An assessment of harmonic filter for indirect field oriented induction motor drives in mining industry

This paper presents a study of the technical feasibility of using harmonic filters with indirect field oriented control - iFOC - in medium voltage induction motor drives such as that in mining industry. The filter consists of a passive second order LC structure connected between a NPC three level converter and the machine. In general, the purpose of the filter is to attenuate the harmonic components generated by the converter in order to make possible to supply old insulation induction machines. Because of the poor insulation technology, these machines do not withstand high dv/dt as those generated by PWM converters. Despite its effectiveness in eliminating harmonic components, the filter introduces challenges to the implementation of the iFOC control because the motor and the converter currents are different. Practical implementation of the iFOC requires careful choice of the controlled variables and location of the current sensors. This work shows that no additional sensor besides that used in regular implementation is required. Only current and speed measurement is employed here, and no voltage measurement is required. Performance comparisons are performed for the system with and without the filter, also at low speeds by means of computer simulations. The results show the feasibility of the iFOC control even if the filter is included.