Unbalance Power Factor at No Load on 3-Phase Motor

At so low power factors, the slightest difference between each phase armature Q factor, usually caused by tiny core inconsistencies, makes a major power consumption difference, i.e. any, even perfectly acceptable differences between phases on motor construction are exaggerated. But that is not a sign of bad quality or any motor problem whatsoever. The figures you give are those of a top quality motor. S.M.

Thank you for your answer. Can you write more about "tiny core inconsistencies" ?

Is it possible that here can be influence a little air-gap excentricity (static / dynamic ? ) or asymmetrical winding too ?

Yes, that kind of inconsistencies I'm talking about. You can't expect all three winding sets and cores to be geometrically and electrically identical, actually, on some winding schemes, this is totally impossible for obvious geometric reasons. An idling motor on normal grid (not VFD, there can be differences there) draws enough current (out of phase of course, but still high enough) for those differences to show, etc etc. S.M.

Why do you care what the power factor is at no load?

I think that no load test is quality index for each motor.

Oh. Nothing to do with single-phase loads connected elsewhere on the supply, then?

I would compare your deviations to the full power draw when mechanically loaded and not unloaded. At this low of a power factor a simple quantizing error of one LSB can appear as a huge error.

Did you record PQ and pf at full load?.

Multimeters are such nice little toys. Digital GIGO in some cases.

If motor manufacturer give on name plate the current,pf,noise,vibration,losses etc both at no-load and at full load it would be useful to test after repairs.

How are you measuring power?

Your meter readings might show equal values but in fact they do not occur at the same instant of time.

If you used an oscilloscope you would see this.

In my opinion, the stress exercised differently on a part of magnetic core -mainly in stator teeth region, I think- it could cause a change in core properties. Losses could be different for the same magnetic flux density in different parts of the core.

If this was observed after rewinding burnt motor winding,due to unbalanced heating of windings in 3phases damaged/overheated core or rotor rotor bars can result in unbalanced noload current

At the beginning no-one measured the no-load losses-it is not an usual test -but the no-load currents. Since Rfe is big [here 300-500 ohm] and Xm is small [25 ohm] the common impedance will be equal to Xm [approx.].

And then, the current depends almost on Xm. Xm=2*pi()*frq*L The inductance L is almost zero in laminate and is concentrated in the gap. As the gap is the same for al phases [if the rotor moves] the Xm is the same in all phases [more or less].And, in conclusion, the absolute value of the currents will be the same in all phases [if no winding issue occurred].

Actually, the magnetic field [intensity] is small in the laminate and elevated in the gap. The the magnetomotive force [ H*leng] is concentrated in the gap then, and the inductance depends on it.