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Considering adding reversing capability to a Porter Cable 690LR (router) universal AC motor for use as a spindle motor. Yes, there are better alternatives but this is to finish someone else's wood-CNC project. 120 VAC, 60Hz, 11A, 27,500RPM, 1.75HP. Typical series-wound motor; reversing the field or brush wires is straightforward.

Porter Cable 690LR Router, partially disassembled

But upon inspecting the (rectangular) brushes, the holders are cylindrical and are retained with a hex lockscrew:

Porter Cable 690LR brushes can be rotated?

The brushes appear to be perfectly aligned to the center of the rotor axis. There is no visible offset or other adjustability. The only thing that can be adjusted is the brush angle:

Porter Cable 690LR brush assembly lockscrew

One of the brushes is rotated more than the other, leading to uneven brush wear:

Porter Cable 690LR brush is unevenly worn

What effect does rotating the brush axially have on operation?

Should this angle be adjusted?

How will this affect forward and reverse operation?

Is it "bad" that one brush has more rotation angle than the other?

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  • \$\begingroup\$ Heated the lockscrew (because it used threadlocker), fashioned a "key" the same size as the brush, and with gentle tapping, was able to eliminate the rotation. Hard to say if this made any improvement as it seemed to run fine before. Hopefully arcing will be less now, after the brush fully wears in. \$\endgroup\$
    – rdtsc
    Commented May 6, 2023 at 23:29

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What effect does rotating the brush axially have on operation?

It mostly depends on the wear of the brushes.
It shorts a part of the inductance armature of the motor, and is "commutating" it.
If it is "turned", it shorts too long which is a bad thing.

Should this angle be adjusted?

Yes, at the minimum angle.

Is it "bad" that one brush has more rotation angle than the other?

Well aligned, it doesn't matter.

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