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Some advice please, Contactor Meltdown
7

Some advice please, Contactor Meltdown

Some advice please, Contactor Meltdown

(OP)




Our contactor supplies 380V off of two legs of three phase system to the primary of a
high voltage transformer. It is for Ultra violet light system on printing press. Previous
to me getting on seen this thing caught fire destroying several contactors and wiring in
cabinet with two other guys replacing components attempting to resurect it.

I put the unit in operation while monitoring current. Secondary output at moment of ocurance
was 10 amps, when contactor opens it starts welding melting contacts as seen in pictures.
Contactor in question is rated 400V @ 30A. I have posted schematic of circuit, contactor with
issue is K13 and capacitor bank is C2. The four capacitors are 30Uf and connected in Paralel with
accuall measurment of 117Uf.

What is funtion of capitors in this circuit and could they be causing the welding action of contacts?
If I were to connect them in Series for 7.5 Uf instead might that correct the problem?

Thank you, Chuck



RE: Some advice please, Contactor Meltdown

A confusing circuit. It seems as if two independently controlled and protected transformers have the secondaries in parallel.
Is the damage on only one pole of the contactor?
I suspect that when the contactor opens and the transformer is disconnected, the transformer becomes an impedance in series with the high voltage developed by T2. The high voltage is arcing across the open contacts. The current is probably fairly small, but is continuous.

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

RE: Some advice please, Contactor Meltdown

(OP)

Hello Bill, Thanks for your reply. Yes two independently controlled transformers in parallel,
one (K12)operates to warm and maintain bulb temperture. K13 engages when press starts printing.

Yes damage on that pole only.

But wouldn't the primary side of transformer be isolated from secondary side? I don't
understand how this contactor would see the high voltage. An inductive kick perhaps?

Chuck

RE: Some advice please, Contactor Meltdown

I have no idea what those caps are doing. Strange circuit, I agree with Bill.

It appears you're using 3 pole contactors for switching just two circuits. Use
that third contact by putting it in series with the second one that keeps burning
up. That will double its breaking voltage capacity and may be enough to cure your
contact-frying machine.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Some advice please, Contactor Meltdown

(OP)

Hot Dogs! That is an excellent idea! I will try that.

Thanks, Keith

RE: Some advice please, Contactor Meltdown

From the connections and the seeming lack of a return connection other than the connection to Ro I surmised that those may be auto-transformers.
I can't see a second high voltage terminal other that the common Ro.

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

RE: Some advice please, Contactor Meltdown

I'm taking a guess that this is an IEC contactor from the style of the drawing and the little bit of the contactor I can see. If it has been sized based on its AC-3 rating then it is almost certainly too small. You should be looking for the AC-6a rating and selecting the contactor based on that. The AC-3 rating is for direct on line induction motor starting. Others are listed here.

What brand is the contactor? Not all brands are equal, even if the rating says they are.

RE: Some advice please, Contactor Meltdown

Further to Keiths suggestion;
It is important in this application that the third contact be in series with the contact that is being damaged.
It may be good also to wire from the bottom of the middle contact to the top of the third contact and from the bottom of the third contact to the transformer. That will keep the voltage between adjacent terminals at less than would be the case with some other wiring arrangements.
Normally it doesn't matter which pole the third pole is in series with. In this case it is important.
If you can do it safely it may be interesting to measure the voltage across that second pole when it is arcing.
It may be more than double the rated 400 Volts of the contactor.

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

RE: Some advice please, Contactor Meltdown

Not my thing, but what I could find...

Is the cap possibly there for if/when the AC coil is swapped for a DC version (i.e., it shouldn't be in there for the current setup)?

Also found this thread on long-length cable capacitance:
http://www.eng-tips.com/viewthread.cfm?qid=287103

All I would ever consider it used for is to smooth out any inductive spikes heading back to whatever is controlling it...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Some advice please, Contactor Meltdown

Looking at the transformer terminals; R, 220, 240, 1160. This looks like a tapped auto-transformer type ballast. The capacitors are connected between 220 and 240. That would be a 20 Volt winding.
Is this a new problem?
What has changed?

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

RE: Some advice please, Contactor Meltdown

I hate people throwing out assumptions involving resonance or harmonics or 'in some way'. And now, that's what I was going to do myself. Sorry for that.

