Re: Power monitoring; ws Re: one wire sensors

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Wed, 08 Aug 2007 09:39:34 -0700, RickH <passport@xxxxxxxxxxxxxxxxxxxxx>
wrote in message  <1186591174.960534.177300@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:

>
>Would your circuit/concept require the homeowner to physically connect
>each branch circuit (breaker) sensor directly to the powerline?
>
>I was thinking more in terms of sensing coils classed by wire gauge
>(10, 14, 12 etc) and type (solid stranded) and power range (rating of
>breaker), that the user would simply "clamp" around each wire after
>identifying the gauge, type of wire and breaker amps.  Sort of like a
>little clamping ampmeters hooked to each branch circuit, each coil
>would have either a hardwire or RF link back to the microcontroller
>unit that times the samples and accumulates/sends a data package back
>to the software.  I've seen the C/H panels installed here at work,
>pretty impressive.  But I was thinking more in terms of a system that
>a homeowner can install without violating electrical codes.  The
>microcontroller would calculate out any irregularities in the induced
>current curve of the sensing coil, and provide a linear measurement of
>amperage for logging.

Yes. That's what I'm working on but it will take a few words for me to begin
to explain my trajectory and rationale. Bear with me ...

It seems to me that the Cutler Hammer panels might be most useful, say, where
decisions need to be made as to which circuits to power from a standby genset
during a power outage, with the transfer switch providing power to CH panel
and not the main entrance panel. Available power might be allocated to
different circuits to stay with in the genset output capabilities.

But the CH panel limitation of only eight branch sensors (+ one main)
immediately raises the question of _which_ eight circuits to monitor. We'd
like to monitor all in the house. Hence a less expensive way is desirable
that doesn't need to provide the largely superfluous (for most situations)
ability to power the circuit on and off as the CH also provides.

In my house, I have been installing hard-wired lighting with circuits going
to two centralized locations (basement and the second floor). Adding up the
lighting circuits and the individual branches, and power to three separate
load centers (basement shop, kitchen and 2nd/3rd floor), there are/will be in
excess of 120 or candidates for monitoring. So the approach(es) has/ve to
'moderately' priced.

The accurate way to measure power (watts) is to independently measure current
(I) and voltage (V) simultaneously and in real time multiply the result (V*I)
many times during each half-cycle (eg several thousand times per second) and
average the value over one or more 50 or 60hz cycles.

This is what the circuit based on the SAMES sa2002x IC that I describe here
www.econtrol.org/power_measurement.htm and here
www.econtrol.org\power_measurement.htm
do. I have a dozen of these ICs but could accomplish the same thing with some
programming effort with (eg) AVR MCUs.

One can readily measure the current (I) in a wire by using a current
transformer. Here's a picture of seven different ones with a US Quarter
Dollar for scale that illustrates several basic types. The frequency response
for all except the one on the bottom right includes the fundamental of the
frequency range of interest (50-60hz).

www.ECOntrol.org/CTs.jpg

The large one on the left is a "split core" rated at 400 amps. I have two of
these in the entrance panel, one for each phase. The fact that these are
"split core" meant that I could install them without pulling the meter or
otherwise disconnecting power to the house in order to pass the conductors
through the whole in the transformer. It has an internal "load" resistor that
is calibrated so that 400 amps (full scale output) is 0.33 VAC. The device is
RU/UL-listed. But whether it would pass inspection would depend on the
discretion of the Authority Having Jurisdiction (aka AHJ; aka The Inspector)
of the US National Electrical Code (NEC) in one's particular jurisdiction.

The donut-shaped (toroidal) transformer to its right is also RU/UL-listed and
rated at maximum of 100 amps with a 300:1 current ratio, meaning that with a
20 amp load, the output is 20/300 = 67 milliamps. With a "load" resistor (a
resistor across the output leads) of 100 ohms, the output voltage is 6.7 VAC
I have about 15 of these. I plan to use these at the branch circuits. AC
compressors and ovens. Note that they waste 1/300th of the power consumed by
the circuit.

The smaller blue rectangle is a current transformer that uses a Hall-Effect
sensor. It has better efficiency (1:2000), accuracy and linearity than the
simple toroidal transformers but requires +/- 15vdc supply to operate. I have
about 30 of these.

What one needs to do with the AC current signal to make it useful depends
both on the load and the use.

If the waveforms were pure sine waves, and the loads purely resistive so that
current and voltage were in phase, we could multiply the  Root-Mean-Squared
(RMS) 'average current measured in this way by a measured average (RMS)
voltage, and compute the RMS power in watts at each transformer. Alas, the
waveform is not perfect and current and voltage are out of phase for
inductive and(or) capacitive loads, so this calculation is but an
approximation in many cases.

But for some loads and uses, this approach is plenty good enough. For
example, the power used by an electric oven can be accurately assessed by
converting the voltage from a current transformer or shunt into a DC signal
using a single OP amp configured with diodes as an active rectifier because
the load is resistive and rectified peak voltage is proportional to current.
Even if one assumed 110VAC instead of measuring actual voltage at the load,
the calculated wattage would typically be within 10% or so depending on
voltage sag. Plenty good enough to remind one to turn off an unused oven,
IMO.

Another way to measure current is by measuring the voltage drop across a
shunt resistor ( a small-valued resistor). This approach has at least two
distinct disadvantages: lack of intrinsic galvanic (electrical) isolation and
the fact that the conductor must be 'cut' so that the resistance inserted.
But high wattage resistors of appropriate value are less expensive and easier
to come by than appropriate current transformers. I recently bought 230+ 25
watt, 0.23 ohm resistors (a convenient value for a 20 amp circuit) for
~twenty cents each on eBay from Don Lancaster
http://stores.ebay.com/Synergetics-Abeja . (Many electronic DIYers will
recognize that name from Lancaster's innumerable books and articles over the
decades.) They also provide the right location to measure voltage which is
also needed for the V*I calculation.

My plan is to use the resistors where I am currently using solid state relays
for control because the conductor is already 'cut' for insertion of the relay
and there are convenient places to mount and heat-sink these aluminum-cased,
screw-mount resistors. My AHJ-approved solid state relay dimmer panel is
here: www.econtrol.org/ssr_panel.htm. The resistors will go in the top part
near the solid-state relays; the auxiliary circuits will be in the
low-voltage compartment at the bottom.

Gotta go. More later as time permits ... Marc
Marc_F_Hult
www.ECOntrol.org