A method for the transferring of 3 phase, asynchronous AC power between a present source supplying power to a load and an alternative source to supply power to the load. The maximum phase displacement is 60 electrical degrees at the load. The transfer can occur between sources which are rotating "forward" or "backward" with respect to each other or jointly with respect to the load. Either present source, alternate source or the load may be used as the timing reference for determining best phase connection of the alternate source to the load.
A method of and apparatus for actuating an automatic transfer switch for disconnecting a motor load from a normal source of voltage and reconnecting the load to an alternate source of voltage at a time when phase differences between the output voltage of the disconnected motor load and of the alternate source voltage are minimized to prevent excessive in-rush currents to the motor upon reconnection. Actuation of the transfer switch is commenced when the difference in phase between the normal source and alternate source is equal to a predetermined number of degrees depending upon whether the alternate source frequency is greater or less than the normal source frequency.
Dual feed power supply system provides high reliability of a dual utility feed, with minimal interruptions in power supplied to critical loads during switching, and compensation for voltage sags occurring on the primary power feed. AC input buses are connected through a transfer switching apparatus to phase lines of a distribution bus. The transfer switching apparatus has a first input terminal connected to one of the phase lines of the first input bus and a second input terminal connected to one of the phase lines of the second input bus, and an output terminal connected to one of the phase lines of the distribution bus. Input switches allow switching from one or the other of the AC input buses to the distribution bus, with a fast transfer switch used to interrupt the supply of power from the input buses to the distribution bus during switching of the input switches.
