Did I melt my transformer, and if so, why?

Discussion in 'General Electronics Chat' started by 20voltzener, Apr 17, 2013.

  1. 20voltzener

    Thread Starter New Member

    Dec 26, 2011
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    When you plug a power transformer into the wall and it starts smoking heavily in a few seconds, is that a bad sign?

    (that's sarcastic, just to catch your interest), (but it happened)

    This is not the first time I have "created smoke" in my electronics projects, and it probably won't be the last (anybody else?)

    BUT, I AM going to start putting a lot more fuses in a lot of places when experimenting with various circuits, even when they seem redundant. If I had done that, I would now have a dead 20 cent fuse, instead of a dead hundred dollar transformer.

    I decided to "improve" my 30 year old Sears battery charger. I was going to add a high quality analog voltmeter as well as an analog ammeter. If I had simply done that and stopped there, everything would have been fine. I understand the very simple procedures in doing that.

    But for reasons I don't fully re-call, I ended up "screwing around" beyond what I would have needed to do that.

    The alternator on my car had failed, and I didn't have enough money to get a new one put in for a few weeks, so I was charging the car battery every couple days, and driving only on small trips. That worked fine. I eventually got a new alternator installed, so that problem is solved.

    Unfortunately, one of my high talents is taking things apart when I don't know how to put them back together. Another of my high talents is deciding to muck about with things at the worst possible time, such as when I am relying on using it every two days to keep my car drivable.

    One thing I have learned, which might be a tip that somebody else can benefit from, is to take numerous high quality digital photo's of something BEFORE you take it apart. Drawing a diagram, even if it is not a 100% full schematic, could also be a good idea. Duh.

    My problem: I think I melted the transformer in the battery charger. But just how, I have no idea.

    I am hoping that somebody who really understands transformers can help me figure out what happened, and also determine if the transformer is now a boat anchor.

    Here are the details and my questions:

    1. My first question regards the primaries and secondaries of power transformers in general. When I went to electronics school (early 1980's), I learned about a transformer having a single primary winding, and one or more secondary windings.
    That a person designs the secondary winding(s) to get the output voltage(s) one desires. Has anybody heard of a transformer being designed in an "opposite" way? In other words, is it possible that my transformer has a single output winding, but that the current availability is controlled by having more than one primary winding, and switching various combinations of those primaries on or off?

    I have studied the wiring of the battery charger, and can't exactly figure out how it works.

    The input is a standard 120 volts AC. The output uses a 5 position selector switch which are labeled: Off, 6 volt charge, 12 volt low charge, 12 volt charge, 12 volt boost/12 volt engine start.

    Also, the front of the charger is labeled as having the following capabilities: 15 amp continuous charge, 30 amp boost (5 minutes on/ 20 minutes off), and 100 amp engine start.

    The built in ammeter is some kind of "non-linear" "inductive" device, where the output cable simply goes through a thin metal "clip" which holds it against the meter assembly itself. The first 60% of the meter goes from 0 to 15 amps or so, and then the last 40% is labeled "start". I have never been terribly impressed with the "accuracy" of this meter, although it does seem to work reasonably well in the 0 to 15 amp portion of it's range.

    When examining the transformer, I find two wires that are of heavier gauge than the others. I am assuming that these are the two "secondary" output wires, based simply on their gauge. There are 5 other wires that go into the transformer, one of which appears to be the black wire from the mains cord. The other 4 wires go through the multi position switch.

    ADDITIONALLY, there are 2 sets of "enameled" wires that come out of the transformer and appear to each go to diodes that are mounted to a large thin "plate", which I think are "heat sinks". These plates are mounted against the laminations of the trans, apparently with a thin insulating layer of something, similar to bakelite or similar.

    These 2 sets of 4 "diodes" are the only things I can find that resemble a rectification function. There are no "bridge" rectifiers as such. My level of knowledge is just slightly below being able to understand how things work in this charger.

    There is a multi-position switch. One input position and four output positions. The "input" of this switch appears to be the black "hot" lead from the mains cord. This switch controls 4 different winding wires that go into the transformer. I have disassembled the switch (AFTER the trans went up in smoke). It appears to me that it is of a "make before break" design. I am halfway through making a drawing that will indicate which of the four outputs are closed when the switch is in each of it's four positions. From examing the "heights" of the lands on this switch, and how they ride against each of four copper conductors, I am quite certain there is some "overlap" of more than one of the switch outputs being on at the same time. This is why I have a theory that there are several Primary windings which cumulatively "add" to the primary current at various switch positions.

    One of the things that I am trying to determine is if the output of the trans is simply a single pair of wires, and that the current control is done by energizing one or more windings on the input (primary) side.

    I've never heard of this, but it seems like it would work fine. Is anybody familiar with this?

    It does not appear that the switch is directly switching amongst various output taps. Now that I think about it, this theory seems to make more sense, because the switch is obviously not able to handle 30 or 60 amps. It is simply not heavy enough in design.

    The other main question I am trying to answer is why, after I had done a variety of work in the charger, including undoing wires, reconnecting them (one or more probably wrongly, I assume) etc, when I plugged the charger into the wall outlet WITH THE SWITCH OFF, the transformer started smoking fairly heavily within about 3 seconds?

    I am assuming that I reassembled it wrong, and that one or more wires was attached to "the wrong thing". Perhaps something was connected straight to Ground, that should not have been? I am thinking that the simple dc winding resistance plus some reactance from the core is the only thing that limited current?

