First Turn Failure on submersible pumps
First Turn Failure on submersible pumps
(OP)
Hi
We are currently hearing stories of a number of first turn failure on submersible pump motors. I know that some of these stories are totally incorrect and the first turn failure is a description given by the pump suppliers for other modes of failure, with no evidence of such, it seems to be a good excuse in some cases.
There are a number of quoted remedies for reducing first turn failure, and most involve the use of dv/dt filters on the output of the VFD to reduce the VFD to a quoted figure when the reality is that it is the peak insulation voltage of the insulation that is the problem.
Reducing the dv/dt will reduce the peak voltage for a given cable length, but from my understanding, quoting a required maximum dv/dt needs to be at a particular cable length and cable velocity factor to have any real meaning. - It is looking like a trend towards proving that the maximum dv/dt is 500V/uS or there will be no warranty on the motor.
Questions are being asked of suppliers about the guaranteed maximum dv/dt of the drive with no information on the installation, cable length etc.
One solution commonly used is a three phase reactor in series with the output of the VFD, sometimes with resistors and or capacitors, often without.
I have not seen any reference to the use of common mode chokes on the output of the VFD being used as a means of reducing first turn failure, but to my warped understanding, we use common mode filters (often referred to as Ferrite Rings) to reduce common mode leakage currents in EMC sensitive environments and also to reduce bearing currents and EDM, both of these are related to the high frequency energy in the output waveform as is the peak voltage impressed on the motor windings.
I expect that the common mode filter as used to reduce EDM would also be effective in reducing the peak voltage at the beginning of the motor windings and therefore reduce the probability first turn failure.
Any comments??
We are currently hearing stories of a number of first turn failure on submersible pump motors. I know that some of these stories are totally incorrect and the first turn failure is a description given by the pump suppliers for other modes of failure, with no evidence of such, it seems to be a good excuse in some cases.
There are a number of quoted remedies for reducing first turn failure, and most involve the use of dv/dt filters on the output of the VFD to reduce the VFD to a quoted figure when the reality is that it is the peak insulation voltage of the insulation that is the problem.
Reducing the dv/dt will reduce the peak voltage for a given cable length, but from my understanding, quoting a required maximum dv/dt needs to be at a particular cable length and cable velocity factor to have any real meaning. - It is looking like a trend towards proving that the maximum dv/dt is 500V/uS or there will be no warranty on the motor.
Questions are being asked of suppliers about the guaranteed maximum dv/dt of the drive with no information on the installation, cable length etc.
One solution commonly used is a three phase reactor in series with the output of the VFD, sometimes with resistors and or capacitors, often without.
I have not seen any reference to the use of common mode chokes on the output of the VFD being used as a means of reducing first turn failure, but to my warped understanding, we use common mode filters (often referred to as Ferrite Rings) to reduce common mode leakage currents in EMC sensitive environments and also to reduce bearing currents and EDM, both of these are related to the high frequency energy in the output waveform as is the peak voltage impressed on the motor windings.
I expect that the common mode filter as used to reduce EDM would also be effective in reducing the peak voltage at the beginning of the motor windings and therefore reduce the probability first turn failure.
Any comments??
Mark Empson
Advanced Motor Control Ltd





RE: First Turn Failure on submersible pumps
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: First Turn Failure on submersible pumps
The NEMA MG1 standard for motors has specific recommendations for running motors on VFD's, but these recommendations do not apply to submersible type motors. It is recommended that you follow the manufacturer's recommendations for filtering. A typical dv/dt filter will limit voltage peaks to 1.5 times DC bus voltage at 1000ft, and limit dv/dt to 200V/us, which will meet the requirements of most submersible motors.
There are exceptions, however. I'm currently on a project where the 575V Grundfos well pump motors require a maximum voltage spike of 850V peak phase-phase, which will require a sine-wave filter to achieve, despite a relatively short cable length.
RE: First Turn Failure on submersible pumps
X49: That is a low voltage requirement. I would respond more about it, but I am afraid it would sound like I was talking bad about Grundfos.
RE: First Turn Failure on submersible pumps
Can you explain a little more about the impedance mismatch at the water / air interface? I had assumed that the cable was screened and / or armoured. Is this not the case, or is there a mismatch even if there is a screen? This latter state is harder to understand.
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
Are we talking about the same type of cable? The submersible cables I am used to seeing are composed of three fairly heavily insulated single conductors twisted together.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: First Turn Failure on submersible pumps
There is that potential impedance mismatch at motor terminals which increases voltage (and therefore dv/dt). But it is bounded to a factor of 2.
So it seems that specifying a dv/dt at the drive output will limit the dv/dt at the motor to no more than twice as much, regardless of cable configuration (and excluding resonant amplification from synchronized reflections of multiple pulses which Gunnar has said is unlikely).
