Author Topic: DC Permanent Magnet Brush Motor - alignment between brushes and magnets  (Read 258 times)

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Offline EEBlackSheep

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On a certain motorcycle, from 2012-2016 the start motors are notorious for working poorly.  We have investigated numerous causes (batteries, grounding, cables, relays), but have found that swapping starters with known good starters usually fixes the issues.  So I have disassembled a few starters to see if I can find the internal problem.  One interesting thing I have found is that there are no alignment marks on the case so that the orientation of the brushes to the permanent magnets is not consistently aligned.

I do not know enough about DC brushed PM motors to know if this is really an issue or not.  Is it?  Could this be our culprit?

I have attached a couple of photos of either end of the motor.

 

Offline amyk

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Re: DC Permanent Magnet Brush Motor - alignment between brushes and magnets
« Reply #1 on: October 17, 2018, 11:03:41 pm »
Yes, the alignment certainly matters because the commutation needs to happen in alignment with the position of the field:

https://en.wikipedia.org/wiki/Brushed_DC_electric_motor#The_commutating_plane

Early large motors had an explicit adjustment for this. You will have to experiment to find the best position.
 

Offline EEBlackSheep

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Re: DC Permanent Magnet Brush Motor - alignment between brushes and magnets
« Reply #2 on: October 18, 2018, 03:31:43 am »
Great info - thanks very much.
 

Online james_s

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Re: DC Permanent Magnet Brush Motor - alignment between brushes and magnets
« Reply #3 on: October 18, 2018, 04:26:41 am »
Back when RC cars used brushed motors, most of the better motors had adjustable brush timing. The effect is similar to adjusting the spark timing on a gasoline engine.
 

Offline Kleinstein

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Re: DC Permanent Magnet Brush Motor - alignment between brushes and magnets
« Reply #4 on: October 18, 2018, 06:46:08 am »
The adjustment has some effect on the motor. Though not DC anymore, but similar, in brushed motors used in electric drills some shift the brushes when changing the direction, as the best position depends on direction of rotation. Those with a fixed position get lower power from a given motor or need to run at reduced voltage in reverse.

However there is no direct universally correct angle, as there can be an additional effective rotation where the windings are connected to the collector. So it is more a question of mechanical space the brushes are often just in the the center between the magnets.
 

Online james_s

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Re: DC Permanent Magnet Brush Motor - alignment between brushes and magnets
« Reply #5 on: October 19, 2018, 11:47:27 am »
Just like the camshaft or ignition timing in a gasoline engine, the timing is only ideal for one specific set of circumstances, for all others it's a compromise.
 


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