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 $\pmb{V}_{\mathbf{DC}}=\mathbf{560\ V}$ 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 $(\pmb{n} < \mathbf{900\ rpm})$ resistive bearing currents, at medium speed (900 rpm $< \pmb{n} < \mathbf{2250}$ 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 $\pmb{I}_{\mathbf{b},\mathbf{pk}-\mathbf{pk},\mathbf{NDE}}\approx 1$ A at non-drive end (NDE) and medium speed. With a Hertz'ian contact area $\pmb{A}_{\mathbf{Hertz}}=\mathbf{0.21\ mm}^{2}$ the apparent bearing current density $\pmb{J}_{\mathbf{b}}$ is 2.4 A/mm2• 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 $(\mathbf{0} < \pmb{n} < \mathbf{3000}$ 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 $\pmb{A}_{\mathbf{Hertz}}=\mathbf{0.04\ mm}^{2}$ the apparent bearing current density there is $\pmb{J}_{\mathbf{b}}=\mathbf{4.7\ A}/\mathbf{mm}^{2}$. This high value will also reduce significantly the resolver bearing life time.