Inverter apparatus and method of reducing speed of alternating current motor

A method of reducing a speed of a motor of an inverter apparatus including:

• a converter portion for converting an alternating current voltage into a direct current voltage, a capacitor for smoothing the direct current voltage, an inverter portion for driving an alternating current motor by converting the direct current voltage into an alternating current, a current converting portion for generating a current detecting value, and a PWM calculating portion for generating a signal for driving a gate of a power element of the inverter portion;
• the method comprising:
• a step of detecting the direct current voltage;
• a step of calculating the current detecting value;
• a step of filtering the direct current voltage by a first filter portion;
• a step of filtering the direct current voltage by a second filter portion by a time constant the same as a time constant of the first filter portion in normal operation and by a time constant larger than the time constant of the first filter portion by a multiplication factor of 10 through 10000 in speed reducing;
• a step of calculating a frequency of driving the motor by a speed reducing rate set by a speed reducing rate setting portion for setting a speed reducing time period of the motor and an output of the first filter portion;
• and
• a step of correcting a voltage command from an output of the second filter portion and a given voltage command.