There must be a manufacturer? Or did the thing breed itself into being? That's even worse than assuming resonance. Apart from the 'funny' transformer with a R mid-tap (R could stand for that R-word, hmm...), there is also a partly unconnected secondary winding. Can't the manufacturer be contacted to get some kind of explanation?

BTW, 30 uF is not what you use for 'smoothing out' AC. But it could very well be part of a resonating (I said I was sorry!) circuit. And once you have that - anything can happen.

As Harry Martinson put it in his book Aniara: "The inventor was himself completely dumbstruck the day he found that one half of the Mima he'd invented lay beyond analysis. That the Mima had invented half herself."

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

RE: Some advice please, Contactor Meltdown

(OP)
Thank you Gentelmen! I really appreciate the input. Point well taken Bill I will connect in series as suggested by you and
Keith I feel this should be a significant improvement also noting Scotty's suggestion about AC6a contactor rating, that
makes sence, addionally I will wire K12 the same way. The contactor in question is (ABB standard motor rated I think, not at work currently)
I do find this circuit quite unusall, it's the first time I have come across switching of a transformer like that. I was thinking
they were possibly pulsing the current with the capacitor bank but really don't know for sure. Dan mentioned capacitor possibly
being there for a DC version but I don't think so, however Dan's second suggestion about soothing inductive spikes makes sence.

Bill I walked into this after about four other guy's chiped away at it, first issue lights were not reaching high temp enough to set ink.
Three other times for meltdown thus releasing the magic smoke burning up several contactors and leads, being a Legacy machine compared
to today's standards OEM parts were obsolete and contactor rating could be one major factor. Also the schematic is general to the
production of other machines from OEM, so not very specific to this unit. You are correct about your description of transformer, when I
get to work later I will rewire everything and search for properly rated contactor replacement. I will monitor it's behavior and risk
losing another standard contactor while waiting for properly rated ones to arrive. I will post a picture of the control box later as well.

Laugh out loud Gunner, thanks for adding a little levity to the conversation, in actuality it's four 30Uf caps parallel @ 117Uf, that said
what do you think considering this value, very interested in your thoughts I have great respect for your expertise.

Chuck

RE: Some advice please, Contactor Meltdown

Thanks, Chuck.
I could talk to the manufacturer. It looks like it is a German thing. And German is my third language. Any name or address? I am getting curious and want to know if I shall drop that R word altogether.

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

RE: Some advice please, Contactor Meltdown

I'm assuming these UV lights each have ballasts? And those ballasts would be at the burned end of the contactors?

You said the system is quite old... could the ballasts be giving up the ghost, creating higher power draws?

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Some advice please, Contactor Meltdown

I think Bill was right in the first post. If those are autotransformers then opening K13 will put the 1160V from T2 across the contact via the 1160V terminal of T2 to the 1160V terminal of T1, through the T1 winding, across the open K13 contact, through fuses F6 and F8, through K12 and to the R of the other tranformer.

So, putting 2 contacts in series might make it work, but you are still applying too much voltage to the contactor terminals assuming they are rated for 690V.

RE: Some advice please, Contactor Meltdown

Is it possible that the fuse F5 has blown or the transformer T1 primary opened to allow the full secondary Voltage (1160) to appear at the primary
I think it would act like a choke rather than a transformer.

It is an unusual circuit for sure.

RE: Some advice please, Contactor Meltdown

I found this on Wiki: https://en.wikipedia.org/wiki/Fluorescent_lamp

About half-way down, there is a circuit with some similarity to yours. The accompanying text says:

"The semi-resonant start circuit was invented by Thorn Lighting for use with T12 fluorescent tubes. This method uses a double wound transformer and a capacitor. With no arc current, the transformer and capacitor resonate at line frequency and generate about twice the supply voltage across the tube, and a small electrode heating current.[36] This tube voltage is too low to strike the arc with cold electrodes, but as the electrodes heat up to thermionic emission temperature, the tube striking voltage falls below that of the ringing voltage, and the arc strikes. As the electrodes heat, the lamp slowly, over three to five seconds, reaches full brightness. As the arc current increases and tube voltage drops, the circuit provides current limiting"

A UV lamp is a fluorescent lamp without coating.

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

RE: Some advice please, Contactor Meltdown

(OP)
Ah, that explains the cap and answers Dan's next question about ballast. Thank you Gunner!

