Compensation of a.c. control systems subjected to carrier-frequency fluctuations

A simple tracking filter is described that enables a parallel-T notch filter to be used for stabilising a.c.-control systems in which the frequency of the supply is subject to large variations. It is shown that the null parallel-T network, with the notch sharpened by feedback, has an effect in an a.c.-control system very similar to the effect of a phase-advance network in a d.c.-control system. This analysis simplifies the design of a.c.-compensating networks. Experimental work is included.