Analysis of the drag-cup A-C tachometer

DRAG-CUP a-c tachometers have very important uses as velocity feedback elements in computing machines, automatic pilots for planes and missiles, automatic fire-control mechanisms, and in other military or industrial control applications. The tachometer is essentially a 2-phase induction generator designed so that when one phase of the stator is energized from a constant-amplitude constant-frequency voltage source the other stator phase gives a voltage of amplitude closely proportional to the speed at which the cup is driven and a phase nearly fixed with respect to the supply voltage, over a substantial speed range.