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From: jeffw@midas.WR.TEK.COM (Jeff Winslow)
Newsgroups: sci.electronics
Subject: Re: Power Factor
Keywords: Capacitive
Message-ID: <1347@wrgate.WR.TEK.COM>
Date: 12 Jan 90 20:32:27 GMT
References: <12525@cbnewse.ATT.COM>
Sender: nobody@wrgate.WR.TEK.COM
Reply-To: jeffw@midas.WR.TEK.COM (Jeff Winslow)
Distribution: usa
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 22

In article <12525@cbnewse.ATT.COM> sherwood@cbnewse.ATT.COM (charles.a.sherwood) writes:
>Someone stated that they did not know of any equipment that caused a capacitive
>power factor. One of the engineers in our power supply group told me that
>switch mode power supplies have a capacitive factor of about .65 due to the
>low conduction angle. This would imply that any electronic equipment that uses 
>a switch mode power supply would have a has a capacitive power factor. 

Power factor in switchers is due to a fundamentally different phenomenon than
in inductive loads. As you say, the low conduction angle translates into a much
higher RMS input current that one would expect from the power consumed, hence
P/VA deviates from unity. However in my experience it's a tossup whether the
(fundamental of the) current leads or lags the voltage. It's true there would
be a tendency to lead, since you're charging partially discharged capacitors,
but usually there's enough impedance in series (EMI filters, etc.) that there
is significant current conduction on both sides of the voltage peak. Most of
that .65 figure is due to the small conduction angle, not phase shift. So 
all those PC's aren't going to help balance much of an inductive power factor.

Phase shift or not, it's considered enough of a problem that there's a fair
amount of research going into how to cheaply and reliably correct it.

							Jeff Winslow