LionelHutz, forgive my ignorance here but in my mind the primary and secondary are isolated
from each other. I'm not understanding how the 1160 can get to the 380 side. What am I missing?
How does an auto transformer differ in this way? But yes it does have many taps like an
autotransformer.

Roy, the fuses are good and this issue has never caused them to open.

A picture of the control box below, I am going to monitor it after making new connections and
my Dept. big cheese is looking into properly rated contactors. I will post any further developements.

Thanks all, Chuck

RE: Some advice please, Contactor Meltdown

(OP)
[quote Waross][/quote If you can do it safely it may be interesting to measure the voltage across that second pole when it is arcing.
It may be more than double the rated 400 Volts of the contactor.]

Due to Bill and LionelHutz theory about high voltage I was second guessing myself so I checked again.
Using high voltage probe just in case. I measured across the second pole from "R" to top of contactor
which gives me a voltage drop of 2.7v, phase to ground gives me 215v and phase to phase gives me 380v.
Currently operation is good ie. not arking, I will monitor it while in operation later, the machine is
down for setup now. Hopefully it does not arch but if it does I plan to have my meter in min/max to see
what I get.

RE: Some advice please, Contactor Meltdown

Basically an auto-transformer has one shared winding.
In this case the winding will be rated for 1160 Volts between R and 1160.
The winding is tapped at 220 Volts and at 240 Volts.
The R-220-240 Volt portion of the winding is often but not always a heavier gauge wire.
With the contactor open, there will be a direct connection through the winding from 1160 to R.
The transformer will now act as a choke in series with 1160 Volts fed from the other transformer.
The current will be limited by the impedance of the winding but even a small current will do a lot of damage if the arc is continuous.
But a more important issue is:
Why does a circuit that has worked for years suddenly develop these issues.
Something must have changed.

Quote (OP)

Bill I walked into this after about four other guy's chipped away at it, first issue lights were not reaching high temp enough to set ink.
Considering this, quite possibly one of the ballasts/transformers is failing.
The circuit was somewhat strange.
With a component failing the circuit is now somewhat strange with added arcing.
We have been concerned with the arcing.
I suspect that we are treating a symptom rather than the root cause.
My first rule of trouble shooting is:
"Find out what the root problem is first."
We may be concentrating on a symptom and not the root cause.

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

RE: Some advice please, Contactor Meltdown

(OP)

That is what you said the first time Sir! except you used a few more fonts to get it through this thick head of mine. I did megger
these things before but I suspect I overlooked something, testing for impedance does present it challenges in comparison to simple
resistance testing. I will do some reading and see what I can do to perform a more deffinative test on both of these.

I did confirm what you said, with contact open there is HV on "R" terminal, the appearance of this thing even had me fooled as it
appears to have two windings. I would like to get hold of a diagram of it unfortunatly it does not have a data plate for more info.
Thank you Bill for the eloqent correction.

Chuck

RE: Some advice please, Contactor Meltdown

So what was the first issue that started the whole mess? Is it possible a contactor failed due to use and the replacement picked was not high enough voltage rated?

You measured when K13 was open and K12 was closed?

I'm also curious why the transformer tap is labelled 220 implying it is a 220V rated tap, but the setup is applying 380V to it. It just seems odd.

RE: Some advice please, Contactor Meltdown

(OP)
Yes Lionel very possible, and just as important not rated for transformer switching (ac-6) I was not involved when that occurred and as I
understand it auto transformers like these are susceptible to short circuit damage.

Yes k13 open and k12 closed, many of our machines are purchased used and some come over from Europe so reassigning tap locations to
compensate for voltage is common around here. Forgot to add that the schematic is general to all similar machines manufactured by
Thiemer if you were to zoom in on picture of control box you could see the tap is actually on 400V input so 380 is within acceptable
10% allowance I think.

