AN UNUSUAL MOTOR

CAN ANYBODY EXPLAIN HOW IT WORKS? I made it after following the instructions from a video in the net and actually it works. I cannot understand how it happens.
In this motor there are no coils , magnetic fields, magnets, and brushes.There are only two bearings connected by a metal axis.
The current flows through the first bearing, the axis and then the other bearing. The electrical power supply is connected to the outer rings of the bearings.

MOTOR PROPERTIES:
1. the engine must be first started. Cannot start alone with the current.
2. the motor rotates in either direction.
3. the engine is working both direct current and alternating current.The power is approximately 5 to 12 Volts with very strong current , about 20 or more Amps.(this is a short circuit and after a few seconds everything is too hot.)
There are different explanation. One of them is :
"In the ball bearing Motor" the current passes through inner race of one side ball bearing , then to the shaft , outer race of other ball bearing , then to inner race , causes a temporally expansion of each ball , deforms it to elliptical and adds pressure between inner & outer race then cause a moment of rotation , for that reason , it needs a prime movement to sustain rotation , by the way it doesn't matter whether the current is DC or AC , it just used to heat up the balls. WATCC THE VIDEO HERE: https://youtu.be/PO1dWIgSmjs

Ah yes, the ball bearing motor -- a most unusual motor, and one that works purely through electromagnetic forces. The thermal expansion theory given by S. Marinov is wrong, but so were the first several attempts to explain it with electromagnetism. If the thermal mechanism were true, then simply passing electric current through a lone ball bearing (given an initial spin) should create a motor; I invite you to connect one lead to the inner race of a bearing, use a brush to make contact with the outer race, and see what happens.

The actual mechanism is a rather complex interaction between the magnetic field of the axle (because you cannot pass a current through a conductor without creating a magnetic field) and the induced currents (via Lorentz forces) in the rolling elements of the bearing. Here's a pdf of a simplified version of the explanation, via Princeton. The full explanation gets into some very heavy math, but the calculations predict forces that closely match those observed experimentally, making it the most likely explanation of how this motor works.

The strange thing of it all is that it works the same also with AC.

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Originally Posted by mariost

The strange thing of it all is that it works the same also with AC.

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I don't think that it is strange. AC is just DC changing direction. So if with DC the direction of rotation is only dependent on the initial physically forced rotation, and not supply polarity then it follows that AC should work just as well. QED.

A question.
Does the speed of rotation depend on how fast you physically spin the disc at startup or is it only necessary to start it in a given direction and then it speeds up to some equilibrium RPM?

Does the speed depend on voltage and hence current?

This type of motor has no starting torque, so the initial push is just to get things going, and it will speed up until it draws the maximum current available from the power supply and/or it hits the frictional limit of the bearings. There's a roughly linear relation between speed and current, up until the motor seizes when the bearings overheat.

Here's a pdf of a research article comparing AC and DC operation -- the speeds are fairly consistent for a given current, and slightly faster with DC, though DC killed the bearings faster. Their test motors could run for up to a minute before they overheated and seized, even in a kerosene bath, but some of them maxed out near 10k rpm (25A DC pumped through a motor with 1/2" bearings).