Electric power conversion device, motor control device, and electric power steering device

1 . An electric power conversion device comprising:

a polyphase inverter including series connections of an upper-side arm switching element and a lower-side arm switching element of a plurality of phases;

a voltage command value generation unit configured to generate a q-axis voltage command value and a d-axis voltage command value for driving the polyphase inverter;

a voltage command value limiting unit configured to limit the q-axis voltage command value and the d-axis voltage command value;

a voltage command value conversion unit configured to convert the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit to a polyphase voltage command value;

a switching control unit configured to drive, based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit, a first switching element, the first switching element being one switching element of the upper-side arm switching element and the lower-side arm switching element, and a second switching element, the second switching element being a switching element of the upper-side arm switching element and the lower-side arm switching element other than the first switching element, by PWM control; and

a current measurement unit configured to measure current flowing through the second switching element, based on a voltage drop across a resistance element connected in series to the second switching element,

wherein the switching control unit is configured to:

set a target duty ratio, the target duty ratio being a target value of a duty ratio of a period during which the first switching element is turned on to a PWM period, according to a target voltage to be applied to a load of the polyphase inverter;

when the target duty ratio of a first phase, the first phase being a phase having the highest target duty ratio among a plurality of phases of the polyphase inverter, is less than or equal to a predetermined first threshold value set to less than 100%, drive the first switching element of the first phase at the target duty ratio; and

when the target duty ratio of the first phase is greater than the first threshold value, drive the first switching element of the first phase at the duty ratio of 100% and also drive the first switching element of a second phase, the second phase being a phase of the plurality of phases other than the first phase, at a corrected duty ratio obtained by correcting to increase the target duty ratio of the second phase, and

the voltage command value limiting unit is configured to:

set a limiting gain, based on a ratio obtained by dividing a difference obtained by subtracting a dead time compensation value from a predetermined upper limit set to less than 100% by magnitude of a voltage vector of the q-axis voltage command value and the d-axis voltage command value; and

by multiplying the q-axis voltage command value and the d-axis voltage command value by the limiting gain, limit the q-axis voltage command value and the d-axis voltage command value in such a way that the corrected duty ratio does not exceed a predetermined second threshold value set to less than 100%.

2 . The electric power conversion device according to claim 1 , wherein the second threshold value is set to a value smaller than the first threshold value.

3 . The electric power conversion device according to claim 1 , wherein the second threshold value is set according to a stabilization time, the stabilization time being a time after switching of the second switching element is performed until a measured value of the voltage drop across the resistance element detected by the current measurement unit stabilizes.

4 . The electric power conversion device according to claim 3 , wherein the second threshold value is set in such a manner that length of a period during which a second switching element of the second phase is turned on is not less than the stabilization time.

5 . The electric power conversion device according to claim 1 comprising

a third harmonic compensation unit configured to superimpose a third harmonic component on the polyphase voltage command value,

wherein the voltage command value limiting unit is configured to set a limiting gain, based on a product obtained by multiplying the ratio, the ratio being obtained by dividing a difference obtained by subtracting a dead time compensation value from a predetermined upper limit set to less than 100% by magnitude of a voltage vector of the q-axis voltage command value and the d-axis voltage command value, by a coefficient, the coefficient matching a rate of decrease in amplitude of the polyphase voltage command value due to superimposition of the third harmonic component.

6 . The electric power conversion device according to claim 1 , wherein the voltage command value limiting unit is configured to limit the q-axis voltage command value and the d-axis voltage command value in such a way that the corrected duty ratio in a phase having the second highest target duty ratio among a plurality of phases of the polyphase inverter does not exceed the second threshold value.

7 . The electric power conversion device according to claim 1 , wherein the switching control unit is configured to set a sum obtained by adding a difference obtained by subtracting the target duty ratio of the first phase from 100% to the target duty ratio of the second phase, as the corrected duty ratio.

8 . A motor control device comprising:

the electric power conversion device according to claim 1 ; and

a controller configured to control the polyphase inverter driving an electric motor, based on a measurement result of the current measurement unit.

9 . An electric power steering device comprising:

the motor control device according to claim 8 ; and

a polyphase motor controlled by the motor control device,

wherein the electric power steering device provides a steering system of a vehicle with steering assist force by the polyphase motor.