Patent ID: 7868580

Claim:
A method for turning a three-phase AC motor on again, after it has been separated from a supply voltage, if a residual field voltage induced by a rotor residual field is present, comprising the following steps: recording time progressions of the residual field voltage and of the supply voltage; determining time points of zero crossings of the residual field voltage; determining angular velocity as well as a change in angular velocity of the residual field voltage from the time intervals between the zero crossings; calculating a phase difference between the residual field voltage and the supply voltage in advance from said time progressions; determining a time point t s , at which the phase difference goes below a predetermined maximum value Δφ max ; and triggering a switching command for turning the motor on again at a time interval that corresponds to a predetermined switching time delay τ s , before said time point t s is reached, so that the motor is connected to the supply voltage approximately at said time point t s .