I have wheel that is on an axle running through two supported bearings. In one quadrant of this wheel, there is a (small) mass. As expected, the mass has a bias of being as close to the ground as possible (because of gravity). However, I have noticed that if I connect the axle that the wheel is on to a DC motor, this bias goes away. I was wondering what the source of this frictional force in a DC motor is. Is this force still present when the motor is running (will the wheel not have bias while spinning, resulting in an approximately constant speed as it spins)? Thank you!


DC brush motors make electrical contact with the rotating armature with spring-loaded carbon brushes. The brush contact is a major source of friction. The armature bearings (at each end of the motor shell) also generate friction, but far less than brush friction. In the photo below, the red and blue brushes are pushed against the commutator:
brush DC motor
The armature frame containing wire windings is quite heavy. While rotating, it acts as a flywheel, smoothing the rotating motion. Any offsetting weight on the motor shaft will likely cause vibration, and bearing friction will increase, since rotating speed of these motors is quite high. A high speed also keeps rotational rate nearly constant, although the extra friction will likely cause rotational speed to be somewhat slower, and current drain somewhat higher.

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  • \$\begingroup\$ Thank you so much for the response! Do you have any resources that further explains why a higher speed would keep rotational rate approximately constant? \$\endgroup\$ – dts Jan 24 '17 at 1:08
  • \$\begingroup\$ @dts A purely mechanical effect of flywheel action - a rotating weight tends to keep rotating. \$\endgroup\$ – glen_geek Jan 24 '17 at 1:12
  • \$\begingroup\$ Awesome! I will definitely look into that some more. Thank you again! \$\endgroup\$ – dts Jan 24 '17 at 1:15

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