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SCR faults on DC Drive
5

SCR faults on DC Drive

SCR faults on DC Drive

(OP)
Situation:

A 9 blocks wire drawing machine. Each block has one 12,6 kW DC Motor. Each block has one DC Drive for each motor. All the motors and drives are equal.

DC Drive on block 4 is having a repetitive fault (4 times in one year). 2 SCR bridge tyristors on the drive explote (see picture).
We checked motor and block`s mechanical many times and never found anything wrong so I am suspecting that there is happening something from the drive`s input side that is damaging the SCRs.

What can be causing this? What things should I check from drive`s input side?

RE: SCR faults on DC Drive

(OP)
After the 2nd fault I installed a new Drive and same fault again. Last time it only blown one of the 3 SCRs

RE: SCR faults on DC Drive

I would have a detailed look at snubbers and commutating reactors.

Snubbers need to be checked for R and C values as well as wiring integrity.

Commutating reactors: Check voltage drop at load. It should be the same (give or take 10%) for each individual reactor.

Also, check arcing at full load. If there is bad arcing, you have to find the reason and remedy.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

dV/dt is one of usual suspects, aka "line spikes" that cause the SCRs to self commutate (fire) when they are not supposed to. I find a big culprit for that is nearby open transition Wye-Delta motor starters. People often use them with no concern for what they can do to other equipment nearby, especially anything with thyristors. I once had a meat plant that experienced multiple failures in succession and they swore it was nothing at their site. We put a Dranetz recording meter on it and sure enough, there was a huge spike right at the time units failed the next time. Then by looking at their process and the time of day, we found a separator that had a 4 speed motor controller that used open transition Wye and Delta changes in a complicated system involving something like a dozen contactors. It was one of the most convoluted systems I'd ever seen, made in Germany in the 50s. They had no idea it was even like that. The spike it threw out into the line every time they changed speeds were often as much as 2,000V. Not every time, but some times, likely due to differences in phase angles at the exact moment of transition.


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

RE: SCR faults on DC Drive

What Jeff says is definitely true. But, if there are nine blocks and only one fails.I think that it is easier to do a quick check on the block that fails. A mains transient very rarely hits just one drive. Snubbers and commutating reactors are there to prevent high dv/dt (from internal turn-off recovery and commutating transients) so Jeff's observation that du/dt can kill makes it even more interesting to check them out thoroughly.

That twelve-contactor contraption from the fifties is a typical example of the philosophy* that we found so many examples of back in the older days.

*"Warum einfach, wenn mann es so schön komplizieren kann" (Why keep it simple when it can be so beautifully complicated).

The Americans kept it simple, and working. Fewer and easier-to-handle problems in their equipment.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

Hmmm, yeah I missed that it was the same unit failing repeatedly and only that one. I agree then, likely a bad snubber board for that unit. Without snubbers they will work for a while, but failure is inevitable. I forgot that when I wrote ownersanuals for soft starters and power supplies, I always included the warning to replace snubbers whenever replacing SCRs, because often the same event that damages an SCR also damages the snubber, but it's difficult to tell.

Gunnar, when I worked for Klockner Moeller back in the late 70s, that, or something like it, was a favorite saying among the Germans who we worked for. I had forgotten it, thanks for that!


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

RE: SCR faults on DC Drive

(OP)
There are no snubbers and no input reactor installed
Please see the schematic drawing I made.
L201 output reactor 18,5 mH
R1 Brake resistor for emergency stop
K204 Emergency stop contact
F201, F202, F203 input fuses

All 9 Drives are feed the same way from R-S-T Bar but only Block 4 keeps failing

Can loose terminals on fuses connection be causing the fault? I am not saying we found that, just asking
What about a mistiming between K204 and Drive Disable operation?
Skogsgurra: Are you meaning to arcing on motor commutator? We have checked this many times while in operation. There is not abnormal arcing


RE: SCR faults on DC Drive

Bad or damaged contacts on K204?.

Chuck

RE: SCR faults on DC Drive

My first thought also. Take a good look K204, R1 and all the associated wiring and connections.
Another option may be to swap the motor with one of the other motors and see if the fault follows the motor or stays with K204.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: SCR faults on DC Drive

(OP)
K204: Will check that too. The thing is that the fault occurs while the machine is running not when braking
About swapping motors. Already done that. Fault stays on block 4

RE: SCR faults on DC Drive

Hello lukin1977

What make and model is that Drive?

Chuck

RE: SCR faults on DC Drive

(OP)

RE: SCR faults on DC Drive

Please investigate also the cabinet temp of block 4. If you have 9 of them, no. 4 should be in the middle of a row! Just a hunch though.

