OT - Intermittent control voltage transformer?

rguimond

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Join Date
Jul 2009
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Escuminac
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Last night I came across a weird situation at the plant. A hydraulic unit wasn't running, but it should have been. Closer inspection revealed smoke curling up from small openings in an enclosure. When I opened the enclosure, the control transformer burst into flames. Closing the door extinguished the flames.

What's strange is that we've been having issues with the hydraulic unit dropping offline intermittently all week. We thought it was a bad e-stop, but now I'm thinking the control transformer may have been intermittent. I've seen lots of transformers fail, but not in this way. Has anyone else?

The hydraulic unit has two pumps. Only one can run at a time. Interestingly, the starter for one of the pumps was found to be stuck in last week. Contacts were completely fried. I'm guessing this was from rapid pulsation of the starter coil, probably due to the transformer dropping out.
 
Not sure if there is an actual question to answer, but could it be the other way round...are the 2 pump contactors mechanically interlocked? I've had it before where the mech interlock has jammed and overheated the control transformer...

Just a thought.

Regards,

Rob
 
Kid OOPS got names wrong
Rguimond


Transformers catch fire sure but only if a failure inside or a severe overload.

You may have something shorting or overloading transformer. THis causes transformer voltage to droop which may cause dropout of relays.

The first thing I would do is a rought check of the wiring looking for loose connections overheat problems etc.
2. I would add up the power (or amps) of all the loads supplied by the control transformer
3. Once I know total load I would order new transformer.
4. Then I would go thru the design and as hooked up and make dang double sure something is not shorting out and cooking transformer.

Dan Bentler
 
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The only load on the transformer is a pilot light and a starter coil. It has worked that way for 20+ years. We rung all the wires out and none are shorted to ground. megger test is OK, too.

BTW, the two hydraulic pumps are electrically interlocked. Two limit switches are mounted so they are only closed when the handle of a three-way valve is in the correct position. As an added degree of safety, the NC contacts in one switch are used to supply the NO contacts of the switch for the pump that is "armed".

Going in tomorrow morning to install new transformer. Will keep you posted...
 
Double check both primary and secondary transformer fusing. 20+ years is a lot of time during which something may have been compromised. It's probably a chain of events that led to the fire, but you want make sure that the next time a starter hangs, all you find is a blown fuse.

I have seen less than 10 transformer failures in 17 years, and they were all the result of marginal or downright poor designs or someone cheating the fuse type or size, or ignoring the cabinet cooler and compromising the temp rating.

When a big HP contactor hangs it's usually a bad result, and mechanical interlocking is a good idea too. I have seen three or four contactors hang because of auxiliary contacts that mechanically failed (thinked brittle baked plastic chipping or cracking), and a couple due to welded main poles, and all the redundant electrical interlocks won't help you there if it moves enough to actuate them. I have not seen a mechanical interlock fail to prevent this problem though...sure it could happen, but in my experience, it's the most reliable form of interlocking, and the electrical interlocking is the back-up method.

We recently lost a small pump panel control transformer and found (apparently by design) no secondary fuse or breaker at all! The primary was sized right and the fuses were as specified but had probably been replaced too many times with the secondary circuit in a faulty condition hence the release of the magic smoke...The panel was cramped, but to me there's no excuse for omitting secondary fusing.

When any transformer smokes thorough investigation needs to happen for sure, something in the design or implementation probably needs correction, and if you prove nothing is wrong, then chalk it up to a twenty year old winding.
 
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Both the primary and secondaries were fused. All three legs on the 575 VAC primary were fused with 1A fuses and a 1.5A protected the secondaries. We rung all the wires out again and didn't find any dead shorts, so we installed a new xfmr. It burnt out within a minute.

I'm stumped. The "start" signal isn't momentary - it's latched in on the PLC that controls it. The PLC actually power the coil of an interposing relay. The E-stop has two NC contacts. One breaks the "start" line between the PLC and the coil on the motor starter and the other breaks power to the PLC output card.

I was hoping that the interposing relay between the PLC output and the coil had arcing contacts that were causing the coil to energize and de-energive rapidly, but that doesn't seem to be the case.

Suggestions?
 
On primary you say 3 legs of 575. That means 3 phase to me and if so a 1.5 kW transformer. Something is really wrong if you fry that in one minute. Which winding(s) did you fry primary or secondary?

Send hookup drawing and label of transformer and maybe we can help. Otherwise it is all crystal ball work. All kinds of guesses can be made most of which will just be an exercise in futility and mostly frustration for you.

Dan Bentler
 
On primary you say 3 legs of 575. That means 3 phase to me and if so a 1.5 kW transformer. Something is really wrong if you fry that in one minute. Which winding(s) did you fry primary or secondary?

Send hookup drawing and label of transformer and maybe we can help. Otherwise it is all crystal ball work. All kinds of guesses can be made most of which will just be an exercise in futility and mostly frustration for you.

