Abstract: The inverter comprises a plurality of branch circuits connected across a DC source. Each branch circuit includes a pair of main controlled semiconductor switching elements, a diode and a smoothing reactor which are connected between the pair of main controlled semiconductor switching elements. A commutating circuit including a commutating capacitor, a commutating reactor and an auxiliary reverse conduction type controlled semiconductor switching element is connected in parallel with each main controlled semiconductor switching element. A pair of circulating diodes are provided for each branch circuit and connected between respective commutating capacitors thereof and the positive and negative sides of the DC source.

Abstract: In an inverter apparatus including a plurality of parallel branch circuits connected across a D.C. source, each including a pair of serially connected controllable switching elements, and a plurality of semiconductor elements connected in parallel opposition with respective switching elements, a D.C. reactor is connected on the D.C. side or A.C. side of the inverter and a circuit is connected across the D.C. reactor for preventing attenuation of the current flowing through the D.C. reactor during the OFF period of the switching elements. A source of control pulses is provided for rendering ON respective controllable switching elements each for a 120.degree. interval of the output alternating current. The source of control pulse includes means for chopping the current flowing through each controllable switching element at a predetermined modulation frequency for the former or latter 60.degree. of the 120.degree. interval.

Abstract: In a method and apparatus for controlling the speed of an induction motor energized from a source of variable frequency, e.g. a controllable inverter, a control signal which is a function of the primary current of the induction motor is produced, a predetermined slip frequency pattern signal is generated in accordance with the control signal, and the source is controlled in accordance with the slip frequency pattern signal for controlling the operating frequency. By suitably determining the shape of the slip frequency pattern signal it is possible to operate the induction motor with either one of normal induction motor characteristic, DC series motor characteristic, DC shunt motor characteristic, DC weak field motor characteristic and DC compound motor characteristic, and to operate the induction motor for smooth and continuous power regeneration.