Detecting insulation condition changes of inverter Fed AC machines based on two current sensors and indirect phase step excitation

A high number of breakdowns of adjustable speed drives (ASD) is caused by a fault of the machine, especially stator related insulation faults frequently occur. The evaluation of the insulation condition is possible with the current sensor signals only. The analyses of the transient part of the current response give evidence if a change in the insulation system occurred. The degradation of the insulation state is noticeable through a capacitive change, which influences the high frequency behavior of the machine and as a consequence the transient part of the current signal. The proposed method is capable to detect insulation degradation through analyzing the transient current response of the sensors already used for the control of the machine. Additionally, the profitable capital expenditure demands the design with minimal number of components. To reduce the costs only two current sensors are available. The non-measured transient phase current response can be reconstructed by a special excitation sequence and signal processing without significant deterioration of sensitivity.