Inverter and overload protection method

Inverters have built-in electron thermal time limit characteristics with the aim of protecting against the overload of alternating-current motors, and there is a need for protection that takes into account the actual temperature of the motor. Provided are an inverter and overload protection method having accurate electron thermal energy time limit characteristics that take the actual increases and decreases in temperature of a motor into consideration during overload and under normal load. The power converter is provided with: a rectifier that converts an alternating current into a direct current; a power inverter that converts a direct current into alternating-current electric power with a variable voltage and variable frequency; an alternating-current motor, the speed of which is controlled by the alternating-current power; a current detector that detects the current of the alternating-current motor; and a control unit that controls each of the aforementioned parts according to the current detected by the current detector. The control unit comprises: an adder/subtracter that adds electronically when the alternating-current motor is operating in overload, and when the alternating-current motor is not operating in overload, subtracts a weighting commensurate with the square time product of the current detected by the current detector during overload, from the cumulative value obtained by the addition during overload; and an electronic thermal heat timing unit that stops the alternating-current motor in the event of an overload operation, based on the integrated value in the adder/subtracter and preset electron thermal heat time limit characteristics.