Changing an induction motor voltage

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Changing an induction motor voltage

Post by weber »

acmotor wrote:Just sitting back with a coffee.
This little 0.37kW 1500RPM motor is pushing 4.44kW at 6000RPM with pretty well the data sheet efficiency.
That's 12 x nominal power at 4 x nominal revs at Tm/Tn =3 !
Have I missed something ?

Yep. You missed the chocky bickie. Image

Seriously. Don't you find it fantastically unlikely that the efficiency would be the same as the datasheet efficiency to two significant digits over such a wide range of torques or power levels? Don't you just have a nagging suspicion that the drive might be doing a very simple-minded calculation of mechanical power by simply assuming the efficiency you put into it?
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Changing an induction motor voltage

Post by Tritium_James »

The VF drive will NOT have 'an efficiency' anyway. It will vary all over the place with output current and voltage. For an example of what's likely going on with efficiency, see the plots on pg7 and 8 of our solarcar motor drive here: http://www.tritium.com.au/products/TRI5 ... eet_v6.pdf The X axis corresponds to 0 - 100% output voltage, and the Y axis is 0 - 100% output current.

ACmotor, we expect to have our new drive available for sale towards the end of the year. Expected pricing is around $10k in one-offs, and $6k in 10-off. Group buy, anyone?
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Changing an induction motor voltage

Post by acmotor »

TJ, keep us posted.

weber, I noted the consistent efficiency in the post. The agreement on 3 of the power levels just happend to be good. It surprised me too. Take 3 more readings and it would likely vary though!
I did point out that the mech. power is calculated and how I calculated it. (it will of course line up to two DP as it was a calc.) The VFD does not pretend to present the mech. power as it does not know motor eff. You follow me there ?
What I presented was a power flow scenario and you are welcome to pull it apart. The in and out are the real knowns. But I feel the mech. power is good to within a few %. Really, I am interested in x 4 or times 8 etc. not a few % !
The two motors may have varied inversely in efficiency ? I have no way of telling. But I read the net result. (I did ask for a shaft torque meter in an earlier post. I checked the letter box and none arrived ! Image )

TJ, agreed on the likely variation of VFD efficiency at varying power levels, however more than a percent or so one way or the other was not evident in the data.
In fact, given that the VFD draws 20mA at coast and the max loss would be at max current with things fairly linear in between (largely IGBT volt drop, so linear with current, P=VI), I would expect very much the performance as measured. Remember here, that these are extremely well researched industrial drives and they would not throw away even 1% if they didn't have to. It would make a mockery of MEPS if they did !
VFD loss would be a % of the kW that the drive was handling. Greater if the torque (current) was higher just as your graphs show. But as I'm saying to weber.... were talking a few % change, however, I'm looking at the x4 or x12 magnitudes !

This is live data. Theory must fit to it unless we can prove an error in measurement.   Image
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Changing an induction motor voltage

Post by weber »

Tritium_James wrote: The VF drive will NOT have 'an efficiency' anyway. It will vary all over the place with output current and voltage.

TJ, while i agree with what you say, I don't see how it is relevant to these experiments since we can surely trust the two drives to correctly measure the electrical power to/from their motors. It is the expectation of large variations in the mechanical/electrical efficiencies of the motors that I was concerned about.
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Changing an induction motor voltage

Post by Mesuge »

acmotor wrote: But as I'm saying to weber.... were talking a few % change, however,
I'm looking at the *(x4 or x12) magnitudes !


