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+ or - ground whats the difference?

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otis

01-24-2005 21:13:45

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I recently purchased a AC CA it is a 6 volt generator tractor. someone has switched it to negative ground and added a regulator. It is not charging but runs well. I would like to put the cut out back on and return it to positive ground. How do I go about this and are there any down sides to a positive ground system?

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dr.sportster

01-25-2005 14:12:53

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Re: + or - ground whats the difference? in reply to otis, 01-24-2005 21:13:45

Otis, just curius.Does it have a magneto?

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jdemaris

01-25-2005 05:48:15

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Re: + or - ground whats the difference? in reply to otis, 01-24-2005 21:13:45

There's no difference in performance, just convention. If you were to put a modern charging system on, e.g. a 12 volt alternator, you'd be hard pressed to find one that's positive ground since negative ground is the current accepted standard. Many voltage regulators have a polarity, ignition coil hookups, diodes/rectifiers, radios, etc. In the electroncs "academic" world, the electron theory is just that - theory - an unprovable idea at its time of inseption an no one really knows which way electrons flow. Way back some experts felt positive went toward negative (the Electron Theory), and other's felt negative went towards postive (the Hole Theory). Thus the reasons whey some engineers liked postive ground, and others liked negative ground. Now that sub-atomic particles are being studied, downs to neutrinos, quarks, bosons, etc. the guessing game is even more convoluted. Back to the real world, your tractor doesn't care which ground you use, just wire it correctly.

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Dave (IL)

01-25-2005 06:22:33

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Re: + or - ground whats the difference? in reply to jdemaris, 01-25-2005 05:48:15

Now that you've brought it up --

I've seen some posts in here that say the winding count and/or the winding material (CU/AL?) is different for pos ground versus neg ground regulators.

Also, why would an ignition coil care which way the polarity is?

I know my H ran pretty well before I discovered the PO had turned the battery around.

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jdemaris

01-25-2005 06:41:13

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Re: + or - ground whats the difference? in reply to Dave (IL), 01-25-2005 06:22:33

The polarity matters with some ignition coils because some have internal ballast resistors and/or diodes and you need the primary current coming in at the correct end of it all. So, some coils it makes no difference at all, and some it makes a big difference. I've got some charts around for various coils and their test output (for use on an Eiseman Coil tester). Quite a few coils are polarity sensitive and show a 30% drop in secondary output if not polarized correctly. A non scientific approach that I use, since many older coils don't have much if any information printed on them - is to simply try the coil both ways and observe the spark gap it will jump. As far a differences in winding counts and AL versus CU with different ground systems? I haven't got a clue. If there are no diodes involved - and there ARE sometimes - then it shouldn't make a difference unless I'm missing something. Take a starter motor and try it both ways. I've seen absolutely no difference in performance or current draw - positive or negative ground. Same goes for generators - you just need to polarize them correctly. So, it seems the field strength does not vary. Different story if you're fooling around with starters with permanent magnets.

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Steve(OR)

01-25-2005 10:13:54

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Re: + or - ground whats the difference? in reply to jdemaris, 01-25-2005 06:41:13

You wrote: "A non scientific approach that I use ... is to simply try the coil both ways and observe the spark gap it will jump."

That is actually the scientific approach - experimentation and observation.

Congratulations - I dub thee "Mr. Science Guy"

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jdemaris

01-25-2005 12:09:45

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Re: + or - ground whats the difference? in reply to Steve(OR), 01-25-2005 10:13:54

What you and I call a "scientific approach" may not be the same as some erudite college professor might argue. In many academic settings, my simple test might be deemed "anecdotal" with few controls, i.e., invalid. I had the unfortunate (or is it fortunate) experience of being a skilled-grunt and grease-monkey for most of my life - then got badly injured - started college in my upper 40s and got a few degrees thinking it would make me smarter. Well, what I DID learn is that, just like any other avenue of life, the academic society has lots of smart people along with a lot of ignorant, closed-minded buffoons who could not survive outside of the classroom or lecture hall. But, I guess this has little to do with tractors. I am once again, a blue-collar grunt.

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Jon H

01-25-2005 08:56:09

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Re: + or - ground whats the difference? in reply to jdemaris, 01-25-2005 06:41:13

There is another factor to consider concerning coil polarity. Electricity flows from a hot electrode to a cooler electrode very easily,while it can take 50% higher voltage to get the same flow from a cool electrode to a hot one. A sparkplug is designed so the center electrode and insulator runs hot enough to burn off any combustion deposits that would cause a short and foul the plug. The center electrode runs much hotter than the side electrode for this reason. Spark has a much harder time jumping from the cooler side electrode to the hot center plug electrode than it does jumping from center to side,so coil polarity/spark flow direction is very important.
This effect of electricity flowing easily from a hot to a cool electrode,but not the reverse is so pronounced that it is the basis for the working of a radio rectifier tube or an old tungar/tunger bulb battery charger which convert AC to DC.

