Phase angle firing into a transformer/hot wire cutter sanity check

Discussion in 'General Electronics Chat' started by strantor, Jan 18, 2013.

  1. strantor

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    This thread is about an existing hot wire cutter that I need to duplicate and improve upon. The existing cutter appears to have been thrown together from random things so I have doubts about some of it.

    Here is the diagram that I made from tracing out the wires. I know I missed something because it shouldn't work as I've drawn it, assuming the transformers are isolated.

    [​IMG]

    The 2 resistors are lengths of steel wire ("cutters"), about 3" long.

    My first doubt is about the "adjuster" black box (pictured below). I don't know what it is exactly but I suspect that it's some kind of phase angle firing power controller. I can't recall ever seeing phase angle control used on the primary side of a transformer; Every instance I can think of, it has directly controlled the load. Is it OK to phase angle fire into the primary of a transformer?

    "adjuster":
    [​IMG]

    The back side of the adjuster has a large rheostat and simple-ish looking PCB.

    My second doubt is about the whole "high/low" series/parallel transformer switch. Is there any good reason to have it set up this way? As far as I could tell, the power to the hot wire was adjustable all the way to zero, even in the "high" (parallel) setting.

    Here are the transformers (both same):

    [​IMG]

    Here is the "hot wire cutter":
    [​IMG]

    Quick explanation of operation: The strands of nylon/polypropylene (whatever) on the right slide to the left along the bar in infinite succession, being in turn seared in two by the piece of wire that is passing through the bar. You can see the wire, suspended between the two bolts. The bolt on the far side is earthed.

    The only operational problem with this setup besides being fugly, is that the hot wire burns out frequently. There is no science behind the selection of this hot wire; it is just pulled from a scrap bin and cut to length. Also, bits of molten strands build up on the wire and prevent the following strands from actually contacting the hot wire. These following strands end up being pulled apart with no help from the hot wire (causes big problems), and sometimes end up breaking the hot wire.

    Quick explanation of my planned "improvements" (if they happen to actually be improvements): I want to use one transformer and power controller that doesn't look like a refrigerator thermostat. I want to substitute the "hot wire" for a "hot knife" like these. I want to add an amp meter.

    Thanks!
     
  2. strantor

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    Also if anybody knows where to get a 240V>2.6V .52A>26A transformer (or anything even close), let me know.

    Edit: actually a transformer with 60+ amps on the secondary could be better.
     
  3. thedoc8

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    Nov 28, 2012
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    What else is connected to the hi low switch...? Running high as written. Looks simple enough, I would see how well one of the knives work for a test. Amp gage would be a nice touch. Looks like if the sw is in low position both legs of the transformer connect to nothing.?
     
    Last edited: Jan 18, 2013
  4. strantor

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    One more question/curiosity. I noticed that my amp readings with the clamp meter maxed out at 52A, exactly the max spec of the 2 combined transformers. I saw noting in the system to limit current. Are these special current limiting transformers?
     
  5. strantor

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    They connect to eachother, putting them in series
     
  6. gerty

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    Closest thing I can think of for a xfmr with that low voltage and high current is a Soldering gun (not iron). We replaced the tip on a weller d550 240w soldering gun with a 10" long piece of #12 solid wire. We formed it into a big loop and used an amprobe to measure current.I don't remember the exact number but it was something like 120 amps. Disconnected wire and checked open circuit voltage, 1 volt.
    Weller makes a 100 watt gun and the current ought to b in line with your needs.
    Is this on all the time? Not sure about duty cycle of leaving a soldering gun on all the time.
    And of course there are other brands than Weller.
     
  7. strantor

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    Yes, continuous duty. soldering gun is a good tree to bark up I suppose. If I can find a high wattage continuous duty cycle soldering gun and toss it inside an enclosure with a button and a dial, I'm pretty sure that could work. My customer wants this to be as cheap as possible. Those hot knife blades I linked to are meant to go with a hot knife that costs $1800+ (X 2, one for each side). They don't want to pay that much. While you get what you pay for, I would like this to look & be as professional as possible, considering it's going to forever have my name on it.
     
  8. strantor

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    any idea how one might bring out 2 leads from this type of gun? it's the 300/550W continuous duty model
     
  9. BillB3857

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    That unit from post #8 looks like a standard iron with a pistol grip. The Weller guns are simply low voltage, high current transformers feeding a short circuit. The single loop secondary is VERY heavy as compared to the wire that makes up the tip, therefore the heat is generated at the higher resistance section. If you were to go with the Weller gun and extended the feed to your cutting wire, welding cable would be a good choice if any motion is involved.
     
