Effects of inductance nonlinearities in a transformer-rectifier DC motor drive system on the AC side harmonic distortion using a time-stepping coupled finite element-circuit technique

The effects of the periodic nature of changes in the magnetic saturation in cores of transformers connected to rectifiers feeding DC motor type loads on performance of the AC side of the system are investigated. Performance issues pertaining to the AC side include harmonic contents of the AC current waveform and total harmonic distortion and their relationship to magnetic saturation effects in transformer cores, whose windings are connected to these rectifiers. The findings were obtained using the rigorous approach of time stepping coupled finite element-state space (TSCFE-SS) modeling and simulation technique, which couples the time-varying magnetic saturation conditions within cores of transformers to their external circuits. The results include TSCFE-SS simulation of the transformer-rectifier-DC-motor load and comparison to laboratory test results on a case study system. The system comprises a 1.5 kVA, 120/277 Volts transformer feeding a full-wave single-phase rectifier bridge which is supplying a 240 V, 7.75 A, 2-pole, separately excited DC motor. The simulation and test data confirm and quantify the significant impact of transformer time-variant core saturation on system currents and harmonic distortion.