Inverter and overload protection method

Abstract

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.

Description

technical field [0001] The invention relates to a power conversion device for correctly judging an overload state and an overload protection method. Background technique [0002] In the power conversion device (inverter) that outputs AC power with variable voltage and variable frequency to control the speed of the AC motor, in order to protect the AC motor from heat due to overload, the thermal time limit characteristic of the electronic thermal relay is built in advance. [0003] The electronic thermal relay detects the current of the AC motor, judges the overload state of the AC motor according to the magnitude of the current value, and protects the motor through an electronically accumulated cumulative thermal time-limit characteristic. The accumulative thermal time-limit feature is an accumulative value for any time, such as 10 minutes, and clears the accumulative value if it exceeds 10 minutes. In addition, in the existing electronic thermal relay, there is only accumu...

AI Technical Summary

Problems solved by technology

[0004] Therefore, when the time interval between the first overload and the second overload in the above-mentioned AC motor is long (the above-mentioned 10 minutes), at the time of light load between the first overload and the second overload, although the above-mentioned AC The motor actually cools down due to heat dissipation, and the temperature drops, but the above-mentioned AC motor is judged to be overloaded and abnormal by using the accumulated (N1+N2) value, so the overload detection action will be advanced (over-protection), and the sudden stop of the motor will occur.

During the stop of the overheating protection, the operation of the equipment is also stopped, so it becomes the cause of a large damage to the production efficiency.

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