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I got a bargain on a few small universal motor that run on 120VAC and I'm trying to reverse the direction. I don't have a lot of experience with these types of motors but I do know they run on AC or DC and the direction can usually be reversed by simply swapping the wires on the brushes. On this particular motor swapping the wires to the brushes does reverse the direction but the RPM is slowed to about 1/3 of that in the forward direction. I have read that manufacturers sometimes tune the placement of the brushes to reduce inductive reactance but I'm not sure how to confirm if this is the case or what to do if it is. The schematic below shows my current test setup. I added a DPDT switch to be able to reverse the direction easily but my goal is to run it in reverse only. There are two small inductors on the outputs of the field windings just before the brushes but I assume these are just for tuning and would not be the cause of the issue but I could be wrong ( they appear to be identical on both sides but I have not measured them).

Is there anything I'm missing that would cause the motor to run slower in reverse or anything I can try to resolve the issue?

Motor Schematicenter image description here

Motor view 1 enter image description here

Motor view 2 enter image description here

Motor view 3enter image description here

Motor view 4 (tuning inductors) enter image description here

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Your motor is 'timed' to operate in one direction by advancing the brushes. This is clearly visible in picture #2, where the brushes look to be rotated anticlockwise by about 12º relative to the stator.

Advanced timing increases rpm in one direction, but reduces it in the other direction. A motor that is 'neutral' timed will go the same speed in both directions.

The timing on your motor does not appear to be adjustable. You could try removing the brush holders and springs from the board and drilling new holes for them, but it may not be worth the effort.

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  • \$\begingroup\$ Thanks for the help. The brushes appear to be 180° apart however do you mean 12° off the center line? Would that matter since the armature is spinning or is it relative to the field windings? Moving the entire brush plate assembly is easy but moving just one brush would be tricky. I tried to remove one of the brass brush holders but they used some kind of phenolic board which cracked when I tried to pry it. \$\endgroup\$ – Neelix Jun 14 '17 at 4:30
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    \$\begingroup\$ Yes, 12º off the 'center line' relative to the field coils. For neutral timing the The entire brush plate assembly would need to be rotated 12º clockwise. \$\endgroup\$ – Bruce Abbott Jun 14 '17 at 5:43
  • \$\begingroup\$ @BruceAbbott: Can you expand your answer a bit? The question caused me to do some research and I couldn't find a satisfactory answer. What timing is involved? Surely the torque is proportional to the instantaneous relative angle between the rotor coil and the stator field - so how is direction significant? I'd like to understand more. \$\endgroup\$ – Transistor Jun 14 '17 at 6:38
  • \$\begingroup\$ The advanced brushes connect to the 'leading' segments of the commutator earlier, so the coils are switched in earlier which increases current and speed. In reverse the brushes are retarded so the coils get switched in later and it runs slower. The bad news is that in reverse the motor still draws more current, so it is less efficient (and usually also produces more brush arcing). \$\endgroup\$ – Bruce Abbott Jun 14 '17 at 7:11
  • \$\begingroup\$ I just realized there might be a (relatively) easy way to reverse the timing. Take the brush and spring holders off the phenolic plate, turn the plate over, and reinstall them on the other side. \$\endgroup\$ – Bruce Abbott Jun 14 '17 at 7:18
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The motor is not constructed for bi-directional rotation. You can see, that brushes are not exactly 90 degrees aligned with respect to the stator. This is due to the compensation, it reduces sparking and EMI.

Further, if you switch the direction of rotation then the brushes might also zap because they are trapezoidal shaped at the end.

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Those are likely just the Tuning Inductors, used because it is intended to be fed with AC and operate at higher speeds. You're right, that would not affect the direction.

Most likely it's because this is a used motor (from the looks of the commutator) and it was never reversed in its previous life, so the brushes have worn into a shape that, when you reverse the rotation, makes poor contact.

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  • \$\begingroup\$ Yes these are used but mostly they look like they were sitting in a box for 30 years and are in pretty good shape otherwise. I tried polishing the commutators on one motor but I didn't do anything to the brushes. They appear to be worn in a concentric curve with the same radius of the armature so I'm not sure how reversing the direction would effect contact but I can try ruffing up the brushes a bit to see if that makes a difference. \$\endgroup\$ – Neelix Jun 14 '17 at 4:35

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