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Need help with wiring

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terry callaghan04/09/2019 14:45:55
211 forum posts
8 photos

Hi, I am trying to wire a controller to this motor. The controller as a live and negative outlet wire . But the motor as three wires. Can anyone please let me know which wire connects to which. Thanksf1f905c0-df76-4eb8-aa29-47e10b6b51e7.jpeg

Andrew Tinsley04/09/2019 14:53:34
1169 forum posts

Well for a start, it isn't a motor. It is a 12Volt car dynamo (or possibly 24volts if it is off an old lorry). So none of your standard mains or dc controllers are likely to work.

In the distant past, I used a dynamo in place of a 12 Volt motor and although it worked, I was a bit dubious as to long term reliability.

I would recommend that you don't be a cheapskate and get a proper 12 Volt DC motor. There are plenty around, you just need to know what you are looking for.


terry callaghan04/09/2019 14:59:30
211 forum posts
8 photos

Hi, thanks for getting back. But it’s not about being a cheapskate, it’s about knowing how to wire it up to a two wire feed. It did work on 12v. So I just need to know which two wires to use.

Andy Carruthers04/09/2019 15:28:57
279 forum posts
23 photos

Assuming these are the 3 wires:

Brown - Probably +12v

Black - Probably 0v

Yellow / Green - Earth

Measure resistance between Brown and Black, should be a few (tens?) ohms

Assuming low - but NOT zero - resistance, try putting a car battery across Brown and Black, just touch the terminals and see if the motor twitches in the right direction, if not, reverse the wires and retry

Howard Lewis04/09/2019 15:37:39
3536 forum posts
2 photos

Looks like a Lucas C40 Dynamo. This is a shunt wound machine, where the supply to the field windings is used to control the output.

This would not be really suitable for traction purposes.

A Series wound motor would be needed, to obtain the torque needed for starting.


SillyOldDuffer04/09/2019 16:01:28
6175 forum posts
1335 photos

A dynamo that's had a few adventures by the look of it!

It's possible it's a positive earth dynamo, ie the + and - are reversed, but assuming it's negative earth:

The Brown Wire is on the D Terminal, this is the main Plus winding.

Judging by it's length, and contrary to Andy's suggestion, I suspect the Green/Yellow wire once connected to the F terminal,. This is missing from the rectangular hole just below D.

F is the Field connection used to magnetise the dynamo; it's normally connected to a Regulator which alters the field current as necessary to maintain a steady 12V out whatever rpm the engine is turning.

I've never tried running a dynamo as a motor but guess the F wire and D terminal should both be connected to +12V from the battery. The dynamo might motor without energising the field, but at low power.

Three wires is a bit odd because dynamos are normally earthed by bolting the metal case to the chassis. I guess someone has bodged this example in the past and the black wire is earth, ie to be connected to -12V from the battery.

Do you have a multimeter? A cheap one would soon remove all doubt.


Ron Laden04/09/2019 16:36:28
1985 forum posts
393 photos

Terry, is this what you are going to use to power the 7 1/4 inch loco, I thought you said you had ordered 3 batteries, a 36 volt 500 watt motor and a 4QD controller..? I hope I am wrong but if the old dynamo is the way you are going with it now good luck with it.

Nick Clarke 304/09/2019 16:51:43
853 forum posts
28 photos

Older 3 brush dynamos can be converted into motors quite easily. As has already been pointed out this id a Lucas 2 brush dynamo which cannot be used as a motor without major changes, and even then they are not very good as motors.

As the dynamo appears to be in a loco chassis - Has it run in the past or is it a new installation? If a new installation I would not start with trying to use this dynamo as a motor. If it has run successfully in the past to bring three wires out (+the body as earth, which could be either positive or negative) you need to know exactly what was done to the device, up to possibly a full rewind and separate excitation of the field coils, before you can attempt to find the correct connections.

Alternately try a few different ones and be prepared for it to do nothing, burn out the device or controller, cause a fire or work - those are the options.

terry callaghan04/09/2019 17:08:58
211 forum posts
8 photos

Hi, yes I do have a multi meter. There are two of these dynamos wired so that one works forward and the other for reverse. I only wish to wire up one dynamo. So Dave if I am reading you correctly.both brown and green/yellow wires to positive and black to earth.

Ron I do have a 2nd loco that I am going to fit in the 500w motor and 36v. I need to purchase a L/H tap tomorrow from Tracy tools to be able to fit the old chain gear to the new motor. But that’s a difference project from this one. I am keeping the old dynamo on this one as I don’t have time available to make or find new fittings for a new motor, so would just like to get this working to use over the next few months.

SillyOldDuffer04/09/2019 18:13:20
6175 forum posts
1335 photos
Posted by terry callaghan on 04/09/2019 17:08:58:

Hi, yes I do have a multi meter. There are two of these dynamos wired so that one works forward and the other for reverse. I only wish to wire up one dynamo. So Dave if I am reading you correctly.both brown and green/yellow wires to positive and black to earth.


Yes, I think this is it.


But check with a multimeter in case I'm wrong and Andy is right.

