Abstract: An electric power converter having an AC input and AC output, and comprising a power rectifier and a power inverter having a DC three-wire circuit, wherein means for detecting a current or a voltage is connected between the AC power source and ground. When a current or a voltage is detected, an alarm is produced or the apparatus is halted. This makes it possible to judge whether the power inverter is in a state capable of producing an excessively large potential.

Abstract: A sine wave input converter provided with a phase sequence computation unit which inputs two line voltages or two phase voltages of the three phase ac power supply, computes the phase of a sine wave voltage of one of these two inputs, and determines on the basis of computation using these two input voltages and the phase whether the phase sequence is positive or antiphase, which is determined depending on whether the result of computation assumes a constant value, or becomes a sine wave oscillating between positive and negative values. Power supply interruption can be determined from the same computation result.

Abstract: In a power converter apparatus having a three-phase AC power supply connecting unit, a positive side capacitor, a negative side capacitor, first-phase and third-phase converting circuits each including switching elements connected in series, and a common phase for connecting a neutral polarity to a second phase, there are provided a common phase converting circuit including two switching elements connected in series between positive and negative polarities, a change detector for detecting a temporal change of an electric current flowing through the common phase, and a duty controller for changing the on-off duty of the switching elements in the common phase converting circuit on the basis of the temporal change.