Thread: Positive Earth - Negative Earth

It works 'cos the generator and starter have field coils, not permanent magnets.

With the polarity reversed, the field coils are also reversed so all works OK. The only thing is that there has to be some magnetic remanence for them to charge. Usually when you connect them backwards, the "flash" of current remgnetises the poles enough for them to work - but not always.

Ron

To amplify Ron's explanation, the (any) starter will turn the same way regardless of the polarity, since changing the polarity for the armature also changes the polarity for the field.

The generator will operate with either polarity in the same way, but it "decides" which polarity it will start generating according to the remnant magnetism in the field poles. (Occasionally a generator will not work because there is no remnant magnetism, especially if the engine has been run with it not charging.

Remnant magnetism is restored in the correct sense by momentarily connecting the battery live terminal to the field or momentarily closing the cutout points.

The only polarity sensitive thing in the Series 1 is the ammeter, and to get it reading in the correct sense (if you reverse the polarity) you need to reverse its connections. Strictly speaking the coil connections should be reversed as well, but it is unlikely that you will see any difference, and it is just as likely it already has a coil designed for negative earth anyway.

John

Well - you have both blown away my ideas on how a car generator/starting system worked. And here I was thinking I knew everything - well that is what my old mum says anyway.

Thanks for that - it further confirms I have a very original if somewhat battered car as it is still positive earth (as highlighted by a working ammeter).

All I need to do is clean out the garage and start work on it. First task is simply to get it roadworthy and registered with some minor cosmetics with a full restoration after I retire - the challenge will be to not to try and "improve" it as I use it, so destroying its originality.

Cheers

Garry

G'day Garrycol

You might even find that with a little cleaning of the terminals and a bit of gentle coaxing, the fuel guage might work as well

cheers

Originally Posted by JDNSW

To amplify Ron's explanation, the (any) starter will turn the same way regardless of the polarity, since changing the polarity for the armature also changes the polarity for the field.

not to nit pick but....

in the case of really really old starters with permanant magnets this is not the case...

but since AFAIK except for really really old cars that went the way of the dodo and even then most of the ones that might still be around today have probabley been converted over.

But it would be just my luck that IF i sat here and said nothing some smartie would find one of these old starters and try hooking it up backwards....

Originally Posted by Blknight.aus

not to nit pick but....

in the case of really really old starters with permanant magnets this is not the case...
.......

I have a collection of old automotive workshop encyclopaedias going back to about 1910, and none of them makes any mention of permanent magnet starters - and since sufficiently strong permanent magnets to make permanent magnet motors practical date only from the invention of alnico magnets in the 1940s, you will excuse my skepticism, I hope. In fact, permanent magnet motors did not become at all common until cheaper strong permanent magnets became available in the seventies. In close to sixty years of taking a close interest in anything automotive I have never even heard of a permanent magnet starter. If you have a reference, I would be very interested to see it.

I suspect you may have starters mixed up with magnetos (a permanent magnet generator combined with a coil, points and distributor), which were commonplace in car, truck and aeroplane engines from around 1900, being replaced by coil ignition in cars and trucks around 1920 in the US and a decade later elsewhere, and still standard in aeroplane piston engines.

John

nahh Magnetos I know about I have one on the AKB3 that im going to get round to rebuilding at some point but the last time I saw a permanant magnet motor used as a starter was actually on a traction type engine, driven through a reduction gear set. you held the decompression levers let the motor spin up the fly wheel then dropped the decompression levers the motor then turned into a generator to run the lights (think carnival ride type setup)

Im fairly certian tho the since reverse on that particular engine was selected by shifting the cam onto a second set of lobes to flip the timing over that it was probabley also setup to spin backwards...

John's description was far better than mine.

Ron