....

When the short is lifted, we get what appears to me as core saturation. I don't know why it would be going into saturation though.

Hi poynt99,

To answer this, we have to consider the AC impedances of the components and their combined resulting Z impedance as is known for a series RLC circuit. Luc used 25 uF capacitor, this has about 106 Ohm reactance at 60 Hz, this would (in itself) draw 249/106=2.34 Amper from his 249V mains.

But as per Luc's scope shots, the current draw from the 249V source was 3.27 A for the shorted secondary and 7.68 A for the open secondary MOT circuit (a 10x multiplier should be used for the CH2 scopeshots as Luc wrote).

It is clear that for the shorted secondary case, the primary coil of the MOT must have a lower X

_{L} inductive reactance than for the unshorted secondary case of course.

So in the shorted case, the 106 Ohm capacitive reactance was reduced only by a smaller amount of inductive reactance than in the unshorted case, thus the resulting total Z impedance of the series RLC network was higher in the shorted case than in the unshorted case, this is because the absolute value of the difference of (X

_{L}-X

_{C}) remains higher when X

_{L} becomes smaller and X

_{C} is constant.

For the open secondary case, the X

_{L} inductive reactance of the primary coil surely was higher than in the shorted case so that the difference of the (X

_{L}-X

_{C}) value becomes lower, this means the resulting Z impedance also becomes lower vs the shorted case: this means a higher current draw from the mains (I=V/Z), than in the shorted case. And a higher current draw inherently biases the MOT core more heavily than a smaller current, this is why we can see the sharp current peaks in the open secondary scopeshot, indicating the beginning of core saturation.

Putting all this otherwise: by connecting any coil with an inductive reactance between 0 and 106 Ohm (at 60 Hz) in series with a 106 Ohm capacitive reactance, then the resulting Z impedance of this series RLC would change like this: the closer the coil reactance approaches the 106 Ohm capacitive reactance, the lower the Z impedance becomes. When the coil reaches 106 Ohm reactance, then series resonance occurs at 60 Hz and the maximum current flows into the series circuit, limited mainly by the DC resistance of the coil and that of the AC source (and by the series load in Luc's circuit).

Gyula