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Brush cap condictivity  Rate Topic 
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 Posted: Tue Mar 13th, 2018 04:36 am
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Tom Nordin
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When current flows through a brush holder to the rotor/commutator, is the current intended flow through the brush spring to the brush cap metal, or is it intended to possibly flow through the spring and/or brush contacting the sides of the brush holder?

Last edited on Tue Mar 13th, 2018 04:38 am by Tom Nordin

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 Posted: Tue Mar 13th, 2018 06:28 pm
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David Allen
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Tom Nordin wrote: When current flows through a brush holder to the rotor/commutator, is the current intended flow through the brush spring to the brush cap metal, or is it intended to possibly flow through the spring and/or brush contacting the sides of the brush holder?


Depending on the design of the motor, it can work either way.

The motors that don't have a high armature current may conduct current through the brush spring to a metal plate between the cap and the brush holder; OR the spring may contact the cap its self.

The more common way it's done is to have a flexible woven copper strap (pigtail) coming out of the rear of the brush, ending in a metal plate. The strap routes through the center of the spring, bypassing the current around the spring. Then, the metal plate on the end of the pigtail is trapped by the brush cap. This traps the metal plate against a contact point inside the brush holder.

Most brush holders and caps are nonconductive for safety reasons.  The inside of the holder is lined with a copper sleeve which is the conductor. The lead wire is connected to this sleeve.  The brush contacts the sleeve on the inside, and also the brush pigtail has its plate clamped against the end of the sleeve, providing a solid connection.

The reason it's somewhat risky to have the spring conduct power to the brush (without pigtail) is due to resistance of spring steel being too high. The spring will become a resistor and will heat its self up. Then it will become annealed and no longer keep tension on the brush.

Hope this helps!





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 Posted: Wed Mar 14th, 2018 11:55 am
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Tom Nordin
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Thank you for your insight, David.  Yes, this helps...   :D

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