In terms of the "big guns" did you attempt to *pre-consult the specific model selection with your "contact" at ABB for this goal, or are you just going completely experimental route to select the motor? If I recall it correctly there seems to be some buzz about "HSE" 2-pole product line in the other AC thread, but that's different not so much ambitious avenue right (< x4)? Btw. I do remember you have brand new 22kW ABB ACIM and perhaps many ~11kW older ones around your shop.
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Changing an induction motor voltage

Post by weber »

acmotor wrote: TJ, keep us posted.

weber, I noted the consistent efficiency in the post. The agreement on 3 of the power levels just happend to be good. It surprised me too. Take 3 more readings and it would likely vary though!
I did point out that the mech. power is calculated and how I calculated it. (it will of course line up to two DP as it was a calc.) The VFD does not pretend to present the mech. power as it does not know motor eff. You follow me there ?
Ah. I was assuming the calculation was being done by the Danfoss drive using a sophisticated induction motor model. I still don't understand your explanation of how you calculated it. You give two equations. One would calculate Pshaft := Pdanfoss * 0.72, the other Pshaft := Pdelta / 0.82. The first line could only have used the first formula, the second line could only have used the second formula. The other three lines could have used either formula.
What I presented was a power flow scenario and you are welcome to pull it apart. The in and out are the real knowns. But I feel the mech. power is good to within a few %.
Is there any rational basis to this feeling? Image
Really, I am interested in x 4 or times 8 etc. not a few % !
I'm not interested in a few percent either, but so far we have no way of knowing if it is only a few percent.
The two motors may have varied inversely in efficiency ? I have no way of telling.
Exactly! There are an infinite number of pairs of efficiency-vs.-power curves for these two motors that would give the measured results. You can't assume a specific curve for one of them and then claim you have experimentally determined the other one. Particularly when the curve you're assuming for the braking motor is unlikely even on basic theory: Constant efficiency over a 4:1 range of power while (necessarily) going to zero at zero output power?
But I read the net result. (I did ask for a shaft torque meter in an earlier post. I checked the letter box and none arrived ! Image )
Well we're just going to have to figure out how to get you a torque meter. Image

If you fit an encoder so one of these drives is operating closed loop, can't it then use a sophisticated model to calculate the torque? Doesn't it have to do that when it operates in torque-control mode?

Another option is to figure some way of mounting one of the motors so the body of the motor can rotate by a few degrees, then using a lever arm and spring balance to measure the torque.
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Changing an induction motor voltage

Post by coulomb »

weber wrote: .. we can surely trust the two drives to correctly measure the electrical power to/from their motors. It is the expectation of large variations in the mechanical/electrical efficiencies of the motors that I was concerned about.

Yes, Ok, so we can trust that we are getting close to 3.64 kW of electrical power from the load motor, and that close to 6.23 kW is going into the rewired motor. So we know that the elec -> mech -> elec path has a combined efficiency of about 3.64 / 6.23 = 58%. We've been assuming that this is about 72% and 82%, which multiply to 59%. I would not expect the efficiency of the load motor to exceed (wild guess) 92%, given that it's 82% efficient at full power. So the test motor's efficiency is at worst 58% / 92% = 63%, for a mechanical output of 6.23 * .63 = 3.9 kW. That's over 10x its nominal output of 0.37 kW.

Granted, 63% is pretty poor, but this thing has a potato sized rotor, and it's only 72% efficient at full load. Plus, this is a worst-case scenario.

I think we should give both motors half a percent inefficiency each, so that's 6.23 * .715 * .815 = 3.63 kW, very close to what was measured. So that means the mechanical output is actually 6.23 * 71.5% = 4.45 kW, or spot on 12.0 x (to 3 decimal places) the nominal power. But that's by design; the assumed effieciencies multiplied out so close.

Remember that 6000 RPM is more than 3x overvoltaging, because the nominal speed is 1330 rpm. So this is really 4.5x overvoltaging, for a max of "only" 12/4.5 = 2.67 overcurrenting.

Ok, so here's another wish: Let's figure out the Tmax / Tnom ratio at 1330 (not 1500) rpm, and compare that with the same for either 6000 RPM, or more usefully, 1330 * 4 = 5320. After all, 6000 rpm is well past the nominal speed for this motor. (Though to find the actual nominal speed, we might need to know the nominal power first... interesting.)

