Don Stanley wrote: > On Thu, Jul 29, 2010 at 1:17 AM, Jon Elson <[email protected]> wrote: > > > > that is a common-mode choke, > > I understand common-mode in cabling and assume the same principal but > not sure of all this means. > > > and there the main motor currents all cancel out, > This is a ?? for me. > OK, if all three phase wires enter the core window in the same direction, then any motor current is supposed to flow in one wire and return through one or two of the others. Therefore, the net flux in the core MUST be zero, as the sum of currents in all three wires has to be zero. > so laminated cores should be fine. > > My question is, "in what circumstance would it not work", > so I can avoid that in the future. > Well, putting a laminated core on a single phase wire would subject it to the full PWM frequency spectrum of the VFD, as the VFD puts out 340 V square waves of varying pulse width at some high frequency of several KHz. Core losses are roughly proportional to frequency. So, core laminations that are fine at 60 Hz will heat up 100 times more at 6 KHz! It will not be quite that bad as the higher frequency causes less peak magnetization, but it will still be a dramatic increase.
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