignition load.

In the "Side" position one of the resistors was shorted out, increasing the charge rate.

In the "Head" position, both resistors were shorted out giving the maximum charge rate (Which was pretty minimal, probably about 10 -12 Amps) to cope with the Side, Tail and "Dip and Switch" headlamps

Some similar method, or variable resistor would suffice for controlling the output voltage.

(My father made up a small low voltage generator set, to light a vicarage way out in the country, and the control for this was a large linear wire wound rheostat. The generator was actually a motor which had once rotated the turret on an American tank!


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