    I had the output cables connected to a 5 ohm power resistor, rated at perhaps 10 or 20 watts. I pulled the cord out of the wall about 6 seconds after I plugged it in. 3 seconds of nothing happening, the next three seconds of smoking (which continued after pulling the cord from the wall, of course).

    The transformer itself is riveted to the chassis. I am planning to drill out the rivets so I can remove the trans and carefully inspect it. Using a small inspection mirror, it appears that something on the bottom (insulation?) is, ah, well, ..... melted.

    I have spent hours doing ohm measurements on all different combinations of the windings, and have come to no conclusions. All of the measurements are very small, no more than about 3 ohms. Some are in the .6 to .8 ohm region. I am using the Relative function on my Fluke 87 so that the resistance of the leads is eliminated, to try to get the best measurements I can.

    I am not sure if one or more of the windings became "shorted", either to another winding, or to the laminations. I am sort of "praying" that some of the wire insulation started melting, but that all the wires are still properly insulated. Long shot, probably.

    Wish I had a Variac. Maybe a "dim bulb tester" would be useful, when I get back to trying to work on this thing again?

    I know this is a long posting, and I'm not sure I'm explaining things very well. I am tentatively assuming that the transformer is ruined, but I am hoping it might not be. Maybe the smoke was just a "limited" amount of insulation burning, and that if I get things wired back up correctly, I might get it working again.

    New power transformers, capable of delivering 100 to 200 amps of output, even if just for 5 to 10 seconds (for starting a car), are extremely expensive. I will be trying to find various possibilities for finding something workable in the surplus world, maybe re-purposing a trans from some other original function.

    I would be happy, maybe even eager, to design and build a high current charger from scratch, the only problem is I don't have nearly the money to buy all the components. I would want to build it in a traditional way, using large filtering caps, heavy bridge rectifiers, 4 gauge output cables, etc.

    Thank You for any advice you can give me, and if there is any other information I can provide to help you guys figure this out, please let me know.

    Sincerely,
    Douglas
    Minneapolis MN
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
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    I'm sure you know that smoke is the key to all things electrical and electronic. The important thing to be aware of is that you must keep the smoke inside the component because, since it is what makes them all work, if you let it out it will quit working. Letting it out is NOT GOOD!
     
  3. 20voltzener

    Thread Starter New Member

    Dec 26, 2011
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    Thanks wmo, ..............I'll try to remember that. Must keep smoke inside, must keep smoke inside,......

    Yikes! this is the first post I ever made here, and it's only been up for like 8 hours, and 172 people have viewed it? Yowza. There's a lotta people here!
     
  4. #12

    Expert

    Nov 30, 2010
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    We're good, but almost nobody will read a 1000 word post by a person that might never come back and read the answer. Try to trim it down.
     
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  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I'm a little rusty on transformer theory, but here goes:
    When a transformer's secondary is open-circuited, the main thing limiting primary current is the primary (magnetizing) inductance. The reactance(X(L)) will be designed to limit primary current to a safe value when unloaded. This reactance will generally be much greater than copper resistance, so the current will be approximately I=V/X(L). The heat produced when the transformer is unloaded is the sum of I^2*R losses plus core losses.

    When you (apparently) applied line voltage to the secondary, which has fewer turns and therefore lower inductance, the reactance was too low to limit the current to a safe value, and the secondary winding I^2*R dissipation caused overheating.
     
  6. #12

    Expert

    Nov 30, 2010
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    What he said: You can't drive a secondary winding with more voltage than it was designed to output.
     
  7. wmodavis

    Well-Known Member

    Oct 23, 2010
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    I agree with #12 - It was way too long. In fact I only read the first little bit before responding with the smokin' theory of electronics.

    You're best off to clearly communicate the important facts that will help someone understand your situation without them having to ask 29 questions to find out what you really mean.

    Clear communication is one of the most important skills of a good tech/engineer but it seems to be sadly lacking these days. Just read enough posts here and you will see what I mean.
     
  8. 20voltzener

    Thread Starter New Member

    Dec 26, 2011
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    I came, I saw, I learned (hopefully).

    Just to let #12 know, I did come back. Thanks

    Yeah, I gotta work on my "communication skills". The post didn't start out to be so long.

    I'll be checking back to see if anybody else reads it or answers. For now, I'll just read through the forums, without posting questions.

    Many Thanks to Ron H. His answer was quite helpful. Knowing that the open circuit reactance would be "designed in" to limit current flow is something I had not thought of.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This reminds me that I once bought a Triad isolation transformer. It ran so hot with no load that I couldn't touch it. I contacted Triad and complained. I received a terse reply that they were designed that way.
    While researching magnetizing (primary) inductance, I found one discussion that pointed out that, the higher the primary reactance, the worse the regulation would be, which makes sense. My transformer has low reactance, which keeps the output impedance low, but causes the transformer to run hot. With a transformer, hot is relative, since it has no semiconductor components.
    Of course, it also saves on copper, which is expensive.:)
     
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  10. #12

    Expert

    Nov 30, 2010
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    So is line voltage. I have a lot of trouble here because the power company delivers 250 volts RMS. Their "spec" is 240V +/-5%, so they are within spec, but my 1970 clothes dryer will not keep an 1800 watt nichrome heater for more than 3 weeks, my HPS light bulbs die in 3 years (rated for 7 years) and transformers designed for 208/230..well you get the drift.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Man, that's a PITA.
     
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  12. #12

    Expert

    Nov 30, 2010
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    I changed the heater in the clothes dryer to a Calrod from a Jenn-Aire kitchen range, I used buck transformers on the Sodium lights, but The air conditioning store still sells transformers rated for 208/230.

    Some things I can fix, some I can't.
     
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