Vector-controlled unrestricted frequency changer (UFC) system and variable speed AC motor drive using such a system

Claim 1

In a UFC system including a plurality of static bilateral switches controlled for conduction to successively interconnect selected pairs of the input terminals of a three-phase AC voltage source with corresponding pairs of output terminals belonging to a three-phase load; said selected pairs being selected in accordance with a predetermined sequential order in each load phase and for a predetermined succession of said load phases; the combination of: means responsive to said AC voltage source for deriving a signal representative of actual electrical degree run by said AC voltage source in a cyclic succession from a zero angle reference; means for providing a signal representative of an output voltage vector demand angle for said AC load; comparator means responsive to said actual electrical degree representative signal and to said vector demand angle representative signal for providing an address signal representative of the phase shift between said vector demand angle and said electrical degree; ROM means for storing electrical angle values related to said zero angle reference as addressed values, and for storing in relation thereto predetermined switch combinations commands; said ROM means being responsive to said comparator means for selecting a combination command in accordance with said address signal; and means responsive to said ROM means for establishing with said switches a switch combination for conduction in accordance with said address signal and the corresponding switch combination command.