Posted by John B. on April 09, 2009 at 19:07:08 from (38.114.64.150):
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.
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