Control method and motorstarter device

A softstarter device adapted for controlling an electric motor connected to the device, said softstarter device comprising:

• determining means for determining an electromagnetic motor torque of the motor,
• a difference calculator configured to calculate a difference between the determined electromagnetic motor torque and a reference torque value for said electromagnetic motor torque,
• an error calculator configured to calculate an error signal from the difference between said determined motor torque and the reference torque value,
• a regulator configured to control the motor torque, in dependence of said torque error signal, so that the motor torque has a rate of change with respect to time, during a stopping or starting time interval, that is adapted to follow a rate of change of the reference torque value, wherein the rate of change of the reference torque value varies between at least a first part of the time interval and a second part of the time interval, and thereby changing the speed of said motor, and
• a voltage measurer configured to measure the input voltage supplied to the motor, wherein the voltage measurer is arranged to measure the voltage at a point before the softstarter device and the motor, and wherein the determining means for determining the electromagnetic torque of the motor are arranged to use said input voltage.

A computer program product, comprising:

• a non-transitory computer readable medium;
• and
• computer program instructions recorded on the computer readable medium and executable by a processor for controlling a computer to carry out a method for controlling an electric motor with a softstarter device, the method comprising
• determining an electromagnetic motor torque of said motor in motion,
• calculating a difference between said determined electromagnetic motor torque and a reference torque value for said electromagnetic motor torque,
• calculating a torque error signal from the calculated difference between said determined motor torque and the reference torque value,
• controlling the motor torque in dependence of said torque error signal, so that the motor torque has a rate of change with respect to time, during a stopping or starting time interval, that is adapted to follow a rate of change of the reference torque value, wherein the rate of change of the reference torque value varies between at least a first part of the time interval and a second part of the time interval, thereby changing the speed of said motor,
• calculating the rate of change of the motor torque based on a measurement of an input voltage supplied to the motor, and
• taking the measurement of the input voltage supplied to the electric motor from an inline position before the softstarter device.