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I have rewound a microwave oven transformer to produce a low voltage on the secondary. I'm not sure what the ampere rating of the transformer is but its about 4mm thick wire on the primary. I wound the transformer to be 93:3 turns, yielding about 3 volts on the secondary. If the input max is 20A, that gives me around 600A on the secondary. Still about 2200W. Trouble is, the secondary appears to have very little current running through it. Why is this?

Only reason I can think of is the inductance of the primary being high enough to limit the current to less than 20A or something. 93 turns, 80 mm loop diameter, 4mm wire diameter, and relative permeability of 4000 (transformer steel) yields 5.35 H, which is huge. I'll bet if I plugged that into the impedance formula of 2 x pi x f x H (60Hz) it would be a huge resistance. Yeah, its 2kOhm, which makes the current max on the primary 54 mA @ 110V I'm guessing? One thing I may have confused is the reactance for a air core coil vs an iron core coil.

Anyways question is, what's wrong with my transformer? Why isn't the secondary high current? What can I do to get high current? I realize I'm probably using the wrong wire size for 600A, yes.

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  • \$\begingroup\$ have you left the magnetic shunts in, or taken them out? Post a picture of it, that could help. How are you measuring the secondary current? 20A sounds very high, even for a MOT, where do you get that figure from? \$\endgroup\$
    – Neil_UK
    Commented Jul 12, 2018 at 18:52
  • \$\begingroup\$ Is that the bars of steel separating the coils? Theyre in. \$\endgroup\$
    – Shorin
    Commented Jul 12, 2018 at 18:52
  • \$\begingroup\$ I see how that would screw with the flux... \$\endgroup\$
    – Shorin
    Commented Jul 12, 2018 at 18:53
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    \$\begingroup\$ The magnetic shunts increase the leakage inductance, to tune out the series capacitor in the voltage doubler. They will limit the current severely, you need to remove them. \$\endgroup\$
    – Neil_UK
    Commented Jul 12, 2018 at 18:54
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    \$\begingroup\$ Russell's answer here is useful. \$\endgroup\$
    – jonk
    Commented Jul 12, 2018 at 19:00

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Primary inductance, if it's coupled completely to the load via the secondary, does not limit the current through the transformer, it limits the magnetising current.

Leakage inductance, enhanced in the case of a microwave oven transformer (MOT) by the magnetic shunts, decouples the primary and secondary coils, and limits the current.

In a MOT, the purpose of those shunts is to make the feed inductive, to resonate out some of the series impedance of the capacitor in the voltage doubler. This improves its efficiency.

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    \$\begingroup\$ As a follow-up, removing the shunts and re-winding the primary lead to higher current on the secondary! (19:3 turns primary:secondary) \$\endgroup\$
    – Shorin
    Commented Jul 16, 2018 at 14:34

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