If memory serves, Magnetic Amplifiers were used before the advent of power electronics to generate really big power supplies to shift large items with motors. The example we were shown was of the gun turrets on a WWII German battleship. Scharnhorst, I think?
Are you sure you're not thinking of an "amplidyne"?
This is an AMPLIfying DYNamo - a big piece of rotating machinery.
Basic idea:
- You have a BIG motor attached to a BIG specialized generator - which looks something like a wound-field DC generator. The motor provides all the output power as mechanical energy.
- The input signal drives the field magnetization of the generator. Very little power is required here: It's just a field winding with a bunch of turns of moderately fine wire. Magnetization is proportional to current. Power consumed is the resistive loss plus the momentary burst of energy input required to build the field (or it gives back such a burst if you're collapsing the field.)
- The spinning rotor generates an output voltage, proportional to the magnetization (and thus to the input current). The power all comes from the mechanical motion and the amount produced is limited only by the magnetization, the rotor, brush, and load wiring resistance, and the input horsepower, not by the input power. Output power can be enormous. Gain is set by the windings and the (typically constant) rotation speed.
- However: The wiring resistance of a winding needed to generate a field DOES put SOME limit on the amount of gain you can get from a single stage of amplification, given a low-energy input signal source. So (at least) one additional stage of amplification is desirable. There's a convenient hack:
- Instead of going to drive an output, the output of the FIRST set of brushes is directly connected to a second field coil, which produces a second field magnetization at 90 degrees to the one produced by the original input signal (or the multipole equivalent of a 90-degree field). The winding resistance produces a current directly proportional to the input voltage generated at the brushes. This additional field generates another voltage on the SAME ROTOR WINDING, which appears at 90 degrees from the first one (or the multipole equivalent) on the same commutator and is picked off by a SECOND pair of brushes.
Two stages of this sort of amplification can turn the tiny signal voltage you get from a pair of selsyns (another motor/generator/rotary transformer hack) you can twist with your fingers into hundreds of kilowatts, suitable for flinging the guns on battleships around. (If you really want more gain you can hang a baby-version preamplifier on the end of the shaft of the main machine and get two more stages - but it's not really necessary.) One dynamo for elevation, a second for yaw. Both can be powered by the same motor, all three pieces built into the same frame and sharing a shaft (and a cooling fan B-) ).
And if something makes the guns stick or the motor stall (like getting a shell through the gearing), feeding an error signal to the amplidyne that the stalled motor can't move to correct will cause a LOT of magic smoke to escape VERY quickly, from the amplidyne, the motor, and maybe the wiring between them. B-)
