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04-05-2011, 03:45 PM #1
Harmonic filtering in VFDs. Input and output
Curious what is common in drives to buffer the harmonics both on the input and output side or if this is even common in drives unless specifically ordered? Will a simple 1:1 transformer do the trick?
04-05-2011, 05:42 PM #2
Output harmonics on a VFD are for the most part meaningless, the only thing it's going to is the motor and it doesn't really matter there. There are output filters that should be considered on some installations, but not harmonics filters. The output of the VFD can be protected from motor circuit shorts by having a reactor, and the motor can be protected from HV spikes resulting from the PWM output by using filters, but neither of those is a "harmonics" issue per se.
The input situation is where harmonics filters are used, but typically in response to a need to keep the Total Harmonic Distortion of an entire plant under control. So for example if your voltage THD is more than 5% a the "point of common coupling", i.e. the load terminals of your main circuit breaker where everything that could be contributing distortion is going to come together, then you may want to consider mitigating it. The least painful method in many cases is to mitigate it at the source point, i.e. the VFD that is contributing the most distortion.
A 1:1 transformer does virtually nothing for harmonics by the way. They used to recommend these types of transformers for DC drives and really old AC drives that used SCRs, but that was for noise isolation, not harmonics. Line noise isn't much of a problem any more though. A Line Reactor, which is sort of like a 1:1 transformer, slows down the rise time of the higher harmonic components and mitigates some of the total distortion, and often times that's enough. Sometimes it isn't though and you have to step it up a notch to a filter.