Insulation monitoring of three phase inverter-fed ac machines based on two current sensors only

The demand of monitoring systems to prevent a breakdown of electrical machines is continuously increasing. Regarding economic issues, usage of system resources and additional components to establish monitoring is restricted. With the method proposed, the evaluation of the stator insulation condition of all three phases is possible using only the two current sensors already available in modern drive systems. The analyses of the transient part of the current response to voltage step excitation give evidence of a possible change in the insulation system. The degradation of the insulation health state is linked with a change in its capacitance, which in turn influences the electrical high frequency properties of the machine and as a consequence the transient part of the current signal resulting from a voltage step. To enable the monitoring of the phase with the missing sensor, a simple summation of all measured currents does not deliver enough accuracy for insulation monitoring. The non-measured phase however, can still be monitored by a special excitation sequence and signal processing without significant deterioration of sensitivity compared to the results if a sensor is available. Further, the influence of temperature and moisture on the proposed method is analyzed.