Commutator Bar Count-Armatures

Commutator Bar Count
Could This Be Causing Your Problem?
By Richard R. Vensel
Let’s start with a little trivia:
Hippolyte Pixii, based on a suggestion by Andre-Marie Ampere, built the first commutator
type AC motor in 1832.
I think we can all say that we have had times when there was just to much torque in a
starter and also the opposite, times when the starter just did not have the torque needed.
The Commutator Bar Count might be the reason for these problems. It may have to go up
or down depending on the issue.
So now you are asking “ How Could That Happen?” Does everyone put a new or rewound
armature in every rebuild? All someone would have to do is change the shaft (based on
the Lester number) and not look at the Commutator Bar Count. The shaft will fit and the
armature will work, but not correctly. It is like switching a 12V for a 24V armature or vice
What is a commutator?
A rotary electrical switch that reverses the current in a motors’ armature producing a steady
rotating force (torque).
There is a difference in torque between a 25 bar and a 29 bar and they both have been
used in the same family of Starter MT numbers units. One example of this would be the
40MT and the 41MT. It looks like they came out with the 29 bar commutator and reverted
to the 25 bar commutator due to excess torque.
So be careful....check the commutator bars.
Special Thanks to Tim and George at Superior Distribution for reminding me of this issue.