Chuck

RE: Some advice please, Contactor Meltdown

Good catch Lionel. I missed that.
However in the latest photo it appears that, wiring diagram to the contrary, the ballast is actually connected to the 380 Volt tap and the capacitors to the 400 Volt tap. That connection will put the same 20 Volts across the capacitors.
flexoprinting: You may want to try measuring the primary and secondary currents of both ballasts when both are energized.
Have you checked for an open or shorted capacitor?
You may wish to try a "Poor man's" impedance check.
Disconnect all leads to a ballast.
Check for continuity between R and 1160.
Use terminals R and 1160.
Connect a 100 Watt lamp or other suitable resistor in series with the ballast (R-1160).
Apply 220 Volts to the series combination. Measure the current and the voltage drop across each ballast in turn.
The currents and voltages of the two ballasts should match closely.
If the results are indeterminate, vary the value of the series lamp or resistor.
Try this test with the capacitors connected and again with the capacitors disconnected.
Shorted turns will often give more of an indication with this test than with a resistance test.
You probably want a resistance that will cause a minimum of about 10% to 20% at least of normal secondary current to get into the more linear area of the core magnetizing curve.

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

RE: Some advice please, Contactor Meltdown

(OP)
I did check caps with meter it's supposed to see ~120Uf I get 116Uf on one bank and 117Uf on the other.
Thank you for that tip,I will do exactly as stated and check results. I was kind of racking my
brain to see what I could do for test and thinking I could bring my function generator and scope
to work if I had to. Not sure I would get acceptable results though.

Thanks again Bill

Chuck

RE: Some advice please, Contactor Meltdown

(OP)
Hi Bill

I think it looks good but will wait for your thoughts on findings.
I moved the tap from 400V to 380V and gained 67V on the 1160V output currently at 1117V.

Caps checked.

There is continuity between R and 1160 on both ballast.

My greatest challenge was finding a 100 watt bulb in an industrial setting, first test was 60watt.
All test were performed at 217V

Voltage drop across R and 1160>>>>>T1= 1.4mv T2= 1.3mv

Current through 60watt=24.2ohms>>>T1=0.38ma T2= 0.54ma
----------------------
next two test performed with halogen lamp for resistance of 3.9ohms

Voltage drop.>>> T1= 1.4mv T2= 1.3mv
Current>>>>>>> T1= 0.50ma T2= 0.46ma
----------------------
this test with caps installed and same 3.9 ohm bulb

Voltage drop>>>T1= 0.9mv T2= 1.4mv
Current >>>>>>T1= 0.36ma T2= 0.41ma

Chuck

RE: Some advice please, Contactor Meltdown

We didn't have high enough current. Those results are somewhat indeterminate.
However they are not that good.
I hope that both transformers were at the same temperature when the tests were done.
Another suggestion.
Apply 217 Volts across R and 220. Check the voltages at the other terminals for the proper ratio and compare with the second transformer.
With one transformer hot and one transformer cold we may see a 10% difference but not with both transformers at the same temperature.
I note also that the caps cause a 28% change in current on one transformer and only a 5.8% change on the other transformer.
Is it possible to measure the current that the caps are drawing in circuit after they have warmed up? That is a better test than a meter test. It often will show a problem due to breakdown under working voltage.
Are those the original contactors or have they recently been changed?

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

RE: Some advice please, Contactor Meltdown

(OP)
As a precaution we are moving on a quote for new ballast. My manager was refered
to a Theimer rep in Florida who basiclly is a parts salesman with no knoweledge of electrical system and could offer no intel. I really
like the quality of German equipment communicating with OEM can be a little challenging sometimes though.

Yes both were @ same temp and I waited for my meter to settle on all measurements, also the resistance reading is of the entire series rig
I put together. I wanted to be as accurate as possible.

I will perform test on Monday and post results, Yes the whole circuit drops two amps when bulb/transformer heat up. Also I need to measure
transformer amps individually yet as you requested so will do that. Press needs to be in motion though or I cook the product belt, I'm
at the mercy of schedualing of runs.

I will attemp cap current also, no these are underrated brand new abb contactors (working on that) I think the temperary California conection will
suffice for testing purposes as long as I'm here to shut it down.

Thank you Bill

Chuck

RE: Some advice please, Contactor Meltdown

Will the ballast under consideration be an exact replacement for the existing ballast?
We can have new issues with circulating currents if the ballasts are not matched.
If you can not get an exact replacement it will be well to replace both ballasts with a pair of identical ballasts.
But first, do you know the manner of the failure of the original contactor?
It may be that the contactor was able to withstand the high feedback voltage and failed due to age.
A worn or broken component may have reduced the clearances and led to the first arc-over.
If the issue is the contactor rating, then itsmoked's suggestion of contacts in series may be the solution.
However I would be more confident had the test measurements been more closely matched.
What to do next;
1> Disconnect the high voltage leads.
2> Apply 217 Volts to the R and 220 Volt terminals and check the voltages on all taps of both transformers. Disconnect the capacitors before this test.
3> Connect the high voltage jumper between the two 1160 terminals and energize both transformers. Check for a circulating current between the transformers.
4> Check the capacitor current of each group of capacitors. If the currents are within 15% to 20% there may be no need to check the individual capacitors.
5> If all the tests give closely matching results it is a strong indication that the original issue was the failure of the original contactor and the replacement with a contactor with a lower voltage interrupting capability.
Note: when checking the current drawn by the capacitor groups it is well to also check the voltage across the capacitors.

