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Interpretation of reading.

Interpretation of reading.

Interpretation of reading.

Good morning

We are experiencing catastrophic failure of drives and controller and have been unsuccessful locating the Gremlin which necessitated a Data logger, the event shown here took out two capacitors
on one of the DC drives Monday morning.

The logger is connected at machine on 400V supply side (it's a European machine, step down Xfmr)
We do have power factor correction at switch gear.

Based on the readings could you give me a brief interpretation on what may have caused this?, We
will have a conference call with three EE's , Data logger service, and two power company reps. later today.

The Data logger service is stating this is an upstream event, while the power company per conversation with my boss is not in agreement and also denies that there was a switch over at this time, We already know holding the power company responsible would be a waste of time, This machine is the only one with problems, we have many drives AC/DC with no issues, so we have
a unique machine and need to prevent further interruptions. I would appreciate your thoughts on this.

Thank you for your time, Chuck

RE: Interpretation of reading.

Are all the machines seeing this event but the others are surviving?
Is there an unusually long feeder to this machine?
Is it possible that another machine is responsible for the event.
Is a phase to phase flash-over possible? Can you arrange to data log another machine or the main panel?
If you have an ungrounded system it opens up more possibilities.

"Why not the best?"
Jimmy Carter

RE: Interpretation of reading.

What's the step-down transformer? Delta on the 480 (guessing) side and solidly grounded wye on the 400 side or is it something else? Voltage monitoring phase-neutral would have been more enlightening, reference for next time.

RE: Interpretation of reading.

Hello gentleman

“Are all the machines seeing this event but the others are surviving?”
“Is there an unusually long feeder to this machine?”
“Is it possible that another machine is responsible for the event.”

At first we had this machine connected to our north service which was close to overcapacity and thoughts were that this could be causing our reoccurring problem due to voltage drop And other machines on that service as you mentioned Bill. The other machine/drives had no issues. However we relocated supply to our east to rule out disturbances from other equipment per request of OEM developer of this machine. The east service has plenty of room for expansion while the only other machines on that service are continuous running vacuum pumps. The supply is 220 feet from machine, this supply also has power factor correction and is not indicating a bad capacitor, so I think we Can rule out a bad cap.Vacuum pumps are happy, no issues.

“Is a phase to phase flash-over possible? Can you arrange to data log another machine or the main panel?”

Yes it’s possible I think due to maybe, the IGBT’s firing out of sequence, but I’m not smart enough to make that determination. There have been other events, (not recorded) which have let magic smoke out of two IGBT’s at a time and blown many 80 amp fuses.

“If you have an ungrounded system it opens up more possibilities.”

I have this thing grounded to China Bill, additionally as Davidbeach nailed it, the Delta/wye step down transformer is an isolation unit with ground per OEM specifications.

We did just speak to data logger Engineer who stated that this event could be either switching from power company or caused from our machine shorting, sadly the other
Piece of our puzzle missing is the information from power company who had their own data logger on mains to switch gear. They could not open the file to give us more info and since it is a Co-opt supplier they say getting info from Excell energy if difficult. We are going to have our own logger on the 480V switch gear and wait for another event,That’s the plan going forward now.

RE: Interpretation of reading.

It does appear to be a line transient, but could it be coming from inside of your facility? A common culprit in my experience is something with a Wye-Delta motor starting scheme, sometimes 2-Speed 1-Winding (aka "Dahlander") motor starters as well. If there is a ringing transient, as that one appears to be, the rectifier in the drive can experience extremely high current spikes if the motor is running when the line transient takes place. The solution is usually to have a source of sufficient impedance ahead of the drive because the inductive time constant it represents slows the rise time of the current spike. You do say that you have an isolation transformer so that usually suffices, but maybe that transformer is too big and doesn't provide enough impedance. You might want to add a line reactor as well.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Interpretation of reading.

Hello jraef

Thank you for your reply, You have my attention. I’m interested in your theory. You say it appears to be a line transient, with that said. How can I be sure?, will our plan of adding another data logger to switch gear clarify this? Our transformer is 75 kVA and has an impedance 4.7 as listed on data plate. The machine has a 40 kVA listing.

How Sir would I determine my requirements, is there a formula to use or measurements I can use to supplement the formula? A line reactor would be economically a much easier solution than the other two suggestions mentioned from our associates here. One suggestion is to have a motored generator, the other was an UPS system, not something I want to propose to the bean counters.

Thank you, Chuck

RE: Interpretation of reading.


A little Googling supplied me with more information, it appears that 4.7 impedance I already have on the transformer would suffice, what complicates
using the chart I found is that it only gives information for single Drives. We have four drives on one DC bus and one controller, basically making this a parallel
circuit, since the chart selection requires HP and V ratings for selection process is it safe to just add Hp of all drives or do I need to use a parallel equation?
Just curious, the EE will determine any recommended needs on our end.

RE: Interpretation of reading.

I would use the rated HP of the front end. That is what is seeing the impedance of the transformer.

"Why not the best?"
Jimmy Carter

RE: Interpretation of reading.


Ah! So impedance of transformer does not cover me after current leaves it. Got it, Thank you Bill and thanks gentleman!


RE: Interpretation of reading.

I think it's an internal short in rectifier followed by an LC oscillation.
In first moment seem that iL1=iL2+iL3, that mean L2 short ti L3 and have an equivalent LC paralel scheme with L=3/2Ls and C=2Cp where Ls in secondary xfer and Cp capacity value of PF compensating caps.
This LC lead to an oscilation of f = 1/2pisqrt(LC) that seem to be 400Hz from data logged.
This oscilation apear superimposed on L3-L1 voltage.
Maybe all 3 IGBT are demaged from upper or lower side of bridge rectifier.

RE: Interpretation of reading.


That's an awesome diagnosis! we had a second trigger reading about one hour later which was almost a mirror image of this one, only at a lessor magnitude.
What happened was one of the pressman attempted a reboot of machine while the other pressman just happened to see a flash from under the drive units. I found two
capacitors blown off the board, a further autopsy was unnecessary since as always' in production we are pressed for time and knew we had to order a new drive. However,
my guess would be at least one of the IGBT's were damaged as well. If anyone were interested I can post second reading on Monday, our next challenge is to figure out
why they are blowing, I talked to the EE Friday and he has many questions for Drive manufacturer. First question is voltage rating of caps, they were rated 400V which
leaves little to no head space depending on how they are connected. The other significant fact is our supply to machine shows not much Harmonics to cause this.

Thanks, Chuck

RE: Interpretation of reading.

I think drive rectifier capacitors can't blowup for such short voltage spikes of 1-2ms as apear in data logger.
Need more details regading bridge type, half/fully controlled, how many filter caps and how are connected and what is DC bus maxim voltage.
May be a false control signal command to IGBTs that lead to short-circuit in bridge or "connect" AC from Xfmr to caps that lead to caps blow-up...

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