The insulation system of electric machines is subject to a variety of stresses that have a decisive influence on its lifetime, especially at inverter operation [
7]. In addition to electrical stresses, mechanical and thermal loads, but also certain environmental influences can lead to a reduction in the insulation quality [
2]. In extreme cases, this can lead to shutdown of machines and equipment, resulting in costly production downtime. An insulation fault in the motor winding leads to severe overheating at the defect [
6,
8] which may show up as a discoloration of the insulating material or as a melted winding. The insulation fault spreads further across the winding and leads to destruction of the electric motor. In order to investigate the cause of failure of a stator, it is necessary to switch it off as early as possible in case of failure. Based on the defect [
1], the analysis can be carried out. The use of inverter-fed machines continues to increase due to rising requirements for system efficiency [
5]. With increasing efficiency higher power densities are targeted for more compact drive solutions [
9]. In novel power electronics, fast-switching semiconductors based on silicon carbide (SiC) are used, enabling increased switching frequencies and shorter voltage risetimes (above 70 kV
\(\upmu\text{s}^{-1}\)). To reduce the power dissipation of electric converters, electric motors are operated at increased voltage slope dU/dt. In addition, to increase charging power in electric cars, DC link voltages increases. However, these optimizations impose novel loads on the insulation system of electric motors [
7]. For the design of electrical machines, knowledge of the electrical stress and the resulting damage is of great interest. To be able to investigate the stress on the insulation system of electric motors in detail, lifetime tests have to be done. For this purpose, test rigs are required, enabling continuous condition monitoring of the stator winding insulation system. In the insulation lifetime tests carried out, the rotor is left out. This is due to the fact that the test effort can be significantly reduced, as no load machines are required to brake the tested stators. The stators are set to the test temperature by ohmic losses in the motor winding. As a result, the heat is generated directly at the insulation system. The inductance of the tested motor (external rotor motors with concentrated winding, see Sect.
2.1), changes only slightly by leaving out the rotor. Therefore the voltage transients will also behave similar to real operation. In real operation the induced voltage of the rotor in a windung with interturn fault (ITF) accelerates aging and leads to rapid failure. The target of the test is to determine the failure mode, therefore the acceleration of a failure is rather of secondary importance. The complete structure of the test is shown in Fig.
15.
The paper is structured as follows. Sect.
2 introduces the test specimen under consideration and the available detection methods. The is examined in more detail in Sect.
3 by creating an equivalent circuit diagram and an FEM simulation. In addition, the transient operation of the method is examined with of a equivalent circuit simulation and the results are compared with measurements. In Sect.
4, a test bench is presented in which the detection method was implemented and which is used for the development of the insulation system of inverter-fed electric machines. In Sect.
5 the content of the paper is resumed.