RE: SCR faults on DC Drive

(OP)
flexoprinting: I dont think that the cause of the fault is internal to the Drive. Like I said before, we installed a new drive and same fault occurred

Next week I will be checking K204 and connections from input and output of drive


RE: SCR faults on DC Drive

I have a feeling that you want a discussion, where you win, instead of a solution to the problem.

There is not much need to check the braking resistor and the contactor. If the contactor is closed most of the time, the resistor gets hot - which you will notice - and if it doesn't close, all that happens is that you can't brake, which the operators would notice and alert you.

So, forget about that.

My suggestion (1 Jun 16 21:01) to check snubbers and commutating reactors were turned down because there were no such things (snubbers and reactors). Lukin, flexoprinter and parchie have all contributed with facts that you also turned down as irrelevant. I do not understand why you ask for help and then tell us you don't need it or that the ideas are wrong. Cannot understand that at all.

Facts:
There are snubbers, flexo actually told you where to find them.

Figure 4.8.3: "Typical connections" shows that there should be commutating reactors. The L1 in the diagram.

Parchie notes that block 4 probably is in the middle of the cubicle, so temperature may be high.

Now, add these contributions and what do you get?

1. Commutating reactors are there to keep di/dt below maximum allowed value. If you don't have them, the thyristors will overheat locally due to time needed to spread the conduction zone across the Si crystal.
2. Heat kills.
3. If ambient temperature is higher for drive #4, which Parchie suggests, then the drive with the highest temperature fails.
4. Now, which drive fails? Is it number 4? Is that a coincidence? Or can you accept that heat and absence of the necessary commutating reactors may have something to do with the failure?

Just askin'

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

(OP)
I dont want a discussion where I win. I respect all of you very much and I am very thankful for your help. I am just saying what I think and trying to reply to all the suggestions made

I forgot to answer Parchie`s suggestion about the middle position of Drive been the problem. Sorry about that, but, that is not the case. (again, not trying to win anything)

The 9 drives are installed in pairs inside individual fan ventilated cabinets

1st cabinet: Command circuit and logic components
2nd: blocks 1 and 2
3rd: block 3 and 4
4th: block 5 and 6
5th: block 7 and 8
6th: block 9

Non of the 9 drives have input reactors installed. They only have one reactor at the output side. L201

How do I check if output reactor (L201) on block 4 is fine?

Thanks!




RE: SCR faults on DC Drive

There shall be commutating reactors. They are shown in the manual. Failing thyristors are typical when commutating reactors are ignored. It is not only ambient temperature, but also current drawn that determines if thyristors fail or not. Fast fuses do not protect thyristors if there isn't any commutating reactor.If you are serious about this, then measure di/dt and compare to the thysistor's specs. People make mistakes and I have seen that on refurbished Arboga machines where Philips drives were replaced with new ones.

The DC reactor cannot make the thyristors fail. It is there to reduce armature current ripple. Arcing in the motor is typical if there is a problem with the DC reactor. Many drives are fine without one. But not without commutating reactors.

You do not need to check the DC reactor. But if you really want to, a bad DC reactor is either burnt or mechanically defect. That latter, I have never experienced. It is also possible to check a DC reactor with a car battery and see if it reacts like a reactor (current increasing according to 1- exp[-t/T]). You can also apply an AC voltage and calculate L from current and frequency. An RCL bridge seldom works when used on heavy reactors, frequency is usually kHz, which is too high to measure a DC reactor. But don't bother about that. The DC reactor is not the problem.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

(OP)
OK. Thanks
Machine is now in operation but only using blocks 5 to 9. I measured voltage drop on DC reactors of the working blocks while in operation and they all read 1 V +/- 0,5 V

I understand the importance of commutating reactors but the situation is like it is, and there is no way that we can install the reactors in all the blocks at the present moment.

By the way. This is an Arboga machine with new drives smile

Can loose terminals on fuses connections be causing the fault? Fault is happening only on block 4
An AC input choke is a commutating reactor?

RE: SCR faults on DC Drive

Loose connections are never good. Don't speculate. Tighten them.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

(OP)
I checked all the terminal connections in cabinet. Nothing loose
Will be installing AC reactors on block 4 ASAP. This will the first time I have to select AC reactors. Any help please? Local companies around here do not help at all

RE: SCR faults on DC Drive

Fine! I like that.

For the prudent, select a reactor that drops around 4% at rated current. For the more adventurous, select one that drops 2%. If these drives run without reactors now, I think that 2% drop will be sufficient. It usually is.