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

RE: Some advice please, Contactor Meltdown

(OP)
They will take three weeks to build and come from Germany, so probably not worth the risk of a mismatch. I will have them order as set,
good point. Thank you Sir!

The first issue they had was bulb not getting hot enough which was caused by improper cycling of cooling fan. My guess maybe a month later the OEM contactor cooked possibly due to age but maybe due to ballast failure as you suspect and was replaced with improperly rated one which cooked again and again. There were two owners of an electrical company from down the road working on it, very smart guys but they were fooled by the readings and such as well. The bulb is a voltage hog and drops 600V across it so simply probing xfmr output with HV probe you only see something like 284V. I have the typical HV probe from Fluke like they used, I was not clever enough to read voltage across "R" and 1160 for fear of manipulating that tiny alligator clip to the "R" terminal and taking a hit. ( I should have powered it down and made my connections, that reading would have help me understand the output more )I suspect they did the same thing because they had the same dumb look on their face that I have.

Currently itsmoked suggestion (which I have dubbed the "California connection" Keith lives there I think) has been working great however, getting to the root cause as you suggest is our goal and I may still use Keith's idea on new contactor anyway. Should the test prove ballast acceptable and we figure out it was just a retiring contactor is fine. Thanks to you folks I will know how to troubleshoot it next time.

I will post the results of test hopefully on Monday schedual permiting, Thank you Bill.

Chuck

RE: Some advice please, Contactor Meltdown

Good luck. Glad to help.

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

RE: Some advice please, Contactor Meltdown

LOL That's what I guessed when I saw the "CA-connection".
Yep, Santa Cruz ~ 70mi South of San Francisco on the ocean.

You might as well use two contacts if one is unused.

Since this beast is only single phase lets hope that middle phase isn't a stinger phase..

Keith Cress
kcress - http://www.flaminsystems.com

RE: Some advice please, Contactor Meltdown

Stinger phase?

You 'murcans sure do some strange things to your electrons. smile

We don't have any equivalent to that in Europe.


On a slightly more serious note, if this is a 50Hz machine running on 60Hz then the L1 reactor is going to drop 20% more voltage than it is intended to. The transformers will be OK running with 80% flux density in the core which will make up for the 20% increase in hysteresis losses.

RE: Some advice please, Contactor Meltdown

You know one when you see one. I was asked to have a look at a spot welding machine that mysteriously failed in some installations and worked well when moved to Another Place.

The stinger phase caused the problem. I mailed the producer (European) and, at first, they didn't believe me.

Problem eventually solved by adding a note on the first page saying that the welder needed a full and symmetrical three-phase supply.

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

RE: Some advice please, Contactor Meltdown

quod erat demonstrandum!

I run into that problem with CNC machines quite frequently. They all have a control transformer that
provide all the power for the PLCs, controls, trajectory planners, etc. Frequently there's a stinger
or "Wild Leg" then, inevitably, someone picks the stinger for the control transformer. Other times the
VFDs pick the stinger to monitor the voltage and the drives frequently trip off-line on 'bad voltage'.
Rolling the phases often solves these problems or I select a different phase for the single phase control
transformer. I've never been real comfortable with open delta feeding drives. Motors don't care at all
but I'm not sure the rectifier front-ends can happily distribute their loading correctly.

That's why I bring up the 'stinger' on the cockamamie ballasts used in Chuck's machine.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Some advice please, Contactor Meltdown

Right, Keith.
You need a delta winding to leave a low-impedance route for the triplenes (3rd, 9th etc) harmonics. If the grid cannot deliver that, you get exactly that problem with "bad voltage" messages.
But not at all sure that it causes a problem with a single phase load applied line-line. There, the impedance should be OK. Triplene impedance is already low enough and it would be a lousy transformer if it cannot handle the light (pun!) load of a lamp circuit.

