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.