# Siemens simoreg dc master failure



## Safari (Jul 9, 2013)

over a span of one month we had two of these DC drive fail. 

the rating is;arm input: 3 phase 
400v
2487 A
50/60 hz
output DC +_ 420v 3000A. 

























the said drive is used in the cold rolling mill.
in both occasions the operator complained on the roll gap suddenly opening during rolling.checking the drive no fault or alarm is displayed.
this would continue until a fault is displayed (high armature current). on checking thyristors we found to have shorted.
what could be the cause?


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## Safari (Jul 9, 2013)

fault is F030


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## Safari (Jul 9, 2013)

just 12 hrs after replacing thyristors again the drive faulted.
suspecting harmonics issues.we dont have an input reactor installed.

the 850kw dc motor is ok.Took megger values( with drive disconnected)


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## Safari (Jul 9, 2013)

44601[/attach]


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## oliquir (Jan 13, 2011)

fault will probably appear after thyristor shorts, are the cooling fans ok in the drive, is it always the same thyristor that faults


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## Bootss (Dec 30, 2011)

Paging Mr. Rejaef


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## Bootss (Dec 30, 2011)

Nickson, is it really hot humid there or is it a dry weather ?
I wonder if ambient atmosphere condition affects that stuff a lot?


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## Safari (Jul 9, 2013)

oliquir said:


> fault will probably appear after thyristor shorts, are the cooling fans ok in the drive, is it always the same thyristor that faults


different blocks fail.not the same thyristor.cooling fan is working ok.


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## Safari (Jul 9, 2013)

Lep said:


> Nickson, is it really hot humid there or is it a dry weather ?
> I wonder if ambient atmosphere condition affects that stuff a lot?


its hot and humid yes lep! however the drives are inside a control room and air conditioned.

just left the company am so tired after doing 18hrs continuous. thats a major breakdown for us if the mill is down.

we are trying to do a locked rotor current test.Decided to decouple the motor from the gear box


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## Bootss (Dec 30, 2011)

I wish your brain trust all the best in troubleshooting this problem.
At least the electric motor looked clean ,commentator looked halfway decent condition.

let the engineers figure it out and you can go home and have a few cold iced teas.

:help::drink::drink::laughing:


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## Moonshot180 (Apr 1, 2012)

so how long was the 1st drive in service before it failed? 

Looking back on one of them pictures, those brushes don't look so good. May want to inspect them for damage and or degradation.


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## Bootss (Dec 30, 2011)

Moonshot180 said:


> so how long was the 1st drive in service before it failed?
> 
> Looking back on one of them pictures, those brushes don't look so good. May want to inspect them for damage and or degradation.


doesn't seem to be much brush material ,the bars an undercut on the commentator kind of hard to tell.


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## sparkywannabee (Jan 29, 2013)

you just had to buy a motor from Bangalore.


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## Safari (Jul 9, 2013)

sparkywannabee said:


> you just had to buy a motor from Bangalore.


lol! its cost effective


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## Safari (Jul 9, 2013)

Moonshot180 said:


> so how long was the 1st drive in service before it failed?
> 
> Looking back on one of them pictures, those brushes don't look so good. May want to inspect them for damage and or degradation.


5yrs of service for the drive. and two years of service for the motor

we have a monthly maintenance schedule of that motor so brushes are all in good shape.


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## sparkywannabee (Jan 29, 2013)

Is it possible you are mechanically overloading the motor. Don't forget to check spring tension on the brush holders. looks like you have a pretty good undercut on the commutator. Do you do the pm yourself. If somebody else is doing them, make sure they are actually checking things out and not just ticking them off on a sheet. Not sure I would put too much faith in the megger test in this case, sometimes even the motor shop can't find the problem with all the diagnostic tools they have. Seems to me that a drive usually tries to supply the motor with what it is demanding. You will probably end up changing more drives unless you replace or rebuild the motor. good luck and stay safe, long hours can be deadly in this kind of work.


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## JRaef (Mar 23, 2009)

Thyristors short from primarily one cause, too rapid of a rise in current, referred to a dI/dt (d = delta, meaning change; I = current,; t = time; so change in current over a change in time). Secondarily, a rapid rate of change in voltage (dV/dt) is said to cause thyristor damage, but what really happens is that dV/dt causes thyristors to self-commutate (turn on by themselves), which LEADS to dI/dt, which shorts them. So from what little I know of rolling mills, and it is very little, I know that there are often safety systems that will release the rollers if the pressure is too great. In your initial description, you mention the operator observing that first. That, combined with the outcome of failed thyristors, would make me think the primary cause is something mechanical in the mill.

"Harmonics" would not likely cause thyristors to fail, they are very rugged devices. Common mode noise can end up becoming so bad as to present the dV/dt scenario, so if everyone else is absolutely convinced there is no mechanical issue, you might try looking there. 

Reactors will not really help much with severe noise spikes in terms of the overall amplitude of the spikes, but the inductance does slow down the rate of rise; that is their value. So if you do have some severe moise issues, reactors may help. But it's better to find and elimiate the source of the noise. One big clue when you have equipment that has worked fine for years, then suddenly starts having issues, is to look for anything else that has changed. I once had to go troubleshoot a meat packing plant in Colorado at which 20+ of our soft starters had failed in less than a year, sometimes more than once in the same installation. What turned out to be the change that started it was that they had installed an old centrifuge separator they bought surplus, and it had a 250HP Y-Delta starter on it. Whenever it transitioned from Y to Delta, it send out a massive voltage spike and any soft starter that was not running at the time, self commutated and blew it's SCRs.


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## Safari (Jul 9, 2013)

thanks jraef. what we actually did, we reduced the voltage and current limits so that any rise in voltage would produce a fault.


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