Charging currents (three phase)

Before we start two things: I made a significant error about ripple current in the previous post, corrected now. Also, we have a lot less data for three phase operation, so this post is more speculative. If you think I made significant errors, please comment. Before going on, I assume you’ve read at least the post Charger Design.

Theory: Consider the following set of graphs, the first one representing the voltage between each of the phase lines (La, Lb, Lc) and Neutral (N).

The output voltage Vdc of a three-phase full-wave rectifier as shown in that post looks like the second graph. Side note: The 0 potential in the upper graph (the N line) is very much not on the same potential as the 0 in the middle graph, which represents the minus output of the rectifier. For a 400 volt line to line system, the average Vdc will be almost 540 volt, peaking 562 and bottoming 487.

Assuming for the moment a simple resistive load on that Vdc, the input current on the wire of phase A Ila will look like the third graph. So there we have it. It looks “somewhat sinusoidal”, but there will be quite a few harmonics in that current. But we can imagine this waveform a bit smoothed out by the filter contraption of the charger.

I have exactly one voltage-current graph of a ZOE charging in 3 phase operation. It is for a Q model on an 11 kW (3 x 16 A) charger, but the battery was already quite full, so it is running on a lower current.

Looks familiar? Some phase shift and smoothing by the filter and that’s it. Just like with the single phase charging, the phase shift is noticeable because of the relatively low charge current. The dips in the middle of the tops are quite noticeable. The current can’t be shaped like the the line voltage simply because in a three-phase full-bridge rectifier, 1/3rd of the time a wire simply can’t provide current. For the remaining 2/3rd, the charger seems to simply follow the voltage curve of the raw DC output, just like it does when running in single phase mode.

A good article about rectifying 3 phases can be found here.

Only slightly related: I received power intake graphs from both a Q210 and an R240 when throttling down. It seems the Q model steps down in roughly 600 Watt steps, while the R model uses smaller 300 Watt steps.

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