Power factor correction
in [Design]
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From: MooseFET on 4 Jan 2010 21:21 On Jan 4, 8:16 am, "Tim Williams" <tmoran...(a)charter.net> wrote: > "MooseFET" <kensm...(a)rahul.net> wrote in message > > news:2562a20f-e214-4701-8f38-50366424ad9a(a)a15g2000yqm.googlegroups.com... > > > With the coil, the current is near constant. This makes the harmonic > > content less > > but still significant. Adding a moderate sized transformer at each > > phase and doubling > > the number of diodes will lower the harmonics quite a bit. > > > Basically it is 3 of these > > ... > > And of course, since you have two phases on two wires (let the other be > common), you have a linearly independent basis (not orthonormal, but so > what) from which you can span the entire vector space of voltage and phase. > So you could, for example, produce a 5-phase system with 10 pulses, or 120 > if you wanted. You spend a lot on transformers, though. > > What does that do for current, anyway? Current through each phase is > essentially a parabolic pulse. All those added up, in the ratios from which > they are generated, should distribute to a fairly constant current, > shouldn't they? Well, the sum of (three phase) currents is always zero, but > the sum of magnitudes isn't: that must oscillate at the 6th harmonic. So it > should be that, in the same way as a choke input filter causes relatively > more current draw on the flanks of the sine wave than overall, this > arrangement also causes more current draw on the flanks, resulting in an > inversely phased 6th harmonic. The curious part is, this nonlinear > conclusion was based on the linear construction of vectors: of course the > diodes, being nonlinear elements, are the reason, but the voltages don't > seem like they should do that. When the number of poles becomes infinite, the harmonics all drop to zero. Beyond the infinite number, there is no further improvement. I hope this helps. > > Tim > > -- > Deep Friar: a very philosophical monk. > Website:http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 4 Jan 2010 21:37 "MooseFET" <kensmith(a)rahul.net> wrote in message news:415e4301-e21b-45c5-a78d-20cd9b09748e(a)v25g2000yqk.googlegroups.com... > When the number of poles becomes infinite, the harmonics all drop to > zero. > Beyond the infinite number, there is no further improvement. > I hope this helps. How can there ever not be harmonics, either voltage or current, input or output, when transforming three phase to DC? I think a simple energy budget shows this is necessary. Ignoring the three phase source, if you start with n equal voltage, equally spaced phases, you will get harmonics of 2n and higher, with a current pulse roughly 1/n of the waveform per phase. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Paul Keinanen on 5 Jan 2010 04:22 On Tue, 29 Dec 2009 13:44:36 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >For much higher power 3 phase rectifiers are used. >That leaves only a small ripple. >I am not sure if you need PFC in such a case. Some issues regarding 3 phase rectifier PFC are discussed in http://scholar.lib.vt.edu/theses/available/etd-08142002-075617/unrestricted/Barbosa_ETD.pdf It also contains a long list of references, which might be useful when doing Google searches.
From: Jan Panteltje on 5 Jan 2010 08:06 On a sunny day (Mon, 4 Jan 2010 18:19:15 -0800 (PST)) it happened MooseFET <kensmith(a)rahul.net> wrote in <b4f80fb0-b390-4a51-949b-00073266a8f3(a)s31g2000yqs.googlegroups.com>: >On Jan 4, 7:35�am, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: >> On a sunny day (Mon, 4 Jan 2010 06:26:08 -0800 (PST)) it happened MooseFE= >T >> <kensm...(a)rahul.net> wrote in >> <2562a20f-e214-4701-8f38-50366424a...(a)a15g2000yqm.googlegroups.com>: >> >> >> >> >On Jan 4, 4:29�am, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: >> >> On a sunny day (Mon, 04 Jan 2010 07:53:30 +0200) it happened Paul Kein= >ane= >> >n >> >> <keina...(a)sci.fi> wrote in <q4v2k5hqkr88dmmja3fssrc7o7cdrnp...(a)4ax.com= >>: >> >> >> >On Tue, 29 Dec 2009 13:44:36 GMT, Jan Panteltje >> >> ><pNaonStpealm...(a)yahoo.com> wrote: >> >> >> >>On a sunny day (Tue, 29 Dec 2009 07:17:31 -0600) it happened "Tim Wi= >lli= >> >ams" >> >> >><tmoran...(a)charter.net> wrote in <Yhn_m.18015$wC3.5...