Softstarter on Pump2

[powerjunx]

Please share your views for failure analysis:

http://img168.imageshack.us/img168/5364/logicboardfg1.jpg

http://img134.imageshack.us/img134/406/scrassmblyjz9.jpg

I dont intend to depict a product inferiority in here or criticize its manufacturer. If anyone had known this, please keep it in yourselves.
This post is to provide a root cause failure views in this forum only .


Thanks,
bil

 

[ScottyUK]

I'm suspicious of the lug broken away from the thyristor gate as being the initiator. As a hypothesis, the lug failed due to mechanical stress and made contact with the anode heatsink, driving a high voltage back through the gate drive transformer and destroying the components on the control circuit side of the transformer.

Can you examine the point at which the lug broke away and see if it has the characteristic crystalline appearance of a brittle fracture?


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

The board looks like some serious tracking across there where current was running across the surface for a moment. Is there any chance something got on the board? Water or a piece of wire or foil or?

Scotty's thought is a good one too!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[powerjunx]

ScottyUK, this unit was under pre-installation, that is; setting up the torque limit and time. After such testing, the motor terminals were disconnected. And bang! ... it's suddenly burst to flame. i can't give you any comment on SCR lug failure, but it's probable. I'll let you know the lug fracture soon after i examine it thoroughly.

Itsmoked, logic board from OEM has protective cover on it as specified. But in actual installation it's nowhere to be found. I guess you're right with your guess as mine. There is high probability that a conducting pieces might get contact with PCB.

Any more wise guess of this failure?


Thanks,

bill

 

[RalphChristie]

Bill:

I have seen this kind of behaviour a few times, we use a similar patent on some of our older rectifier stations. It is used as a source of dc supply to our overhead pantograph systems. The difference is that we are using normal diodes, while you are using SCRs.

Such flashovers on our stacks normally started after
:some rain or with high humidity present.
:failure of the bolt insulation (in your picture it would be the blue insulation studs next to each SCR)
iodes not been torqued correctly.

Is there a way that you can open some of those chips and take some pictures? I have a list with causes for typical failures - eg. misaligned heatsinks, incorrect clamping, blocking voltage breakdown, overclamping, high surge current (di/dt), dV/dt failure, etc, etc.

Regards
Ralph



[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[jraef]

I kind of agree with ScottyUK and itsmoked both, although it is more likely that there was some installation debris resting on that gate lead when you energized it. It has all the classic signs of conductive debris as itsmoked said, but the damage to the board indicates that a high voltage traveled up the gate lead and flashed the traces. If it were only the gate lead shorting to the heat sink, the damage would have been less severe to the SCR stack assembly, but the burning on the board would have been similar. All the gray dust indicated vaporization of something more substantial than just a little crimp on terminal at the end of the gate lead. Probably a drill tailing or maybe even a small screw that fell down and rested right there. I've seen that dozens of times, looks just like that.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[powerjunx]

Ralph, installation factor is also accountable as you enumerated above. But i classify it as less probable.

Jraef, i'm much convince as what itsmoked pointed out. Noting that this board is brand new as purchase.

Other way around;
1.) Since Control board is new, is there a possiblity of ESD (electrostatic Discharge) issue here?
2.) How about missing the cautions upon adjusting the torque limit and time rheostats where carried out while the board is energized?
3.) How about some possibilities like carelessness, safety precautions, etc? I appreciate if anyone could add..

However, i can't disregard Scot's view. As preventive measure, i would recommend to insulate the Gate terminal lugs like a sort of shrinkable tube insulator to prevent the shorting path between gate and anode terminal. Would you agree with it, Jraef?

Thanks,
bill

 

[itsmoked]

Well now that I have looked at the stack picture again... Clearly that entire end of the stack was involved in a massive 'ugly' event. The terminal that is broken looks more to me like it got caught in the conflagration. It melted off, it didn't just 'break'. This no doubt conducted the mains stuff up onto that logic board. Yes it looks like tracking across the board, but it was probably just driven by a very large voltage that ran up the control wires. I mean look it's the middle stack that had the melt off and the middle of the control board's three control outputs.
I think something along the lines of RalphChristie's scenario is correct.




Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

Looks like there was a large 3-phase flashover involving that whole end of the SCR phases and obviously a flashover on the card.

Check the SCR leads where they're twisted together. I've seen it before where there was an insulation failure that shorted the lead to the heatsink or each other and caused that.

Scotty's description is quite possible. Once it flashed during the fault it would be hard to tell if it burned off or broke off too.

Otherwise, I would suspect the flashover started at the phases and then the card card failed which caused phase-phase circuit tracking on the board which then blew off the gate lead.

The other possibility is that the SCR failed and put a large gate to cathode voltage out of that lead which overvoltaged the gate circuitry on the card and caused it to flashover and also blew off the gate lead at the same time causing the 3-phase flashover on the scr phases. I can't see this happening if there was no motor connected though.

You say the motor was disconnected at the time? The unit was powered back up again though? I can't see the insulators, water or dirt on the clamp between the phase or SCR torque having an effect if there was no motor connected. In this case, the line and motor heatsinks would be at the same voltage due to SCR leakage resistance. Unfortunately, this also somewhat shoots down SkottyUK fault description. If both heatsinks were at the same potential then the lead shorting to the anode side would not cause a flashover.

 

[powerjunx]

LionelHutz,
"you say the motor was disconnected at the time? The unit was powered back up again though?" I guess you're right, this would be the possible.

Prior to the occurence, this board (new) was installed while SCR assembly is already installed and operational. The installation was carried out in order to test the functionality of the board. Adjustment were made on the board (rheostat) such as torque limt, time setting, current limit as well as the dip switch for motor FLA. Consequently, start and stop of motor (dont know how many times) is obvious while refining the board's adjusments until a adjustment was attained. Motor terminals were disconnected in order to pull out the tested board for safekeeping as spare. Then, whoosh! it ignites!

I suspect that a redundant adjustment of rheostat was carried out while main power supply is energized and noting that motor cable was disconnected. Any comment?

With above description, i never disregard the probability of poor workmanship, carelessness, safety awareness to the board adjustment, etc.

Thanks,
bill

 

[powerjunx]

Here's mounting board of SCR and Control Board:

http://img167.imageshack.us/img167/1939/panelboard2db3.jpg

See, what you got..

 

[armandh]

last one I had like that [early 1980s]the leads to the motor were disconected at the motor but not insulated from ground. trashed a controler that looked a lot like this one.