Acmotor, can you actually measure the speed of the coupling with the strobe, or is it more to check for wobbles or something?
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Changing an induction motor voltage

Post by coulomb »

coulomb wrote: I think we should give both motors half a percent inefficiency each, so that's 6.23 * .715 * .815 = 3.63 kW, very close to what was measured.

Actually, thinking about this, it seems to me that the load motor is also a bit beyond its comfort zone. Its nominal speed is ~3000 rpm, so it's already overspeeded by 2:1. Yet not overvoltaged, so as a generator, it's in its field weakening zone. (Does this apply in generator mode? I think so.) It's running at 2x its nominal power, so about its nominal torque. Does this give us a clue as to what its efficiency would be?

Remember that if the load motor's efficiency is actually lower than the assumed 82%, then the test motor must be more efficient than the assumed 72%, by about the same factor, since they must multiply to 58%.
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Changing an induction motor voltage

Post by coulomb »

weber wrote:
acmotor wrote:The two motors may have varied inversely in efficiency ? I have no way of telling.
Exactly! There are an infinite number of pairs of efficiency-vs.-power curves for these two motors that would give the measured results.

Well, yes, in the same sense that there are an infinite number of real numbers between 0.58 and 1.0. Neither motor can be more efficient than 100%, and the product of the two has to be 58%, so the worst efficiency of either motor is 58%.

In the absence of information to the contrary, I'd expect the test motor to be a little worse than assumed, and the load motor (whose losses are supplied by the test motor, at slightly lower speed than if the load motor was supplying them itself) would be slightly better than assumed. So maybe 83% for the load motor, hence 58%/83% = 70% for the test motor. But that means that the power was in fact 6.23 * .7 = 4.36 kW, or 11.8 x Tnom. But how much more efficient can an induction generator be, compared to its motor efficiency? I don't know, and there is no real reason to guess anything between 82% and maybe 92%. And really > 92% only seems unlikely based on a gut feeling.
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Changing an induction motor voltage

Post by Tritium_James »

weber wrote:TJ, while i agree with what you say, I don't see how it is relevant to these experiments since we can surely trust the two drives to correctly measure the electrical power to/from their motors. It is the expectation of large variations in the mechanical/electrical efficiencies of the motors that I was concerned about.
Sorry, I guess I misread what you wrote! Yes, certainly I think you should be able to trust the electrical power measurements.
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Changing an induction motor voltage

Post by acmotor »

I'll try to strobe the coupling for speed.

mesuge, what I've found is that if I went on what people think they know about 3PIMs in overload, I'd never have built red suzi. Proof of the pudding e.g. this test, is the best way to go. Mind you, there are probably plenty of people e.g. metric mind and azure who laugh at this thread ! They have been there many years ago and in a professional way.Image

weber, Yes to the spring balance (or at least load cell with no movement). I was going to do a stall torque measurement in a similar way.

Does this help....
I've think I've sussed the VFD torque measurement. It is % of the nominated kW motor nominal torque at that RPM. Thus confusing (me) number for the little motor.
A quick check then at ~6000RPM, 1.48kW Mech. by previous data and knowing I 'told' the VFD it was a 5.5kW motor (smallest it would accept)

5.5kW so torque at 6000RPM is 8.75Nm

Torque (live data at ~2.06kW) reads 27% so 8.75 x 0.27 = 2.3625Nm (plenty of DP so weber doesn't complain !)
That's just what it is. Would we expect any different ?
Does than confirm the shaft power I've calculated ?
I can't say how independent this number is from inside the VFD.

Strange it works out given the mismatch of motor and controller size. Maybe the algorithms are just so much smarter than me ! ( No comment thanks   Image )

PS, weber why do you ASSUME efficiency cannot be similar over 4:1 ????
My 22kW ABB comes with a data sheet showing..
load % efficiency %
25        91.8%
50        93.6%
75        93.9%
100       93.3%              
read: whatever basic theory blown clean out of the water ! Image
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Changing an induction motor voltage

Post by acmotor »

coulomb wrote: But how much more efficient can an induction generator be, compared to its motor efficiency?


