Abstract: A controller having a first timer and a second timer for controlling a duration of a blowdown valve. A coincidence circuit including a second set of timers determines whether an operational overlap interval exists, and properly extends operation of the blowdown valve.

Abstract: An adjustable undervoltage trip fault interrupt circuit is used to prevent "nuisance" undervoltage fault trips caused by instantaneous sags in the AC power lines of less than a selectable time. The interrupt circuit includes a time delay circuit (IC4) responsive to a detected signal for generating a pulse signal having a selectable duration. A gating circuit (IC3) is responsive to the detected signal and the pulse signal for generating a fault trip signal only when the detected signal is longer than the selected duration of the pulse signal. In this manner, instantaneous line sags shorter than the selected duration of the pulse signal are eliminated from causing the "nuisance" interruption of a variable frequency drive controller.