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dr.sportster

01-25-2005 13:47:02

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Re: + or - ground whats the difference? in reply to Jon H, 01-25-2005 08:56:09

Yes Jon,But most would have a mag and batt. polarity is meaningless to the mag.

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dr.sportster

01-25-2005 13:56:35

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Re: + or - ground whats the difference? in reply to dr.sportster, 01-25-2005 13:47:02

I guess not everyone has a magneto.I have positve ground but it works with the battery box,it looks original.I can deal with battery charging situation no problem.What advantage is there to changing to neg ground?

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Jon H

01-25-2005 15:16:47

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Re: + or - ground whats the difference? in reply to dr.sportster, 01-25-2005 13:56:35

I agree DR,If the machine has a mag,the rest of the electrical system has no effect on ignition. My comments were directed at battery ignition systems. The main disadvantage I see to a pos ground system is that it is an odd ball in a world of neg ground systems. This can really get you into trouble if you try to jump start a neg ground machine to one with pos ground by connecting one jumper cable to the "hot" battery/starter connection on each machine,and the other jumper cable to the frame/engine block of each machine. This will cause a dead short equal to the combined voltage of both batteries. The same goes if you let any metal parts of both machines touch while jumper cables are correctly connected.

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dr.sportster

01-25-2005 14:06:13

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Re: + or - ground whats the difference? in reply to dr.sportster, 01-25-2005 13:56:35

Normally the wire from the points[batt coil ign]goes to th negative terminal on the coil.In a positive ground batt coil ign would that stay the same and still give the hotter spark?

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Jon H

01-25-2005 14:24:37

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Re: + or - ground whats the difference? in reply to dr.sportster, 01-25-2005 14:06:13

On a tractor with neg ground battery/charging system ,the neg coil terminal would connect to ground through the distributor. On a tractor with pos ground battery/charging system ,the pos coil terminal would connect to ground through the distributor so the electrical flow through the coil primary is in the same direction for either a pos or neg ground system. This insures that the flow of current in the coil secondary winding is always correct to flow from the sparkplug hot center electrode to the cooler side electrode.

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dr.sportster

01-25-2005 17:16:04

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Re: + or - ground whats the difference? in reply to Jon H, 01-25-2005 14:24:37

Makes sense.Now about the generator.I was thinking you must flash the field terminal with the negative off the battery or else its not really doing anything.Im talking about in the positive ground system.Does that sound right?

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Jon H

01-25-2005 20:52:49

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Re: + or - ground whats the difference? in reply to dr.sportster, 01-25-2005 17:16:04

To polorize/field flash an externally grounded field generator,you need to install the battery in your choice of neg/pos ground connection. When you jumper the BAT terminal to the ARM or GEN terminal of the regulator or cut out,it connects the correct polarity power to the generator "HOT" brush.One end of the field coil/coils are connected to the "HOT" brush while the other end is connected to ground through the regulator or light switch resistor. This will polorize the field for what ever polarity you have the battery connected,and make the generator charge with the same polarity.

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dr.sportster

01-26-2005 13:35:10

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Re: + or - ground whats the difference? in reply to Jon H, 01-25-2005 20:52:49

Jon,Thanks for clearing this up for me.Otis is probably done his conversion by now.

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jdemaris

01-25-2005 09:37:21

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Re: + or - ground whats the difference? in reply to Jon H, 01-25-2005 08:56:09

I wonder if the bulb you mention is what my old magneto magnet charger once had. It has what looks like light bulb sockets where the AD/DC conversion takes place - but whatever was in there originally was long ago changed over to a more modern retrofit rectifiers.

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Bob

01-25-2005 11:23:23

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Re: + or - ground whats the difference? in reply to jdemaris, 01-25-2005 09:37:21

That would likely be it... Tungar rectifiers were mercury vapor rectifiers capable of some SERIOUS amperage, for a vacuum tube!

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Jon H

01-25-2005 14:07:03

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Re: + or - ground whats the difference? in reply to Bob, 01-25-2005 11:23:23

Those in the picture Bob posted are exactly what I was talking about. I have an old Allen generator test stand from the 40's? that has a bank of 4 of them to produce power for the variable speed(DC?) electric motor that turns the generator,atleast that is my best guess as the tester was not in working condition when I got it. Also one of my childhood memories is of being in the shop of the Minniapolis Moline dealer back in the 50's and that I was facinated by those glowing bulbs when they were using that old battery charger. seems I remember the owner complaining that they had a short life.

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Bob

01-24-2005 21:29:21

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Re: + or - ground whats the difference? in reply to otis, 01-24-2005 21:13:45

Do you have battery ignition (distributor) or magneto? If ignition is a magneto, just set everything up the way you want it, ground and generator-wise, polarize the generator, and then start it up.

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