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I can understand how a DC motor works, and keeps rotating in the same direction with the split design of the commutator. The following (simplified) picture nicely explains it:

enter image description here From here.

I know we can use some simple circuits like H-bridges etc. to control the direction of the rotation, but I would like to know if it is possible to modify the existing commutator (like shorting neighbors, or every other, etc.), and make the motor oscillate at half rotations? In real life the commutator is not two-piece only, but quite a large number of slots.

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  • \$\begingroup\$ not the commutator ... the rotor ... have it hit a spring ... that might reverse it ... the motor probably will not live very long though \$\endgroup\$
    – jsotola
    Oct 3, 2022 at 17:49
  • \$\begingroup\$ Actually the question is very simple. Think of the normal case. You flip the polarity (with split commutator) just at the time the electromagnetic field is reversed. SO the reverse of reverse is again forward. Now what I am asking is not to flip. \$\endgroup\$
    – user2800464
    Oct 3, 2022 at 17:56
  • \$\begingroup\$ If you don't commutate, the motor will rotate to a particular orientation and then stop. \$\endgroup\$
    – Hearth
    Oct 3, 2022 at 19:27

2 Answers 2

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Instead of a commutator, use slip rings, and the armature will turn only until the magnetic field is opposite that of the stator. Then reverse the voltage when the armature has started to oscillate back, due to inertia taking it past the centered position, and the armature will turn back.

Using timed pulses, or reversing current flow, the armature can be made to move forward or backwards. This is the principle of the stepping motor, where multiple poles allow for smooth motion.

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  • \$\begingroup\$ So are slip rings continuous 360 contactos as opposed to split commutator; or are they something like 90 degrees out-of-phase with a normal commutator. \$\endgroup\$
    – user2800464
    Oct 4, 2022 at 7:50
  • \$\begingroup\$ @user2800464, see the link. \$\endgroup\$
    – DrMoishe Pippik
    Oct 4, 2022 at 15:09
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The rotation of a DC brushed motor or a universal motor can be changed by rotating the orientation of the brushes with respect to the stator. I have an electric drill that uses this method to provide reversal. So, for automatic reversal, the armature could have a mechanism that rotates the brush assembly by the proper angle to reverse the phase and the armature should then rotate in the opposite direction. I don't have a good picture of my drill but these might help visualizing it:

B&D Drill

B&D Drill

B&D Drill

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  • \$\begingroup\$ I could not really get the idea from the pictures. Do you mean brushes are rotatable; or do you mean brushes are say 90 degrees out of phase from their normal position.? Or may be you can explain the manual procedure how you reverse, like you stop first. clik on reverse button, brushes flip, then go... something like that.. \$\endgroup\$
    – user2800464
    Oct 4, 2022 at 7:54
  • \$\begingroup\$ There are about 16 contacts on the commutator, or 8 pole pairs, spaced at 22.5 degrees, each of which reverses the polarity, and thus direction. The pictures don't show the mechanism clearly, and I'm not going to re-open the drill, but the FWD-OFF-REV control can be seen toward the rear of the drill. I think there is a mechanical interlock with the trigger. Here is some discussion of this: eng-tips.com/viewthread.cfm?qid=44786 \$\endgroup\$
    – PStechPaul
    Oct 5, 2022 at 4:15

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