CLAIMS

1. A power conversion device comprising:
a modulation-wave generation unit to generate a modulation wave;
a carrier-wave generation unit to generate a first carrier wave with a frequency higher than the modulation wave, and a second carrier wave with a frequency lower than the first carrier wave;
a switching-signal generation unit to generate a switching signal by comparing either the first carrier wave or the second carrier wave to the modulation wave; and
a power conversion unit to include a switching element that performs a switching operation based on the switching signal, wherein
the carrier-wave generation unit outputs the second carrier wave when a modulation factor of the power conversion unit is lower than a threshold value, and outputs the first carrier wave when the modulation factor is equal to or higher than the threshold value, and
when the modulation factor is equal to or higher than the threshold value, the power conversion unit operates in an overmodulation mode, in which the switching operation is stopped during a period longer than one cycle of the second carrier wave.
2. The power conversion device according to claim 1,
wherein the carrier-wave generation unit switches between
the first carrier wave and the second carrier wave in a
restart period when the power conversion device performs
restart in a frequency range higher than a frequency that
corresponds to the threshold value of the modulation factor
3. The power conversion device according to claim 1,

wherein the carrier-wave generation unit switches between the first carrier wave and the second carrier wave in a notch-off period when the power conversion device performs notch-off in a frequency range higher than a frequency that 5 corresponds to the threshold value of the modulation factor
4. The power conversion device according to any one of
claims 1 to 3, wherein the first carrier wave is
synchronized with the modulation wave, and the second
3 carrier wave is asynchronous to the modulation wave.
5. The power conversion device according to any one of
claims 1 to 3, wherein the first carrier wave and the
second carrier wave are both synchronized with the
) modulation wave.
6. The power conversion device according to any one of
claims 1 to 3, wherein the modulation-wave generation unit
generates a modulation wave whose fundamental wave
) corresponds to one cycle of the output-voltage command.
7. The power conversion device according to claim 6,
wherein the threshold value has a value of less than n/A.
> 8. The power conversion device according to any one of claims 1 to 3, wherein the modulation-wave generation unit generates a modulation wave that includes one cycle of the output-voltage command as a fundamental wave, where a 3n-th-order (n is a positive integer) harmonic of the fundamental wave is superimposed on the modulation wave.
9. The power conversion device according to claim 8, wherein the threshold value has a value of less than

10. The power conversion device according to claim 1,
wherein when a modulation factor of the output-voltage
command is lower than a threshold value, the second carrier
wave is output asynchronously, and when the modulation
factor is maintained in a state of being lower than a
threshold value, and when a frequency of the output-voltage
command is equal to or higher than a threshold value, the
second carrier wave is output in synchronization with the
output-voltage command.
11. The power conversion device according to claim 1,
wherein when a modulation factor of the output-voltage
command is lower than a threshold value, the second carrier
wave is output asynchronously, and although the modulation
factor becomes equal to or higher than a threshold value,
when a frequency of the output-voltage command is lower
than a threshold value, the first carrier wave is output
asynchronously.
12. The power conversion device according to claim 1,
wherein at a time of at least one of restart and notch off,
the power conversion device changes a mode to a mode in
which number of pulses in one cycle of a carrier wave is
fixed to a set value.
13. The power conversion device according to any one of
claims 1 to 3, wherein the switching element included in
the power conversion unit is formed of a wide bandgap
semiconductor.
14. The power conversion device according to claim 13,

wherein the wide bandgap semiconductor is a semiconductor using silicon carbide, a gallium nitride-based material, or diamond.
15. A vehicle drive system comprising:
the power conversion device according to any one of claims 1 to 3;
an input circuit to generate power to be input to the power conversion device; and
an electric motor to be driven by the power conversion device.