Unexplained torque output from large VFDs
Unexplained torque output from large VFDs
(OP)
I'm working on a project where we are planning to replace an older, non-supported VFD with a new VFD of similar 12-pulse LCI-type archtecture. The motor and driven equipment (centrifugal pump...>10,000 HP) remain unchanged. Reading through the original test report for the motor and VFD issued by the motor vendor, I have discovered mention of huge unexplained torque spikes exceeding 100% FLT, and other excitation of the 1st TNF. As a result, I am concerned about the likelihood of encoutering similar unexplained torque spikes after installing the new VFD supplied by another manufacturer. Ater all, how can a torsional analysis account for huge unexplained torque spikes? (rhetorical question)
I'd be grateful for any insights as to causes and possible mitigations to prevent the occurrence of such potentially damaging torque excitation after installing the new VFD. Are we planning to use the best possible VFD design to prevent such anomalies?
I have read recent technical papers on subsynchronous torsional interaction (SSTI) when using PWM-type VFDs, but nothing in those papers metions unexplained torque spikes as large as 100% FLT. I'm doubtful any of the VFD manufacturers will share this kind of information with us, so am hoping to elicit some comments from folks who have dealt with these issues first hand.
I'd be grateful for any insights as to causes and possible mitigations to prevent the occurrence of such potentially damaging torque excitation after installing the new VFD. Are we planning to use the best possible VFD design to prevent such anomalies?
I have read recent technical papers on subsynchronous torsional interaction (SSTI) when using PWM-type VFDs, but nothing in those papers metions unexplained torque spikes as large as 100% FLT. I'm doubtful any of the VFD manufacturers will share this kind of information with us, so am hoping to elicit some comments from folks who have dealt with these issues first hand.





RE: Unexplained torque output from large VFDs
I expect that there will also be torque transients.
If the time for a pressure impulse to travel in your fluid to the first point of reflection and back to the pump is equal to or a small multiple of the period between the passing of one impeller blade to the next impeller blade, you are in a forbidden zone. The effect is related to the speed of the pump and most VFDs have provision to program in forbidden speeds. The drive will not stay at a forbidden speed but will accelerate past it to a safe speed. There is a similar action as the drive frequency drops.
This may or may not be the issue.
Bill
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"Why not the best?"
Jimmy Carter
RE: Unexplained torque output from large VFDs
You do not say if the torque transients are periodic (if TNF doesn't mean Torsional Natural Frequency - which I start to realize that it may) or if they are starting transients.
If they are starting transients, there may be an explanation in the way voltage is applied to the motor. When the three phase voltage is applied to the motor, there is a transient that "sucks" the rotor into the resulting zero speed excitation field. This can usually very easily be seen and can result in short torque peaks that exceed rated torque, but not starting torque.
Many PWM inverters have a parameter that can be set so that voltage is ramped up in one or two seconds and, therefore, avoid that starting transient. The same thing should be possible to do in a CSI inverter by ramping up the DC current. But I don't think that I have ever seen that used in the few CSI inverters that I have met in my life.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unexplained torque output from large VFDs
We had one 60hp GE spindle motor that would shake on way thru 900rpm on accel; if left to run at 900rpm it would osc at near FLA. Nothing wrong with the vfd or motor, just combination of the total LRC values in the system.
RE: Unexplained torque output from large VFDs
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unexplained torque output from large VFDs
Numerous failures have been reported in the literature attributable to SSTI or white noise from the VFD. Sometimes it's attributed to the controller of the VFD.
I've recently done a torsional analysis on a fairly complicated VFD motor driven machinery train. The VFD drive vendor will provide a torsional output signature describing the motor air-gap torque which is used as the torsional excitation to the torsional analysis. The torsional output signature desribes the frequency and amplitude of the various harmonic components of the pulsating torque. However, these harmonics rarely exceed a few percent in magnitude...which even at this level can be enough to cause fatigue failure when operating at or near a TNF.
My main concern are these very large unexplained pulsations..20%, 66%, and over 100% of FLT.
The situation we have now is that we're planning to replace only the VFD and not the motor. The VFD supplier can proivide the voltage and current output characteristic, but this needs to be converted into torque...and somebody needs to provide a guarantee on noise to enusre we do not have anything like the unexplained torques mentioned above.
RE: Unexplained torque output from large VFDs
"Will work for (the memory of) salami"
RE: Unexplained torque output from large VFDs
As mentioned in the OP, the existing VFD is a 12-pulse LCI drive.
As a mechanical engineer, I lean towards using a VFD with the lowest achievable torque ripple and have done a torsional analysis for a large PWM driven compressor in the past. However, others on the team lean towards using a 12-step LCI drive.
