Patent ID: 8390238

Claim:
A softstarter for starting and stopping an asynchronous motor having three phases, wherein only two of the three phases are controlled by the softstarter, the softstarter comprising two pairs 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 associated with said two pairs 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 for measuring voltages across the two pairs of semiconductor devices, and a first zero-crossing detecting unit configured for detecting zero-crossings of the measured voltages across the two pairs of semiconductor devices and providing zero-crossing signals to the DC reducing unit, and characterized in that the softstarter further comprises, a second voltage measuring unit for measuring input voltages in a first and second of the phases or between the first and second phases, a second zero-crossing detecting unit configured to detect zero-crossings of the input voltages between the first and second phases and providing zero-crossing signals to the DC reducing unit, the DC-reducing unit is configured to calculate phase delays between the zero-crossings of the measured/calculated input voltages between the first and second phases and the zero-crossings of the voltages across the two pairs of semiconductor devices, to calculate an average phase delay based on the two latest phase delays or a multiple of two of the latest phase delays, and to adjust firing angles of the semi conductor devices based on the zero-crossings of the measured/calculated input voltages between the first and second phases and the calculated average phase delay during start and stop of the motor, a current measuring unit for measuring currents in at least two of the phases, a voltage estimating unit configured to estimate an input voltage between a third phase and one of the other phases based on at least one of the measured input voltages measured by the second voltage measuring unit, a power computing unit configured to calculate an instantaneous power supplied to the motor based on at least one estimated input voltage and the measured currents, and a torque control unit including a torque computing unit configured to calculate an electromagnetic torque of said motor based on the calculated instantaneous power, and the torque control unit being adapted to control the motor torque during start and stop of the motor based on the calculated electromagnetic torque and a reference torque value for the electromagnetic torque.