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I have this smallish VFD, for a 2.2kW motor, single phase. So it's smallish as VFD but as a single phase load is quite big; this is Europe so it's 230/50Hz land.

However, it need to pass EMC testing with a quite restrictive standard for current harmonics (EN12015; it's based on the EN 61000-3-12). In short, you sample the current, take the RMS value as a reference, do an FFT on the sample and look at the harmonic content (very shortly, it takes three different standards to explain the whole procedure!). For each harmonic you simply divide the RMS by the reference current (i.e. the resulting RMS) and you need to stay below the stated limit.

Now: I have these values (I don't have a dedicated power meter so the measurement conditions are not exactly as required but it's a good approximation). This is how the VFD works out of the box.

I3 (150Hz) ~55%
I5 (250Hz) ~40%
I7 (350Hz) ~20%

The current shape is typical of a double wave rectifier (as expected). By the way it's at about 50% load, at low load they get really bad.

The limits are:

I3 (150Hz) 35%
I5 (250Hz) 20%
I7 (350Hz) 13%

(and that's an epic fail, as expected); manufacturer says "put a line reactor and/or a DC link choke" (it's explicitly told in the manual that you need it for EMC, by the way).

Two beefy inductors later: scope image

I3 (150Hz) ~55%
I5 (250Hz) ~26%
I7 (350Hz) ~7%

I5 and I7 are getting better (actually with even bigger inductor they can be defeated). However I3 is substantially not affected. By the way the ringing only occurs with the line choke attached and only with that model of VFD; maybe the choke resonates with something in the input stage.

If it were a 3-phase system I could use an harmonic filter (beefy capacitors and even bigger inductors). Did that in the past, work as advertised. The fact that in 3-phase systems there is substantially no third harmonic of course helps, the filters act from the fifth, usually.

How do you tame the third harmonic from a single phase VFD? I read about the Ziogas filter (and I am studying the paper) but I never heard of it before; however it seems quite reasonable: DC link reactor, the line choke and a suitable capacitor to resonate out the third harmonic.

Any other idea? how do you handle these? (apart of a huge and expensive active compensator, I mean)

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  • \$\begingroup\$ What are the waveforms meant to be in the scope image? Has the VFD got a power factor correction stage built in? I believe it should have to be legal in Europe but, if I assume the upper waveform is the input current then it surely cannot have PFC. Please add a diagram showing where the inductor is connected and please give details of the inductor. \$\endgroup\$
    – Andy aka
    Commented Apr 3 at 11:25
  • \$\begingroup\$ The VFD has no PFC stage since they are subject to their own very liberal EMC directive. It's the machine builder (my) job to make it comply with the EU final product directive. Yes, the upper trace is the current after 2mH line reactor and 2mH DC link choke \$\endgroup\$ Commented Apr 3 at 11:50
  • \$\begingroup\$ What frequency range is the EMC standard applicable at? \$\endgroup\$
    – Lundin
    Commented Apr 3 at 13:29
  • \$\begingroup\$ IIRC the basic emission limits are the usual CISPR 11; my issue is with current harmonics however \$\endgroup\$ Commented Apr 3 at 17:48

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An option to reduce inductor size could be PFC with partial switching, but need many efforts that probably make no sense against common filters. See figure from Toshiba PFC circuits

An active PFC (commercial), with inrush limit, could supply directly DC bus, as normally control circuits are supplied from DC. You need also to disable monitoring of entry voltage. Again, a difficult option.

enter image description here

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  • \$\begingroup\$ You mean like when you make a shared DC link with modular inverters; I don't actually know if this unit could go in shared bus mode (maybe it can) but I doubt that a PFC corrected link feeder would be much cheaper than an external compensator. Also the problem isn't with inductor size, the issue is that even a bigger inductor (I tried) has almost no effect on the third harmonic \$\endgroup\$ Commented Apr 3 at 11:50
  • \$\begingroup\$ @Lorenzo Marcantonio: many single drives share hardware with multi-drive, they add rectifier, precharging and AC monitoring, so it could be done. As you say, not cheaper, unless you already have some feeder available in the installation. \$\endgroup\$
    – Bravale
    Commented Apr 3 at 11:59

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