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Analog dynamic range, accuracy and number of bits

  • Thread starter Habib Bouaziz-Viallet
  • Start date
J

josephkk

Jan 1, 1970
0
I don't follow. If you add a digital back end to a rotating disk meter
you can add any of these features. So what features can't you add to a
rotating disk meter with a digital back end? Is the advantage that the
digital meters are less expensive?

Rick

Certainly both of these and providing for co-generation (e.g. solar) is
much easier by just going all digital direct in the first place (also
cheaper which the utilities DO care about).

?-)
 
J

josephkk

Jan 1, 1970
0
Three watts seems pretty extreme to me. This is something that should
be doable for well under a quarter of a watt before power supply
inefficiencies are considered. I have no idea how they deal with that
however. In fact, the more efficient, switched power supplies will have
poor PF, lol! I believe I've read that there will be a requirement to
manage PF even on low wattage power supplies.

Rick

Actually low watt PF corrected power supplies can be efficient, it is just
a different design. What is really difficult is near zero idle power and
short pulses of a fractional to a few watts. MPS430 based designs are
famous for this. Many sleepable uCs and uPs have this issue.

?-)
 
J

Jeroen Belleman

Jan 1, 1970
0
Arrgh. It is the same phase in 120/240 V systems, NOT phases. Just
opposite ends of a single center tapped winding. Opposite polarity is
correct, and other hot ( / leg) is acceptable. I get twitchy on this
because i have to correct sparkies and dumball "engineers" on this all the
time.

If we're considering sine waves with odd harmonics, why would it matter
if one talks of opposite polarity or 180 degrees phase difference?
120/208 V "single phase systems" are different. Other leg still applies.

As John pointed out, if the load consists of rectifiers without
PF correction, the currents do not cancel in the neutral wire.
In fact, the current may end up to be the *sum* of the currents
in both hot wires.

Jeroen Belleman
 
The classic wire sizing assumes that current from the three phases
will cancel into the neutral. When the current is dominated by higher
harmonics, namely big current spikes at the peak of each sine wave,
they add up; they don't cancel because they are not simultaneous in
time.

While I very much doubt that the harmonics will actually burn down the
house, the (possibly urban legends) that the harmonics have burned the
neutral conductor in a cable, causing hazardous voltages appears to be
believable.

At higher distribution levels, it is assumed that the neutral current
will cancel out in a three phase system and thus, the neutral
connector might quite a lot weaker than the phase connector. In a
ground cable, the armoring around the phase conductors might act as
the neutral conductor. In a pole mounted cable system, three aluminum
phase connectors plus a steel cable servicing as the mechanical
strength as well as the neutral condition (assuming neutral conductor
canceling out).

However, if this canceling assumption is not true with electronics
load, the third harmonic current is far larger than initially assumed,
causing burn through of weak neutral conductors.
 
R

rickman

Jan 1, 1970
0
Certainly both of these and providing for co-generation (e.g. solar) is
much easier by just going all digital direct in the first place (also
cheaper which the utilities DO care about).

That's what I am asking. I haven't heard anyone say the digital meters
are cheaper than the disk meters. Is that true?

The issue of solar is moot. When you add your solar collectors, if you
want to feed into the grid you have to coordinate with the utility and
they give you a special meter. My house *still* has the same disk meter
it had 50 years ago.

Rick
 
R

rickman

Jan 1, 1970
0
Actually low watt PF corrected power supplies can be efficient, it is just
a different design. What is really difficult is near zero idle power and
short pulses of a fractional to a few watts. MPS430 based designs are
famous for this. Many sleepable uCs and uPs have this issue.

?-)

How does that work exactly?

Rick
 
J

John S

Jan 1, 1970
0
On 11/14/2012 4:39 PM, rickman wrote:
My house *still* has the same disk meter
it had 50 years ago.

Rick

Probably because they work so well.
 
