Modeling of the Nonlinear Characteristics of Voltage Source Inverters for Motor Self-Commissioning

In self-commissioning, motor parameters are estimated by analyzing the measured currents and voltages. At standstill, the obtained voltages are low because the motor back EMF is zero. Therefore, the inverter nonlinearities are comparable with the estimated voltages and it is necessary to compensate for the nonlinearities. In order to model the inverters nonlinear characteristics, dc current test is conventionally used. However, this test results in a nonlinear equation that contains nonlinearities of two phases. Hence, a numerical solution must be carried out to achieve each inverter phase nonlinearity. Moreover, three-phase symmetry of the inverter is necessary in conventional methods, making them inappropriate to characterize the nonlinearities of multilevel inverters with bypassed cells. This paper proposes a novel characterization algorithm with no numerical solutions applied for motor self-commissioning of VSI-fed drives. Not only the proposed method improves the fundamental and the THU of the calculated phase voltage by 42% and 6% at low currents, respectively, but also it is suitable for characterizing the nonlinearities of multilevel inverters with bypassed cells. A prototype of a five-level cascaded H-bridge inverter and a two-level inverter feeding a 2.2-kW threephase induction motor is provided to verify the superiority of the proposed method specifically at very low currents.