I expect the 12-step LCI drive to be noisier than an equivalent PWM drive...possibly a lot noisier. But do not know how the two drives compare in terms of cost and resistance to obsolescence.
RE: Unexplained torque output from large VFDs
Anyhow, they produce torque ripple and I would not use one of them today. The torque problems that Jeff mentions are probably not an issue for a pump application but the power factor is bad (speed dependent) and the torque riple, as said.
They have a good reputation for being reliable. But most VFDs are that today.
I have seen a few Robicon MV drives lately and they don't seem to have any problems at all. Bearing currents, for one thing, were expected to be a problem. They are not. I measured close to zero.
Or just go mainstream and run PWM. Then you can use your old motor.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unexplained torque output from large VFDs
It's possible we'll end up with a PWM design...I'm for the design producing the lowest amount of torque ripple.
But even with PWM, there will still be a concern with SSTI and white noise excitation.
RE: Unexplained torque output from large VFDs
We had SCR 6 pulse systems, we invented 12 pulse systems, but never do I recall some LCI thing.... Perhaps it is time I am put out to pasture as too old to keep up, but please.....
can you make LSI into 3 words please?
RE: Unexplained torque output from large VFDs
RE: Unexplained torque output from large VFDs
RE: Unexplained torque output from large VFDs
mike kilroy...because it's not actually a water pump. It's a turbomachine with low torsional damping. Operating on a need-to-know basis.
RE: Unexplained torque output from large VFDs
"Will work for (the memory of) salami"
RE: Unexplained torque output from large VFDs
RE: Unexplained torque output from large VFDs
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unexplained torque output from large VFDs
You rarely see them used below 5000HP or so.
There is an Australian manufacturer of UPS systems called Thycon, which, until very recently (and still may) made 12 pulse output CSI UPS systems. All thyristor, and what they called "Line Commutated". They built them down to about 5 kVA :)
Admittedly, the output section of the UPS contained a shedload of capacitance, and a 'variable' inductor, so that an output voltage could be set up, and therefore perform the commutation function.
RE: Unexplained torque output from large VFDs
Do you mind if we steer the discssion back to my question regarding noise, SSTI and unexpected torque fluctuations?
Does anyone have 1st hand experience with this, especially if it resulted in mechanical failures?
RE: Unexplained torque output from large VFDs
I think that it is important that we know what we are telling each other. Definitions and acronyms that are not fully understood by the participants need to be defined.
So, to start over. Pleasy explain in some depth what your acronyms stand for, please.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unexplained torque output from large VFDs
Modern CSI drives are using SGCTs, Symetrical Gate Commutated Thyristors, which now allow for a PWM output instead of six step, so are no longer bound to a specific load impedance to facilitate firing. In other words they no longer as similar to LCI tecnology and don't need to be impedance matched to the motor. That facilitates easy motor selection and replacement, something that had been the realm of VSI drives for years and partially lead to the decline in use of CSI technology.
LCI drives were, however, still six step as far as I know, but apparently this one is 12 "step?" Tgmcg, in your original post, you said these are 12 pulse, now you seem to indicate 12 step. I'm unfamiliar with a 12 step output configuration, but 12 pulse would refer to the FRONT END conversion for the purpose of reducing harmonic reflections to the line source, you cannot equivocate it to output issues. So if it is a 12 PULSE drive, that would have nothing to do with torque pulsations. If it is 12 STEP, then I have no idea. I was just relating my personal experience with older 6 step CSI drives having possible issues with low speed torque stability and since the basic technology of an old 6 step CSI drive is very very similar to a six step LCI drive, I felt that information might be relevant and useful to you.
In reality, because of the nature of LCI drive technology, every drive is custom built to match the load and motor it is connected to. So to that point, the most relevant source for information on YOUR drive will be the original mfr. If that's not possible and you are going to replace it anyway, I think your concerns about torque transients are best brought up in your initial discussions with potential vendors for the replacement. Although your concerns are valid, I think it's more likely that this is an issue with your old drive specifically, and could be related to its age rather than the current technology. I have not heard of torque transients being an issue in MV drives of any sort for over a decade now.
"Will work for (the memory of) salami"
RE: Unexplained torque output from large VFDs
The OP deals with noise and unexpected torque pulsations. This is actually a very serious issue with large VFDs, can cause catastrophic failure of drive train components, and loss of millions of $$ in lost production. The problem is becoming more apparent as VFDs are increasingly used in larger critical service applications (e.g. LNG compressor drives). To date, primary awareness of this problem appears to be among the end-users, and primarily among their mechanical engineering specialists...including myself. The adverse consequences are primarily mechanical.
I posted here in the hope that some of the more experienced electrical specialists reading this thread may have first hand experience with the phenomena.
RE: Unexplained torque output from large VFDs
RE: Unexplained torque output from large VFDs
The original manufacturer is out of business. Parts and technical support are no longer available.