Dan Bentler

Please refer to the attached quick'n dirty schematic. Now that I've drawn it, I think that the 120 VAC supply for the interposing relay coil may be from another source - not that it should make any difference, since the common for the NO contact in the interposing relay is fed from the transformer.

I can't really say what set of windings burnt. The varnish melted and caught fire, just like it did with the original transformer.
 
Some thoughts on troubleshooting this problem, you may have already considered them.
First , disconnect the motor leads from the load side of the starter. Next, use a clamp-on ammeter to measure the current by briefly energizing the starter for 2-3 seconds. If the amps are higher than the rating of the transformer at the end of the 2-3 second period, the starter or coil is bad. It could be the iron laminates or something similar such as the armature not fully seating or parts of the armature missing. It is possible that the armature coil is a DC type and the AC is supposed to be rectified to DC but the rectifer has failed. If you are just using an ohmeter to ring out the wires it may not detect some carbon tracking, hopefully you are using a megger.
 
Please refer to the attached quick'n dirty schematic. Now that I've drawn it, I think that the 120 VAC supply for the interposing relay coil may be from another source

Is the secondary neutral tied to ground? Show that on your next update if so.

Also, I have not seen a single phase control transformer with a three phase input. Not saying it ain't possible, I just haven't seen one in my limited experience.
 
Is the secondary neutral tied to ground? Show that on your next update if so.

Also, I have not seen a single phase control transformer with a three phase input. Not saying it ain't possible, I just haven't seen one in my limited experience.

You're right - only single phase. Two legs of the three-phase 575 VAC supply become the primary. both are fused.

No, the neutral isn't bonded to ground.
 
Okay, so what about specs. What size is the transformer? Motor starter coil? Relay coil? PLC current draw? What exact type of fuses are installed?

Also, is there a surge suppressor on the contactor coil? Should there be one?

Is the incoming power clean? Could there be spikes from say VFDs elsewhere in the plant causing excess heating of the primary? You might need a scope to "see" this.
 
How many people have rewired this thing and how cobbled up is it.
Seems like every time we ask a question we get more confusion different answers. As examples first it is 3 phase transformer now single.
You are confused over what the 3 relays do and who feeds power to them.

OK now here is what I understand
1. This machine has 2 hydraulic pumps.
2. Each hydraulic pump is a separate controller
3. Relays of each hydraulic unit are interlocked (I forget with the confusion if that is both mechanical - think not - or electrical
4. COntrol of which pump is running is done with PLC
5. You keep blowing a transformer on one pump

From memories of equipment that 14 people have modified fixed etc etc I recommend:
1 go thru and cut out all wiring and trash it
2. Identify what you need and the nameplate data on it
3. Add up total amps and watts for control devices fed by a transformer
4. Identify how the secondary and primary of transformer relate ie what is their ratio and can it be connected for other voltages ie does hi side have two winding ie 1 and 2 AND 3 and 4 and what is on secondary same setup or just one winding.
5. Find out which winding on transformer burned out.
6. Check resistance and voltage ratings of all coils - bench test would be best. Are they bound or operaye freely?
7. After you have found compnents that are salvagable
8. Take a break
9. Make up a wiring diagram
10 Rewire.

Dan Bentler
 
This is my first post here but I have to deal with odd things like this daily, chicken plants are hard on wiring. Luckily I haven't had a transformer catch on fire yet.

You noted that the secondary is fused for 1.5A. Its not beyond the realm of reason, but I find it unlikely it is in the control circuit itself. For something to draw enough current to melt winding insulation on two different transformers without blowing the fuse doesn't sound right. You did use a new fuses on the new transformer correct?

Maybe look at the enclosure. Is there anything on the door that can make contact with transformer? Any wires hanging down in the panel? Does the transformer have adequate spacing to stay cool? Are the voltage transients on your power system. Are there any new large inductive devices in or near the enclosure?

MAKE SURE IF YOU COMBINE 110V CONTROL POWER FROM MULTIPLE SOURCES THAT THE TRANSFORMERS ARE CONNECTED TO THE SAME LEGS. IF ONE TAKES FROM LEG A & C AND ANOTHER FROM B & C DON'T HOOK THEM TOGETHER.

Sounds like the transformer was either wired incorrectly or something is shorted between the winding and fuse.

Also don't rule out the possibility of a bad wire. I had a ghost a couple of years back when I was still in school. I kept blowing fuses and couldn't figure out why. Until I realized that a wire had a hairline cut lengthwise, made by the box it was in. When you pull it out the cut closed and everything looked perfect, but when you shoved the switch and everything back in the insulation opened up and the wire made contact wit the metal box.
 
How many wires come out of the transformer on the primary side ?
Do you have a centre tap on the primary or the secondary ?
How many wires come out of the transformer on the secondary side ?
The resistance of the primary will be alot higher than the secondary resistance.

I am guessing that the primary has 4 wires with 2 joined together as a centre tap,
that was miswired by having the two windings opposing each other .

Magic smoke appears quite quickly under those cicumstances.

Give us a new diagram as your old one has in your own description too many errors.
 

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