Same efficiency, no brainer DOL. With VF is up to the VFD how well it handles it though.
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Changing an induction motor voltage

Post by Hemonster »

acmotor wrote: (I did ask for a shaft torque meter in an earlier post. I checked the letter box and none arrived ! Image )


Hi Acmotor,

Can't you figure out the torque of the driving motor by working out the slip bettween the electrical and mechanical rotations of the driving motor? or similarly the negative slip of the regening motor? You'd need some fancy mathematical do-dah-kee formula ... but I'm sure you have it written on the back of your hand somewhere Image

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Changing an induction motor voltage

Post by acmotor »

I did, I washed my hands though ! Image Just waiting on the weber to ratify my last post !
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Changing an induction motor voltage

Post by coulomb »

acmotor wrote:
coulomb wrote: But how much more efficient can an induction generator be, compared to its motor efficiency?


Same efficiency, no brainer DOL. With VF is up to the VFD how well it handles it though.

Um, not according to this paper:
http://www.esat.kuleuven.be/electa/publ ... b_1602.pdf

From the conclusion:
   "This is not the case for low efficiency machines, where the efficiency for generator mode can drop several percent."

I'd say the 2.2 kW 82% motor efficiency load machine qualifies as a "low efficiency machine". So maybe we should assume an efficiency of say 80% for the generator, though in this case would make the test motor 58%/80% = 72.5% efficient. This is not so unreasonable, as larger motors are usually more efficient. Does that apply in this case though? If indeed iron losses are proportional to about the 0.7th power of speed, this could well be the case. Wow, that would be amazing: heaps more power, and a little more efficiency to boot!

BTW, surely both VFDs don't indicate 6000 rpm, do they? I assume they indicate assumed mechanical RPM, not electrical "rpm".
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Post by weber »

acmotor wrote: Mind you, there are probably plenty of people e.g. metric mind and azure who laugh at this thread ! They have been there many years ago and in a professional way.Image
Did they publish their results? If so, where? If not, then you deserve all praise, ACmotor. So long as you can give us some real data on the mechanical power, not just guesses.
Does this help....
I've think I've sussed the VFD torque measurement.
...
I can't say how independent this number is from inside the VFD.
As per your last sentence above, I don't think it helps unless you can find something in the manual that makes it clear how this torque % is obtained. Otherwise, we're just indulging in more guesswork.
PS, weber why do you ASSUME efficiency cannot be similar over 4:1 ????
My 22kW ABB comes with a data sheet showing..
load % efficiency %
25        91.8%
50        93.6%
75        93.9%
100       93.3%              
read: whatever basic theory blown clean out of the water ! Image

Thanks for correcting me there. I was clearly wrong to suggest it couldn't be constant over a 4:1 range.

However I note that the range of loads covered in your test were from about 60% to about 275%. Do you expect efficiency at a 275% overload to be similar to the maximum efficiency? I'm not assuming it couldn't be, I'd just like to see some evidence.
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Post by weber »

P.S. The basic theory I referred to was that torque goes up approx linearly with current, while losses go up as some higher power of the current. Copper losses go up as the square of current.

However, I just realised that we expect the losses at 200 Hz to be dominated by iron losses, which probably don't change with current. So near-constant efficiency seems more likely to me now (even from 60% to 275% load).

But that doesn't alter the need for actual measurement. Just because we agree now on the probability, doesn't make it so.

And it still seems unlikely to me that the efficiency at 6000 rpm would be the same as that at 50 Hz.

If you expect the continuous power rating to go up in direct proportion to the frequency, shouldn't you be expecting the efficiency at 200 Hz (at nominal torque) to be 93% so the kW losses don't increase.

