250HP Induction Motor - Greater Phase to Phase current Imbalance
250HP Induction Motor - Greater Phase to Phase current Imbalance
(OP)
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 ?
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 ?





RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
The input current to the VFD is meaningless with regard to current balance going to the motor, there is no direct connection because the input power is just the raw material from which the VFD makes DC, so the input AC data is otherwise irrelevant. It might only be relevant as an indicator of line voltage imbalance.
"Will work for (the memory of) salami"
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
old field guy
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
If you can find an old Crompton non-electronic moving iron meter, that could be used. Or a Messwandlerbau clamp together with a moving iron Multizet or similar.
I wouldn't trust one of Yokogawa's or Agilents modern meters either - even if they are way above Jeffs USD 3000 limit. I am not against electronics. Not at all. But, for field measurements, I prefer good old transformers and electromechanical principles.
BTW, are the cables separate or "cabled"? If separate, you can easily have an impedance unbalance. Especially when cables are 340 m long.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
The extension cable is a 185 Sq.mm single lead double insulated copper conductor tied along the steel column pipe. 6 Cable leads are tied together as Pairs U1+W2, V1+U2 & W1+V2 to make DELTA connection as 3 Wire in the VFD Output.
If the problem is with the motor either one of phases should show IR value less than others or Winding Resistance should also be differentiated. Both are stable in all 3 phases. And if the Imbalance is caused due to the defect in the motor the load of 540A will not be reflected in the Power Input cable too !!!
This is the fist time i am handling an VFD installation hence some questions may appear stupid, bear with me.
If the unbalance is caused by Impedance how can it be eliminated ? Is it advisable to run the motor with Impedance Imbalance or to be specific the actually current drawn by the motor is really lesser than what has been measured as 540A by the clamp meter ?
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
What frequency are you running at when these measurements are made?
I have seen similar when the cables were mis-identified and one winding was reversed. The measurements were at very low frequency and the drive refused to ramp up.
best regards,
Mark
Mark Empson
Advanced Motor Control Ltd
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
No, absolutely not. The incoming power is just converted to DC, so nothing that happens on the load phases passes through to the individual line phases. The line only sees total power, evenly distributed across the incoming phases by the converter bridge rectifier.
Hmmm... 6 separate unshielded conductors tied to a steel column, no conduit?
"Will work for (the memory of) salami"
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
The cables have been verified for continuty to avoid misconnection of the leads.
Normally in Submersible pump installtion no conduits are in practice. Generally it would be 3 core sheathed cables or double insulated individual leads.
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
So you are getting that level of imbalance when running and the pump is doing some work.
Sounds like something seriously wrong. I would not run the pump like that.
I would suggest that you
a) separate the three windings and measure the resistance and inductance of each winding
b) recheck the polarity of each winding. If the start and finish of one winding are reversed, you can get some interesting currents.
Best regards
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
"Will work for (the memory of) salami"
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
I have seen 80 - 90 amps flowing in the earth cable due to current induced in the riser when one or more cables rotate around the riser on the way up.
Mark Empson
Advanced Motor Control Ltd
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
If separate, the imbalance can be worse than if the cores are in one or two cables.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
Comments?
At that length, the wires don't have to be wrapped around the pipe and wrapping all the wires around the pipe should affect all wires equally.
If one wire is wrapped around the pipe, even a plastic pipe, the separation from the other wires alone will cause problems. The proximity of the iron will help.
If the wires are tied flat on the pipe, the outer wires will not enjoy the same amount of field cancelling as the center wires and will have more impedance. The proximity of the iron will help.
Looking more and more like a cabling issue. Again, the proximity of the iron will help.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
The resistance is good & uniform in all 3 phases. The polarity is also ok. So problem with motor is ruled out being brand new & factory tested.
I have consulated one of our known installer who install biggers pumps like 400HP & above, he suggested that the separate cable leads has to be crossed soon after the drop cable joint to avoid impedence. That is individual cables of U1+V1, V2+W2, W1+U2 & tied flat to column pipe.
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
Lifting the pump is an expensive option, it is in NZ anyway, and I would be looking at what can be achieved before lifting it if possible.
There can be a problem with the motor, I have seen issues with brand new motors, especially operating on VFDs with long cables to the motor. Submersible motors typically are sensitive to voltage transients as the insulation rating is quite low. Many manufacturers require dv/dt filtering to minimise the rate of rise of voltage to less than 500V/uS. Very Long cables may require the use of a sinusoidal filter on the output of the vfd. It is possible for the voltage transients to cause a shorted turn in the motor and if it is only one turn, the motor will continue to operate with a large imbalance and a high temperature rise, so do not discount the motor itself.
You have measured the resistance and that is balanced. Have you measured the inductance? that can give an indication of a shorted turn problem. I would measure each winding separately and look for equal inductances.
There is a possibility that the lay of the cables as the pump has gone down has resulted in an imbalance in the turns around the riser, effectively adding a transformer/reactor in series with one or more conductors. - Hard to determine, but can result in a high current in the earth conductor going down to the motor.
I would suggest that you look at the polarity of the three windings and ensure that you do not have a start and finish transposed.
I have done this before without lifting the pump, the experts were all set to pull the pump, but we did some tests and saved a fortune.
Most of the problems that I have encountered have been with six wire double star motors where the cables have been identified at the top incorrectly.
From your description, this is a six wire delta motor, so I would try using a 24V transformer with about a 10 amp secondary rating.
Connect the 24VAC to one winding, say U1-U2. Then connect the other two windings in series v2 to w1 and measure the voltage between v1 and w2. next revers one winding, i.e. connect v2 to w2 and measure between v1 and w1. If you can measure an appreciable voltage difference, you can use this to determine the polarity of the windings. You need to do the test twice, connecting the 24V to 2 separate windings.
Try it in the workshop on a known motor first so that you can see what affect you have and then know what to look for.
Another way that I have used to confirm the polarity, is to first reconnect the motor in star and use a low voltage three phase applied to the windings. Measure the current in each phase and reverse either the high or the low phase to see if you can balance it up.
Be very careful, submersible pumps are very unforgiving and can be easily damaged by incorrect wiring and high voltage testing.
Good luck,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
As Mark highlights, I'm surprised that no sine wave filter has been incorporated. Most borehole pump manufacturers would specify the dv/dt levels and this would often necessitate an output filter. In doing this, and with the length of motor cable involved, it would also be important to evaluate if a step up transformer would be required. A sine wave filter will 'add' to the volt drop issue at the motor end and needs to be considered for the reliability of the motor.
Regarding borehole pumps. A large Danish pump company are often telling me that the major cause of motor failure and (prior to failure) the resultant odd measurements of volts & current, are due to inferior termination and glanding at the motor end. If not carried out by a (recognised) installer or the manufacturer, this is by far the biggest cause of early life failure.
The use of a VFD will typically 'find' weaknesses in poor termination as it will in motor insulation.
RE: 250HP Induction Motor - Greater Phase to Phase current Imbalance
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