(a)newsfe07.iad>= >: >> >> >> >>>PFC is a flyback thing. �But flybacks suck over 100W, where forwa= >rd = >> >looks >> >> >>>better. �But forward sucks over wide duty cycle ranges. �What d= >o t= >> >hey do, >> >> >>>just bigger flybacks? �Nothing at all? >> >> >> >>>Tim >> >> >> >>>-- >> >> >>>Deep Friar: a very philosophical monk. >> >> >>>Website:http://webpages.charter.net/dawill/tmoranwms >> >> >> >>For much higher power 3 phase rectifiers are used. >> >> >>That leaves only a small ripple. >> >> >>I am not sure if you need PFC in such a case. >> >> >> >The ordinary 3 phase 6 pulse rectifier is a quite nasty polluter of >> >> >the mains. >> >> >> >To reduce the harmonics, 12 pulse rectifiers have been used, with one >> >> >set of rectifiers connected to the wye windings and the other set fro= >m >> >> >separate delta windings. For high power systems, you still are going >> >> >to need a medium/low voltage transformer on site, so it is not a big >> >> >deal having separate wye and delta secondary windings on that >> >> >transformer. >> >> >> >If you want to get away with the heavy 50 Hz transformer, there seems >> >> >to be various more or less patented 3 phase systems using flyback etc= >. >> >> >systems to reduce the PFC, but these seems to be hugely complex and >> >> >have special floating requirements for the DC side. >> >> >> >If floating output or voltages other than those obtainable by simple >> >> >rectifiers are required, it might be easier to simply get three singl= >e >> >> >phase SMPS with PFC at the input and connect the DC outputs in >> >> >parallel. >> >> >> >If the input voltage ratings permit, instead of wye, connect the >> >> >inputs into delta to avoid any mains neutral polluting and also allow >> >> >higher voltage and hence higher power (1.7x) for a specific >> >> >semiconductor amperage. >> >> >> Yes, the ones I have dealt with also had a huge heavy inductor beteen = >the= >> > rectifiers and the filtercaps. >> >> � � � � � � � � � � � � � � � � �L >> >> � � � � � � � � � � � � � � � ==== >== >> >===== >> >> � � �-------------------------^^^^^^^^^---------------- + >> >> � � | � � � | � � � | � � � � � � � � = >�= >> > � � � � � | >> >> � �--- � � --- � � --- � � � � � � � � �= > = >> >� � � � �| >> >> � �/ \ � � / \ � � / \ � � � � � � � � �= > = >> >� � � � �| >> >> � �--- � � --- � � --- � � � � � � � � �= > = >> >� � � � �| >> >> � � | � � � | � � � | � � � � � � � � = >�= >> > � � � � � | >> >> --- | � � � | � � � | � � � � � � � � � = >�= >> > � � � � | >> >> � � | � � � | � � � | � � � � � � � � = >�= >> > � � � � �=== >> >> --- ) ------| � � � | >> >> � � | � � � | � � � | � � � � � � � � = >�= >> > � � � � �--- C >> >> --- ) ----- ) ----- | � � � � � � � � � � � � = >�= >> > � | >> >> � � | � � � | � � � | � � � � � � � � = >�= >> > � � � � � | >> >> � �--- � � --- � � --- � � � � � � � � �= > = >> >� � � � �| >> >> � �/ \ � � / \ � � / \ � � � � � � � � �= > = >> >� � � � �| >> >> � �--- � � --- � � --- � � � � � � � � �= > = >> >� � � � �| >> >> � � | � � � | � � � | � � � � � � � � = >�= >> > � � � � � | >> >> � � �-------------------------------------------------- - >> >> The inductor smoothes the main current, and reduces capacitor ripple c= >urr= >> >ent. >> >> For something like 4kVA and up. >> >> >With the coil, the current is near constant. �This makes the harmonic >> >content less >> >but still significant. �Adding a moderate sized transformer at each >> >phase and doubling >> >the number of diodes will lower the harmonics quite a bit. >> >> >Basically it is 3 of these >> >> > � � � � � ------------------ To bridge >> > � � � � �( >> >A---------- >> > � � � � �( >> > � � � � � ------------------ To bridge >> >> >B----------- >> > � � � � � ) >> > � � � � � ) >> > � � � � � ) >> >C----------- >> >> >Since the voltage between B and C is at 90 degrees to the voltage from >> >A >> >to ground, the voltage on the secondary only needs to be the tan() of >> >the >> >angle you are shifting the phase by. >> >> >tan(15)=0.27 >> >> An interesting solution, never seen that before, maybe I am too old:-) >... Or maybe too young. Back when rectifiers couldn't handle the >power, >you needed more diodes and a way to share the current so doing this >didn't >add parts. mmm I dunno, designed something for the army that a had selenium rectifier in it..
From: Jan Panteltje on 5 Jan 2010 08:43
On a sunny day (Tue, 05 Jan 2010 11:22:10 +0200) it happened Paul Keinanen <keinanen(a)sci.fi> wrote in <bt06k5ta0ku0ma6t6umbkhou0pp867cd9t(a)4ax.com>: >On Tue, 29 Dec 2009 13:44:36 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>For much higher power 3 phase rectifiers are used. >>That leaves only a small ripple. >>I am not sure if you need PFC in such a case. > >Some issues regarding 3 phase rectifier PFC are discussed in >http://scholar.lib.vt.edu/theses/available/etd-08142002-075617/unrestricted/Barbosa_ETD.pdf I am impressed, felt back at school for a while. A rather complete design guide! >It also contains a long list of references, which might be useful when >doing Google searches. After reading 240 or so pages... My head is full :-) |