Comparison of a 1.1 kW-induction machine and a 1.5 kW-PMSM regarding common-mode bearing currents

An 1.1 kW-induction machine and an 1.5 kW-permanent magnet synchronous machine (PMSM) are investigated, measuring the common-mode bearing currents in dependence of rotor speed. Both machines are fed by an IGBT-voltage source inverter at \pmbVDC=560 V DC link voltage. The parasitic high frequency machine capacitances are analytically calculated. The stator winding-to-frame capacitances are also measured. The 1.1 kW-induction machine shows at low speed resistive bearing currents, at medium speed (900 rpm rpm) electrical discharge machining (EDM) bearing currents and at high speed above 2250 rpm capacitive dvldt bearing currents. The highest average peak-to-peak EDM bearing current is \pmbIb,pk-pk,NDE ≈ 1 A at non-drive end (NDE) and medium speed. With a Hertz'ian contact area \pmbAHertz=0.21 mm
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the apparent bearing current density \pmbJb is 2.4 A/mm
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Thus it will probably reduce significantly the bearing life time. The 1.5 kW-PMSM shows mainly EDM bearing currents over almost the complete speed range rprn ) with average peak-to-peak bearing currents of 0.1 A at drive end. The peak-to-peak bearing current of the resolver bearina is 300% higher. With a Hertz'ian contact area \pmbAHertz=0.04 mm
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the apparent bearing current density there is \pmbJb=4.7 A/mm
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. This high value will also reduce significantly the resolver bearing life time.