Pover factor (was genset trailer)

[Victor Tikhonov <vtikhono@lsil.com>]

Peter VanDerWal wrote:
> 
> A bridge rectifier on single phase AC creates a pulsating DC.  Current only
> flows when the voltage rises above the battery voltage.  (warning crude
> ASCII art)
> 
> Voltage wave form
> 
>     ^        ^
> ---/-\------/-\----------------------------  Battery Voltage
>   /   \    /   \
>  /     \  /     \
> /       \/       \
> 
> Current wave form
> 
>     ^       ^
>    / \     / \
>    | |     | |
> ___| |_____| |__
> 
> The spiky current wave form doesn't match the voltage waveform (hence poor
> power factor).

Peter, what you described technically applies to 3 phase as well:

 
               
 ---/\-/\-/\-/\-/\----------------------------  Battery Voltage
   /  X  X  X  X  \
  /  /  X  X  X \  \
 /  /  /  x  X \ \  \
/  /  /  /  / \ \  


 Current wave form
 
           
   || || || || ||
   || || || || ||
 __||_||_||_||_||__
 
Still peaks of the current, but at very top of each sign wave.
Plateau of the top of the wave is large and 3 phase overlap so
often, that if battery voltage gets lower, indeed pure DC starts
flowing.


 
> With three phase, because of the way the phases interact, when you rectify
> it with a 6 diode bridge you get pure DC out.  Pure DC voltage, pure DC
> current and both current and voltage are perfectly in phase.
> 

In your single phase picture they are also perfectly in phase -
there is no leading or lagging. The max current peak (in time) coincide
with max voltage point, so they are in phase. Technically it's not
the same "Power factor" as when you supply AC to the light bulb
through a capacitor. In this case current and voltage are perfect
sin waves, no peaks, and still poor power factor because there is
phase shift between the two.

THese are two distinctive cases and in both we call it "power factor",
but one should realize the fundamentals (and thus solutions) are very
different.

> > Who says there are no power factor control problems with
> > three phase?   Tell us more about that.

I believe the original author meant the voltage spikes (because
poor brushes contact, whatever), not peaks, are problems
for 3 phase generators, and so just as dangerous for PFC-50
as spikes (from lightening, not because bay-boy-type current peaks)
in single phase supply.

Victor

> > Tom Shay
> >
> > ----- Original Message -----
> > From: "VanDerWal, Peter MSgt" <vanderwp@fhu.disa.mil>
> > To: "'EV List'" <ev@listproc.sjsu.edu>
> > Sent: Friday, July 26, 2002 2:26 PM
> > Subject: Re: no-man's-land genset trailer
> >
> >
> > (stuff snipped out)
> >
> > > Or save the cost of the PFC charge and buy a three phase 10kw generator
> > for
> > > $3700 and just use a 6way bridge (no PFC problems with three phase).
> 10kw
> > > (approx 80 amps) will almost let you drive continuously. 350lbs and it
> > says
> > > it's "Super Quite".
> > > http://www.electricpowergenerator.com/threephase.html
> > >
> > >
> >
> >