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

RE: Some advice please, Contactor Meltdown

(OP)
We do have machines in house w/high leg but not this one and those were my thoughts the empty contact is present why not use it for additional prevention.

Interesting observation by Scotty it is a 50Hz machine, not sure I'm smart enough to understand Hysteresis losses and the associated effects but do those loses matter since the transformers are also coils and would see the same losses? making the whole circuit equally functional whether on 50Hz or 60Hz? Reactors are for current limiting doesn't this introduce impedance into the circuit exacerbating the potential for contactor to arc as Bill described? Forgive my lack of knowledge fella's I'm looking at a forest and you folks have the ability to see trees within the forest.

Chuck

RE: Some advice please, Contactor Meltdown

Transformers and motors are concerned with the "Volta per Hertz" ratio.
Your 50 Hz machine, on the 380 Volt tap has a 380V/50Hz = 7.6 Volts per Hertz.
At 60 Hz and the 380 Volt tap the applied V/Hz ratio is 6.3 Volts per Hertz. It is lower than the machine wants so you are okay.
Now if the machine was intended for 380 Volts and 60 Hz the V/Hz ratio at 50 Hz would be too high and the motor or transformer would be in danger of saturation and overheating.
This is a whole other topic for discussion that fortunately you mostly don't have to worry about going to a higher frequency at the same voltage. Reactors will have a higher impedance and capacitors will have a lower impedance but if the machine worked for a number of years before failure there is probably not an issue.

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

RE: Some advice please, Contactor Meltdown

(OP)
So if it were designed for 60 Hz and needs to see 7.6V per Hz it would need a supply of 456V and at 50Hz
that would be 9.12V I think I understand. And yes another topic, I don't want to get off on a tangent.

Thank you Sir!

RE: Some advice please, Contactor Meltdown

Don't exceed the V/Hz ratio that a device is designed for. You may either lower the voltage or increase the frequency without exceeding the ratio.
A transformer may be operated at a lower voltage than it was designed for or a higher frequency than it was designed for.
A motor does not like low voltages. Raising the frequency on a motor is equivalent to dropping the voltage as far as torque is concerned.
hen we convert motors between 50 Hz and 60 Hz, we try to adjust the voltage in the same ratio so as to keep the same Volts per Hertz ratio.

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

RE: Some advice please, Contactor Meltdown

If it's a 50Hz designed system operating on 60Hz then that would be messing up the T1-C2 resonance circuit that was tuned for operation on 50Hz.

RE: Some advice please, Contactor Meltdown

(OP)
Thanks Bill, I assume these rules apply to AC applications only while DC drives might be completely different.

219V across R and 220 on T2 xfmr- all terminals 1 Through 7 reading 219V terminals 8 through 10 @ 148V

219 across R and 220 on T1 xfmr- all terminals 1 Through 7 reading 219V terminals 8 through 10 @ 158V

219 across R and 220 on both xfmrs and jumper across both 1160 terminals I could see no current moving
or circulating between transformers.

I started the system up and allowed both transformers and bulb warm up.
T2 had 5.1 amps
T1 had 4.7 amps
Voltage across capacitor bank measurment taken at 420 and R = 412V on T1 and T2
Current draw on T1 caps = 19.4Ma
Current draw on T2 caps = 19.2Ma

Lionel, most of our German equipment has 50/60 Hz on data plate but not this one. It's 50Hz.
I assume this affects the frequeny of resonance, by how much? Really wish I had a HV probe
for my scope. Those portable Flukes are nice but spendy.

I added another picture first one not very clear, sorry about that.

Chuck


RE: Some advice please, Contactor Meltdown

I don't quite understand your readings.
I'll give the sequence; Note: Let's round up the 219 Volts to 220 Volts for simplicity.
Apply 220 Volts across R and 220.
You should see the following readings with the capacitors disconnected:
Terminal R to terminal 360,- Should Read 360 Volts
Terminal R to terminal 380,- Should Read 380 Volts
Terminal R to terminal 400,- Should Read 400 Volts
Terminal R to terminal 420,- Should Read 420 Volts
Terminal R to terminal 1160,- Should Read 1160 Volts

No circulating current is a good sign. Many of the common faults would result in a circulating current.

