Importance

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Abstract

An uninterruptible power system having a main power path between input and output terminals monitors the phase of the voltages at the input and output terminals while power is applied to the input terminals from an AC power source. The phase difference between the voltages at the input and output terminals is determined and, upon occurrence of a fault in the power supplied to the input terminals, an inverter is turned on to provide power to the load at a frequency which matches the frequency of the input voltage before the fault and at a phase which is the phase of the voltage at the input terminals before the fault corrected in accordance with the phase difference determined before the fault. Prior to transfer of the supply of power back to the main AC power source when the fault ends, the phase and frequency of the voltage at the input terminals is again measured. The frequency of the voltage from the inverter is adjusted to match that of the voltage at the input terminals, while the phase is adjusted to the phase of the input voltage corrected by a selected phase difference. The selected phase difference may be the last phase difference determined before occurrence of the fault, or a phase difference which is corrected based on the current drawn by the load where the phase shift between the input and output terminals in the uninterruptible power system changes as a function of load current.

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First Claim