Experimental Assessment of the Non-Characteristic Harmonics Generated by Adjustable Speed Drives

Abstract Adjustable speed drives (ASD) are widely used in industry. A typical voltage-source ASD is comprised of a three-phase rectifier, a low-pass filter and an inverter stage, as shown in Fig. 1. This figure also shows, from left to right, the typical current waveforms at the a.c. input, the d.c. link, and the output. An important issue in this kind of apparatus is the harmonic content in the current drawn from the a.c. line, since the maximum amplitudes that can be tolerated are tightly regulated by standards. It is a common practice to analyse the harmonic generation considering isolated stages but, due to the complexity of the expression involved, the analysis is performed under several simplifying assumptions. Quite often, however, the analysis fails to predict accurately the harmonic content obtained in a particular application. Especially troublesome are the so-called non-characteristic harmonics (that is, harmonics whose frequency is not an integer multiple of the mains frequency). For example, harmonics related to the output waveform that propagate through the filter, to the mains. This paper is aimed at the experimental investigation of the non-characteristic harmonics generated by a typical voltage-fed ASD. The investigation is performed taking into account parameters such as the modulation index and the low-pass filter cutoff frequency. Three different cases are considered: a single ASD, two inverters sharing the same d.c. bus, and two ASD’s connected to the grid at the same point. The test performed show that, although the generation of non-characteristic harmonics does occur, in most cases the amplitudes involved are small. An exception to this occurs when the low-pass filter has not been properly designed, or the supply is unbalanced.