I also expect the efficiency to go up with frequency, but not as much. I expect it to be around 83% at 200 Hz (at nominal torque).

If it really is still only 72% then it's even worse than I thought.
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Post by weber »

coulomb wrote:If indeed iron losses are proportional to about the 0.7th power of speed, this could well be the case. Wow, that would be amazing: heaps more power, and a little more efficiency to boot!
Iron losses are nowhere near proportional to the 0.7th power of speed (or frequency). You can re-read my original post on the topic here:
viewtopic.php?t=585&p=13171#p13171

However, increased efficiency is exactly what we expect when we overvoltage and overfrequency like this. I expect the kW losses to increase but the % losses to decrease. I think I wrote that elsewhere on this forum some weeks ago.

Also let me restate that I fully expect the peak power (i.e. the power at breakdown torque) to go up in proportion to the overvoltage/overfrequency ratio. Perhaps ACmotor would tell us the electrical frequency as well as the rpm in future.

I fully expect the 4-times rewired 0.37 kW motor to have a peak power at 200 Hz that is 4 times its peak power at 50 Hz, if fed 415 Vac. What is the Tmax/Tn for this motor? If it's fed an even higher AC voltage then it should have proportionally higher peak power. But that has nothing to do with efficiency or continuous power.
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Post by acmotor »

Firstly, that paper. Give me a break !

What it really said was that for any reasonable quality motor that the efficiency may change by up to about 1% motor to gen at working torque, assuming the accuracy of the instrumentation used could resolve that !
Image

weber, efficiency at 275% overload would depend on the shape of the torque RPM curve for the motor. Current increases linearly up to maybe 70% of pullout torque ...but pullout torque curve is established at typically 50Hz. What is looks like at 200Hz I am guessing. The same ?
Given the above, it is quite likely that efficiency hasn't dropped much if at all. pf continues to improve very slightly as you approach pullout then falls away.
I think the data already presented agrees with this efficiency situation. i.e. evidence.
Maybe overlapped your post there, sorry.
weber wrote:
If you expect the continuous power rating to go up in direct proportion to the frequency, shouldn't you be expecting the efficiency at 200 Hz (at nominal torque) to be 93% so the kW losses don't increase.

I also expect the efficiency to go up with frequency, but not as much. I expect it to be around 83% at 200 Hz (at nominal torque).

If it really is still only 72% then it's even worse than I thought.


Can you re-word that for me ? the 93% numbers are for the 22kW motor.

Also breaking news....
The Delta knows what is going on with torque too. (just operator problem). There is a list of readings available on the fly and if the number of poles, nominal Hz, voltage, slip RPM and operating current have all been entered (as they have) then the '2.4' that comes up amongst the numbers is in fact shaft torque (when at Tn according to all the other readings. This will be at the slipped RPM but the VFD of course knows the torque as it is putting it there. Motor efficiency just slips the RPM. Different method to the Danfoss.... same result.
It is amazing.

So no question. Shaft torque is known and efficiency stays essentially the same over the test range.

Another temperature test coming... Image
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Post by acmotor »

BTW, I used the strobe on sync to mains to see when the shaft was at 6000RPM. I trimmed VFDs to hold this 6000RPM. Danfoss on little motor was around 170RPM high and Delta was around 135RPM lower than sync revs.
Maybe confused things ? should just have let them both slip ? Not much in it at 6000RPM anyway.

Here is the temperature graph....

Image


Ambient started at 20°C and dropped to 18 by the end of the test.
Both DC and shaft fans were on. (DC fan on after motor stopped as well)
Temperature had topped out and started to follow ambient down.
Motor was stopped at 50 minutes.
So temperature rose 63°C.
This still leaves you in safe territory for class F windings and ambient of 40 or more degrees.
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Post by coulomb »

acmotor wrote: Firstly, that paper. Give me a break !

What it really said was that for any reasonable quality motor that the efficiency may change by up to about 1% motor to gen at working torque, assuming the accuracy of the instrumentation used could resolve that !

