A motor drive system comprising: an ac motor; a multi-output dc power source including first, second and source terminals providing three output potentials (Vdc_a, Vdc_b, GND); a switching circuit including first, second and third switching devices (SW1, SW2, SW3) which are connected, respectively, with the first, second and third source terminals, and which are driven, respectively, by first, second and third PWM pulse signals (Vu_pwm1, Vu_pwm2, Vu_pwm3, Vv_pwm1, Vv_pwm2, Vv_pwm3, Vw_pwm1, Vw_pwm2, Vw_pwm3, A, B, C, D, E) to connect one of the source terminals with the motor to drive the motor with power of the multi-output dc power source; and a controller to produce the first, second and third PWM pulse signals, characterized in that the controller configured to produce the first PWM pulse signal by comparing a first desired modulation factor (mu_a_c*) with a first periodic carrier signal, to produce the second PWM pulse signal by comparing a second desired modulation factor (mu_b_c*) with a second periodic carrier signal, and to control a ratio between an amount of power (Pa) supplied from the first source terminal and an amount of power (Pb) supplied from the second source terminal, by varying the first desired modulation factor (mu_a_c*) and the second desired modulation factor (mu_b_c*); wherein the first and second periodic carrier signals alternate between an upper limit and a lower limit periodically so that a point at which the first periodic carrier signal reaches the upper limit is coincident with a point at which the second periodic carrier signal reaches the lower limit; wherein the controller is configured to determine the first and second desired modulation factors (mu_a_c*, mu_b_c*) in accordance with the first output potential (Vdc_a) and the second output potential (Vdc_b); and wherein the controller is configured to produce the third PWM pulse signal from the first and second PWM pulse signals. an ac motor; a multi-output dc power source including first, second and source terminals providing three output potentials (Vdc_a, Vdc_b, GND); a switching circuit including first, second and third switching devices (SW1, SW2, SW3) which are connected, respectively, with the first, second and third source terminals, and which are driven, respectively, by first, second and third PWM pulse signals (Vu_pwm1, Vu_pwm2, Vu_pwm3, Vv_pwm1, Vv_pwm2, Vv_pwm3, Vw_pwm1, Vw_pwm2, Vw_pwm3, A, B, C, D, E) to connect one of the source terminals with the motor to drive the motor with power of the multi-output dc power source; and a controller to produce the first, second and third PWM pulse signals, characterized in that the controller configured to produce the first PWM pulse signal by comparing a first desired modulation factor (mu_a_c*) with a first periodic carrier signal, to produce the second PWM pulse signal by comparing a second desired modulation factor (mu_b_c*) with a second periodic carrier signal, and to control a ratio between an amount of power (Pa) supplied from the first source terminal and an amount of power (Pb) supplied from the second source terminal, by varying the first desired modulation factor (mu_a_c*) and the second desired modulation factor (mu_b_c*); wherein the first and second periodic carrier signals alternate between an upper limit and a lower limit periodically so that a point at which the first periodic carrier signal reaches the upper limit is coincident with a point at which the second periodic carrier signal reaches the lower limit; wherein the controller is configured to determine the first and second desired modulation factors (mu_a_c*, mu_b_c*) in accordance with the first output potential (Vdc_a) and the second output potential (Vdc_b); and wherein the controller is configured to produce the third PWM pulse signal from the first and second PWM pulse signals.