You may want to do this test with the lamps disconnected.
I suggest energizing both ballasts and letting them sit, energized for long enough for the temperature to stabilize.
Then check that the voltages are what they should be and check for circulating current.
We want to rule out a ballast breaking down when hot.
Do this test with the capacitors disconnected. If the ballasts pass, do the test over with the capacitors

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

RE: Some advice please, Contactor Meltdown

All -

I think we're over-thinking this one. Capacitors, transformers and discharge lighting are all horrible loads to switch. Most NEMA contactors will handle this, but most IEC contactors won't unless they're chosen for severe service. This is an AC-5a, 6a or 6b switching duty - discharge lighting; transformer switching; capacitor switching respectively. If you use an AC-1 or AC-3 rated contactor in this sort of service it will have a short violent life. Look at the de-rates applied to IEC contactors for the more severe duties: it normally forces a jump of at least one frame size and occasionally two. I really think this contactor is simply under-rated for the job it is being asked to do.

Last year we picked up on a similar problem with a group of AC-3 rated starters being used in inching duty, and every now and then we'd see a starter bucket burn up. They were only built for DOL starting, and we were hammering the life out of them. I've had them rebuilt as AC-4 starters using a much heavier contactor and the problem has gone away.

RE: Some advice please, Contactor Meltdown

Quote (flexopinting)

I assume this affects the frequeny of resonance, by how much?

By 6/5ths.

RE: Some advice please, Contactor Meltdown

(OP)
Good Morning

Ah yes the challenges of communication with interpretation differences, Sorry Bill my bad. I interpreted across as one of the three legs @ 220v across those two terminals, I was thinking you would have said 220v through "R" and "220" so what your saying is I should have a neutral on "R' and one leg of 220v to the terminal. Which presents another problem I need to correct to make this thing safe. Whoever installed this machine only brought in four wires and the fourth wire is connected to ground of cabinet!! There is no connection for neutral!?, I followed the supply conduit to a 480v Delta to 380v wye transformer which should have five wires going to machine, I need to open supply panel when I get to work and see where that forth wire is really connected and don't know may need to check xfmr connections
also. If that fourth wire is connected to neutral in xfmr and I get impedance on any of the three leads this would make the cabinet hot correct?

Yes Scotty I think your correct It's looking like the contactor is probably the cause because when I checked voltage ratios a while back with it connected 380, they seemed reasonable. I do want to make sure these things are balanced though and will perform the test as Bill suggested to remove all doubt I will try to get it right this time though. Thank you for pointing out the NEMA/IEC differences which could be substantial.

Thank you Lionel, is this difference enough that I should be concerned about?

Chuck

RE: Some advice please, Contactor Meltdown

My bad. Your original diagram showed a 220 Volt feed. I overlooked (forgot) The picture showing the 380 Volt feed.
You can supply 220 Volts between R and the 220 Volt terminal or 380 Volts between the R and the 380 Volt terminal.
Either way you should see the correct voltages across R and the other terminals.
Scotty. I agree with you.
But it took a little while to get here. Once here, I thought it prudent to check that there was no problems with the ballasts.
I have been thinking that it is probably a contactor issue but not saying it. Once it has been said that;
"It's most likely a contactor issue." There is a great tendency to overlook signs that it is actually something else.
But now it has been said, (I was very close to saying it myself.)
If the final round of tests look good, then we can adress the contactor issue.
At that point I will happily defer to your advice. You are a lot closer to IEC land than I am.
Two issues with the contactor;
1. The high voltage backfeed. Keith's suggestion may solve that issue.
2. The contactor may still be underrated swithcing 380 Volts to a ballast.
Back to you.

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

RE: Some advice please, Contactor Meltdown

Ha-ha, yes it is very easy to focus on the 'obvious' problem and then miss something hiding in the dark corners. Got that T-shirt. lookaround

RE: Some advice please, Contactor Meltdown

Oh? That T-shirt. Got dozens of them!

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

RE: Some advice please, Contactor Meltdown

(OP)
Lol, well I must have both shirt and hat on the way then. I performed another test after pulling my head
out of you know what.


With 380 L1 connected across transformers 1&2 on the 380 terminals.
L2 connected across terminals "R" for a 380v supply to both transformers.

