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#130785 - 02/05/07 10:07 AM Watt/KVA
aldav53 Offline
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Registered: 08/22/01
Posts: 545
Loc: Chandler, AZ USA
How close is KVA to wattage when reading on a transformer or machine, etc.
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#130786 - 02/05/07 10:10 AM Re: Watt/KVA
SteveFehr Offline
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Registered: 03/19/05
Posts: 1195
Loc: Chesapeake, VA
It depends on the power factor, which depends on the load. Generally speaking, PF is between 0.8 and 1 which means kW is 80-100% kVA. But this isn't always the case; you may be able to calculate it based on nameplate data, but it ultimately will require a measurement to know for sure.
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#130787 - 02/05/07 10:26 AM Re: Watt/KVA
aldav53 Offline
Member
Registered: 08/22/01
Posts: 545
Loc: Chandler, AZ USA
So the PF would be different according to the load and/or if its a 120/208 or 277/480v.
if that makes sense
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#130788 - 02/05/07 06:48 PM Re: Watt/KVA
WFO Offline
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Registered: 09/03/05
Posts: 202
Loc: Cat Spring, TX
The voltage would not determine the power factor.
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#130789 - 02/05/07 07:05 PM Re: Watt/KVA
aldav53 Offline
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Registered: 08/22/01
Posts: 545
Loc: Chandler, AZ USA
This is for a 400 volt machine 3 ph 63 amp.
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#130790 - 02/05/07 10:05 PM Re: Watt/KVA
ScubaDan Offline
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Registered: 08/23/06
Posts: 21
Loc: Sacramento, CA, USA
It might help to review the power triangle to see the relationship of Real Power(P), Apparent Power(kVA) and Imaginary Power(kVAR).

The angle opposite the Imaginary Power side is known as the angle of phase offset, of which its cosine is power factor.
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#130791 - 02/07/07 01:32 PM Re: Watt/KVA
Scott35 Offline

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Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
Here is a simple example of the Power Triangle, which will assist with the Power Factor conclusion:
(it also doubles as the Basic Impedance Triangle, and the Basic Pythagoris Theorym)

============================================

4 Watts (True Power),
3 VARs (Reactive Power),
5 VAs (Apparent Power).

============================================

Power Factor = 0.8 (80%) lagging
4 Watts is 80% of 5 VA.
---------------------------------------------

Some detail to these items:

*** WATTS ***
Watts = True Power,
KW = Kilo Watts = 1,000 Watts;

*** VA ***
VA = Volt-Amps = Apparent Power = Reactive Power and True Power compiled,
KVA = Kilo Volt-Amps = 1,000 Volt-Amps = 1,000 VA;

*** VAR ***
VAR = Volt-Amps Reactive = Reactive Power = Scuttle Power,
KVAR = Kilo Volt-Amps Reactive = 1,000 Volt-Amps Reactive = 1,000 VAR.

In the example values listed above, the preceeding relate to listed Trigonomic Terms:

<OL TYPE=A>


[*] 4 Watts = "Sine" (Base of the Triangle),


[*] 3 VARs = "Cosine" (Opposite side of Triangle),


[*] 5 VAs = "Tangent" (Hypotenuse)
</OL>

You can use this formula to find any single unknown value, as long as you have two known values.

Scott35
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#130792 - 02/11/07 07:24 PM Re: Watt/KVA
Scott35 Offline

Broom Pusher and
Member
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
Bump....

Any update on this?
(Did the examples make sense + help?)

Scott35
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Just Say NO To Green Eggs And Ham!
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#130793 - 02/12/07 03:40 AM Re: Watt/KVA
LarryC Offline
Member
Registered: 07/05/04
Posts: 780
Loc: Winchester, NH, US
Scuttle Power?

Is that supposed to mean the "power" that is scuttling back and forth between the load and the source, that doesn't do any real work, but requires us to upsize conductors and such, in order to accommodate its journey?

Larry C
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#130794 - 02/12/07 09:07 AM Re: Watt/KVA
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: boston, ma
I've never heard the term 'scuttle power', but seeing as how it is used as a synonym for reactive power, then 'power going back and forth between source and load' is exactly how I read it.

If you look at a sinusoidal load with a power factor less than 1, you will find that the sine wave representing current is not in phase with the sine wave representing voltage. This means that there will be portions of the AC cycle in which current has one polarity, and voltage the other, meaning that the load is supplying power back to the source. Over the entire AC cycle, the net power delivery is from source to load, but over part of the cycle there is a reverse flow of power.

-Jon
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