It might be this:


Measure ohms from Red to Black and then Green/Yellow to Black. If I'm right, resistance Red to Black will only be the value of the main winding, and Green to Black will only be value of the Field coil. If wrong, and green is earth and black the field, then the resistance will be that of both coils in series, which checking the other combinations should reveal is higher than two lower readings. Sorry if that's clear as mud!



Edited By SillyOldDuffer on 04/09/2019 18:18:25

terry callaghan04/09/2019 18:49:58
211 forum posts
8 photos

Hi Dave. Ok these are the figures. Black to green/yellow=8 ohms. Brown to green/yellow 4.5 decreasing. So with this new info. How or should I wire my two controller wires. Thanks, michael

Robert Atkinson 204/09/2019 19:35:25
751 forum posts
17 photos

We need more measurements than that

in addition to the two readings you have can you do

Black to Brown

Case "ground" to Green / Yellow

If there is a reading between case and Green / Yellow also measure case to brown and case to black.

The machine probably originally a dynamo may have been modiifed in the past, we can't be sure.

Basically here are two ways to wire the basic motor windings- series or parallel (Shunt) as you only have 3 wires it appears that only parallel is possible unless there is also connection to the case (as there would have been in the original dynamo).
The other thing to sort ou is the direction of running, if you reverse ONE of the windings the motor will run the other way.
There is an extra decision which is where to connect the speed controller. while it may seem opvious to connect it to both field and armature, there is no reasion why it should not be connected to just one with the other connected to the supply all the time the motor is running.

If you want to try different combinations without fireworks get a couple of 55W headlight bulbs and put one in series with two of the (positive) motor connections. This will limit the current to safe levels. The motor shaft will have to be disconnecte from any load of course.

Robert G8RPI.

J Hancock04/09/2019 19:45:41
432 forum posts

Good effort but prepare for being disappointed at what it can achieve.

You really need a proper motor.

J Hancock04/09/2019 19:46:00
432 forum posts

Good effort but prepare for being disappointed at what it can achieve.

You really need a proper motor.

terry callaghan04/09/2019 19:50:28
211 forum posts
8 photos

Hi Robert, here are further readings.

Black to brown. 3.9

ground to green/brown. Ol

case to black. 1.3

case to brown. Ol.

thanks, michael

Diogenes05/09/2019 08:46:54
37 forum posts
6 photos

If you wish to pursue using this motor/dynamo in the engine, there is a short passage in "Electric Motors in the Home Workshop" (p.50) by Jim Cox (Workshop Practice Series No.24) that deals with this very subject, although only from the perspective of obtaining a constant speed motor for workshop use.

I'm not much of an electrician, but the essence of it is that;

As Richard A. suggests above, you will need to separate the supplies to field and the armature so that they can be supplied independently.

Cox claims 12v in the field and 18v (to account for losses in the windings/across the brushes) at the armature are needed to achieve about 2000rpm (the original min. "cut-in" speed) and 12v in the field and 24v at the armature resulted in a 2800rpm constant speed motor giving c.3/4hp continuously rated - he advises that the original rating of the field should be adhered to, i.e. the maximum design voltage should not be exceeded if one wishes to avoid overheating.

Generally these dynamo's gave about 30amps at 13.5v, and should be safe to 6000rpm.

Hopefully you'll receive some further assistance to help you get it sorted out.

Diogenes05/09/2019 08:56:30
37 forum posts
6 photos

..Although looking again at the photo (particularly the flange), are we sure it isn't a starter motor? ..say, Morris 1000, for the sake of further discussion?..

Diogenes05/09/2019 09:06:18
37 forum posts
6 photos

No, I see it's a dynamo!

SillyOldDuffer05/09/2019 10:30:47
6175 forum posts
1335 photos
Posted by terry callaghan on 04/09/2019 19:50:28:

Hi Robert, here are further readings.

Black to brown. 3.9

ground to green/brown. Ol

case to black. 1.3

case to brown. Ol.

thanks, michael

Hi Michael,

I'm just on my way out and haven't had time to think over breakfast, but can you clarify what's meant by:

ground to green/brown - is that a typo for "case to green/yellow"?


Ol. (is that zero ohms?)

For the avoidance of doubt would you do the full set of measurements

Black to Green/Yellow,
Black to Brown,
Black to Case,
Gr/Yell to Brown,
Gr Yell to Case,
Brown to Case,

Got a few busy days ahead but should be able to look at the results this evening.



Howard Lewis05/09/2019 11:07:15
3536 forum posts
2 photos

The Lucas C40 dynamo fitted to the original Mini was rated at a maximum of 22 amps at 13.5 volts.

This means that it is capable of producing 297 watts

The one shown is possibly one of these.

The unventilated version (C40L?? ), with a solid brush end backplate was used on tractors and construction equipment, in dusty environments, and was only rated at about 15 amps, because of the lack of cooling air flow through the machine..

Running the dynamo, as a motor, on a higher voltage (Applying 24 volts would imply a current draw of 44 amps ) is bound to result in overheating and a shorter life.. You may get away with it, if it is not used at full load continuously. But eventually, it will fail because of the cumulative effects of overloading and overheating.

If you seek reliability, fit a motor designed for the work.


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