From: Tony Williams
Subject: Re: power factor
Date: Tue, 03 Sep 2002 08:13:50 +0100
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article ,
R. Lewis wrote:
> Is there an *easy* way of measuring the wattage of a reactive
> circuit (mains frequency, PF about 0.3) with bog standard lab
> gear - storage scopes and multimeters ?
Iin | Iin
\ |I0 /
\ | /
\ /|\ /
\ | /
For an applied voltage, Vin, the currents will
split into two components, an inphase component,
I0, and a quadrature component, I90. The reactive
component could be capacitive or inductive, so
I90 could be leading or lagging. Iin is the
resultant of the two current components and Iin
is what is measured by an ammeter.
2 2 2
(Iin) = (I0) + (I90).
Now add a known load resistor across the circuit,
which adds an extra inphase component IR. The new
value of Iin (call it Iin2) will be given by;
2 2 2
(Iin2) = (I0 + IR) + (I90).
Two equations, two unknowns. Simple algebra will
give values for I0 and |I90|.
Now do the same thing with an added reactive load,
probably a capacitor. Choose the value so that IC
is less than the calculated value of |I90|, say
roughly half of it. This allows you to determine
whether |I90| is a leading or lagging I90.
Repeat with a few values of IR and IC to verify
the method and calculations.