Hi Chris,
thanks for you answer!
My system must read the real power that a big Auxiliary converter take
from a hv generator.The converter can use 3000Vcc and 1500V 50 Hz and
1000v 16 2/3 Hz.
So,for monitorig tension and current, with an DC panel, both ac and DC
source generator i convert with AD637(true rms to DC) Ac or DC .And
this part of circuit goes well.
The tension trasducer is an Hall effect ABB transducer wich can take up
to 5000v AC or DC and give 20 mA for end of scale.
Now,the other problem is reading average real power.
I multiple the V and I with AD632 .......but i must integrate.So i
tought to filter the out of multiplexer with an active low pass filter
with a 10 Hz(very low) Fs and read his output on digital panel meter in
DC.
If the supply is DC i think that there aren't poroble also without the
filter........bat when the supply is AC???
You say that without filter the aout of multiplier is the real power
that load take also if is supply with AC?
The value of multiplier should be about 2 or 3 volt max when load takes
max power.
What do you think?
By,
MAX
Hi, Max. It sounds like you're really on the right track. Considering
the voltages involved, I'm sure you're going the extra step to be safe.
It sounds like you're not positively sure about real power and apparent
power.
Real power is the average of the instantaneous voltage-current product
over a line cycle.
Apparent power is just the product of the rms voltage and rms current.
Period. You're going about this the right way by not just multiplying
the outputs of your RMS-to-DC converters. It sounded like that was
what you were possibly doing on your first post. If you want an analog
representation of real power, you have to directly multiply the direct
outputs of your transducers, and then average that over the line cycle.
Don't believe me -- look it up.
Now you may have some difficulties with the digital panel meter. Most
all of them are based on an ICL-7107 or ICL-7135 IC, which does dual
slope integrating D-to-A conversion. Here's a reference that might
give you an intro:
http://www.intersil.com/data/an/an017.pdf
Now you're going to have some difficulty using a DPM by itself, because
they're usually adjusted to achieve maximum noise rejection at a
particular line frequency. Your 50Hz rejection will be good, but your
16-2/3Hz regection maybe not so much. You're going to have to
carefully evaluate, and make compromises between noise reduction and
response time. Either that, or you may also be able to have one of
these meters custom-modified for the lower frequency. It's just math
and component selection -- the chip can handle it. The 7107/7135 IC is
capable of slower conversion rates, and has inherently great line
frequency reject at that one particular frequency.
But I'm wondering, for example, if it's necessary to have a 10 Hz f(c).
Possibly a lower cutoff frequency might be more useful. After all, no
one is capable of visually absorbing 10 readings per second, anyway.
Of course, since you're talking about a total project accuracy of a
couple of percent, you should remember that an analog meter does the
averaging automatically. You also don't have to worry about power.
Some taut band analog meters with mirrored backing can get you pretty
close, too. But if you use an op amp filter, watch for offset and
drift. Get premium op amps that will stay put.
When doing this project, I would think your biggest difficulty will be
offset and drift in the multiplier itself. This is going to be a
particular problem if your circuitry is in an industrial environment
with major changes in temperature. Five percent accuracy plus annoying
offset is easy. Getting it down below two percent, and getting the
offset at a minimum is going to take some looking at to do well.
Something you may want to look at is periodically doing an offset null
like a low frequency chopper. Many ways to get this better than a
couple of percent accuracy, if that's your need.
It sounds like you've got a genuinely interesting project on your
hands. Take the time to read up on things. I remember doing one of
these years back with an old Motorola analog multiplier. I had a lot
of fun, and learned quite a bit about practical instrumentation tricks
in the process. You've got a much better IC that will try to make you
look good. The old Motorola IC I used needed half a dozen tweaker pots
and seemed like a spirited but rather willful pony that always wanted
to drift off and roll in the grass when it felt like it (sorry, Mr.
Thompson). This is the type of project that can lead you to real
professional growth.
Good luck, and be as safe as you can
Chris
