Turn-off control means for an ac-to-dc electric power converter

Claim 1

Improved means for controlling an electric power converter used to supply direct current to at least one electric motor, said converter comprising the combination of a plurality of rectifying circuits each having a-c input terminals, a pair of d-c output terminals, and controllable means including at least one pair of alternately conducting main electric valves for interconnecting said input and output terminals so as to convert alternating voltage at said input terminals to direct voltage of variable average magnitude at said output terminals, the pair of main valves in a first one of said rectifying circuits having cyclically operative valve turn off means respectively associated therewith, means for respectively applying a plurality of in-phase alternating voltages from an a-c power source to corresponding input terminals of said rectifying circuits, means for serially connecting the output terminal pairs of said rectifying circuits to an electric load circuit including said motor, whereby the voltage applied to said load circuit is the sum of the output voltages of all of said rectifying circuits, means for providing speed and current feedback signals respectively representative of the speed of said motor and of the magnitude of current that said converter supplies thereto, means responsive to said feedback signals and to a given command signal for producing a control signal having a value that varies between predetermined first and second limits as a function of any error between actual and desired magnitudes of motor current, and turn on control means responsive to said control signal for generating a first set of firing signals capable of periodically turning on said main valves in said first rectifying circuit in synchronism with the alternating voltage applied to the input terminals thereof and for generating a second set of firing signals capable of periodically turning on the main valves in a second one of said rectifying circuits in synchronism with the alternating voltage applied to the input terminals thereof, the firing signals of said first set being nominally generated at an ignition angle (α 1 ) that varies from fully retarded to fully advanced as said control signal varies over a first predetermined range of values extending from said first limit to a preselected intermediate value of said control signal and the firing signals of said second set being nominally generated at an ignition angle (α 2 ) that varies from fully retarded to fully advanced as said control signal varies over a second predetermined range of values extending from approximately said preselected intermediate value to another control signal value outside of said first range, wherein the improvement comprises turn off control means responsive to said speed feedback signal and operative to control said valve turn off means so that normally said main valves in said first rectifying circuit are periodically turned off in synchronism with the alternating voltage applied to the input terminals thereof at a desired extinction angle (β) which varies as a function of motor speed.