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From: amdx on 25 Jul 2010 17:42
Hi Guys,

Here's a power monitor that uses clamp on current transformers.
I don't see where it monitors voltage, so wouldn't it measure reactive
power at an incorrect rate?
http://www.currentcost.net/howitworks.html

Mikek


From: Paul Hovnanian P.E. on 25 Jul 2010 22:56
amdx wrote:
>
> Hi Guys,
>
> Here's a power monitor that uses clamp on current transformers.
> I don't see where it monitors voltage, so wouldn't it measure reactive
> power at an incorrect rate?
> http://www.currentcost.net/howitworks.html
>
> Mikek

I didn't see where they claim to measure reactive power. Or how
accurately they measure real power for that matter.

There are a couple of possibilities:

This thing measures amps only and makes an assumption about power factor
and voltage to calculate watts.

Its possible to measure the phase angle of the conductor's voltage using
the capacitive coupling between the conductor and CT secondaty winding.
The voltage itself can't be measured accurately, but the phase angle
between V and I can, so power factor can be calculated. An asumed value
for voltage magnitude is used.

The web page installation instructions are too simplistic and there are
a couple of voltage inputs into the metering.

--
Paul Hovnanian mailto:Paul(a)Hovnanian.com
------------------------------------------------------------------
If life was fair, Elvis would be alive and all the impersonators
would be dead. -- Johnny Carson
From: whit3rd on 25 Jul 2010 23:21
On Jul 25, 2:42 pm, "amdx" <a...(a)knology.net> wrote:

> Here's a power monitor that uses clamp on current transformers.
> I don't see where it monitors voltage, so wouldn't it measure reactive
> power at an incorrect rate?

Yes. Power will be the reading multiplied by some constant in the
range (+1, -1). Probably, for typical household appliances
(water heater, refrigerator, incandescent lamps) the constant will
be very near +1.
From: Robert Baer on 26 Jul 2010 00:40
amdx wrote:
> Hi Guys,
>
> Here's a power monitor that uses clamp on current transformers.
> I don't see where it monitors voltage, so wouldn't it measure reactive
> power at an incorrect rate?
> http://www.currentcost.net/howitworks.html
>
> Mikek
>
>
By definition, a CURRENT transformer cannot measure or indicate power.
From: Phil Allison on 26 Jul 2010 00:53

"Robert Baer"
>
> By definition, a CURRENT transformer cannot measure or indicate power.


** Under circumstances where the AC supply voltage is known and PF is likely
to have only a minor effect on the total energy consumption - it most
certainly can give useful indications.



..... Phil


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