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#130785 - 02/05/07 02:07 PM Watt/KVA  
aldav53  Offline
Member
Joined: Aug 2001
Posts: 545
Chandler, AZ USA
How close is KVA to wattage when reading on a transformer or machine, etc.


The Golden Rule - "The man with the gold makes the rule"

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#130786 - 02/05/07 02:10 PM Re: Watt/KVA  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,208
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.


#130787 - 02/05/07 02:26 PM Re: Watt/KVA  
aldav53  Offline
Member
Joined: Aug 2001
Posts: 545
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


The Golden Rule - "The man with the gold makes the rule"

#130788 - 02/05/07 10:48 PM Re: Watt/KVA  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
The voltage would not determine the power factor.


#130789 - 02/05/07 11:05 PM Re: Watt/KVA  
aldav53  Offline
Member
Joined: Aug 2001
Posts: 545
Chandler, AZ USA
This is for a 400 volt machine 3 ph 63 amp.


The Golden Rule - "The man with the gold makes the rule"

#130790 - 02/06/07 02:05 AM Re: Watt/KVA  
ScubaDan  Offline
Member
Joined: Aug 2006
Posts: 21
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.


#130791 - 02/07/07 05:32 PM Re: Watt/KVA  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
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


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Bump....

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

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#130793 - 02/12/07 07:40 AM Re: Watt/KVA  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 783
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


#130794 - 02/12/07 01:07 PM Re: Watt/KVA  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
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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