Discharge Bearing Currents in Small Inverter-Fed Induction Machines with Rated Power of 90 W

This paper focuses on parasitic bearing currents in small inverter-fed induction machines. Therefore we investigate five identical four-pole squirrel cage induction machines with rated power of 90 W each and rated rotor speed of 1360 min(exp -1). Both motor ball bearings, type 6000, are lubricated with mineral oil based grease, type Arcanol MULTI3. The axial iron core length is 55 mm and each motor has 24 stator slots. The three-phase round wire stator windings are star connected with rated line-to-line voltage of 400 V. The motors are connected in parallel to one two-level PWM voltage source inverter, type FC 302 from Company Danfoss. One of the five motors is specially prepared for bearing current measurements with an electrical insulation between outer bearing rings and bearing shield at drive end and non-drive end. With a variable three phase voltage for the input of the inverter up to 560 V we operate the inverter at three different dc-link voltages (low, normal and high) to measure the influence of different common-mode voltage values on discharge bearing currents and rotor-to-ground bearing currents. In short-term measurements the rotor speed is varied between 0 min-1 and 3000 min(exp -1) in steps of 150 min(exp -1) to measure bearing currents with and without rotor grounding at different lubrication film thicknesses. One long-term test with discharge bearing currents over 1000 hours at increased dc link voltage of 800 V is investigated.