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From: michael@m-net.UUCP (Michael McClary)
Newsgroups: sci.electronics
Subject: Re: Domestic KiloWatt Hour Meters
Message-ID: <1228@m-net.UUCP>
Date: Sun, 26-Apr-87 23:44:47 EDT
Article-I.D.: m-net.1228
Posted: Sun Apr 26 23:44:47 1987
Date-Received: Sat, 2-May-87 09:35:04 EDT
References: <923@mhuxh.UUCP> <1715@kitty.UUCP> <243@omssw1.UUCP>
Reply-To: michael@node.UUCP (Michael McClary)
Organization: McClary Associates, Ann Arbor, MI
Lines: 55

In article <243@omssw1.UUCP> sdp@omssw1.UUCP (Scott Peterson) writes:
>A while back, someone told me you could make watt-hour meters run
>slow, or even backwards by messing with the power factor angle.
>I think it involved putting a huge inductive load on the line.
>This came up when someone was explaining why large industrial motors
>have large capacitors in parallel with them.

Well, you can, but that's not why the capacitors are there.  (If that
were then only reason for the capacitors, the big companies would leave
them out, reducing both their motor costs and electric bill.)

If you pull your current way out of phase, the meter gets inaccurate,
but not seriously.  If you pull a LOT of current 90 degrees out-of-phase
(but I forget whether it's leading or lagging), you can get the meter to
back up just a tad.  It's not enough to worry about.

The power company must provide current to supply whatever load is
presented.  This means it must provide both "real" power (in-phase current
times voltage) plus "reactive" or "imaginary" power (90 degrees leading
current times voltage).  If you hang an inductor across the line, it will
pull "imaginary" power, and if you hang a capacitor across the line, it will
"supply" it.  (An inductor is called a "load" and a capacitor a "source" of
imaginary power, rather than the other way around, because big loads, such
as induction motors, tend to be inductive rather than capacitive.)

The power company must feed this "reactive power" to its customers.  The
amount that isn't "generated" by capacitors somewhere will come from the
generators, causing them to run out-of-phase, providing a mix of in-phase
and reactive current.  The generators are limited by the total current,
not the total power, they produce, so this lowers the amount of real power
they can generate.  Also, the resistive losses in the transmission lines
are proportional to the square of the current, regardless of the current's
phase, and this is real energy that eventually comes from fuel.

Therefore it costs the power company to provide this "reactive" power, and
it charges its large industrial customers a surcharge if they run too far
out-of-phase.  So the customers buy capacitors to generate their own, keeping
their bills down and everybody happy.  Sometimes they buy motors which have
their own power-factor correction capacitors, so the capacitors and the motor
will be switched on and off together, but usually they just buy a few big
banks of capacitors and turn them on and off to keep the plant under the
surcharge limit.

(Most cpacitors that you find with a motor are "starting" capacitors,
 used to provide an out-of-phase current during motor start-up to give
 it extra torque.)

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