Abstract: When an inverter is disconnected from a rotating motor due to power failure or motor accident, it is necessary to connect the inverter power again to the rotating motor by the inertia force in order to keep the motor rotating. To achieve the above reconnection, the inverter is controlled in accordance with open-loop control method, without use of any motor speed detecting means, thus improving control response speed. In reconnection, the inverter is started at a frequency higher than the rotating motor speed and at a voltage lower than the rated value; when a predetermined time has elapsed after the starting voltage was applied to the motor, only the frequency is decreased; when inverter driving current drops below a predetermined value, the frequency is held and only the voltage is increased until the voltage-to-frequency ratio reaches a predetermined value.

Abstract: When an inverter is disconnected from a rotating motor due to power failure or motor accident, it is necessary to connect the inverter power again to the motor rotating by the inertia force in order to keep the motor rotating. To achieve the above reconnection, the inverter is controlled in accordance with the open-loop control method, without use of any motor speed detecting means, thus improving control response speed. In reconnection, the inverter is started at a frequency higher than the rotating motor speed and at a voltage lower than the rated value; only the frequency is decreased gradually but increased gradually again when the detected motor driving current reaches zero or the motor regenerated current starts flowing; and the voltage is raised when the detected motor regenerated current reaches its maximum value.