Hi,
I need some advice please. Generator putting out negative volts.

Scenario: 1954 motorcycle with a simple 6V negative ground generator.

For testing .. I have everything but the generator out of the circuit. No battery or voltage regulator. Just using the brush leads and turning the generator. When I spin the generator in the proper direction... it puts out negative volts.

Note: it has new brushes (had the same symptom before with old brushes). Contact ring is clean and in good shape. Field coil is to spec ( static amp test.. 2.1 amps on 6 volt battery)

History: when I found the bike.. the battery was hooked up backwards. Who knows if the “mechanic” used a 12v car battery to jump the poor thing but nothing would surprise me.

So... I assume that the field polarity is switched. I have tried flashing it in the proper direction but no change. I am concerned about flashing it with higher voltage or holding power on it because the windings are 70 years old.

Any ideas or advise? Maybe demagnetize totally and reflash? How do you do that?

I am capable but not an expert at electronics so I thought you all might be able to help me through this.

Many thanks for your thoughts ,
P

 

Interesting problem statement. Following this thread.

 

How did you determine it was negative ground? It was not unusual in the early 1950's for cars and motorcycles to be positive ground.

What model motorcycle?

 

The FAQ section might give some answers.
http://www.batteryfloatchargers.com/battery_charging_safety.htm

 

Thanks.. it is a ‘54 BMW R25/3. I didn’t want to taint any answer by saying it was weird and foreign. It is still a simple 6V generator.. basically on a bench.
It is 100% supposed to be negative ground. British stuff was positive ground but this wasn’t. For sure.

It seems useful to know that they ran it backwards.

Btw, all of the electrics work fine.. except the charging. I don’t want to confuse things though. To diagnose this problem I think it is best to just image a generator on a bench with two brushes and two field wires.

My guess is that flashing it with much more power might fix the polarity but.. if it burns those windings.. the parts are very hard to find and $$$$$

Thanks for your interest and advice.

 

Oh, by the way...
it has a new battery but again..
I think the problem is at its simplest with the generator not hooked up to anything.
It has no battery power for this test. Just using the armature and residual magnetic in the fields. Spin it and it puts out negative volts. Crazy.

Is there a way to completely demagnetize the field? It has to be the field polarity.. right?

What do you think?

Thanks!!

 

Most 1950s bikes used Lucas Dynamos, which gave out Positive Earth, so
Swap field windings round to change polarity output, the windings are in Series with two wires, one to Earth, one comes out to a Terminal .

 

Hi,
So... I assume that the field polarity is switched. I have tried flashing it in the proper direction but no change.

You said ... flashing it in the proper direction
Where did you get your information from?
The Service Manual?
Exactly, what process did you follow?

There may have been two different style Generators used,
and the Field connections are 180° opposite.

Type #1 - One side of the Field is connected to +6V and the Regulator grounds the other Field Terminal
Type #2 - One side of the Field is connected to Gnd and the Regulator applies +6V to the other Field Terminal

This diagram for the BMW R25/3 shows a (-) ground electrical system
https://zeitmaschinen-shop.de/data/gfx/pictures/large/4/2/324_2.jpg

 

Mvas... brilliant!
I will repeat.. mvas is brilliant.

I would have never thought to flash the positive to ground. It worked!!!

I am super grateful for everyone’s input.

The manuals are very sparse with info other than specs.

So excited to put it back together and see if it works. This has been going on for weeks. One hour at a time. Took forever to find that it was charging backwards. I will have to see how it behaves with battery and regulator hooked up but I am confident that this was the major problem.

Thanks again to all.