Fred, as far as I can tell, remembering I have no circuits of this AC servo drive, two of the three phases are brought into this board, rectified, and chopped, the chopper / oscillator using this transformer as a load (and possibly feedback on one winding). As you can see from its size its a high frequency transformer with I suspect a ferrite core rather than iron. I agree Meggering it is a good idea that I hadn't thought of as presumably the primary is at a few hundred volts.