Current-limiting transformer using capacitor

[FHR]

Hi!
I have a microwave oven transformer connected to the mains (240V 50Hz) with 4uF capacitor in series with phase/live wire.

The thing I don't understand is, why primary current with not-connected secondary wiring is 0.40A and when I short-circuit the secondary it actually lowers to something like 0.25A.

Does anyone have any clue why this happens?

[T3sl4co1l]

If you measure a few voltages and currents in the circuit, you should be able to come up with a reasonable value for the primary inductance.  If you draw up the phasor diagram of circuit impedances (complex values), you should see just what you're looking at.

Probably it's above resonance, so you'd get considerably more power with a larger capacitor.  Eventually, you'll apply so much voltage to the primary that it starts to saturate, pushing it back above resonance.  This is essentially how a ferroresonant transformer regulates voltage (or more precisely, flux == V/freq), although the circuit isn't quite the same.

A more linear example of resonance would start with a low voltage, maybe 6.3 or 12.6V, and running the primary in series with the capacitor.  Then you'll see a good bit of voltage and current multiplication.

As far as I know, the general operating curve is elliptical -- whereas a pure series resistance or inductance has a linear relationship (between O/C voltage and S/C amperage).

Tim

[FHR]

Didn't know this ferroresonant transformer existed. It kinda makes sense now. Thanks