Simulation and Analysis of Common Mode Voltage in 2-level and Multilevel Inverter Fed Induction Motor Drive with Long Cable
Sharana Reddy1, B.Basavarajat2
1Sharnareddy, Electrical and Electronics Engineering, Ballari Institute of Technology and Management, Bellary, Karnataka, (Andhra Pradesh), India.
2Dr. B.Basavaraja, Electrical and Electronics Engineering, Gitam University Hyderabad Section, (Andhra Pradesh), India.
Manuscript received on November 22, 2012. | Revised Manuscript received on December 04, 2012. | Manuscript published on December 30, 2012. | PP: 406-410 | Volume-2, Issue-2, December 2012.  | Retrieval Number: B0966112212 /2012©BEIESP

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Abstract: The development of high frequency, Pulse Width Modulation (PWM), based Adjustable Speed Drives (ASDs) has increased the energy efficiency, performance and controllability in the induction motor applications. But high speed switching device such as Insulated Gate Bipolar Transistors (IGBTs) used in ASDs having rise time of 0.1µSec.,that generate fast switching transients (high dv/dt) about 6000V/µSec for 400V system and common mode voltage. This common mode voltage causes unwanted shaft voltage and resulting bearing currents. Parasitic capacitive couplings create a path to discharge current in the rotor and bearings results in premature bearing failure. In many new and retrofit industrial applications the PWM inverters and motors must be at separate locations thus requiring long motor cable, which contributes over voltage at the motor terminal due to voltage reflection phenomenon. In 480V application, inverter output common mode dv/dt can be as high as 7000V/µsec. and at motor terminals in the presence of long cable (20ft) can reach11000V/µSec. Higher common mode dv/dt (nearly double) at the motor terminals results in higher induced shaft voltage and bearing currents. Multilevel inverter generates smaller Common-Mode (CM) voltage, thus reducing the stress in the motor bearings. In addition, using sophisticated modulation methods, common mode voltage can be eliminated. 
Keywords: Common mode voltage, induction motor drive, multilevel inverter, voltage reflection.