The AM Forum

a recent qso with W2CQH has added some information to the info available on microwave oven transformers (mot) .... a few system considerations are in order ... to make this seem cohesive will discuss 'old' stuff then new stuff

'old' stuff :  take an unloaded mot and test the primary current draw ... a good one does not draw much current until you reach 90 Volts or so .... the current draw then rises rapidly until the applied voltage approaches 125 to 130 Volts at which it may draw nearly full load power .... this behavior is due to the cost cutting measures applied at the factory in terms of reducing the amount of copper and steel used in the mot .... reducing this unwanted excess current draw is done in amateur radio practice by removing the magnetic shunts and filament winding and adding turns to the primary .... this alone will help somewhat (generally about 1/2) and does not seem to reduce the output voltage by much .... a further reduction in apparent current draw can be seen by paralleling a 10 Ufd oil cap with the primary winding and seems to do a power factor correction .... the mot seems to be tolerable now at least looking at the primary side ..... the secondary side is made with one end grounded to the frame and the other end well insulated for hV generation ... this is the classic half wave voltage doubler topology .... I missed this until talking with Reed when he pointed out that the magnetron is the second diode of the doubler .... up until this point I thought it was just a half wave rectifier .... this is important when core saturation effects are considered .... using the pair of mots in paralleled primary / full wave center tapped secondary topology is a misapplication of the transformer secondary from this point of view and would lead to an unloaded secondary on 1/2 of the waveform .... could get ugly

'new' stuff:  with these ideas in mind it would appear that a better way to use mots could be like this .... use 2 identical mots for the hV supply powered from 120 V as series primary / half wave doubler secondary, each half .... most mots are rated around 800 W and a pair applied in this manner could easily provide 800 W of power with no rewinding of the mot (magnetic shunts should still be removed) and 2.5 to 3 kVdc output

just some ideas for thought

If you mean to use each one with a half wave rectifier, then its CCS rating is probably less, like 60% of the full wave rating.

no, the half wave doubler keeps a load on the secondary for the full cycle as each diode conducts ... if only a single diode the secondary is unloaded for half cycle .... more current would be available but mind core saturation as well as these xfmrs tend to saturate easily

to use 2 mot as described and remembering that magnetic flux in one mot is not shared by the other then use the half wave doubler on each secondary and combine the outputs ....