VFD Tripping

Sounds like a bad motor....

...or bad cabling. You've already proven that the VFD works properly, so it must be associated with something that "touches" the motor.

Does unbalance inductance plays a role in this ?

Winding resistance: balance 0.08ohm.

Inductance : 0.03mH,0.1mH,0.1mH with power cable

Kindly share your opinion.

For 0.1mH @ 50 Hz....

X = 2πfL = 2 x 3.142 x 50 x 10^-4 = 0.03 ohm

R = 0.08 ohm, ∴ X < R

You have measured wrong or given wrong units - shaft power/voltage you give would have X ≅ 5 ohm line to star point (L ∼ 15 mH). However, if inductance ratio is correct, something is wrong with motor, maybe shorted turns.

If that "earth fault" is derived from residual of 3 line CTs, you might have insufficient time delay set or too low a current setting to ignore CT saturation imbalance.

Suspecting earth fault, have you done 2.5 kV insulation test on stator??

Hi,

Can you be more detailed about shaft/power ratio.

I will once again check and let you the exact values.

We have done 1000V insulation test on the this motor of 415V.Do I need to check with 2.5kV ?

Thanks.

Apologies for my wrong figures, one has to use locked rotor values!

110 kW shaft power motor @ ξ = 0.9, cosΦ = 0.9, 415V 3 ph

I_ph = 110/[0.415√3x 0.9 x 0.9] = 189 amps full load x 6 locked-rotor = 1134 amps.

Z = 240V/1134A = 0.212 ohms, hence X = 0.2 ohms using your resistance measured 0.08 ohm N.B. 0.2² + 0.08² = 0.0464 ≅ 0.212² = Z²

X = 0.2 = 2πfL ∴ L = 0.2/(100π) = 0.64 milliHenry. [a line-line measurement would be higher].

Your measurement of 0.1 & 0.03 mH still seems low.

2.5 kV would be excessive & you have written that motor was rewound so insulation should be OK.

Thanks for the clarification. In my case , i have checked it again.The values has come as below. R-B Y-R B-Y 0.96mH 0.06mH 0.96mH Resistance values are as below R-B Y-R B-Y 0.084ohm 0.108ohm 0.096 Does this indicates inter turn short?

A short may be reason for low value of L, however Y-R seems to have highest resistance.

You should be aware that stator windings are coupled magnetically to rotor. A cage rotor is like a short circuit secondary on a transformer, but rotation of rotor does affect coupling a little since it has copper/aluminium bars in slots in rotor iron circumference. Try different rotor positions.

If possible, I would try applying 12V/30A 50Hz AC supply to each phase & measure current taken with clamp-on meter, to show that this is not just a low level instrument effect.

Apologies, I meant to add. You have had a number of parts changed on VFD, but have any filter capacitors to earth between IGBTs & motor been checked or changed? They could connect to earth if faulty & "self-healing" property give intermittent problem.

Thank you very much for your inputs and for the time you have spent on my questions.

Regarding the issue, we will further check the motor in detail and will revert .

Is this an older motor that is not listed as VFD compatible?

I received a replacement motor for a mixer on a tank about 10 years ago, that someone found sitting in a corner of an old warehouse. It must have been 30 years old, was only about 60% efficient, but it had never been run before. That took me and phone calls to technicians from the VFD OEM about 3 days to get the current vs voltage curves just right to allow this antique to run on a VFD. Initially it was as you describe what your VFD/motor combination is doing.

It is 8year old motor but was rewounded on last year !

What effects the unbalance inductance causes the vfd to trip overcurrent/earth fault.

It is 8year old motor but was rewounded on last year !

What effects the unbalance inductance causes the vfd to trip overcurrent/earth fault.

The motor is most likely VFD compatible. This is not the source of your problem. I am not a motor expert.