The Existence of Circular Currents in Electrical Motors

Interesting phenomenon. First time it comes to my mind.

Electric motors, by some means need a circular oriented force to make them spin around:

like a 3 phase net is generated that way, just to accomplish it,

and most other motors have it created artificially by different techniques.

THIS IS NOT THE QUESTION: In what kind of order (strength) these currents are generated and how do they flow?

I expect it as being a sort of cage rotor reaction, from remaining magnetism, affecting the next windings of the stator?

The volume of iron content in the cage and the mass and composition of the shaft might play a roll in it?

This is a reverse thought and certainly food for the specialists here.

It's not really my area but if there is a current there is obviously a voltage force that drives it. My understanding is that 3rd harmonics in 3 phase systems do/can generate large currents. I would guess that VFD's have more than the odd harmonic and that the same might apply to them.

Hi,

capacitive and inductive coupling from coils to stator and rotor are not equal so a voltage between stator (housing) and rotor (shaft) will exist.

This is dependent on frequency and especially a problem with switched power.

Any nonlinearity will generate some DC.

AC and DC will damage any roller-bearing if above 5V and/or 5 µA.

So accept the isolating mounting of the bearings. If selected properly it will smooth a bit the often much too bad machining of the stator-side of the bearing-seats.

Bearings are usually degraded by a factor of 10 by bad mounting quality.

Who ever saw a ball (or roller)-bearing with an error-movement of the shaft of only 0.1 µm? This is possible! Most cannot achieve 1 µm!

RHABE

Can I see it this way. The rotor outer surface and the stator inner surface are the two plates of a capacitor. The air split between stator and rotor is the dielectricum. The rotor shaft and bearing are the electrical conductors.

Now where does the current comes form ? Is it because the rotor is geometrically not perfectly in the middle of the stator , because it is turning excentric in the stator. In this way the thickness of the air split is changing when the shaft is turning.

Or is it coming form currents that are inducted in the rotor and do not flow through the rotor cage but flow through rotor shaft and bearings ?

Or is it coming form currents that are inducted in the rotor and do not flow through the rotor cage but flow through rotor shaft and bearings ?

Hi,

The rotor outer surface...

this is correct but only one part of the puzzle.

In this way the thickness of the air split is changing when the shaft is turning.

This is not generating errors as a capacitor but as a magnetic generator. Any moving (rotating) magnetic field is generating voltages and currents in nearby electrically conducting materials. Perfect geometrical roundness and concentricity is not sufficient to have these down to negligible as also magnetic out of roundness will generate the same problems. Manual centering helps but a high cost. Extreme example are scanner spindles for the printing industry.

Or is it coming form currents that are inducted in the rotor and do not flow through the rotor cage but flow through rotor shaft and bearings ?

This is true too, the current will be divided depending on impedances.

Worst part is often the fast dU/dt from switched or PWM power supplies, as fast rising or falling voltages couple capacitively to everywhere and this coupling is different to rotor and stator.

RHABE

As mentioned, problem is worse whit a VFD. Sliding earth contacts to the shaft may help avoiding the high frequency current components flowing thru the bearings.

brgds

Snel