The invention relates to a device and a method for estimating the rotational speed ω of an electric motor when freewheeling, the method being carried out in a variable speed drive for controlling the motor by generating reference alternating voltages Va, Vb. The method comprises a step for measuring the measured flux and torque currents Id and Iq of the motor in an orthogonal two-phase marker d, q, a step for determining said reference voltages Va, Vb, by carrying out a current regulation on the basis of said measured currents, and on the basis of zero reference currents Idref and Iqref in said marker d, q, and a step for calculating the rotational speed ω by evaluating the angle of rotation of the voltage vector, the components of which are said voltages Va, Vb in a fixed orthogonal two-phase marker a, b.