Converters with an active front end (AFE)

Keywords: front end converters  semiconductor switches  rectifiers  generated harmonics 

In contrast to conventional converters, active front end (AFE) converters use semiconductor switches (such as IGBTs) as rectifiers instead of the diode bridges which are otherwise generally used. The switches can be turned on or off at any time. If they are controlled in an appropriate way, the amplitude of the generated harmonics is reduced and the current drawn by the converter is approximately sinusoidal.

Another advantage is that the DC link voltage can be varied up to the peak value of the line voltage. Some AFE converters can also feed energy back into the power supply network from the dc link circuit, for instance when a motor is being braked.

In this document, two different application examples of AFE converters from EPCOS are discussed. The first example shows a converter with a feed-in/return-feed unit that can support up to twelve drive modules with a total motor cable length of up to 500m. The common mode currents occurring in this case drive every conventional filter choke to saturation. To this must be added the extra stress caused by return-feed operation. We have elaborated several solutions in order to offer a suitable filter for such extreme applications. Some of these have already been patented while in other cases the patents are currently pending.

The second example was developed for a single AFE converter of the kind frequently used to drive elevators, for example. In this case, motor cables of up to 20m length are typically used. Nevertheless, the particular interference characteristics of this converter type must also be considered. In order to suppress the interference of this unit, a conventional ring core choke with a highly saturable core material was used and a sophisticated capacitor circuit was selected.

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