Abstract: The present invention relates to an inverter apparatus which controls a rotation speed of an electric motor by varying a frequency of voltage being applied to an induction electric motor. A pulse width modulating signal is generated by comparing a modulated wave signal with a triangular wave signal. The modulated wave signal is obtained by amplifying an error between a magnetic flux command signal having a frequency proportional to a frequency command from an outside source and an integral of a voltage signal being applied to a motor, the voltage signal being an output voltage of an inverter part. The triangular wave signal is a carrier frequency signal of a non-audible frequency, but even if the carrier frequency is made to be an audible frequency, distortion of the output voltage can be minimized. Therefore, the inverter apparatus can drive the motor with low noise.

Abstract: Disclosed is a variable DC voltage device wherein one terminal of a DC power supply, a switch, a first coil of a transformer, a second coil mounted on a core of the transformer with the same polarities with those of the first coil, a diode for preventing a shortcircuiting of the DC power supply when the switch is closed and the other terminal of the DC power supply are connected in series in the order named; a load is interconnected between the junction between the first and second coils and the other terminal of the power supply; and a compensation circuit is provided for restoring to an initial state the flux variation caused when the switch is closed, whereby the load is controlled by the alternate opening and closing of the switch.