Patent ID: 7468593

Claim:
In a three phase power supply network wherein a main source of power and an auxiliary source of power are alternatively connected to a motor bus, a digital system for transferring power from said auxiliary source to said motor bus when the supply from said main source is interrupted, a method comprising: a. taking samples of said motor bus waveform at a selected rate of samples per cycle of an operating power frequency; b. utilizing data from said samples to calculate the instantaneous difference in voltage magnitude and phase angle between said motor bus waveform and auxiliary waveform; c. sensing the decaying frequency on said motor bus; d. increasing the number of samples per frequency cycle as the motor bus frequency decreases to obtain an increased number of data samples per cycle; e. calculating the projected closing point or actual transfer of voltage source dependent on the data obtainable from said total number of samples; and f. actuating said transfer relay to close at the closing point.