Motor shaft voltages and bearing currents and their reduction in multi-level medium voltage PWM voltage source inverter drive applications

This paper presents test results on problems and solutions of motor shaft voltages and bearing currents in medium voltage PWM drive systems. Tests show that multi-level medium voltage PWM voltage source inverter drives can cause motor bearing currents, similar to a low voltage PWM drive, even with one motor bearing insulated. Common mode voltages generated as results of PWM switching are observed on motor windings and being capacitively coupled to the motor shaft, leading to bearing currents. Potential solutions, including altering common mode circuitry, changing grounding scheme, providing common mode filtering, and grounding motor shaft, are investigated. Test results on their effectiveness are presented