Patent ID: 8018747

Claim:
A pulse-width modulated (PWM) rectifier for converting a three-phase alternating current input into a direct current output, comprising: a semiconductor device configured to receive the three-phase alternating current input and to output the direct current output according to a set of PWM signals; a control unit configured to generate said set of PWM signals in accordance with a three-phase modulation scheme in which a first set of PWM voltage commands synchronized to said three-phase alternating current input is created based on a difference between an output voltage of said PWM rectifier and a target value thereof, said set of PWM signals is generated by comparing said first set of PWM voltage commands with a PWM carrier having a constant amplitude and constant frequency, or a modified two-phase modulation scheme in which said set of PWM signals is generated by comparing a second set of PWM voltage commands with said PWM carrier, the second set of PWM voltage commands being created by saturating one phase selected from among three phases constituting said first set of PWM voltage commands in said three-phase modulation scheme to a maximum or minimum value of said PWM carrier and by applying an increase or decrease, required to achieve said saturation, to the other two phases of said first set of PWM voltage commands; a detecting unit configured to detect at least one parameter selected from among an input current, output current, input power, and output power of said PWM rectifier or a temperature of said semiconductor device; and a modulation scheme switching unit configured to compare a detection value from said detecting unit with a predetermined threshold value and, if said detection value is larger than said threshold value, to switch the modulation scheme to be used in said control unit from said three-phase modulation scheme to said modified two-phase modulation scheme, said modulation scheme switching unit being configured to select said three-phase modulation scheme as the modulation scheme to be used in said control unit irrespective of the result of the comparison between said detection value and said threshold value if a PWM modulation percentage is equal to or larger than a predetermined value, where the PWM modulation percentage is a ratio of an amplitude of said first set of PWM voltage commands in said three-phase modulation scheme to the amplitude of said PWM carrier.