Positive Ground converted to Negative Ground

Over the 35 years as being a mechanic and around old tractors I have only run across two people who were worried that their starter would reverse rotation if they converted the tractor from positive ground to negative ground. They thought that since it's a DC motor that it should reverse direction.

I explained to them that this only happens when permanent magnets are used in a DC motor. With permanent magnets acting as the field their polarity is not changed only the current going thru the armature is changed.

But in the old tractor starters the field windings and armature windings both get reversed current thru them so it still turns the same direction as before it was converted from positive ground to negative ground.

I'm suprised of all those asking questions on converting their tractors from 6 volt pos ground to 12 neg ground that this question or worry hasn't come up here or maybe I have missed it.

The starter will turn the same direction if current changes in both the field and armature.

Changing the current flow in only the field or the armature will change the starter rotation. This I have only had to do one time and it required some rewiring inside the starter. Not an easy job. I wouldn't recommend it either.

Actually Im NOT surprised so many non electrical persons are concerned about it. It just seems sensible and intuitive to many a lay person that if you reverse polarity on that DC motor it should turn the opposite direction ALTHOUGH WE (many of us at least) KNOW ITS NOT TRUE ON OUR TYPICAL TRACTOR STARTER MOTORS. Hey, I remember wayyyyyyyyyyy back when I was a young green kid thinking the same until my Grandfather n Uncles set me straight lol That same uncle also gave me an education on single phase split phase AC motors and what direction they turned or could be easily made to turn. He had one he had disabled the start up winding on and turned it on and it, of course, just sat there n hummed and DID NOT TURN soooooooo he takes his hand n gives the pulley a good spin in a direction n sure enought it took off n ran in the direciton he started it I WAS AMAZED. Manyyyyyyy years later after graduating in EE and starting to work for Century Electric Motor Company I still never (and to this day many years after his death) forgot that although I now understood it..

John T

Hi John,

I sure enjoy reading your replies. I believe you get tired of having to repeat yourself so often. I enjoy electricity my self and get an education reading your explanations.

Keep up the good work!!

John B.
Highland, IL.

You really must run with some BLEMS!

One must always be mindful that electricity is just a theory.

I don't agree with what all my friends say, but we're all dumb just on different things.

Well, when they reversed the batteries in their slot cars, they ran backwards!!!!!!! Not such a difficult problem. Just don't understand DC motors where fields and armatures both are powered!

This is something that is not intuitively obvious, even to someone with an electrical background. I imagine that for most folks who know that a starter turns the same way regardless of polarity, they know from experience not from theory.

What"s a BLEMS?

Have you ever seen a second-rate tire or battery marked "BLEM"?

Well, some folks (yes, I'll include myself) should have "BLEM" stamped on their foreheads from time to time!

Thanks for the kind words, for yearssssssss older gents helped me including my grandpa n uncles etc so now I figure its payback time and glad to help when I can. Im semi retired with a small country law practice so Im in front of this puter a lot anyway so its not all that much work.

John T

Those lil DC motors use a permanent magnet as the Field which has a fixed magnetic North/South Polarity,,,,,,,,,therefore when you reverse the electrical polarity in the armature its resultant generated North South magnetic polarity changes so the magnetic forces turn it the opposite direction....

If you use electric current to create the magnetic field by its turns around a soft iron core, its resulatant North/South magnetic polarity is of one orientation depending on direction of current flow and likewise in the armature and it turns one particualr way buttttttttttttttttt even if you swap polarIty it (CURRENT FLOW) changes in BOTH Armature and Filed so they do not change RELATIVE TO EACH OTHER so it still turns same way. To reverse it you have to swap winding ends so flow is changed in only ONE and then it will indeed reverse

Hope that helps a little, and before someone wants to jump all over this brief explanation, remember I cant put perfectly accurate in a paragraph what it can take a book to fill lol so there

John T

Back in the DARK ages, eight grade,early 60's.I ask my science teacher the same question about an DC electric heater motor I had from a car.
He did not have a clue. I learned why later. BUT remember, almost all small engine starters are PM except the S/G's and some newer car are using PM starter because they are lighter, gas mileage, weight.

Kent

It is the 'rubber band theory".