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This paper evaluates a new induction motor, the electrostatic shielded induction motor (ESIM). An ESIM reduces rotor shaft voltages to levels below the bearing lubricant's electric field intensity breakdown level and offers one solution to accelerated bearing wear caused by fluting induced by pulsewidth modulated (PWM) inverters. The paper begins by reviewing shaft voltages and the resulting bearing currents when operated on PWM voltage source inverters. An example of bearing fluting is shown, and system models are presented and discussed. The construction details and test results for several ESIM's are presented. Experimental results show the ESIM solves the electrostatically induced rotor shaft voltage and bearing problem without degrading the electromagnetic performance of the motor. Index Terms - Bearing capacitance, bearing current, bearing current density, bearing voltage ratio, electric discharge machining, electrostatic shielded induction motor, rotor-shaft-to-ground voltage, stator-to-rotor capacitance.

Modern power inverters provide the industrial control industry with significant advantages. The faster switching devices have increased drive performance, but with some recently discovered disadvantages. One disadvantage, rotor shaft voltage and resulting bearing current, has become an industry concern. The oil film in a bearing acts as a capacitor and provides a charging mechanism for rotor shaft voltage buildup. Electrical breakdown of the film can damage the bearing. This paper examines the mechanical and electrical characteristics of the bearing and converts them into models. The mechanical model for the bearing contact area establishes an allowable bearing current density, which is used to estimate the electrical life of a mechanical bearing. The electrical model for the bearing provides a significant advancement, aids bearing design, and the analysis of electrically induced bearing damage. Finally, the paper presents quantitative results for one solution to the shaft voltage and bearing current problem. Index Terms - Bearing capacitance, bearing current, bearing current density, bearing threshold voltage, electric discharge machining, electrostatic shielded induction motor, rotor shaft-to-ground voltage, stator-to-rotor capacitance.