Patent ID: 8810180

Claim:
A method for operating an electric motor, which has a primary part and a secondary part, the primary part having a polyphase field winding having winding phases, the phase terminals of said field winding each being connected to an output terminal of an output stage, the output stage having controllable semiconductor switches for applying phase voltages to the output terminals, said method comprising the following steps: a) introducing an operating phase by applying the phase voltages to the output terminals of the output stage in such a way that a traveling magnetic field is induced in the field winding which causes a relative movement between the primary part and the secondary part, b) disconnecting the phase voltage at least one of the output terminals in order to introduce a measurement phase, wherein once the phase voltage has been disconnected, the winding current in the winding phase for which the phase voltage has been disconnected is conducted and maintained via at least one freewheeling element having a nonlinear characteristic, c) measuring the electrical back-emf which is induced in the winding phase connected to this output terminal as a result of the relative movement between the primary and secondary parts in order to determine the angular difference between the phase angle of the field current and that of the back-emf, d) characterized in that, an edge in the winding voltage which occurs during the zero crossing of the winding current flowing in said winding phase is detected and acts as a trigger signal for the measurement of the induced back-emf of the relevant winding phase, in that once an acicular voltage peak has decayed, a voltage of the back-emf is measured, and in that an angular difference between the phase angle of the field current and the phase angle of the back-emf is determined from the measured voltage of the back-emf and previously known characteristic variables for the electric motor.