----- Original Message ----- From: <[EMAIL PROTECTED]> To: <[email protected]> Sent: Wednesday, June 07, 2006 6:36 PM Subject: Re: Battery shapes
> > > -----Original Message----- > From: Michel Jullian <[EMAIL PROTECTED]> > To: [email protected] > Sent: Wed, 7 Jun 2006 18:28:40 +0200 > Subject: Re: Battery shapes > > Terry allow me to disagree. I understand the UPS manufacturers rating > game, I > have played it myself, but it is not misleading in fact, it is not even > a little > cheating as you imply: if the 15 kW UPS you mention has to spit out 15 > kW, it > _will do it_ of course, and if it's load has a power factor of 0.75 as > is quite > common, the rms current will _actually be 167 A_ (15000/(120*0.75)), so > it will > _really_ be outputting 167*120=20 kVA. And the wires will heat up as if > the 167A > were real, which they are. > > The UPS rating only misleads the dudes who think that Pavg=Urms*Irms, > and those > deserve to be misled anyway ;) > > Controversy solved? > > Michel > > P.S. As for the UPS's IGBTs or Mosfets, they often paradoxically prefer > this > kind of load (rectifier+filter, PF=0.6 to 0.8) to a purely resistive > load > drawing the same kW. Mine did in any case, as such loads only drew > current when > they were saturated :) > > <><><><><><> > > You're most welcome to disagree; however shifting the phase between the > voltage and the current does not contribute to ohmic heating of the > branch conductor. > > Terry Thanks Terry. The above is quite right but in the case of typical rectifier+capacitor loads the low power factor is due to shape rather than phase of the current (brief capacitor refill current pulses at voltage peaks). But this increases RMS current all the same, and RMS current is what heats the wire: Pw=Rw*Irms^2. This is a fact. If you care to reread my previous post in this light you will realize you have unjustly criticized your electrical dudes :) Michel