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  10. shortbus

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    How about an arc welder for a power supply? http://www.harborfreight.com/welding/arc-welders/70-amp-arc-welder-68888.html This one even has a adjustable amperage output rather than changing "cable jacks".

    Don't pay that much attention to them, but CNCZone.com has a hot wire cutter forum that might give some ideas. Always thought that they used nichrome wire/ribbon for the heat blade.??
     
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  11. strantor

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    I thought about a welder, but it would need to be a pretty substantial welder to maintain 100% duty cycle at my desired amperage. Not sure why a welder almost seems less capable than a soldering gun.
    Thanks, I'll look into it.
     
  12. gerty

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    The welder has a secondary open circuit voltage of 42, way higher than he was looking for.
     
  13. strantor

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    Well I've found several transformers in the 3.3V-5V range with more than enough amps on the secondary, but I am reluctant to pick one because I cannot figure out how the current on the secondary is limited. As I noted about the current installation, When the dial is adjusted all the way, the current goes up to 52A (the exact rating of the transformers) but no more. I am not confident that the same thing will happen with one of these other transformers. Anybody know how the current setup might be limiting current? I've heard that some transformers can limit current, like the ones used for lighting and such, but those are HV, correct?
     
  14. strantor

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    Any ideas how the current might be limited? there are no torroids or intelligent controllers.
     
  15. THE_RB

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    It's sometimes done to phase angle control the mains power into the transformer primary.

    As a test you could just use a light dimmer or cheap "motor speed controller", I think harbour freight had a motor speed controller complete in a box for about $39. Your total power (and primary current) is quite low so even a hardware store lgiht dimmer might be fine.
     
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  16. BillB3857

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    I once had a welder that controlled current by moving an iron core within a very heavy inductor. The more iron in the core, the lower the current.
     
  17. thatoneguy

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    Transformer Shunting

    A magnetic method for limiting current, though I'm unsure how easily you could implement the idea.

    IIRC, there was a method involving a 3rd winding on the core, and the current through that winding would saturate the core/limit current through electromagnetic means, rather than changing the composition of the core. I can't remember what it was called, however, so my google fu is weak.
     
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  18. BillB3857

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  19. thatoneguy

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  20. strantor

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    I thank you very much for answering my question, but transitioning from theory to application, I don't see either of these feasible options.

    Current limiting transformers: I can only find in HV variety for tesla coils and neon signs. I might be able to have one special made, but that could entail months of searching for a transformer builder who knows how to do it.

    Saturable reactors: I cannot find at all, except for made-to-order.

    I also cannot locate the manufacturer of the currently installed transformers, which leads me to this:

    I've already got that harbor freight jobby - they call it a router speed controller I believe; That was one of the things I had in mind when I initially posted this thread. Since then, I've convinced myself that I need something a little more "intelligent." Something with current feedback and proportional control.

    Sure, I could use the harbor freight deal, and adjust it up and up until the amp draw is at the transformer max rating, and then draw a line on it and tell the operator not to exceed the marking, but I know that it would happen.

    I could potentially hack the "speed" pot with a resistor to change the span for a safer max, or I could get one of the phase angle SSRs with pot for phase angle power control and do it a little more legitimately. But there's a problem even with this solution. What if they put a different blade/hotwire in with a different resistance? say it's half the resistance of the blade that was installed when I calibrated the max - they're going to burn up the transformer and wiring.

    So I think I need a smart controller that monitors load current. Something I can set a max ( = transformer rating), and have adjustment from 0 to max. This has proven hard to find in this instance. Reason being, most controllers of this type are serious overkill, designed for big heaters & such. I would prefer to control the load on the secondary side, but being only 3V, I don't think that meets the min voltage spec of the SSRs. So I need to control it from the primary side, but now we are talking about controlling 0-2A with a device that's designed to control 0-1000A (typical) so I'm a little worried about the span and resolution.

    Does there not exist a scalable low power, current feedback proportional current controller? This is the closest thing I've found (25A), currently waiting on a quote for one.

    Alternatively, I'm considering a eurotherm 3208 heater controller. I've used these before and I know that they have a "manual power control" mode where, instead of monitoring temp with a thermocouple and maintaining setpoint with PID, you just set power level (0-100%) and it monitors load with a torroid. However, I'm not sure that was phase angle; it might have been bang/bang.

    any other ideas?
     
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