Re: 208 3PH question


"Robert Swinney" <judybob@xxxxxxxxx> wrote in message
news:PZ6dnb4ncdiY2sfUnZ2dnUVZ_r6dnZ2d@xxxxxxxxxxxxxxx
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

I'm quite aware of the math involved. I'm not confused. Your statement that
full load current is equal to 3x that of current in each leg is still wrong,
no matter how you try to rationalize it, because we don't express current in
a three phase circuit that way. If a three phase motor has a full load
current of 30 amps, that means each leg carries 30 amps, not 10. It's a
convention.

Your other assertion:

"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"

is also wrong, because VA is not the same as watts. VA is apparent power and
it IS measured using ammeters and voltmeters.


"ATP*" <waxwingslain@xxxxxxxxxxxxx> wrote in message
news:49596229$0$4903$607ed4bc@xxxxxxxxx

"Robert Swinney" <judybob@xxxxxxxxx> wrote in message
news:I8Odncw4P7yLw8TUnZ2dnUVZ_rmdnZ2d@xxxxxxxxxxxxxxx
Hmmnnn - - what part of KVA did you find beneath your level of
comprehension.? Actually the concept
is fairly simple for most people.

Bob Swinney

It's pretty funny how you keep pressing a point when it's obvious you
don't
have a clue. Your statement that full load current is equal to 3x that of
current in each leg is dead wrong. Stick to your original statement
instead
of saying something else. How is full load current equal to 3x the current
in one leg?
<waxwingslain@xxxxxxxxxxxxx> wrote in message
news:49595866$0$4884$607ed4bc@xxxxxxxxx

"Robert Swinney" <judybob@xxxxxxxxx> wrote in message
news:yr6dnVVITZX_isTUnZ2dnUVZ_sjinZ2d@xxxxxxxxxxxxxxx
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

KVA is not current, and the formula is not for current. Full load current
is
not equal to 3x that of current in each leg.


"ATP*" <waxwingslain@xxxxxxxxxxxxx> wrote in message
news:495913e1$0$4885$607ed4bc@xxxxxxxxx

"Robert Swinney" <judybob@xxxxxxxxx> wrote in message
news:mKadnbY9PpYUacXUnZ2dnUVZ_szinZ2d@xxxxxxxxxxxxxxx
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)

In the formula above, Amps would be the circuit current. Full load
current
(as in FLA) is the same as the current on one leg, if we're talking
about
the same thing. The only way you can triple the amps in this case is if
you
multiply by 120, which makes sense with 208 wye since it could be
supplying
three 120 V circuits with a neutral. That math, however, doesn't work
with
other three phase voltages.

Bob Swinney
"dan" <notme@xxxxxxxxxxx> wrote in message
news:495832b3.6280232@xxxxxxxxxxxxxxxxxxxxxx
When figuring watts or VA on a three phase circuit, do you triple the
amps measured on one leg?

Thanks.
--

Dan H.









.

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