There's actually been a number of recent failures attributable to subsynchronous torsional ineraction (SSTI)and even white noise from PWM-type VFDs. This has been a fairly hot topic at recent Turbomachinery Symposioums in Houston, as VFD's have gained wider acceptance for critical service applications among the major oil companies.
RE: Unexplained torque output from large VFDs
Interesting, I had not heard this. I'll check with some of my colleagues to see if this is anything making the rounds.
"Will work for (the memory of) salami"
RE: Unexplained torque output from large VFDs
Well...no.
As I stated in an earlier post LCI can also be "Line commutated", and I still have NFI
tgmcg, I'm not trying to have a go at you here, but when a few 'olds and bolds' ask you for a few definitions, claiming that they've already been accurately defined may not be the case. For all I now, they may be accurately defined in the mech world, but I'm not a mech person.
Skoggs asked a simple, and reasonable, request. Now, as this is not the mech section of eng tips, I humbly suggest that you cut us some slack and type out any acronyms that may cause some confusion.
RE: Unexplained torque output from large VFDs
I'm an "old and bold" myself. ;)
TNF was accurately interpreted as torsional natural frequency.
This appears to be one of those cross-discipline issues where there's actually not a whole lot of overlap between the electrical specialists and the mechanical specialists. Traditionally, that interface occurred within the electric motor, which is an electromechanical device. However, with the advent of VFD's, the problem source moves upstream further and deeper into the electrical world, but the problem manifests itself in the mechanical world. The motor is now primarily a participant, but not the cause of the problem. So there is now a bit of knowledge gap, and as you can see, it is a challenge to even find the common language to bridge that gap.
Then again, there's always been such a gap wth MEs and EEs...maybe just a little bit wider now. LOL!
RE: Unexplained torque output from large VFDs
Is there any "popular" description of the phenomenon? An article or a conference paper? I sometimes get involved in odd discussions and I definitely feel that this topic, if it is as common as it seems to be, will pop up anytime, anywhere. Like to see some text.
As to "old and bold": I am twice as old and half as bold as any of you.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unexplained torque output from large VFDs
LCI drives were, however, still six step as far as I know, but apparently this one is 12 "step?" Tgmcg, in your original post, you said these are 12 pulse, now you seem to indicate 12 step.
I've never worked in the drives area, but did work in UPSs for many years. I must admit that I always used 12 pulse / 12 step to mean the same thing. I also worked on many systems which were a 12 pulse (step) output - a CSI (current source inverter) developed specifically for the UPS market. This UPS used 30 thyristors with a 12 pulse input rectifier, or 24 if the rectifier was a base 6 pulse model.
With the PWM world getting better at the output voltage harmonic levels, this company developed a 24 pulse inverter output, so up to 36 thyristors in the Ac - Dc - AC chain.
RE: Unexplained torque output from large VFDs
6 "step" when referring to the output side of a motor drive refers to the output waveform.
Again, apples and oranges.
"Will work for (the memory of) salami"
RE: Unexplained torque output from large VFDs
http://turbolab.tamu.edu/proc/pumpproc/P24/10-kais...
http://www.tmeic.com/Repository/Media/Comparison%2...
Here is my attempt to digest this into something simple enough I can understand:
As mentioned, the current to the motor changes in steps. Seems counterintuitive at first, but not when we realize the current is not interrupted but switched from one phase to another (and taking advantage of machine voltage to help with commutation).
So for 6 pulse converter, the current and mmf vary through 6 discrete states in the course of one electrical cycle, so 60 degree step change in mmf at the moment of switching. The torque is given as cross product of rotor mmf and stator mmf. The rotor mmf is varying smoothly while the stator curent is varying in steps, so it is easy to see the resulting torque will vary over the course of that 60 degrees: highest just after the stator step change, lowest just before the next stator step change.
I gather 12-pulse results in 12 discrete states in the course of one electrical cyle, so 30 degree step in mmf at the moment of switching and smaller torque transients than for 6-pulse, although the two references did not agree on amount of torque oscillation expected.
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(2B)+(2B)' ?
RE: Unexplained torque output from large VFDs
Let theta = angleof stator field minus angle of rotor field = thetaS - thetaR
theta is a sawtooth function. It starts at some value theta0, then linearly increases for a period DeltaTheta = 2*Pi/6 or 2*Pi/12, then drops sharply back to theta0
(this assumes rotor has high enough inertia that rotor angle can be viewed as increasing linearly, not affected by any torque swings).
