Re: integrating true RMS wattmeter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Sparktron01@xxxxxxxxxxx 

Hi Dest!
 -------------- Original message ----------------------
From: "Tesla list" <tesla@xxxxxxxxxx>
> Original poster: dest <dest@xxxxxxxxxxxxx>
>
> Hallo.
>
> i`ve found in archives (97) this thing:
>
> http://himplast.ru/father_dest/temp/killawatt.PNG
>
> the author is David E. Sharpe, TCBOR, this is revision 02, 6/26/97.
> i have a couple of question to you, Dave:
 > a) have you upgraded the thing to 10-14KW range as was intended in the
> past?
>
---------------------------------------------------
I forwarded information to a fellow coiler in Florida (USA), he installed
a CT (50:5 if memory is right) and a 2:1 PT (240:120) and was getting
good results to max of ~ 15kVA.
---------------------------------------------------
> b) is there any more recent revisions here?

> c) why did this thing was so nonlinear: -12.66% error at the begin of
> scale and +2.69% error at the end? what was the reason of
> _nonlinearity_ at all - optocoupler or op amp?
----------------------------------------------------
Reason for non-linearity and inaccuracy is op-amp (741 was used) and
optocoupler.  The circuit configuration as shown in known as a "Pocel's
Cell".  It is very sensitive to air currents around it (so install in 
a aluminum
box for shielding).  Op-amps to use include OP27/OP37 (was originally
the specified part), Maxim MAX427 (used and was much better linearity
and accuracy) and National Semiconductor LF412 (low offset, low noise)
should improve circuit performance.  You are using the optocoupler
at the lower end of its operating range, so some low range nonlinearity
shoulc be expected).
-----------------------------------------------------
>
> d) http://www.pupman.com/listarchives/1997/march/msg00347.html
>
> "Variation in Yokogawa from calculated due to integration times, manual
> data entry, and TCR (temp coeff. of resistance) effects of heater
> element(s)."
>
> what? nippon meter was so SLOW??? what does it mean - "manual data
> entry"?
>
> e) "The experimental design was basically use the controller to power
> a 120VAC,1.56kW floor heater.  Inductance is trival (shaded pole
> blower) and 1500W heater."
>
> so you have tested the thing only at 60Hz? and at LINEAR load???
> what`s the point in that - anybody can go and buy $5 ferrodynamic
> wattmeter for example - it would be more accurate and linear in such
> case : ))) my $3 50Hz _mechanical_ meter accuracy rating is 1.5!
>---------------------------------------------------
Convenient, and a known load that is resistive.  I also measured
with a HV transformer (open circuit) and wattage measured very low
(to be expected).  If characterized with a Yokogawa WT2000 series
power meter (accuracy 0.03% of reading + 0.03% of range).  I have just
purchased the WT3000 for work which is 0.02% accuracy.  If you know
or a more accuracy power meter, let me know...  :^D
----------------------------------------------------------------------
> f) "The differential voltages (Vshunt) and (Vload) are in fact
> MULTIPLIED to the form:
> 39 X Rshunt/Rmultipler * Vload * Iload"
>
> then why do you advice to reduce Rshunt (to 250 uOhm!) instead of
> changing oa gain for example? is it limitation of optocoupler, oa,
> something other? there are many other oa & optos here.
----------------------------------------------------------------------------
Adjusting operational span on optocupler may improve linearity and
response, haven't had a change to investigate further.
----------------------------------------------------------------------------
>
> 10) "The only concern I have with this design is there is minimal circuit
> board clearance through the optocoupler (0.1 and 0.3 inches)."
>
> are you speaking about hv spikes in the mains? so strong, that they can
> cause breakdown of 2.5mm of air? don`t you think that in that case you
> better shut your coil down immediatelly instead of trying to measure
> it`s power consumption? % )
-------------------------------------------------------------
Air clearance of standard dual in-line package is 0.3 inches across
device (photo diodes to transistors).  Was concerned of solder flux
PCB contamination promoting tracking/unintended leakage currents
that could add to non-linearities and could cause circuit failure.

A better solution would be to cut a 0.080" (2mm) slot across length of
optocoupler to allow some air clearance, and prevent PCB tracking if
HV spikes are present.
------------------------------------------------------------
>
> 11) this circuit is darned simple - maybe we can use a couple of
> additional elements to get real _electronic_ accuracy? : )
----------------------------------------------------------
Use the highest accuracy op-amps you can get (low offset, bias currents)
must be +/-15V (unit will swing to +14V ~ 1.4kW ).  Build circuit on a
shielded PCB, add isolation slot under PS2501-4.  Enclose in a shielded
box to prevent air currents and EMI influence.

On my large controller, I using this part from Ohio Semitronics

https://www.ohiosemitronics.com/productDetails.asp?sku=PC5-011E

with small PCB to translate from 4-20mA to voltage.  It is pricey, but
VERY accurate (listed as 0.5%, saleperson said 0.3% is commonly
seen in field).

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA