Patent ID: 8536810

Claim:
A control device for an alternating-current motor, in which an applied voltage is controlled by an inverter, the control device comprising: a pulse width modulation control unit for generating a control command for said inverter by performing pulse width modulation control based on a comparison between a voltage command signal of sinusoidal wave for operating said alternating-current motor in accordance with an operation command and a carrier wave signal, a mode-switching determining unit for indicating which one of an overmodulation mode and a sinusoidal wave modulation mode is used for said pulse width modulation control performed by said pulse width modulation control unit, in said overmodulation mode, a frequency of said carrier wave signal being controlled in accordance with rotation speed of said alternating-current motor such that the frequency of said carrier wave signal is an integral multiple of a rotational frequency of said alternating-current motor, in said sinusoidal wave modulation mode, the frequency of said carrier wave signal being set irrespective of the rotation speed of said alternating-current motor; and a determination value setting unit for variably setting a switching determination value for determining switching from said overmodulation mode to said sinusoidal wave modulation mode, based on a state of a power conversion operation performed by said inverter, when said pulse width modulation control is performed in accordance with said overmodulation mode, said mode-switching determining unit determining whether to switch from said overmodulation mode to said sinusoidal wave modulation mode, based on a comparison between a value concerned with said voltage command signal and said switching determination value.