Getting a second opinion: an electrically reversible repulsion-induction motor?

Thread Starter

Just Another Sparky

Joined Dec 8, 2019

I'm just trying to get a second opinion on the motor above for an old 30's or 40's-ish Burke 126A miniature mill that I recently obtained. I *think* It's a brush-riding type permanent repulsion-induction motor - but with an unusual electric reversing capability. It's a configuration I've never seen or heard mention of before.

The electric reverse feature hasn't been spoken of anywhere on the internet to my knowledge. It's not shown in any of my modern textbooks or in my period correct copy of Audel's Handy Book of Practical Electricity. It's been my observation that surviving examples of repulsion-induction motors are designed exclusively to be reversed by shifting the shorting brushes back and forth across either 'hard' or 'soft' neutral, thereby changing the relative positions of the armature poles in relation to the stator during startup.

But this one is reversed by switching two of the four (unmarked ;D ) leads in the termination box. From my experiments it appears that one pair of leads is for the main stator winding, while the other (connected in series with the aforementioned) seems to be some sort of compensating winding - if I have my terminology correct?

There's sometimes a clicking sound that comes from the motor when it reaches full speed, and again once it slows down to a certain point. I can't tell if it's coming from a centrifugal shorting necklace buried somewhere on the shaft side of the rotor (repulsion start, induction run?) or just something making a markedly similar noise in the gear head. Sometimes I think it might even just be my imagination. I can't really tell if there is a squirrel cage cast into the armature either - as would be the case with a permanent repulsion-induction motor. The running speed is uncharacteristically stable for a series brush motor (It barely changes when I put load on and take it off), and it exhibits the god-awful power factor that is mentioned in descriptions of repulsion motors (1100VA, 310W running unloaded). It basically sits at 100% or 110% FLA all the time, dependent on the direction of rotation. It runs at the exact same speed in both directions and there is no sparking on the brushes once it's up to speed.

Am I on the right track thinking this is a novel little RSIR/RI motor with electric reverse, or is there something I'm missing?

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Joined Jul 18, 2013
Consulting the motor rewinding 'Bible' by Robert Rosenberg, the indepth explanation of the R.I. motor shows one that has lifting brushes when up to speed, these drop back again when the rpm drops off, maybe what you are hearing.
There is no description of one that can be electrically reversed.
The number of brushes depends on the pole count/rated RPM of the motor.

Delta prime

Joined Nov 15, 2019
But this one is reversed by switching two of the four (unmarked ;D ) leads in the termination box
Hello there. I think you may have a hybrid. Because you're switching
2 of the 4 for reverse.
I could very well be wrong.
Maybe or perhaps it sounds like a
two-field compensated motor.
Humor me for a moment while I gather my thoughts. ;) A type of universal motor, has a stator coil similar to that of the split-phase motor, and two types are normally available: the single-field compensated motor and the two-field compensated motor. The former has one stator winding, while the latter has two.
The two-pole compensated motor has a stator winding like the main winding of a two-pole split-phase motor, with fields wound into the slots of the stator. Field poles, of course, must be of opposite polarity and con nected in series with the armature. This type of motor may be reversed by interchanging either the armature or field leads and then shifting the brushes against the direction in which the motor will rotate.

Two windings are used in the stator of the two-field compensated motor, similar to the starting and running windings of a split-phase motor; they are located 90 electrical deg from one another. The compen sating winding is used to reduce the reactance voltage present in the armature when it is operating on alternating current, caused by the alter nating flux.
The speed control of this type of motor may be regulated by several methods, some of which include a centrifugal switch, using a tapped field, or inserting resistance in series with the motor.

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
Well I did some more snooping and uncovered a few more hints. It turns out that the brushes are shorted together, which tells me that there is definitely a repulsion component employed in this design.


Looking around a little more, I became suspicious of the copper ring sitting in front of the commutator:


Look real closely through the gap between the copper ring and the commutator. See what's very stealthily hiding inside of the hollow commutator? Looks like a shorting necklace. Clicking noise heard at startup and shutdown. Looks like a duck... Quacks like a duck...

These two details in concert eliminate all other possibilities - this is most definitely a brush-riding repulsion-start induction-run motor - with electric reverse. I didn't think such an animal existed, but... the evidence is right there.

It's fun digging into old equipment like this. The things you learn...
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Thread Starter

Just Another Sparky

Joined Dec 8, 2019
I think the inner necklace is comprised of a series of copper 'fingers' which pivot outwards to contact the commutator when acted upon by the centrifugal mechanism, probably via a pushrod buried somewhere in the armature. (I've heard the flyweights are usually located on the PTO/shaft-end of RSIR motors in contrast to the usual fan-end arrangement seen in cap-start motors).

Here's a picture I found online that shows a slightly different implementation of the shorting necklace to illustrate it's basic concept:

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