At the 6 Stand Cold reduction Mill of the US Steel, Gary Works, they needed a system to control the pairs of 9000 HP motors that drove the top and bottom roller

At the 6 Stand Cold reduction Mill of the US Steel, Gary Works, they needed a system to control the pairs of 9000 HP motors that drove the top and bottom rollers to stretch cold steel inches thick. Solid state electronics, and even tubes, weren't up to the task. Earl Pace was the Westinghouse engineer to figured out how to do it. He used a set of Synchronous AC motors to take the incoming 3 Phase, and drive a set of DC generators. The technology to control the field windings on the DC generators was just within reach. So he used the biggest SCR systems of the day to drive those. Driving the field windings allowed you to zero or even reverse the output of the generator. In effect, it was one of the largest magnetic amplifiers in history. Those generators then drove the DC motors on 6-stand. Some of the control circuitry used Magnetic amplifiers when they need to have multiple isolated inputs. The amazing thing is that they can pass DC signals to about 1 khz. (They had a 10 Khz drive signal) All of this from 1964. I repaired some of the control modules back in the 1980s, which is how I learned about it all.