Dynamo Polarity - drbe
I am connected with a Motor Club. Recently the question was asked:- "Does anyone know how to change the polarity of a dynamo?"

Apparantly, it seems, if you touch one of the leads to earth while the dynamo is running, the polarity is reversed. Allegedly.

Does anyone has any ideas on this subject. I should point out that we are talking about 1930s - or thereabouts - cars.
Dynamo Polarity - bell boy
thats basically it ......its to do with the dynamo fields but i honestly can"t remember,however a true google answer im sure would come up if i tried.
dont forget the battery as fitted when doing this ,,,,,,and any retro tat
Dynamo Polarity - Screwloose

To re-polarize a - mounted but disconnected - dynamo, just flash a live feed [pos or neg, depending on earthing type] from the battery on to the field terminal [the small one marked "F"] a few times. Job done.
Dynamo Polarity - John S
No, not while it's running! Did this job many years back to convert the Minor to negative earth to use a 'modern' radio. Connect the earth terminal of the battery up in the 'new' configuration. Disconnect both dynamo leads. Connect a length of flex (doesn't have to be heavy cable) to the 'live' terminal of the battery. Touch the other end of this lead briefly to the small (field I think) terminal of the dynamo a few times - it will produce a small spark as you do it. Job done - dynamo repolarised. Connect dynamo leads, connect battery and when you start up the ignition light goes out. What you are doing is reversing polarity of the small residual magnetism in the dynamo which is necessary to start it charging.

Dynamo Polarity - ffidrac {P}
As an auto electrician of several years standing (and some sitting as well ;) ) I can confirm that the above by John S is correct.

Under no circumstances try to do it while the engine is running!
Dynamo Polarity - Cliff Pope
That's right. I did it ages ago on my +ve earth Triumph 2000 in order to fit a Luminition kit. Sometimes the dynamo will re-energise itself simply by running, but usually it needs a bit of coaxing.
Remember there may be other things that need their polarity reversing to work in the right direction. But no delicate electronics I think on a 30s car.
Dynamo Polarity - John S

You've reminded me! I'm told that sometimes motors such as heater motors can run backwards, but I had no such trouble. The ignition coil should have the connections swapped as voltage output drops if polarity is changed. Not at all sure why this should be, but they are marked +/CB and -/IGN, so there is a polarity sensitivity there.

Dynamo Polarity - Cliff Pope
I can't see why either, and i've had a long thread argument in the past on this. But I gave up, and concluded that probably the electrician did know best after all. Just remembered the ammeter of course. It's a bit alarming to see a sudden 25 A drain if you have forgotten to swap the wires.
Dynamo Polarity - Number_Cruncher
The coil works just the same whichever way you connect it, but spark plugs work better with a particular polarity of spark, and require a lower voltage to fire - IIRC, the centre electrode should go negative to obtain the best spark.

Dynamo Polarity - drbe
Thank you all for your responses. The information has been passed to the appropriate party.
Dynamo Polarity - bell boy
to throw a spanner in the works it was suggested many moons ago that a positively charged automobile was less sucummed to body rot than those dammed modern negative ones.....................................comments all welcome?
Dynamo Polarity - Altea Ego
I thought the idea was that a postively charged car body suffers from galvanic corrosion, more so than a negativley charged car
TourVanMan TM < Ex RF >
Dynamo Polarity - bell boy
tis late?
are ye for? or against? :-(
Dynamo Polarity - Cliff Pope
Regardless of the possible advantages to plugs, bodywork, etc, the over-riding benefit of negative earth is that then one can fit modern radios etc, or electronic ignition conversions. I imagine that is why the original questioner was asking.
Dynamo Polarity - drbe
The reason for my original post, was because the question had been raised at a Motor Club (West End, Esher, Motor Club) of which I am a member.

Many of the technical discussions at the bar go straight over my head. This was one of them. I know that this was the subject under discussion - but why - I don't know.

The members' cars range from Austin 7 specials to classic Bentleys to 50s and 60s sports cars.
Dynamo Polarity - ffidrac {P}
to throw a spanner in the works it was suggested many
moons ago that a positively charged automobile was less sucummed to
body rot than those dammed modern negative ones.....................................comments all welcome?


It's called the sacrificial anode effect.

NOTE: Electricity is lazy, it will take the shortest route from positive to ground that it can find.

Basically a positive charge encourages corrosion due to the electorns being liberated from the + pole of a power source. (Electron flow being the reverse of conventional current flow)

Connect your bodywork to the '+' side of the battery and the electrons will try to 'throw themselves off' (like a lot of tiny lemmings!)
In damp conditions, rain mist etc, the electrons find an easy path to earth through the water droplets taking molecules of metal from the bodywork.

Connect your bodywork to the '-' side of the battery and the metalwork is a lot closer to earth potential and the electrons will not move away so readily (rubber tyres, concrete, tarmac etc insulate from the ground so a very small potential does still exist) so the metal does not corrode so readily.

I have tried to keep it simple.

Dynamo Polarity - Another John H
I can't find any reference for it at the moment, but it's my understanding/half remembered snippet, that
all the above was known about when the decision was made to make cars positive earth, and it wasn't
done just to increase sales.

The reasoning was to reduce electrolytic failures in the (moisture holding) cloth insulated wiring.


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