How would a net field "build up" when the winding is open-circuit for part of the cycle?
200 turns sounds like overkill. Again, probably 10 or 20% is all that's needed. I'm guessing the original primary is only about 300 to begin with.
Tim
Yes. thinking about it, the flux shoud reset during the off time. However about forty years back I saw a TRIAC dimmer circuit smoke a transformer. It was almost certainly a DIAC based firing circuit, which would only have generated one firing pulse per half cycle in normal operation, and I can only suppose that residual flux caused enough asymmetry during the first cycle that commutation was delayed till after the next half cycle's firing pulse, which as a result led to a rectifying action, and the TRIAC must have been really beefy compared to the primary's current rating, so it didn't simply fuse the TRIAC, restoring a normal full-wave supply to the transformer before it cooked off.
On the 200 turns isssue - yes that does sound excessive. Bsfeechannel dropped in that number as a result of some 'calculation'. It would make more sense to ramp up the supply with a Variac, carefully monitoring the primary current, preferably with an isolated current probe to let you see the waveform, to determine the voltage at onset of saturation, then check the voltage on a 10 turn temporary secondary winding to let you calculate the volts/turn and thus determine how many additional turns would be required to shift the onset of saturation to 10% above the nominal supply voltage.
@Davelectronic,
Yes splitting the core is a last resort, as it almost invariably introduces a slight air gap unless you can clamp it with enough pressure and re-weld it afterwards, or totally grind out the welds and rebuild it with interleaved E laminations from each end with the I laminations in the gaps between them either end, but that always seems to have issues getting the last few laminations back into the primary's coil former, so you end up leaving a couple out making the saturation problem even worse.
If you've still got the one you've split, it would be a good candidate for a core for the DC choke - ideally grind down the center leg to gap it, but if you don't mind a high external field, you could gap it with layers of varnished paper or thin card right across between the E and I.
It may be worth considering a separate buck transformer. e.g. to buck the 240V supply to a 1 KVA MOT by 24V, the buck transformer only has to handle the MOT primary current of a bit over 4A, so a relatively small 120VA transformer with its secondary in series with the supply to the MOT will do that with plenty of margin.