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Understanding three phase line EMI filters

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langwadt:

--- Quote from: T3sl4co1l on June 14, 2019, 07:37:26 pm ---Yeah, you wouldn't want to do that for a substantial load, but for a control, to get more reliable power, that would be okay.  Makes sense.

The problem is neutral currents, which ideally shouldn't be drawn.  It's usually a smaller conductor, to handle leftover unbalanced current, not full load.

Tim

--- End quote ---

? like everyone else around here I have 3 three-phase power, when I use a single phase load like a kettle only one phase and neutral is used so the phases and neutral must be roughly the same size

duak:
As long as there are 2 or 3 phases present, there won't be any neutral current.  I see the rectifier diodes switching to deliver the highest peak dfferential voltage at any instant.  If the load was a bipolar supply with the common center tied to neutral then there could be.  But here, the load shouldn't be referenced to neutral.  I looked at the app note, and it looks like the author used the ground symbol for circuit common but it doesn't mean it should be connected to neutral - that could be exciting!

I've worked with a few 3 phase systems, but they've always had a wye transformer source with the center connection to neutral.  There are apparently other PoCo connections, but I haven't seen them. eg., corner grounded delta (2nd image) and high-leg delta (3rd image).  I think the circuit above will work fine with these configurations.

The only configuration that would cause a problem would be an ungrounded source transformer.  Conceivably, there could be higher phase to neutral voltages than phase to phase voltage.  It would take a particular failure or wiring mistake but it's possible.

Here's a triplen current story:  I traced some down in a delta-wye step-down transformer operated as a step-up.   If memory serves, the line fuses were 150 A and would occasionally open under load.  There was some 50-60 A in the transformer's neutral, even when it was unloaded.  It was pretty damn hot too.  I disconnected the neutral and all became as it should be.  That poor transformer was probably conducting a good bunch of the triplen currents generated in the building.

coppercone2:
for understanding the mysterious inductor resistor parallel combo

http://ecee.colorado.edu/~rwe/papers/APEC99.pdf

The first page explains quite a bit

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