Electronics > Projects, Designs, and Technical Stuff
Understanding three phase line EMI filters
XaviPacheco:
Hi,
I'm reviewing the schematic attached (EMI1) which I found inside an application note on a power supply design. I want to understand how to select the proper filter design for the three phase line. In the schematic, they have used a resistor and inductor in parallel with a set of capacitors in star connection. What kind of filter is this? Why do they put a resistor in series with the varistor? In other designs, I have seen just an inductor in series with each line like shown in the attachment Power1 where they are using individuals 680uH inductors at each line. How do I decide the proper filter?
MosherIV:
Hi. I think the resistors across the inductors are to prevent a spark when you unplug the equipment, gives a path for back emf from the filter inductors. They should have no (little) effect when in circuit, too hugh resistance.
I think the 10 \$\Omega\$ in line with the MOVs is to stop them blowing up when there is a massive pulse. The extra resistance should reduce the energy enough the the MOV will not physically split apart.
That is my guess ;)
T3sl4co1l:
Resistance in series with a MOV defeats its purpose. I would hope those aren't actually 10 ohm resistors, but placeholders for something like a thermal fuse, which disconnects the MOV if it fails (overheats due to excessive leakage current).
The 10ks are probably just for tweaking the response. A CMC has a parallel resonant peak, below which it is inductive, and above which it is capacitive. Resistance in parallel reduces this peak, but also broadens it, and maybe that's useful in combination with whatever capacitances or other parasitics are in that particular circuit.
Large value resistors don't do anything for ESD or surge.
Note that the inductors are coupled, so that mains current is balanced between lines and the core can be a high-mu material. Individual inductors don't have this advantage, and must be rated for saturation > full peak load current, otherwise they stop being effective and some noise gets let through.
The neutral rectification is bizarre, or at least very unusual. I can think of reasons to do it, but it shouldn't be done for most three phase equipment.
Tim
MagicSmoker:
Where the heck did you find that first circuit, OP? It makes no sense whatsoever. The second one appears to be for very low power, so likely for control circuitry or the like.
Regardless, it is common to use a 3-winding common mode choke on 3-phase power followed by 3 capacitors tied to either a virtual, or actual, neutral point. The insertion of additional inductance in series with the neutral makes no sense unless, perhaps, there is a serious problem with 3rd harmonic currents.
EDIT - and paralleling two bridge rectifiers with one supplied by line-to-line and the other supplied by line-to-neutral voltage also makes no sense.
SparkyFX:
The rectifiers do not make much sense, one will have sqrt(3)*Phase Voltage (for 120° 3~ sine wave), the other will have phase voltage. after redrawing it, it looks like the typical 3 phase rectifier, but with a non obvious reason for the connection of the neutral on the second half bridge.
Not all inductors must be EMI filters, they could also serve as inrush current limiters as component protection.
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