#130785  02/05/07 10:07 AM
Watt/KVA

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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

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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 80100% 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

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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

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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

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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

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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

Broom Pusher and
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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 = VoltAmps = Apparent Power = Reactive Power and True Power compiled, KVA = Kilo VoltAmps = 1,000 VoltAmps = 1,000 VA;
*** VAR *** VAR = VoltAmps Reactive = Reactive Power = Scuttle Power, KVAR = Kilo VoltAmps Reactive = 1,000 VoltAmps 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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#130793  02/12/07 03:40 AM
Re: Watt/KVA

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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

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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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