A method of determining a corrected rotational speed signal for an electric motor, comprising determining a periodic rotational angle signal that has a periodic rotational angle error that is dependent on the rotational angle, determining a half period duration of an interference ripple of a raw rotational speed signal, averaging of at least one pair of sampled values of the raw rotational speed signal, converting a rotational angle signal to a digital raw rotational speed signal that includes an interference ripple, determining an average value in response to at least one pair of sampled values of the raw rotational speed signal, forming a correction value dependent on a sampling period duration and on a frequency of the interference ripple of the raw rotational speed signal for consideration of a discretization error, and forming the corrected rotational speed signal from a difference between the average value and the correction value.