When figuring watts or VA on a three phase circuit, do you triple the
amps measured on one leg?

Thanks.

Thanks.

--

Dan H.

Dan H.

Thanks.

--

Dan H.

Dan H.

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.

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.

--

Dan H.

Dan H.

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.

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.

--

Dan H.

Dan H.

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

Bob Swinney

Hmmnnn - - what part of KVA did you find beneath your level of comprehension.?
Actually the concept
is fairly simple for most people.

Bob Swinney

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

Bob Swinney

snipped-for-privacy@privacy.net (dan) wrote:

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

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

Wes

(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?

Wes, does any meter with current transformers measure VA as opposed to a smaller service with an inline wattmeter, which should measure actual watts?

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?

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?

A watt-hour-meter measures true watts.

Some utilities charge power factor penalties.

--

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

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

Click to see the full signature.

Home meters have often not measured true watts. The expensive
ones do but they get close and don't play to much with phase angles.

Martin

ATP* wrote:

Martin

ATP* wrote:

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

Click to see the full signature.

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

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

Click to see the full signature.

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