Torque is
T(theta) = K*sin(theta) ** SEE NOTE 1
where K = |V1*|V2| / (X * w)
Tave/K = int{K*sin(theta), theta = theta0....theta0+DeltaTheta} / DeltaTheta
Tave/K = { cos(theta0) - cos(theta0+DeltaTheta) } / DeltaTheta
Tpk-to-peak/K = sin(theta0+DeltaTheta) - sin(theta0)
Use small angle APPROXIMATION: sin(theta)~theta. cos(theta)~1-theta^2 / 2
Tave/K ~ { (theta0+DeltaTheta)^2 - theta0^2 } / [2*DeltaTheta]
expand and simplify
Tave/K ~ { DeltaTheta^2 +2*theta0*DeltaTheta } / [2*DeltaTheta]
Tave/K ~ { DeltaTheta + 2*theta0 } / 2 [eqn 1]
Find the Ratio pk-to-pk/average Torque:
Tpk-to-pk/Tave = 2 * deltaTheta / { DeltaTheta + 2*theta0 }
Tpk-to-pk/Tave = deltaTheta / { DeltaTheta/2 + theta0 } [eq2]
Assume we want to compare two approaches (6 pulse and twelve pulse) to give the same Tave/K. -> need to eliminate theta0. Solve for theta00 from eq1
Theta0 = Tave/K - DeltaTheta / 2
Plut into eq2
Tpk-to-pk/Tave = deltaTheta / { DeltaTheta/2 + Tave/K - DeltaTheta / 2 }
Tpk-to-pk/Tave = K*deltaTheta / Tave
The above gives us a very crude first way to estimate the torque oscillation as fraction of average torque. Note that halving the deltaTheta (using 12 pulse instead of 6 pulse) would halve the peak torque oscillation.
In addition to the crude small angle approximation, there is an even bigger approxiamtion:
Note 1: T(theta) = K*sin(theta) is a steady state relationship. We are applying it to a transient situation. The approach might be called "quasi-static". There are many dynamic effects neglected. Accuracy of the assumption is unknown. The types of phenomenon described in the thread linked by Gunnar are all things that appear from transient analysis that wouldn't be predicted by quasi-static analysis of induction motor (assuming the steady state torque speed curve applies during transient). The T(theta) = K*sin(theta) assumption is similar simplifying assumption that may lead to big errors.
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(2B)+(2B)' ?
RE: Unexplained torque output from large VFDs
It still requires some more assumptions to figure out what K*deltaTheta migh be in relation to Tave = k*theta. In other words, what is the theta corresponding to steady state operation at average torque. We know that if theta exeeds Pi/2 we experience pole slip.
Let's look at some values of Tave = K*theta
For Tave = K*Pi/3
6 pulse: Tpk-to-pk/Tave = PK*deltaTheta / Tave = {K*Pi/6} / {K*Pi/3} ~ 0.5 **
for 12 pulse: Tpk-to-pk/Tave = K*deltaTheta / Tave = {K*Pi/12} / {K*Pi/3} ~ 0.25
For Tave = K*Pi/4
6 pulse: Tpk-to-pk/Tave = PK*deltaTheta / Tave = {K*Pi/6} / {K*Pi/4} ~ 0.66
for 12 pulse: Tpk-to-pk/Tave = K*deltaTheta / Tave = {K*Pi/12} / {K*Pi/4} ~ 0.33
For Tave = K*Pi/6
6 pulse: Tpk-to-pk/Tave = PK*deltaTheta / Tave = {K*Pi/6} / {K*Pi/6} ~ 1
for 12 pulse: Tpk-to-pk/Tave = K*deltaTheta / Tave = {K*Pi/12} / {K*Pi/6} ~ 0.5
For Tave = K*Pi/8
6 pulse: Tpk-to-pk/Tave = PK*deltaTheta / Tave = {K*Pi/6} / {K*Pi/8} ~ 1.33 ***
for 12 pulse: Tpk-to-pk/Tave = K*deltaTheta / Tave = {K*Pi/12} / {K*Pi/8} ~ 0.67
** note the 6-pulse Tave = K*Pi/3 scenario would have very small margin to pole slip at peak torque: Margin = Pi/2 - Max{theta} = Pi/2 - [Pi/3 + (Pi/6)/2] = Pi/12
*** note 6-pulse Tave = K*Pi/8 scenario would involve torque going to zero since deltaTheta/2 = Tave. Any lower Tave would result in reversal of torque for 6-pulse.
The above encompasses the widest possible range of plausible values for Tave if the assumptions are corrrect.
The assumptions/caveats of previous post still apply.
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(2B)+(2B)' ?
RE: Unexplained torque output from large VFDs
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(2B)+(2B)' ?
RE: Unexplained torque output from large VFDs
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(2B)+(2B)' ?
RE: Unexplained torque output from large VFDs
If the periodic pulsations excite a mechanical resonance, then the torque pulses produced by commutation may increase to the point that mechanical damage results.
Bill
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"Why not the best?"
Jimmy Carter