Two phase power control of a three-phase load circuit

Claim 1

Method for controlling power supply to a three-phase load circuit from a three-phase wire AC line by zero voltage switching using two switches, each connected in series with a corresponding phase wire, the third phase being connected direct to a corresponding pole of the load circuit, under steady state conditions, said switches being alternately closed for an operative series of cycles of voltage of said AC line, said method comprising the steps of: closing one of the switches at a time near a zero crossing of voltage across the one switch, to start an operative series of voltage cycles; said zero crossing of the voltage across the one switch being selected to occur so that the sign of the derivative of said voltage across the one switch, immediately before the closing of said one switch, alternates from one series of cycles to a following series of cycles; closing the other of the switches approximately 90° after the closing of the one switch, and at a time near a zero crossing of voltage across the other switch; allowing said two switches to open, for ending the outstanding operative series of cycles when the duration of said outstanding series of cycles is equal to a minimal duration of 3/4 period of the AC line voltage, increased by a whole number, including zero, of periods of said AC line voltage.