It is true, John. Your assumption that all "PFC-equipped SMPS draw a **sinusoidal** current" is incorrect. It is also incorrect to assume all PFCs totally isolate the SMPS from the line. Note the caveat 'can' in my original statement. If you have a -0.8 PF alternating with a +0.8 PF at a fifty percent duty cycle, when measured with a simple PF meter, the result will average out and be a PF of 1. When measured with a with a quality power line disrubance analyzer it is quite a different story.
In an actual case seen in a 100V, 3 phase system, in Western Japan an existing server farm had a PF-0.8. A new system, with some of the first PFCs, was installed. After bring up of the new system, some of the old systems started tripping off line. The neutrals (wire) were sized at four sizes smaller than the line (wire) under the assuption there is no neutral current with line to line loads. After installation the neutrals were burning up. A check of the new system PF with a simple phase meter showed a PF of 1. A clipon ampmeter showed the neutral current was close to the line current. A distrubance analyzer scope showed a harmonic rich environment. Further analysis showed the new PFC were correcting the source, rather than the load to a PF of 1. (i.e. if the PF was -0.8 before it should still be around -0.8 after adding a resistive load.) This drasticly increased system losses rather than reducing them as intended. I think we have gotten smarter in building systems since then. "Those who can not learn from history are doomed to repeat it." George Santayana Fred Townsend DC to Light John Woodgate wrote: > In message <[email protected]>, dated Tue, 26 May 2009, > Fred Townsend <[email protected]> writes: > >> Also sometimes forgotten is the fact the PFCs can cause neutral >> currents greater than line current in 3 phase SMPS. No good deed goes >> unpunished. > > > This is not true: NON-PFC SMPS cause increased neutral currents > because the triplen harmonic currents from all phases add > arithmetically in the neutral. Since PFC-equipped SMPS draw a > **sinusoidal** current, there are no significant harmonic currents, > triplen or not, so no abnormal neutral current. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
