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In Reply to: RE: Some simple facts posted by Jon Risch on August 08, 2014 at 23:14:50
"One thing that CMC's do tend to encourage, is the use of a VERY large inductance in the filter circuit. However, what a lot of people overlook is that placing 1 mH in the AC line actually degrades the peak current delivery due to the sheer AC reactance at the 60 Hz."
Jon, I think you got 'er backward. The AC power doesn't "see" the CMC because the current is differential so it can be most anything without affecting power delivery.
Regards, Rick
Follow Ups:
Nope. The AC line "sees" the full inductance of the CMC, what you are thinking of is the inductor CORE of a CMC, it sees only the differential current, which is only present when there is a fast spike coming in on the H or N line. Normal AC current draw by the load unit "cancels out" in the CMC core, this is where it can "get by" with a much smaller core than if it were an independent inductor.
The inductance does NOT cancel out, but is effectively increased by the common mode wiring of the inductance elements.
Unfortunately, when a differential voltage spike like this does occur, the spike generates a reverse polarity spike in the other winding of the CMC, and this essentially DOUBLES the spike voltage the load unit sees. With independent inductors, this does not occur.
Since I have measured actual AC line operating conditions as a result of my design time with the Discwasher SpikeMaster surge protector and filter, I have seen how this can actually occur fairly often, usually due to less than perfect AC wiring conditions, and lopsided induction of a transient signal onto/into the AC line.
This is exactly the reason I did not use a CMC type inductor in my AC Line Filter and Surge Suppressor design, and one of the reasons it keeps the output so clean and free from extraneous noise and voltage spikes.
Jon Risch
"The inductance does NOT cancel out, but is effectively increased by the common mode wiring of the inductance elements."
Inductance cancels for differential mode current and increases for common mode current passing through the common mode choke. That is why they are useful. Nothing is perfect, however.
Sorry, yes I have swapped the concept of typical common mode wiring for common mode current, which is incorrect. Been working late a lot, shouldn't post when I am tired.
However, the bottom line is the measurements I took indicates that a voltage spike or surge does NOT always flow through the CMC equally, in fact, more often than not, the return flow does not occur due the the load absorbing the voltage spike, so it does not fully get through to the return leg of the CMC, and the end result of the voltage spike doubling I spoke of does occur.
Jon Risch
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