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The "DC motors" that I'm aware of use a DC power source, but they have a current running through a coil that changes direction based on the angle of the motor. Therefore, they current inside actually alternates as the motor rotates by switching the current (electromechanically with a brushed motor, or electronically with a brushless motor).

But are there any motors which are trully direct-current? By this I mean that the internal currents never change direction.

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Yes, there are Homopolar motors.

They are not very practical, but they do work. You can make one with a permanent magnet, a battery and a paper clip.

enter image description here

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A ball bearing motor can run without alternating current -

enter image description here

https://en.m.wikipedia.org/wiki/Ball_bearing_motor

http://www.electricstuff.co.uk/bbmotor.html

I have built one of these. They can rotate at an impressive speed.

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  • \$\begingroup\$ The life is limited though... :) \$\endgroup\$ – Solar Mike May 1 '18 at 21:18
  • \$\begingroup\$ @SolarMike Isn't that true of all motors? I know what you mean thought. \$\endgroup\$ – HandyHowie May 2 '18 at 6:45
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In the end (likely even with the homopolar motor mentioned), with an electromagnetic motor (the ball bearing motor mentioned is thermal!), there will be AC effects at play, even if polarity is not reversed. Any motor design that would run off a perfectly steady field as generated by a DC driven electromagnet could have that electromagnet replaced with a permanent magnet*, violating accepted scientific facts about energy conservation. A DC electromagnet in motion relative to a permanent magnet will in effect experience changes in reactance, modulating an AC current in sync with the motion onto the circuit.

*A DC electromagnet not interfered with by other magnetic fields uses no actual power to keep the field in existence - all the power usage is by resistive losses. Compare real-world superconducting electromagnets - you energize them, short circuit them, then switch the power off, and they continue being magnets indefinitely.

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    \$\begingroup\$ I don't think it has been proven that the ball bearing motor uses heat to run. \$\endgroup\$ – 12Me21 May 2 '18 at 1:46
  • \$\begingroup\$ @12Me21 Even if it hasn't been proven scientifically, it's the only explanation I have heard that doesn't make a tonne of assumptions. Occam's razor and all that. \$\endgroup\$ – Trotski94 May 2 '18 at 8:13
  • \$\begingroup\$ AFAIK, the wire in a homopolar motor does not act like an electromagnet; instead, the magnetic field directly effects the moving charge carriers in the wire. And I can't really see where there would be any AC effects there, you can get the torque even with a stationary wire and totally DC current. \$\endgroup\$ – jpa May 2 '18 at 10:31
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    \$\begingroup\$ I suspect the mistake here is a lack of physics. AC is one way to change the direction of current. Moving the actual conductor through space is an alternative way. That's why a DC-driven electromagnet can still have a varying field: the magnet itself may rotate. The question already hints at that, but incorrectly assumes that even with rotating electromagnets there may be a need to reverse the direction of current. As homopolar motors show, that's not the case. \$\endgroup\$ – MSalters May 2 '18 at 10:58
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    \$\begingroup\$ @rackandboneman Not all DC electromagnetic effects can be approximated by permanent magnets. See en.wikipedia.org/wiki/Lorentz_force : the force is always perpendicular to the magnetic field, whereas for permanent magnets, the force is always parallel to the magnetic field. \$\endgroup\$ – jpa May 2 '18 at 18:08
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In a DC motor with a commutator, the current does not change direction. The path through the rotor windings changes but the overall direction stays the same if you could observe the path from the outside. The magnetic field produced by the rotor current moves very little as the rotor rotates.

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In certain range some motors may be called DC in your sense. Voice coil is one of them. Like this https://youtu.be/QFMQwspj_6Y

Brushless motor in a sense too: total current of all coils is controlled to be dc for given torque. Here:https://youtu.be/bQYEavwFGy8

But the term comes in fact from the kind of power supply required for the motor. Ac motor, dc motor...

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  • \$\begingroup\$ A brushless motor? But aren't those basically synchronous motors with control circuitry? \$\endgroup\$ – Hearth May 1 '18 at 19:49
  • \$\begingroup\$ Read carefully. The total current. The one you multiply by Ki to get torque. The motor is built and controlled to keep this one constant so that the movement is smooth. \$\endgroup\$ – Gregory Kornblum May 1 '18 at 19:51
  • \$\begingroup\$ Ah, I missed that. \$\endgroup\$ – Hearth May 1 '18 at 20:00
  • \$\begingroup\$ A voice coil fed DC in the strictest sense doesn't move :) \$\endgroup\$ – rackandboneman May 2 '18 at 15:56
  • \$\begingroup\$ Eventually.. But at the beginning it does :D \$\endgroup\$ – Gregory Kornblum May 2 '18 at 15:57
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A solenoid is a linear motor which can be actuated using DC. If you reverse the polarity, it will drive in the opposite direction (assuming it's working against a permanent magnet field).

Solenoids are used as the motive force in relays and speakers, so they're actually very common. Speakers are usually driven using an AC signal, however.

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