A SOFTSTARTER FOR CONTROLLING AN ASYNCHRONOUS THREE-PHASE MOTOR

摘要

The present invention relates to a softstarter (2) for starting and stopping an asynchronous motor (1) having three phases (L1, L2, L3), comprising two pairs (3A, 3C) of semiconductor devices of the type turning off at zero-crossing of the current therethrough, wherein each of said two pairs of semiconductor devices is connected in anti-parallel, and the first pair of the semiconductor devices is adapted to control the voltage of one of the phases of the motor and the second pair of the semiconductor devices being adapted to control the voltage of another of the phases of the motor, a DC reducing unit (16) associated with said two pairs (3A, 3C) of semiconductor devices for reducing a DC component in a load current of the said asynchronous three-phase motor during start of the motor, a first voltage measuring unit (8A, 8C) for measuring voltages across the two pairs of semiconductor devices, and a first zero-crossing detecting unit (14) configured for detecting zero-crossings of the measured voltages across the two pairs (3A, 3C) of semiconductor devices and providing zero-crossing signals (X1, X3) to the DC reducing unit.