Comprehensive Protection Of Windings And Bearings For Asd Fed Motors

AC induction motors controlled by Adjustable Speed Drives (ASD) may be subject to power quality disturbances associated with Pulse Width Modulated (PWM) voltage which can adversely affect the electrical and mechanical life of the motor. PWM pulses can be reflected at the motor end of the cable and increase the peak voltage at the motor terminals. Excessive peak voltage due to voltage reflection, which is exacerbated by cable length and fast IGBT pulse rise times, can cause premature electrical failures in motor windings. Voltage rise, (dv/df) associated with IGBT pulse rise time can also develop shaft voltage that ultimately causes discharge currents to flow through the bearings, leading to premature bearing failure. While these problems can exist in any motor application, they tend to be more pronounced in submersible pumping due to cable length and the high capacitance associated with underground cables. This paper explains multiple effects of PWM voltage on motors and demonstrates a comprehensive approach for protecting both the windings and bearings of ASD fed motors.