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I had the job of repairing some of these. They were made by Westinghouse in 1964 as part of a control system running an 6 stand rolling mill. The nice thing about them is you can have several inputs, all electrically isolated from each other (separate sense windings) and they worked all the way down to DC, which surprised the heck out of me.

The driver was a 10khz square wave, so the frequency response was DC to 1khz... fast enough for control loops.

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Down to DC? That should be a surprise, because it's certainly not true! Faraday's law of induction only applies to varying fields, which means AC or physical movement. Remember, it's perfectly normal and common to have a square wave with a DC component of zero!

I think he means the response frequency, not the frequency of the AC drive (which he states was 10 kHz).

Yeah, they’re almost definitely referring to the control signal. Since it’s controlling the saturation of the core, it’ll happily go down to DC just fine.

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