A common-mode choke is designed to act on the effect of the net current that flows through all of the lines at the same time. This is opposite of the differential receivers that try to ignore the voltages that arrive "common mode" from a cable.
The major reason to use a common mode choke on the output of a VFD is because, the VFD output is PWM modulated which causes large voltage swings even when not much motor current is needed. These swings will produce high frequency current spikes due to capacitive coupling between the motor windings to each other and to the metal used for magnetic poles in the motor. A choke is an inductor, and thus tries to inhibit high frequency current spikes. Inductors suffer from a problem called magnetic saturation. The core material can only support so much magnetic field. Once the current is great enough to cause saturation, the apparent inductance drops dramatically and the filtering effect is reduced. Because motors consume large amounts of current, large magnetic cores would be needed to support these large currents if three individual chokes were used on the 3 phase lines. Common mode chokes take advantage of the nature of the 3 phase power waveform. If you sum the currents in the 3 phases, the net result is zero, assuming no leakage and ignoring the effects of capacitance. Therefore, if you build the choke so that the current in the three phases is adding field strength into the same magnetic circuit in the core, the net field is zero and a smaller inductor can be used. The 3 phase power is effectively differential and causes no net field. Now the inductance only has to deal with the current spikes caused by the PWM voltage waveforms exciting the capacitive coupling and a smaller magnetic core can be used. Regards, Steve Stallings > -----Original Message----- > From: Don Stanley [mailto:[email protected]] > Sent: Friday, July 30, 2010 3:20 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Quieting The Variable Frequency > Drive (VFD) Noise.How To. > > On Thu, Jul 29, 2010 at 1:17 AM, Jon Elson > <[email protected]> wrote: > > Hi Jon; > My 3 phase experience is mostly practical with a basic > knowledge of the principles. I think you are saying much > more than I am getting in the following statements. I will > say what I think I am getting for each and where there is > more could you please elaborate? > > > > If the windings for each phase run parallel, > > Three cores in a row sharing the ends. The coils are > advertised as patented bobbin windings. > > 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. > > so laminated cores should be fine. > > > My question is, "in what circumstance would it not work", so > I can avoid that in the future. > > When I teach technical classes my first statement is "the > only stupid question is the one you never ask". > I know this is not a class but you have knowledge I am lacking. > Jon, I hope this is not a bother. > > Many Thanks > Don (Details) > > > -------------------------------------------------------------- > ---------------- > The Palm PDK Hot Apps Program offers developers who use the > Plug-In Development Kit to bring their C/C++ apps to Palm for > a share of $1 Million in cash or HP Products. Visit us here > for more details: > http://p.sf.net/sfu/dev2dev-palm > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > -------------------------------------------------------------- > ---------------- > The Palm PDK Hot Apps Program offers developers who use the > Plug-In Development Kit to bring their C/C++ apps to Palm for > a share of $1 Million in cash or HP Products. Visit us here > for more details: > http://p.sf.net/sfu/dev2dev-palm > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ The Palm PDK Hot Apps Program offers developers who use the Plug-In Development Kit to bring their C/C++ apps to Palm for a share of $1 Million in cash or HP Products. Visit us here for more details: http://p.sf.net/sfu/dev2dev-palm _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
