Simulation and experimental measurement of shaft voltage, bearing current in induction motor drive

Generally the Induction Motor (IM) is considered to be a constant speed motor. But due to the advances in Power electronics, the IM speed can be controlled within limits. For efficient operation, the 2-level inverter with Pulse Width Modulation (PWM) can be used. However, in this method the inverter output will not be sinusoidal. It will be a stepped square wave, due to that the voltage at the star point of the IM stator winding will not be zero, hence there exists a voltage with respect to the common ground. This is known as Common Mode Voltage (CMV). The CMV will induce a voltage in the rotor shaft; there by a current will flow to the common ground via the bearing. Due to the flow of current through the bearing the electrical discharge machining (EDM) will takes place at the inner race of the bearing. Hence the life of the bearing will get reduced. To minimize the effect of shaft voltage and bearing current a 3-level inverter can be used. The 3-level inverter output will produce less shaft voltage and bearing current when compared to 2-level inverter. In this paper the authors have done both the 2-level and 3-level inverter for running the IM. The Arduino Microcontroller is used for generating the Space Vector Modulation (SVM) signals and the necessary isolation has been done using opto-isolators. Simulation has been done using MATLAB/Simulink and the results are verified with the experimental results. The Fast Fourier Transform (FFT) has been done for the experimentally recorded shaft voltage and the bearing current for the 2-level and 3-level inverter outputs by using signal analysis software. The experimental and simulated results are compared.