What's that Lassie? You say that ATP* fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 28 Dec 2008 21:47:26 -0500:
I just remembered that I will need to know the power factor to figure
out what the watts used is.
Any way to do that without any fancy equipment? It probably changes
as the load changes.
I'm trying to figure out what it costs to run one of the shops air
compressors. They are 25hp screw type. The motor runs constant, and
a valve shunts the output when pressure is reached. I measured about
10A unloaded and 15A loaded on one leg with a clamp on ammeter.
I thought there would be more difference. Must be the power factor.
A quick search on rotary air compressors turned up a 25 HP with an 18.5 KW
motor, which would be over 51 full load amps per phase before efficiencies
are even considered if my calcs are right. 15 amps would be pretty lightly
loaded. Take a look at this chart from WEG:
http://www.galco.com/techdoc/weg/02518ep3e284t_dat.pdf
They give efficiencies and power factors at different loads. I don't get
involved with large motors like this very often, maybe Bruce can comment.
Yes, but !! Be careful in caculating VA from current measured with a clamp-on
type of ammeter,
though. In as much as E and I are out of phase, any leg current measured with
clamp-ons will not be
accurate. Full load current will be equal to 3 x that of current in each leg.
The formula is:
3-phase KVA = (Volts x Amps x Sq. root of 3) / (1000)
Bob Swinney
When figuring watts or VA on a three phase circuit, do you triple the
amps measured on one leg?
Thanks.
The formula as given is for the full load rated current of a 3 phase motor,
either delta or wye;
makes no difference. If one were to actually mesasure the KVA in each leg it
would be 1/3 the KVA
as calculated for full load. The math works with all 3-phase.
Bob Swinney
Some confusion may come from the well known fact that current and voltage are
out of phase in an
inductive circuit. As Wes pointed out ELI is the ICE man, a great memory aid
which has voltage
leading current in L; current leading voltage in C. Power (heat) developed in
an inductor is the
vectorial product of current and voltage. Power, however, is a sort of absolute
quantity. Power
does not add vectorially. If 3 equal quantities comprise the whole, then each
is equal to 1/3 of
the whole. If voltage is constant in 3 equal "packages" of power, then total
power must be 3 x the
current in each package.
Bob Swinney
Apples and oranges. Watts = power = work.
VA is volts x amps but ELI the ICE man comes into effect.
IOW the voltage and current waveforms are not in phase causing power to be lower
than just
volts x amps. The amps that have to be carried down the utility are real to the
utility
so in an industrial setting VA is what one is billed at for demand.
In a resistive circuit V * A * 1.732 or so would be the right number for watts
or VA.
Wes
One may look at 3 phases as originating in 3 separate generators. In fact, some
early 3-phase was
distributed over 6 wires to illustrate the 3 generator concept. Of course the
voltages and currents
in each phase (leg) were identical. The legs (phases) are 120 degrees apart.
The formula for each
leg taken seperately is KVA = (Volts x Amps) / 1000. The three legs combined at
120 degrees have
KVA = (volts x amps x 1.73) / 1000.
Bob Swinney
(dan) wrote:
Wes, does any meter with current transformers measure VA as opposed to a
smaller service with an inline wattmeter, which should measure actual watts?
I'd welcome a cite to same; with the tariff as well.
The Thomson moving coil watt hour meter, the one with the spinning
disk, measures honest-to-gosh watthours. It's the gold standard of the
industry.
Only recently have solid-state meters supplemented them, usually where
time-of-day billing and/or remote metering is wanted. They can also read
VARS I suppose; it's just a bit more firmware code. I expect they will
someday replace all Thomson meters.
Outside of those, I have never seen a residential installation that
even measured VARS, much less billed for them. Some do log a peak load
reading. [They had a needle that was pushed up and had friction against
falling back to zero...]
In some industrial installations, you can be charged a "power factor
penalty" but I've never seen any recording metering of same; merely
the utility tests it every so often. I suppose such is possible on the
largest [steel mill, auto plant] consumers that buy at the 132KV and
above level.
--
A host is a host from coast to snipped-for-privacy@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Apologies to Dan who started this long and annoying thread with an honest
question.
The simple answer is No; it is not practical to measure current in the
individual phases. This
pulsating current cannot easily be measured. I was wrong to try to show that
the current was 1/3 of
rated load current. It can be seen, though, that Power in each phase must be
equal to 1/3 total
Power if the phases are balanced. Simple logic supports this.
Instantaneous current is another matter. In a 3-Phase motor the phases are
separated by 60
electrical degrees. At any point in time instantaneous current will vary in
accordance with which
phase is being considered and with the point of time in the cycle. At say, time
0 of the sine wave,
Phase 1 current is at zero crossing (0 degrees), thus its instantaneous current
would be I total x
sin 0. At this same time Phase 2 is at 60 degrees and its instantaneous current
would be I total I
x sin 60. Also at this same time Phase 3 is at 120 degrees and its
instantaneous current would be I
total x sin 120. Current of the 3 phases adds and the resultant could be
stated: I instantaneous
at time zero = I total x (sin 0 + sin 60 + sin 120). I instantaneous at any
time = I total x [sin
theta + sin (theta + 60) + sin (theta + 120)] These effects can be viewed by
displaying 3 phases
with a common time base on an oscilloscope.
Bob Swinney
writes:
I'd welcome a cite to same; with the tariff as well.
The Thomson moving coil watt hour meter, the one with the spinning
disk, measures honest-to-gosh watthours. It's the gold standard of the
industry.
Only recently have solid-state meters supplemented them, usually where
time-of-day billing and/or remote metering is wanted. They can also read
VARS I suppose; it's just a bit more firmware code. I expect they will
someday replace all Thomson meters.
Outside of those, I have never seen a residential installation that
even measured VARS, much less billed for them. Some do log a peak load
reading. [They had a needle that was pushed up and had friction against
falling back to zero...]
In some industrial installations, you can be charged a "power factor
penalty" but I've never seen any recording metering of same; merely
the utility tests it every so often. I suppose such is possible on the
largest [steel mill, auto plant] consumers that buy at the 132KV and
above level.
--
A host is a host from coast to snipped-for-privacy@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
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