Measuring Device

A method for off-line testing of an electric motor, which comprises stator windings and a rotor arranged along a rotation axis, comprising measuring a physical quantity of the stator winding while the rotor is being rotated about the rotation axis, wherein periodic measuring data concerning the physical quantity are obtained, characterised by;

• applying a periodical test signal to at least one of the stator windings;
• collecting first measuring data from one or more waveform periods of the test signal;
• detecting peak values of the collected first measuring data;
• forming second measuring data based on the detected peak values of the first measuring data;
• comparing the symmetry between at least the fundamentals of two or more half-cycles of the collected second measuring data;
• and
• generating a signal, which indicates the symmetrical relationship between at least the fundamentals of two or more half-cycles of the collected second measuring data.

An apparatus for off-line testing of an electric motor, which motor comprises stator windings and

• a rotor arranged along a rotation axis, the apparatus being 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, characterised in that the apparatus is adapted to;
• apply a periodical test signal to at least one of the stator windings,
• collect first measuring data from one or more waveform periods of the test signal,
• detect peak values of the collected first measuring data,
• form second measuring data based on the detected peak values of the first measuring data;
• compare the symmetry between at least the fundamentals of two or more half-cycles of the collected second measuring data,
• generate a signal, which indicates the symmetrical relationship between at least the fundamentals of two or more half-cycles of the collected second measuring data.