unstable machine

[1lostsoul]

I have a machine in the field exhibiting an erratic behavior, 480VAC/200A. There are multiple VFD’s and servo axises on the machine. The VFD’s are common in one MCC and all the servo amps are located on separate panels in the machine. Half of the servo drives are 230VAC AB 3000 indexing and half are non-indexing amps. Multiple times we have had the non-indexing drives and our Delta-Tau motion controller fail without any reason. Each time the motion controller encoder inputs are flattened, as well as the output encoders in the servo drives, no feedback is given from our vendors what could cause this problem.

We have made many attempts to resolve the issue, isolation of DC power supplies, separation of all communication cables that talk to these devices in a separate conduit, checking all grounding points, and monitoring signals with a scope, everything seems normal. The machine ran for four months without problems, then failed, another time for 2 months then failed. I have tried to form a pattern, and monitor the situation, but everything always looks ok when I throw a scope on things. Any thoughts? Maybe I’m overlooking the oblivious?
Thanks..

 

[ronbert]

1LOSTSOUL:

Have you checked the harmonic distortion on the
480VAC?

How are the VFDs fed? Are the servo drives fed from
the same source?

ron

 

[1lostsoul]

I do not have any means to check the harmonic distortion.

The VFD's are fed directly from the incoming 480VAC, through proper protection, and the servo drives are fed off a step down transformer (480 Primary / 230/120 secondary), which is branched off the main 480VAC source.

 

[ronbert]

How many and what size are the VFDs?

Use scope to look at 480VAC. Sine wave should be "clean";
no dips at 90, 270, only one cross at 0, 180, 360.
Be sure to look at all three phases. The newer VFDs have
filter to reduce HD, BUT.. a lot of newer VFDs on same
feed will produce HD that can get past filters on feed side
of PS, and servos.

 

[itsmoked]

Wow ronbert, a shop guy looking at 480V with a scope.. This could easily be that last thing he ever does.

If you decide to do that Soul be careful, v e r y, careful.


A common killer that takes a while is heat. The system could be marginally designed and in your shop have a high ambient or impaired circulation. This will manifest itself as failing in a few months. I would run the system for several hours then if possible point a temp gun that has a MAX capture,(most do),at the entire system. Hopefully you can do this safely while it's ON as the excessive temps will be gone entirely within about 5 secs. Use a spray painting pattern to cover everything. See what you find. The damnedest little part can be toasting away causing your eventual failures.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sslobodan]

You know that all cables going from VFD must be shielded, and that connection should be as short as possible?

 

[Skogsgurra]

Keith is absolutely right. "A shop guy" usually does as he has been told and assumes that it safe to do so. Seen some of those accidents.

I do it myself. But I use a differental isolation amplifier (like the Fluke 920 or similar). I also use current limiting fused test pins with those irritating, but good, isolating sleeves and heavily isolated alligator clips (not the tiny lab types, more like a real gator). I also never measure directly on high current busbars. Only where fuses are below 100 A. It minimises the risk. The voltage is usually 690 or 500 V.

If you do that, and are careful, the risk is minimal.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[1lostsoul]

Thanks for all the feedback so far, the ideas are changing my thought process some. An answer to some of the above questions.

The machine has 12 VFD's, 4 7.5HP, 2 10-HP, and 6 20HP, and the 120 circuit that feeds the CPU with the delta tau controller in it, is on a 5A control concept filter. The servo's have no filtering, just fused off the transformer, and heat should not be an issue since the MCC has an A/C unit on it and it never gets above 80F.

 

[ronbert]

ITSMOKED & SKOGSGURRA;

In the US "shop guys" are trained to use PPE and
the proper techniques for use of a scope on 480.
My BAD if 1lostsoul is not from US.

1lostsoul:

The size of the VFDs you described are sufficient to
generate enough HD to cause your problem. If you are not trained to use a scope on 480VAC then use scope to look at 120VAC that feeds CPU.
Look at 120 BEFORE CC filter and after CC filter.
Also look at 230/120 that feeds servo drives.

 

[sreid]

What's significant is the Delta Tau controller getting the encoder inputs zapped and the encoder outputs from the servo drives getting zapped too! This has to mean something common to the encoders. I would guess the encoder receiver supply voltage. Check and see if the receivers are powered by a separate power supply.

 

[1lostsoul]

sreid,

Yes that is what I have been zeroing in on, thinking the problem was there. I believe the rest of the machine is fine since none of the other devices are failing in any matter. One change that I have done was to isolate the DC supply that feeds the Delta Tau breakout boards, separate DC power supply, and the rest of the DC needed for the machine is on another. I did see a drop of noise on the encoder channels, thought I nailed down the problem, but it has since failed. I believe the next step I might be taking is installing some OPTO-Isolation board on the PMAC side, though I have never done this on any of or other machines that follow this design, nor had this behavior on any other past machine design using a very similar setup. My thoughts on this though was now I will not destroy the Delta Tau, but might destroy the isolation boards and/or the servo drives still, I guess I will not know until I make the change.

So far all the input has been helpful in providing me to step back and maybe find a different approach to the issue.
thanks.

 

[jraef]

One of the problems I have always warned about (usually to deaf ears) with putting VFDs in MCCs is output conductor routing. While hopefully everyone knows to route output conductors through metallic grounded conduit or use shielded trip[lexed cable, the fact of the output conductors snaking around inside of MCCs with no shielding is an unfortunate reality in a lot of US installations. This is not as much of a problem elsewhere in the world where use of shielded cables is de riguer, but here in the US the practice has yet to achieve universal implementation. What I see a LOT of is that the motor leads are in conduit FROM the MCC, but INSIDE the MCC they are just run as unshielded wires, just like any other motor lead is. This means you get a LOT of RF from them, and some sensitive circuits that get bombarded by this RF on a continuous basis eventually break down.

Just more food for thought.