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I was trying to test the capacity of a battery pack I'm building and was trying to measure the current draw using a 35 turn 540 brushed DC motor (the website I had purchased from said something about it being sensored, but I couldn't find any sort of sensor inside.) I read somewhere that the cuts/notched are related to some sensor.

I connected the DC 540 motor to a 1.2V 7000mAh NiCd battery and measured the current. In one direction it spins a bit faster and reads 1.58A and in other direction it spins a bit slower and draws 1.89A. Any idea what could be causing this?

Both the carbon brushes seem so have even wear on them, too.

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    \$\begingroup\$ One more photo, please, from the end. We're looking for slightly off-axis brushes in relation to the magnetic core. Brushes may be advance or retarded to compensate for field distortion (search for the term) and this will cause a difference in performance between forward and reverse. \$\endgroup\$
    – Transistor
    Apr 19, 2021 at 21:53
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    \$\begingroup\$ It's a bit hard to tell from the new photo. It looks like a symmetrical arrangement and there's no preferred direction indicated. You could eyeball the dimples at the shaft end (which probably locate the magnets) and see if the brushes are inline or 90° off (symmetrical) or some slightly odd angle. They seem to be popular on the RC model sites. See if you can find a datasheet rather than an Banggood ad. \$\endgroup\$
    – Transistor
    Apr 19, 2021 at 22:07
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    \$\begingroup\$ It probably says "sensored" because if you're running a brushless motor in a car, it needs sensors. That, however, is a plain old brushed motor, which commutates the good old fashioned mechanical way. \$\endgroup\$
    – TimWescott
    Apr 19, 2021 at 22:14
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    \$\begingroup\$ In the 2nd & 3rd pic I see some notches in the edge which are lined up with a slot in the body of the motor. They make me think that maybe they're intended as an indicator of how the brush advance/retard is set. The knurled screw heads also look to me as though they're intended to play some part in making adjustments easier. \$\endgroup\$
    – brhans
    Apr 19, 2021 at 22:26
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    \$\begingroup\$ Yes, the notches indicate the timing advance. You can loosen the screws and rotate the bell to adjust the timing. \$\endgroup\$
    – Bruce Abbott
    Apr 20, 2021 at 1:01

3 Answers 3

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Because motor coils are inductors, it takes a bit of time for the field to build up. Thus, for high-speed use in one direction, you can bias the position at which the commutation happens with respect to the magnetic field. If the commutation leads a bit, the motor will generate more torque for the motor current, and it'll go faster -- in that direction.

That's probably what is the case here, either purposefully, or because it's a cheap motor without well-controlled build standards.

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It has been known for over 100 years that the optimum brush angle for minimum sparking is not exactly 90 degrees. This sweet angle is somewhat load-dependent. Many large vintage DC machines had a manual brush angle adjuster to minimise sparking. When the expected load and direction are reasonably well known, like on a car starter motor, a fixed angle is employed. In your case, the DC motor has been optimised for one direction.

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    \$\begingroup\$ Would any motors have been designed with separate brushes for the two directions? If one is using a DPDT reversing switch, using four brushes would require having separate wires from switch contacts to the two sets of brushes, but wouldn't otherwise require any switching hardware. For that matter, would be practical to construct a 3-brush motor that could be reversed using a SPDT switch (connect a common brush and one of the other two, based upon the desired direction)? \$\endgroup\$
    – supercat
    Apr 20, 2021 at 21:05
  • \$\begingroup\$ Interesting question .I have not seen this .Small angle differences would mean that the foward and reverse brush would mechanicaly cram . \$\endgroup\$
    – Autistic
    Apr 21, 2021 at 21:21
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The motor has phase advance and is optimized for spinning in one direction over the other. It isn't symmetrical. Look at the backplate, it might have curved screw grooves to rotate the angle. This is the adjustment for phase advance.

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