PURPOSE: To exactly discriminate troubles, by ON/OFF-controling in the three modes of a zero voltage pattern mode, an output voltage pattern mode, and a normal V/F starting mode at the time of starting.

CONSTITUTION: A voltage type inverter is provided with a rectifying section 1, a main circuit smoothing condenser 2, the overcurrent detector 3 of inverter direct-current, and an inverter section 4, and the AC power drives a motor 5. The excess current detector is composed of an overcurrent detection circuit 6, a latch circuit 7, a one chip microcomputer 8, and the like, and at the time of the generation of troubles, an indicator 13 is lit. From a trouble discriminating circuit, the output of the output waveforms U1∼W1 of the microcomputer 8 is generated in order at the time of starting in the three modes of a zero voltage pattern mode, an output voltage pattern mode, and a normal V/F starting mode, and on a specified timing, a gate block GB1 is controlled, and the display of a display 17 is distinguished from. Then, the troubles of element damage, output short circuiting, overloading, or the like are distinguished from each other, and can be displayed.