How to Wire Plasma Cutter Isolation transformer?

Discussion in 'The Projects Forum' started by intellcity, May 19, 2009.

  1. intellcity

    Thread Starter New Member

    May 19, 2009
    I am building a power supply for a Plasma cutter. Since the design results in high DC voltage on both the torch and work clamp I want to provide an isolation transformer so I can ground the work clamp for operator safety.

    I am starting with a 3-coil 3-phase transformer and rewiring it to 3 single phase coils to feed 3 bridge rectifiers. The coils are mounted on an iron core shaped like 2 "E"s joined together with a coil on each leg of the "E". One "E" faces up and the other "E" faces down forming a solid rectangle with two rectangular holes in it.

    Should the center coil have the same magnetic polarity as the two end coils or should it be reversed? In other words should the center coil have the North pole up at the same time as the two end coils or South when the two end poles are North?
    (^)(^)(^) or (^)(v)(^) ? (^)=North up (v)=North down
  2. DonQ

    Active Member

    May 6, 2009
    Been awhile since I've done this, but I think that I remember that the leads were marked something like X and X'. That would tell you the polarity.

    Also, I can't think of a reason for the center winding (actually 2 windings, primary and secondary in each of the 3 coils) to be any different from the other 2. Since they don't have to be different, it would be wasteful to make them so. The are almost certainly wound on the same machine.

    It sound's like this may not be "new in crate", so the markings my not be on the connection block. Since the windings are identical, you should be able to tell by the location of the wires exiting from the coil which is which.

    Going to have fuses on it when you test it, right? Shouldn't be a problem, but you wouldn't want your neighbors to have to reset their clocks.
  3. intellcity

    Thread Starter New Member

    May 19, 2009
    Yes, each coil has two windings: primary and secondary.
    And Yes – I am using circuit breakers, it is a 40 KVA transformer so melting wires is a possibility. I am trying to avoid this, as well as heating and coupling losses.

    I can identify all of the leads for all (3X2=6) windings. The windings are all electrically isolated. However the coils are magnetically coupled since there is an iron bar across the top and another across the bottom joining all three iron cores. (Actually these are “E” shaped laminations interleaved right and left.)

    I am going to connect the three primary windings to 240VAC. If I connect L1 to X1 and L2 to X2 for the primary windings for all 3 coils this would make the top of each coil magnetically North at the same time (and then reversed to South 60 times a second.) If I wire the center coil primary winding reversed, that is: L1 to X2 and L2 to X1 This would make the top of the coil South when the other two are North.

    So back to my question: should the coils all be magnetically aligned or should the center coil be magnetically opposite the two end coils ?