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brushed induction motor confusion? [answered] Answered

My father and I are currently restoring a metal-working lathe, however, we do not have 3 phase power to power the original 0.5HP 3 phase motor. We do, however, have a slightly smaller 0.25HP motor. At first appearance, it seemed to be a simple AC/DC brushed motor with a field and winding on the armature. There was no wiring diagram supplied with it, but in one configuration (what appeared to be the ameuture and field coils) are wired in parallel for 120V operation. This did not seem right, since the field coils will be simply shorted across the mains, but I assumed that since we are dealing with AC and inductors, this is OK. It works flawlessly, but reversing the polarity of either the fields or armature would cause the motor to sit and buzz. If started manually, it took off slowly but sounded awful. I need the motor to rotate in both directions and do not have enough materials to do this mechanically.

On closer inspection after dismantling the motor, it appears there are 4 brushes, connected together in 2 pairs. They are not directly connected to anything. The 4 wires coming out seem to be just for wiring both field coils in either series of parallel for 110V and 220V operation respectfully. Perhaps even more strange, the 4 brushes are mounted on a centripetal switch, which disengage the brushes from the armature.

I was able to make out on the rust that it said "revolution induction motor." So I reasoned out that after the fields are energized, it will induce an electrical current in a few of the coils on the armature, and the energy would flow to the other 2 sets of brushes, causing a different set of coils on the amature to energize and this would initially start the motor. After it gets going, the switch disconnects all the brushes and the motor operates as a simple induction motor.

My father used to rebuild motors for a small company, and this is in fact one of the motors he repaired. Although he is skilled at this practice, he does not understand the operation of induction motors and can only figure out wiring by strictly following a diagram or trial and error. I am curious about what this type of motor even is, and how it works!

In th meantime, I will research induction motors and how they work, and see what I can learn.

4 Replies

icengBest Answer (author)2014-08-01

GO to my https://www.instructables.com/file/FBVKUCWGPBD4Z7Z/

instructable repulsion-start induction-run and listen to the wind-up and switch over to induction flywheel.

Start torque of the brush motor is higher then the induction run maintain torque of the flywheel after the brushes lift off.



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-max- (author)2014-11-02

[solved] I have figured it all out: It is in fact a repulsion-start induction-run motor. It starts of as a repulsion motor, and the angle of the brushes indication the direction of rotation, there is a centrifugal switch that activates when it reaches speed and the brushes lift off and it runs as a basic induction motor. It is a quite convoluted design, and these are in fact used in special application where high-starting torque is necessary, like in compressors. Because of there design, they are relatively expensive.

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iceng (author)-max-2014-11-02

Exactly :-)

You have a rare machine...!

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-max- (author)2014-08-01

OK, I seem to have stumbled across whats called a repuslion-start motor. Does this sound right?

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