Theory behind electric-reversing type repulsion start induction motors

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
234
Just posting here to see if anyone might be able to shed some light on this little headscratcher for me.

Most repulsion start induction motors still in existence today rely on a mechanical adjustment of the brush cage to reverse rotation. Moving the brush positions changes the angle of repulsion and develops torque in either one direction or the other.

One of the motors in my possession is an exception to this rule. It is an electric-reversing type repulsion start induction motor. It is wound for a single voltage. Correct brush position is at soft neutral. Four leads are exposed in the termination box. Two windings. Correct operation is achieved by connecting the two windings in series. The motor can be electrically reversed by reversing the polarity of one winding with respect to the other.

Can anyone shed some light on the theory behind this very rare and unusual arrangement?

Could it be that the second winding is simply offset from the first such that it creates an upsetting 'push' or 'pull' in the stator field with respect to the armature? I.E. moving the field in relation to the brushes rather than moving the brushes in relation to the field?
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
25,630
There are three distinct classes of repulsion-induction motors, some distinguished by having either two pairs or one pair of brushes, what you are achieving is reversing the phase of one winding WRT the other.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
234
There are three distinct classes of repulsion-induction motors, some distinguished by having either two pairs or one pair of brushes, what you are achieving is reversing the phase of one winding WRT the other.
This is a four pole repulsion-start induction-run motor. Not a repulsion-induction motor. They are distinct technologies in their own right and the nomenclature is not interchangeable.

RSIR starts as a repulsion motor and transitions into a sort of wound-rotor induction motor. R-I motors on the other hand are constructed with a squirrel cage and repulsion windings so as to function as a hybrid of both technologies simultaneously. They have inherently poorer starting and running characteristics + shorter brush life in return for reduced complexity and cost.

This motor has two brush pairs set 90* apart (four pole motor) which are all short-circuited together and adjusted to sit at soft neutral. The motor does not develop any torque with only one of the two windings energized - and draws excessive current in that state. Both windings must be connected in series for the motor to develop torque and consume nameplate current.

Skimming the internet in search of more information, I keep coming across occasional vague references to 'inducing' windings in conjunction with 'field' windings in the context of electrically reversing repulsion, R-I and RSIR motors.



Above is the image I was able to link, but I did see a thumbnail for an image no longer available depicting the same as above sans the second field winding.

Could it be that what I'm seeing with the two separate windings in the termination box are a field winding and one of these so-called 'inducing' windings? Which winding would serve what purpose in this context? A search for the term 'inducing winding' doesn't bring up any hits even remotely related to this subject. Can't find any references within my library of period-correct books either.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
25,630
This is a four pole repulsion-start induction-run motor. Not a repulsion-induction motor. They are distinct technologies in their own right and the nomenclature is not interchangeable.
The Repulsion start - induction run versions I am familiar with had a centrifugal weight mechanism that lifted the brushes once rpm was attained.
 
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