I want to use a MOT (microwave oven transformer) for something it's not intended for. As a consequence, I need to limit the current so it doesn't melt itself. I am planning on connecting the ballast to the primary. The load on the secondary should be capacitive. I understand that a resistive ballast will limit current because V=I•R, so if V is constant, I will be relatively constant.

How does a reactive ballast limit current? Can I use a capacitor instead of the usual inductor? If I use both (in series) to correct the power factor, will that counteract the ballasting? Will connecting two MOTs, primaries in parallel, secondaries in series, change much?

  • \$\begingroup\$ Would this unintended use happen to involve big sparks? Because, that would be awesome. \$\endgroup\$ – Phil Frost Dec 19 '13 at 19:57
  • \$\begingroup\$ I'd like it to be part of a time machine or... maybe a tesla coil. \$\endgroup\$ – Andy aka Dec 19 '13 at 20:01
  • \$\begingroup\$ IIRC people use magnetic shunts -- here is a good website: wiki.4hv.org/index.php/Microwave_oven_transformer Alternatively can't you use a second MOT as an inductor ballast? \$\endgroup\$ – HL-SDK Dec 19 '13 at 20:14
  • \$\begingroup\$ I'd like it to be for a Tesla Coil, though from reading that article (@HL-SDK), that wouldn't be a good idea. I also have more capacitors than inductors. \$\endgroup\$ – Ethan Reesor Mar 18 '14 at 4:16

A reactive ballast limits current the same way a resistive ballast does. V=I*Reactance. Yes, you can use a series capacitor to limit current. Yes, if you cancel out the capacitive reactance with an inductive reactance, the net reactance will be zero, and zero reactance won't limit the current.

If you put two transformers in parallel/series as you described, it's the same as doubling the number of secondary turns. It is doubling the number of secondary turns. And it's the same as making the primary wires twice as thick. And doubling the area of the transformer core, so it can handle twice as much primary current


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