But 82% isn't reasonable. For low efficiency motors, it can be several percent.

You say that the Delta knows the torque "as it is putting it there". I don't follow that. All it can do is two things. First, it adjusts the voltage to the requested speed; in this case, there isn't enough, so it just leaves the voltage full on. Secondly, it arranges the slip such that it according to its model, the motor should end up with the requested torque, negative in this case. But this motor is open loop, so it will be a guess.

Did the delta do an auto tune OK? Can you read out the values of resistance and reactance, for example?
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Changing an induction motor voltage

Post by Mesuge »

OK, pls. allow for one little tangent here, do you guys realize that industrial VFDs are quite sensitive to G forces, I've noticed in several spec sheets by different vendors that after >60Hz it goes to hell (like <<<0.5G only), so upper 60-200Hz territory will be funny for the longevity prospects of these drives, I know we are mostly not about the NEDRA experience, but highway traffic/situations might get you to these levels as well. There is only one diy EV guy (MG Kermitt) who documented VFD box repackaging and so far he is only blowing stuff up..
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Post by coulomb »

G forces? Is this something to do with sensitive inductor cores or something? Other than that, what could be sensitive about industrial drive electronics that doesn't apply to "vehicle grade" drives?

No, at least this guy doesn't realise that.
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Changing an induction motor voltage

Post by Mesuge »

As I mentioned I hate to place a tangent topic and pls. correct me if I'm completely off base here but this thread is also about running ACIMs at <200Hz, right? Now, I'm confident we don't have to start debate about the automotive grade electronics/case design v. stationary & indoor use only of industrial VFDs, the industrial VFDs can crack/brake even by shipping just from a bad glance over the parcel hah.

For example, the Delta spec sheet sais, "Environment: Vibration: If it is below 20Hz, it is 1.0G; if between 20-60Hz, it is then 0.6G" So, to scale it up for even higher freq., go figure. And other manufs./vendors mentioned similar limitations in this regard on their drives..

I'm just playing devil's advocate role here, btw. we can move this debate into that main AC thread..
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Last edited by Mesuge on Sun, 12 Jul 2009, 21:25, edited 1 time in total.
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acmotor
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Changing an induction motor voltage

Post by acmotor »

Yep, Delta AMA'd fine and produced R and Xr. I've not note them. Xr was 4650mohm from memory. I'll look them up if it means anything.

At some point I think we need to accept that both VFDs have got it right. They are both predicting the shaft torque without? knowing the efficiency and we predict the shaft torque and get the same number. The specialty of open loop (sensorless) control is getting it right. So far all these numbers seem to add up to me bar a few percent.
Where in particular do you consider there is an error ?

That paper tested eff1 (high eff) 7.5kW and eff2 5.5kW and eff3 5.5kW. only eff3 showed any difference to take notice of. You couldn't buy an eff3 in Oz ? You have a copy of the ABB datasheets on eff. ratings.

My 2.2kW Teco at 82% is well into eff2 so I'd hardly call it low efficiency for its size !

Mesuge, that topic has been raised.
I've had a close look at the drives I have and they could do with a little attention to stop things flapping in the breeze. But nothing the average tech couldn't do. I agree with you the topic should be addressed though.
Thermal cycling would still be the greatest killer (I think it troubles a lot of PWM DC drives) At least IGBTs are bolted on and bolted to.

I G proof electronics to run in centrifuges for a living. Some has gone to 3000G. You soon get the drift of what matters.Image

Kermit did some things very wrong IMHO. He didn't feel for pullout so cooked the motor. He removed the DC bus caps to "save weight and size" I did email he at the time. WHAT WAS HE THINKING ! There was no local low impedance dump for voltage transients. So he punched through some IGBTs in voltage. None of that surprising unfortunately.

Hang on, mesuge, are you confusing vibration frequency with operating frequency ? Image

   
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