Inverse-time underfrequency relay
Richard Gibson Hoft · Transactions of the American Institute of Electrical Engineers Part I Communication and Electronics · 1957
THIS PAPER describes a magnetic inverse-time underfrequency relay unit. It contains a relay which is operated when the frequency of an aircraft alternator drops below 380 cps (cycles per second). The time required for the relay to pick up is an inverse function of how far the frequency is below 380 cps. The complete unit contains a frequency-sensitive circuit which provides a voltage signal proportional to alternator frequency for driving a magnetic amplifier which has a very long time constant. The output of the magnetic amplifier is the coil of the underfrequency relay.