Inverter controller and method for operating the same

An inverter controller comprising:

• an AC motor as a load,
• a rectifying circuit for converting AC power from an AC power source into DC power,
• a smoothing capacitor for smoothing a DC voltage from the rectifying circuit,
• an inverter circuit for converting DC power supplied via the smoothing capacitor into a desired frequency,
• a current detection circuit for detecting an output current of the inverter circuit,
• a voltage detection circuit for detecting a terminal voltage (Vpn) of the smoothing capacitor,
• a voltage command calculation circuit for calculating a voltage command to the AC motor,
• a speed command calculation circuit for calculating a speed command to be used when it is determined that the AC power source is in a power failure state or when the terminal voltage (Vpn) reaches a certain value,
• wherein the speed command calculation circuit includes:
• a power calculation circuit for calculating a power output value (P) based on the output current and the voltage command, and
• a power target calculation circuit for calculating a power output target value (Pref) based on the terminal voltage (Vpn), a previously-set target voltage (Vpnc) of the terminal voltage (Vpn) for the power failure state, and a capacity (C) of the smoothing capacitor, and
• wherein the speed command calculation circuit calculates the speed command based on the power output target value (Pref) and the power output value (P).