Electric Resonance Effects in Electric Machines – How to Measure Them?

Windings of electric machines contain several series and parallel resonances. With steadily increasing switching frequencies in drive applications, the probability of resonance excitation by converter harmonics increases. For rotating electric machines in resonance condition, abnormally high voltages causing insulation premature ageing can occur within the winding. To safely avoid such resonance conditions, the knowledge of resonance positions is necessary. Consequently, impedance curves have to at least be calculated or measured prior to commissioning. Knowledge of internal voltage gains and transfer functions are also highly desirable. This means that appropriate measuring devices have to be available and suitable measuring configurations defined. This paper deals with the verification of a vector network analyzer for the characterisation of a medium-sized traction motor. For typical saturation levels of the magnetic circuit it is additionally shown that the experimentally obtained transfer functions can be utilised for the recalculation of internal voltages and therefore determining of motor insulation stress.