250HP Induction Motor - Greater Phase to Phase current Imbalance 4

[friend81]

Hi Folks,

Hope someone can help me on the below issue,

We have installed a 250HP, 380V, 60Hz Submersible pumset at 340m deep in a Borewell . The motor is a 6 lead motor & all the 6 leads were taken out of the well & connected in DELTA as a 3 wire motor before conencting to the VFD.

After switch-on the pumpset the Amps measured in each phase of VFD output are 376A / 436A / 540A in running voltage of 374V/376V/373V. The nameplate current of the motor is 430A. When measured through the Power Input cable to VFD the current is stable as 376A/374A/375A.

The current imbalance is measured only in Cables between the VFD output & Motor. After inverting the motor leads in VFD output same phase of the motor continues with the higher current. The insulation resistance of motor is 40M Ohms & Winding resistance 0,66 Ohms are stable in each phase. So problem in the motor is ruled out. Further if the problem is in the Motor the current will be same in Input as well as Output cables.

Could anyone say what could cause this type of Current Imbalance in VFD Output. No filters are installed yet along with VFD.

Could it be caused by Impedence in longer drop cable tied to the Steel column pipe ? Or anything wrong with the VFD ?

 

[Marke]

Hello Friend81

Further to may last post, some indicative figures that I have measured an a motor in the workshop.

1. 24VAC connected between W1 and W2.
Voltage between U1 and U2 2.088VAC
Voltage between V1 and V2 2.12VAC

Connect U2 to V1 (windings in phase and in series)
Voltage U1 to V2 4.206 Volts

Connect U2 to V2 (windings in series antiphase)
Voltage U1 to V1 0.132VAC

This shows that the winding U1 U2 and V1 V2 are correctly marked.

Now repeat the exercise with the 24VAC connected to V1 V2 and prove that U1 U2 and W1 W2 are correctly marked.
If there is an error, as U and V are correct, it must be W that is incorrect.

Note the voltages may vary considerably from the values that I have quoted. What you are looking for is the series connection with the higher voltage.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd