INVERTER AND OVERLOAD PROTECTION METHOD

摘要

Provided are an inverter.and an overload protection method having an accurate electronic thermal heat time limit characteristic in which actual temperature rise and drop of a motor in overload and in normal load are taken into consideration. The inverter includes a power rectifier that converts an alternate current into a direct current, a power inverter that converts the direct current into variable voltage variable frequency AC power, an AC motor that is speed-controlled with the AC power, a current detector that detects an electric current of the AC motor, and a control unit that controls each of the above units in accordance with the electric current detected by the current detector and is characterized in that the control unit has an adding/subtracting unit that electronically performs addition in overload operation of the AC motor and performs subtraction from an integrated value obtained by addition in the overload with a weighted value commensurate with the current squared times time of the electric current detected by the current detector in the overload when the AC motor is not in overload operation, and an electronic thermal heat time limit unit that stops the AC motor in overload operation on the basis of the integrated value in the adding/subtracting unit and a previously set electronic thermal heat time limit characteristic.