The ongoing development of drives for city buses is marked by an increasing hybridization and electrification. A high operation time and the utilization of new drive concepts using permanentmagnet synchronous machines (PMSM) require very durable power electronic and hence power semiconductor technology. First of all the article will initially explain current and future city bus drive concepts and then look at the resultant requirements on power electronics. Starting with state-of-theart drives it will be shown that future drive concepts using PMSM will result in new requirements for power electronics and especially on the load-cycling capability of IGBT. Strategies to fulfill these requirements will be discussed in the second part. The last chapter provides an overview of promising new packaging technologies for power semiconductors. The potential thermal cycling capability of these new technologies will be estimated. Finally the potential of SiC for the lifetime of electric drives will be briefly evaluated.
This paper presents an improved PWM strategy for three-phase inverters, especially suited for use at low ratios of switching frequency to fundamental output frequency. The method of third-harmonic injection is used to ensure maximum utilisation of the DC bus voltage. Implementation issues for the new modulation strategy are discussed. A comparison with a conventional, regularly sampled PWM is presented, based on simulation and experiment. The advantage of the new modulation strategy is evident in the frequency band from zero to half the switching frequency. While distortion below half the switching frequency exists in conventional PWM due to intermodulation, the new modulation strategy can completely avoid any distortion below half the switching frequency, even at low switching frequency. The minimum switching frequency is in the range of twelve times the output frequency, which is only four times the frequency of the injected third harmonic.
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