Transformer 1 Transformer 2
----voltage reading---

1. 199.6----199.3

2. 217.1----217.2

3. 233.3----234.4

4. 249.7----250.2

5. 216.1----216.2

6. 1097----1099

7. 112.4----112.5

8.201.9----204.1

9.212.7----214.5

10. 223.3----225.2


All readings are very close to same, I think variation showing might
be due to other loads in building and time taken to move probes causing voltage to deviate slightly.

I did repeate the current circulation test (correctly this time) and seen some current flow.
R to R lead was @ 410Ma
380 to 380 was @ 400Ma
1160 to 1160 was @ 30Ma

We received our quote for ballast $5,800 .00 Yeow! Hopefully you are going to say they are good Bill.
Was talking to my Manager about it and he remembered saving pictures of OEM contactor that burned up in fire.
It's only rated for 600V, I also included the first time it let the gremlin loose.



RE: Some advice please, Contactor Meltdown

wowwee! That was a nasty gremlin..

BTW Nice testing. You won't find closer matching than that in the real world.

What I think is happening is the contact opens and depending on where in the power
line cycle you get a huge arc as a lot of energy is stored in the magnetics or a not so big arc otherwise.
This causes the very classic "It worked for a year then did this!" result. Replacement and repair results
in some utterly random period before, "It happened again!"

This is the kind of thing often seen with wye-delta motor starters. Runs for a year then trips the breaker.
Runs for a month and trips the breaker, then runs for 2 years and trips the breaker. Lots of head scratching.

A snubber across the contact would limit the voltage available to arc, to something considerably lower AND
would provide someplace to dissipate the energy over a large surface area - the resistor in the snubber circuit.

Any arc that did occur would be shortened in period to something dissipatable by your contacts. What you have
now is a case of if the arc is a peak-energy arc then the contact metal gets so hot as to significantly
vaporize into the gap. It then participate in the arc plasma reducing its normal air-gap resistance down to something
shockingly low. In that state with more and more contact metal joining the party it becomes impossible for the
arc to ever extinguish because now the contact-space may have only 40~70Vac of interruption ability. This
continues until there is actually no further contact metal present to continue the arc. If you're lucky
you get what you just got. If you're not lucky you get what your latest picture depicts.

Even worse, the heavier duty the contactor is the worse the results because there is more contact metal available
to maintain the plasma. A heavy duty relay will give the latest picture results because you had ~2kW dissipating
in that space for maybe 20 seconds whereas the latest failure with perhaps a lesser contactor had exactly the
same power dissipation but ran out of metal sooner (10 seconds?), hence less damage.

Realize that when AC arc welding with a stick welder the substantial arc length is not much less than 690V contactors
can provide and that once the metal of the 'stick' is spewing into the arc the arc voltage drops to only 50~70Vac.
Picture an arc welding arc between your contacts. The final results would look just like what you have.

You need a snubber to limit the arc time and to suck up the stored inductance energy.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Some advice please, Contactor Meltdown

Sorry Keith, I beg to differ.
Remember that when only one contactor opens There is over 1000 volts across that one pole. The current is limited to a low value by the impedance of the ballast, but it does not take much current to do a lot of damage if the arc is not interrupted.
The original contactor had a 660 Volt rating. The replacement may only be rated for 400 Volts.
With 1160 Volts across a 400 Volt rated contactor you don't need inductive kick to maintain an arc.
That last picture looks like sustained arc damage.
I support your first excellent suggestion to connect two p[oles in series.
It could be that there are voltage fluctuations on the grid, and whenever the contactor is opened with the grid voltage on the high side of the tolerance, the 1160 Volt feed back is not interrupted and we have the sustained low current arc.
I find that oversized contactors have more of a cooling effect on the arc as a result of the greater mass of the contacts.

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

RE: Some advice please, Contactor Meltdown

(OP)
Hi Keith

Keith, would a snubber be a problem during normal operation since they are connected across phases? I understand those to divert high frequency signals or high voltage spikes and sense we need the resonant circuit and high voltage a Low pass or high pass filter would not work either way? Or are you thinking a four pole contactor w/nc terminals to bring it into circuit when contactor opens?

Chuck

RE: Some advice please, Contactor Meltdown

(OP)

Thank you everyone for your valueable input on a circuit which was humbling not only for me
but five other Techs as well. I now have greater insight obtained by the comments and doing
a lot of googling even learning about material properties coupled with manufacturing of contact
material.

The solution was simple, getting there not so much. A real smack myself in the forehead moment!

Chuck

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