J

John S

Jan 1, 1970
0
They had a temporary (above-ground) neutral installed within the
hour... a cable about as thick as my wrist ;-)

...Jim Thompson

I do hope you are writing "a cable about as thick as my wrist" as an
embellishment. If so, I can deal with that.
 
J

John S

Jan 1, 1970
0
I do hope you are writing "a cable about as thick as my wrist" as an
embellishment. If so, I can deal with that.

OTOH, maybe you have a really tiny wrist. I didn't consider that.
 
J

josephkk

Jan 1, 1970
0
If we're considering sine waves with odd harmonics, why would it matter
if one talks of opposite polarity or 180 degrees phase difference?

Yes. Correct description of the situation is crucial to understanding
what is correct or incorrect operation.
As John pointed out, if the load consists of rectifiers without
PF correction, the currents do not cancel in the neutral wire.
In fact, the current may end up to be the *sum* of the currents
in both hot wires.

That depends a lot on how the rectifier is connected and just what is
connected for legs. If the rectifier is across 240 from a 120/240 system
no neutral current (harmonic or otherwise) flows; very different from
3-phase systems.
Jeroen Belleman


?-)
 
J

josephkk

Jan 1, 1970
0
That even applies to 120/240 center-tapped systems where the 120 volt
loads are 0 and 180 degrees. If the loads were half-wave diode
rectified, and each line was running at its rated max RMS current, the
neutral could be overloaded. Again, if the line currents don't overlap
in time they add into the neutral, not balance as would happen with
sinewave loads.

You could get the same effect with a resistive load on one side and a
capacitive load on the other. The orthogonal neutral currents wouldn't
cancel.

It has been a long time since i saw normal equipment using a half-wave
rectifier. That pretty well went out with 5 tube radios.
I don't think i have ever seen it from each line directly, always a
transformer in-between.

?-)
 
J

josephkk

Jan 1, 1970
0
That's what I am asking. I haven't heard anyone say the digital meters
are cheaper than the disk meters. Is that true?

Not in initial cost, though that may change in time. It is total cost of
over the lifetime, including things like paying for meter readers.
The issue of solar is moot. When you add your solar collectors, if you
want to feed into the grid you have to coordinate with the utility and
they give you a special meter. My house *still* has the same disk meter
it had 50 years ago.

Not only that it likely can support co-generation if it is of better grade
(can run backwards to reflect your co-generation power) IFF tariffed at
the same rate.
 
G

Grant

Jan 1, 1970
0
It's common to use a diode as a two-step incandescent dimmer or hi/lo
heater controller. I've always wondered what a DC line current
component would do to an electric meter... maybe slow the disk on
mechanical ones, saturate a CT?

Add corrosion to earth points due to DC component running to ground.

Grant.
 
J

Jeroen

Jan 1, 1970
0
Yes. Correct description of the situation is crucial to understanding
what is correct or incorrect operation.

That depends a lot on how the rectifier is connected and just what is
connected for legs. If the rectifier is across 240 from a 120/240 system
no neutral current (harmonic or otherwise) flows; very different from
3-phase systems.



?-)

Well, that's why I said *may*. Figuring out for which conditions this
is actually true is left as an exercise for the assiduous student.

Jeroen Belleman
 
In Europe, tube TV sets were equipped with PY series half wave
rectifiers (later selenium/silicon) in order to produce about +200 V
anode voltage.

With the quite large distribution districts (400-1000 m) from the
distribution transformer, it was quite likely that about 50 % of the
TV sets were plugged one way, while the other 50 % were plugged in the
opposite way, so there was not a large risk to a DC bias or second
harmonics problem.
It's common to use a diode as a two-step incandescent dimmer or hi/lo
heater controller. I've always wondered what a DC line current
component would do to an electric meter... maybe slow the disk on
mechanical ones, saturate a CT?

While I have personally experimented with a rectifier as power
control, I have never heard of any commercial product (in Europe)
doing this (more likely various serial/parallel systems).
 
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