Abstract: An apparatus for testing an electric motor is described. The motor comprises stator windings and a rotor arranged along a rotation axis. The apparatus is adapted to measure a physical quantity of the stator winding while the rotor is being rotated about the rotation axis, whereby periodic measuring data concerning the physical quantity are obtained. The apparatus is adapted to collect measuring data concerning at least two periods of the periodic measuring data, to compare the symmetry between at least the fundamentals of two or more half-cycles of the collected data and to generate a signal that indicates the symmetrical relation between at least the fundamentals of two or more half-cycles of the collected measuring data.