Percent drop of what? Good question. When you run at rated current, the drop measured over the Reactor shall be 2% of applied L-N voltage. In your case, 500 V in and 34 A rated, will need reactors that drop around 10 V at 34 A. RMS in both cases. So, X is 14.7/sqrt(3) = 8.5 ohms. That corresponds to around 13 mH at 50 Hz. Never mind the different waveforms. Just go RMS, everyone else does. Expect delivery of a unit weighing around 5 kg, give or take a few kgs.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

(OP)
Thanks Skogsgurra!
Mains voltage is 3x380V
I have the DC Drive TPD32 EV Instruction Manual and this is what I found:






RE: SCR faults on DC Drive

(OP)
Sorry to bother you again but,
How did you calculate L from 8,5 ohm?

XL=w.L=2 x pi x f x L=2 x 3,1415 x 50 x L

8,5=100 x pi x L

L= 0,027 H

L=27mH

RE: SCR faults on DC Drive

I used a slide ruler. Use your value if you feel more comfortable with it. Any reactor in the right ball-park is better than no reactor at all. You were most unwilling to put any reactor there only yesterday - remember? Use anything that isn't ridiculously small or large.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

(OP)
I wrote to Gefran for a quotation for the reactor code S7D03 . What still puzzles me is why only block 4 keeps falling

The only suspicious thing that I found different from the other blocks is a joint in the PE cable to the drive. See picture

RE: SCR faults on DC Drive

You can also opt to put in one larger reactor for each pair of drives if there is a single feed to them, that might make it easier. Same inductance, but sized for the total current of both motors.

Another thing that might be happening here is relating to that K204 contactor again. Your sketch doesn't show the control circuit, but assuming that contactor is controlled by an output of the drive, and assuming you tapped L-N for 230V control power, it would be IMPERATIVE for the coil of that contactor to have surge suppression. It could be that in all of your cabinets, the contactor coils did have surge suppression installed initially, but on this particular unit it has burned out, allowing a spike to get back onto the AC line every time that contactor changes state. Distance of the circuit may be preventing that spike from causing self commutation in any adjacent drives, but because it is right there on #4, it affects only that one and because you likely left that contactor in place when you swapped out the drive, you killed the replacement eventually too.


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

RE: SCR faults on DC Drive

Can you swap components to see if the problem moves to another motor or block?

RE: SCR faults on DC Drive

Need to apply 2-4% factor, so L will be about 0.5 - 1mH.

RE: SCR faults on DC Drive

There might be a problem with L201. This may have been placed in the circuit to limit di/dt. If current rises too fast through a SCR, device damage will occur. Inductors in either the AC side or the DC side will lower the current rise.

RE: SCR faults on DC Drive

Dear iop, there you go again.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

dja
A reactor in the DC side does NOT reduce di/dt. Only AC reactors do. Have a closer look at the commutation between thyristors and you will understand why.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: SCR faults on DC Drive

(OP)
BrianE22. Yes, We already tried that. We swapped motors and problem stayed in #4. 3rd time it failed I installed new DC Drive also failed after 2 weeks of operation. On previous fails I changed the drive`s 3 SCRs with identical spares (IXYS MCC26 - 16IO1B - ND)

jraef:
Good idea about a single reactor for each cabinet. Thanks!
yes, control circuit is 220V

I will try to explain the logic of the ES (emergency stop) control circuit

On each wire drawing block there are emergency push bottoms that are series connected. When one of them is pushed it closes a relay in cabinet Nº1 (relay GA.K11). This relay close a contact that goes to all of the rest of the cabinets. ES signal

On each cabinet there is another ES relay (A.K1) that opens drive´s enable signal and closes K204 (brake resistor contactor)

Normal operation:

There is a ramp generator card on cabinet 1 that feed all the rest. When start bottom is pushed the drives are enabled and ramp goes up. When stop is pushed ramp goes down and enable signal is cut off BUT there is an programable relay contact on each drive (Relay 2) that is used to keep each drive enable until speed gets down to a certain low value (speed zero level)

I been taking to machine operator and he told me that he never uses the ES while at full speed but he uses to stop the machine while he is in threading stage (this is when the wire is pulled from block to block until 9th at low speed) because is easy for him to push the ES than the normal stop bottom. So on each day ES may be pushed many times

I think this relay 2 is causing the K204 to close while drive is still enable. This is what I was referring to on 2 Jun 16 14:53 when I said: mistiming between K204 and Drive Disable operation

Will have to check the speed zero level on relay 2 on drive #4. They are not the same on every block due to different speed reducer gears

Also checked for surge suppressors and there are none. Will have to install them too

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