Doyle F. Busse

Bearing currents and their relationship to PWM drives

This paper examines AC motor shaft voltages and the resulting bearing currents when operated under pulse width modulation (PWM) voltage source inverters. The paper reviews the mechanical and electrical characteristics of the bearings and motor in relation to shaft voltages and bearing currents. A brief review of previous work is addressed, including the system model and experimental results. The theory of electric discharge machining (EDM) is presented, including component calculations of the system elements. The effect of system elements on shaft voltages and bearing currents are evaluated experimentally and the results compared to theory. A design calculation is proposed that provides the relative potential for EDM. Finally, the paper presents quantitative results on one solution to the shaft voltage and bearing current problem.

System electrical parameters and their effects on bearing currents

This paper examines AC motor shaft voltages and resulting bearing currents when operated under pulse width modulation (PWM) voltage source inverters. The paper reviews the electrical characteristics of bearings and motors that cause shaft voltages and bearing currents. A brief review of previous work is presented, including a system model for electrical analysis of bearing currents. Relying on the work of a companion paper, the propensity for electric discharge machining (EDM) is determined by a design equation that is a function of system components. Pertinent machine parameters and their formulas are presented and values calculated for machines from 5 to 1000 HP. The effects of system elements on shaft voltages and bearing currents are evaluated experimentally and the results compared to theory. Finally, the paper presents quantitative results for one solution to the shaft voltage and bearing current problem.

The effects of PWM voltage source inverters on the mechanical performance of rolling bearings

Modern power inverters provide the industrial control industry with significant advantages. The faster switching devices have increased drive performance, but with some 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.

An evaluation of the electrostatic shielded induction motor: a solution for rotor shaft voltage buildup and bearing current

This paper evaluates a new induction motor, the electrostatic shielded induction motor (ESIM). An ESIM reduces rotor shaft voltages to levels below the bearing lubricants electric field intensity breakdown level and offers one solution to accelerated bearing wear caused by fluting induced by pulse width 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 ESIMs 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.

Characteristics of shaft voltage and bearing currents

This article reviews investigations into reduced bearing life due to voltage source adjustable speed drive (ASD) AC motor operation. Relevant bearing failure mechanisms and indicators are discussed. dv/dt and electric discharge machining (EDM) contributions are discussed and experimental data presented showing the voltage levels on motor shafts when operating with ASDs. Finally, techniques to reduce shaft voltage are discussed, along with the electrical characteristics and interaction of system components. The example chosen is electrostatic shielded induction motors.

Mitigation of the effects of common-mode current on the operation of SCR-based rectifiers for AC drives

Common-mode current generated by the PWM AC drive at low modulation index and higher switching frequency results in overheating of the pre-charge resistor of the SCR based rectifier. In this paper, two modifications are proposed for the operation of SCR based rectifiers to significantly reduce the resistor overheating issue. The solution is cost and size effective when compared with alternative solutions. Experimental results are provided to show the reduction of common mode current flow through the pre-charge resistor and the consequent reduction in its temperature.

Mitigation of the Effects of Common-Mode Current on the Operation of SCR-Based Rectifiers for AC Drives

Common-mode current generated by the pulse-width modulated (PWM) ac drive at low modulation index and higher switching frequency results in overheating of the precharge resistor of the silicon-controlled rectifier (SCR)-based rectifier. In this paper, two modifications are proposed for the operation of SCR-based rectifiers to significantly reduce the resistor overheating issue. The solution is cost and size effective when compared with alternative solutions. Experimental results are provided to show the reduction of common-mode current flow through the precharge resistor and the consequent reduction in its temperature.