A Modeling Technique to Analyze the Impact of Inverter Supply Voltage and Cable Length on Industrial Motor-Drives

This paper presents a precise modeling technique to evaluate the impact of high frequency overvoltage in long cable pulsewidth modulation drives. This modeling technique results from the need to correlate the inverter´s supply voltage and cable length with the motor´s dielectric insulation class. The mathematical formulas describing the transient voltage and current in the cable are developed in the frequency domain. The inverse Laplace transform of the voltage is then applied to obtain the time domain sets of equations for the computation of the overvoltage not only at the motor terminals, but at any point along the cable. The proposed technique is precise and very suitable to adjustable speed drive (ASD) systems. Contrary to existing methods, it does not require any representation of the ASD-system components. This paper includes simulations and experiments that were carried out on an industrial 5 kVA ASD prototype using a four-wire braided-shielded long cable. The measurements of the cable, inverter and motor required characteristics are investigated in detail. The simulation results are then compared with the obtained experimental waveforms so as to validate the modeling technique. At the end, some recommendations, aimed at protecting the motor, are formulated based upon the proposed technique.