Also note that max dv/dt and max voltage are both important, but for different reasons. The first tends to stress the turn insulation and the 2nd tends to stress the ground insulation. So it is logical to have a separate limit each.
Sorry if I'm misunderstanding your point/questions.
=====================================
(2B)+(2B)' ?
RE: First Turn Failure on submersible pumps
The air/water interface is an abrupt change, as is the cable to motor. It is the fact of having two impedence mismatches that makes it so difficult to deal with. We have found that keeping the carrier frequency as low as possible and proper output filtering has been the best way for submersible motors to have a decent life span when on VFDs.
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
The supply was low voltage and the motors were high voltage (600 V supply, 4160 V motors).
Some pads used high voltage VFDs and filters.
Some pads used low voltage VFDs and transformers.
These pads were all at the same location.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
Actually, long cables between a VFD and a motor can act as a pulse forming network, significantly increasing the dV/dt of the PWM waveform. This causes significant AC current to flow through the insulation, resulting in dielectric heating, sometimes to the point that the insulation softens and fails.
As mentioned before, an LC filter on the VFD output can help tremendously. I would not expect a common mode filter to help much unless the failures observed are always between the stator windings and ground (that said, it is perfectly reasonable to precede the LC filter with a common mode filter - which is often just a ferrite toroid through which all of the motor cables pass through).
I primarily design motor drives, and the IGBTs in the one I am working on now can slew 650V in 1.0us, and that is *with* ~400nH of inductance in series with each IGBT's output terminal inside the drive (the inductance is for wave-shaping and dynamic current sharing between paralleled modules). This particular drive is for a dedicated purpose and won't be more than 10m or so from its motor, but if that distance were extended out to 30m or more I would totally expect problems with winding insulation and bearings to occur.
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
This is a recognized means of reducing EMC emissions on the output of the VFD and EDM bearing currents within the motor.
The EMC leakage is a function of the dv/dt across the capacitance between the stator winding and the stator frame, and one mode of EDM is dependent on the dv/dt across the capacitance between the stator winding and the rotor of the motor.
This suggests to me that the dv/dt at the motor is somewhat reduced and therefore I would expect that the first turn failure would also be reduced.
I have not seen this put forward as a possible solution, rather I have seen dv/dt filters and sinusoidal filters recommended.
Is this because the common mode filter does not work in this instance, or because no one has tried it out?
I guess it is an out of the square type question.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
If you want to shape the dV/dt of the PWM waveform from the drive and you don't have the luxury of customizing the gate driver circuit, then you pretty much have to put in some series inductance and shunt capacitance (ie - an LC filter) on each phase leg. I usually turn to the good folks at Magnetics, Inc. for high performance inductor/transformer applications first (or Micrometals, Inc. for applications which are, shall we say, more cost-sensitive).
RE: First Turn Failure on submersible pumps
Thank you for your comments.
My observations here have been that certain drives are more prone to causing 1st turn failure than others.
With two families, I have yet to see genuine first turn failures, although I have analysed pumps with incorrectly reported first turn failures.
One motor I looked at, which was reported to have failed with a first turn failure, had greater than 1MOhm to earth and perfect winding resistances. The rotor would not turn. The ends of the can did not show any sign of swelling or heating.
The problem was found to be plastic extruding through the stator slots into the gap, jamming the rotor.
The problem was due to the iron loss heating the iron with insufficient cooling, causing the plastic insulation to soften and as it expanded, to extrude into the gap.
Cause, extended periods of very low flow. This was due to a change made in the VFD settings a couple of weeks before the failure, and was proven by the Scada.
My experience has been that the drives with good common mode filtering have been less prone to first turn failures. I do not know if this is coincidence or a legitimate byproduct. From the postings here, it would appear to be coincidence.
Thank you and best regards,
Mark Empson
Advanced Motor Control Ltd
RE: First Turn Failure on submersible pumps
I have seen many three phase reactors used on the output of the VFD for this purpose, but not single phase reactors.
Any comments?
Mark Empson
Advanced Motor Control Ltd
RE: First Turn Failure on submersible pumps
RE: First Turn Failure on submersible pumps
The three phase reactors are wound on a three phase core and the single phase reactors are wound on a single phase core. There will be coupling between the windings of the three phase reactors but none with the single phase reactors.
Mark Empson
Advanced Motor Control Ltd
RE: First Turn Failure on submersible pumps
Re: relationship of common mode noise filter to less winding damage:
Could simply be that VFD mfrs who are conscientious enough to think about putting in common mode noise filters are going to tend to be those who will also address standing wave propagation in their designs. Those who are out to make a quick buck by spewing out cheap drives and don't bother with common mode noise issues are highly unlikely to pay any attention to any other issues either.
"Will work for (the memory of) salami"