Power conversion device and electric power steering device

1. A power conversion device, comprising:

an inverter, which includes a plurality of semiconductor switching elements, and is configured to invert a direct current (DC) voltage output from a DC power supply into three-phase alternating current (AC) voltages by the respective semiconductor switching elements being switched on and off, to output the three-phase AC voltages;

a capacitor provided in parallel between the DC power supply and the inverter; and

a controller configured to calculate voltage command values, which are command values for the three-phase AC voltages to be output from the inverter, based on an input control command value, and to output on/off signals for switching on and off the respective semiconductor switching elements of the inverter in accordance with the calculated voltage command values,

wherein the controller is configured to identify, among a plurality of voltage vectors that correspond to patterns of the on/off signals, a first voltage vector, a second voltage vector, and a third voltage vector that are most closest, second closest, and third closest to a current vector in phase, respectively, and generate the on/off signals based on the second voltage vector and the third voltage vector.

2. The power conversion device according to claim 1 ,

wherein the controller is configured to:

compare a carrier wave with each of the voltage command values, to thereby output the on/off signal;

employ a lower solid two-phase modulation as a voltage modulation method when the first voltage vector closest in phase to the current vector is a voltage vector formed by, out of the plurality of semiconductor switching elements, turning on two semiconductor switching elements on a high electric potential side and turning on one semiconductor switching element on a low electric potential side; and

employ an upper solid two-phase modulation as the voltage modulation method when the first voltage vector closest in phase to the current vector is a voltage vector formed by, out of the plurality of semiconductor switching elements, turning on one semiconductor switching element on the high electric potential side and turning on two semiconductor switching elements on the low electric potential side, and

wherein the controller is configured to:

when the lower solid two-phase modulation is employed, equally shift the voltage command values so that a minimum voltage command value out of the voltage command values matches a minimum value of a plurality of carrier waves, set one phase of remaining two phases other than a minimum phase in which the voltage command value is minimum out of three phases as a carrier wave inverted phase, and invert the carrier wave in the carrier wave inverted phase about a center value as a reference with respect to the carrier wave in another phase of the remaining two phases; and

when the upper solid two-phase modulation is employed, equally shift the voltage command values so that a maximum voltage command value out of the voltage command values matches a maximum value of the plurality of carrier waves, set one phase of remaining two phases other than a maximum phase in which the voltage command value is maximum out of the three phases as a carrier wave inverted phase, and invert the carrier wave in the carrier wave inverted phase about a center value as a reference with respect to the carrier wave in another phase of the remaining two phases.

3. The power conversion device according to claim 2 , wherein the controller is configured to:

set the carrier wave as a sawtooth wave;

switch the carrier wave inverted phase from the one phase to the another phase at a center of a lower solid two-phase modulation section, in which the lower solid two-phase modulation is employed; and

switch the carrier wave inverted phase from the one phase to the another phase at a center of an upper solid two-phase modulation section, in which the upper solid two-phase modulation is employed.

4. The power conversion device according to claim 1 , wherein the controller is configured to:

compare a carrier wave and each of the voltage command values, to thereby output the on/off signal; and

set a frequency of the carrier wave in a middle phase having a second largest voltage command value out of three phases so as to be twice as high as a frequency of the carrier waves in remaining two phases other than the middle phase.

5. The power conversion device according to claim 2 ,

wherein the control command value is a current command value of current to be supplied to a motor to which the three-phase AC voltages are to be applied, and

wherein the controller is configured to, when the current command value is equal to or smaller than a threshold value set in advance:

set phases of the plurality of carrier waves so as to be in phase in the remaining two phases other than the minimum phase in a lower solid two-phase modulation section, in which the lower solid two-phase modulation is employed; and

set phases of the plurality of carrier waves so as to be in phase in the remaining two phases other than the maximum phase in an upper solid two-phase modulation section, in which the upper solid two-phase modulation is employed.

6. The power conversion device according to claim 4 ,

wherein the control command value is a current command value of current to be supplied to a motor to which the three-phase AC voltages are to be applied, and

wherein when the current command value is equal to or smaller than a threshold value set in advance, the controller sets the frequency of the carrier wave in the middle phase so as to match the frequency of the carrier waves in the remaining two phases other than the middle phase.

7. The power conversion device according to claim 5 , wherein, in a case in which the current command value is set to a rated current value of current to be supplied to the motor, a maximum value of a current flowing through the capacitor when the carrier wave is inverted is set as a first maximum value and a maximum value of the current flowing through the capacitor when the plurality of carrier waves are in phase is set as a second maximum value, the threshold value is set based on a ratio of the first maximum value to the second maximum value and on the rated current value.

8. The power conversion device according to claim 1 ,

wherein the control command value is a control command value directed to a motor to which the three-phase AC voltages are to be applied,

wherein the motor is a permanent-magnet synchronous motor; and

wherein the controller is configured to obtain the phase of the current vector based on a rotation position of the permanent-magnet synchronous motor.

9. An electric power steering device, comprising:

the power conversion device of claim 1 , and further comprising a motor;

a torque detector configured to detect a steering torque of the electric power steering device; and

wherein the control command value controls the motor based on the detected steering torque.