ECN Forum
Posted By: aldav53 Watt/KVA - 02/05/07 06:07 PM
How close is KVA to wattage when reading on a transformer or machine, etc.
Posted By: SteveFehr Re: Watt/KVA - 02/05/07 06:10 PM
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.
Posted By: aldav53 Re: Watt/KVA - 02/05/07 06:26 PM
So the PF would be different according to the load and/or if its a 120/208 or 277/480v.
if that makes sense
Posted By: WFO Re: Watt/KVA - 02/06/07 02:48 AM
The voltage would not determine the power factor.
Posted By: aldav53 Re: Watt/KVA - 02/06/07 03:05 AM
This is for a 400 volt machine 3 ph 63 amp.
Posted By: ScubaDan Re: Watt/KVA - 02/06/07 06:05 AM
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.
Posted By: Scott35 Re: Watt/KVA - 02/07/07 09:32 PM
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
Posted By: Scott35 Re: Watt/KVA - 02/12/07 03:24 AM
Bump....

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

Scott35
Posted By: LarryC Re: Watt/KVA - 02/12/07 11:40 AM
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
Posted By: winnie Re: Watt/KVA - 02/12/07 05:07 PM
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
Posted By: Scott35 Re: Watt/KVA - 02/12/07 10:07 PM
Quote

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?

Ding, Ding, Ding, Ding, Ding!!!!!!!

Yes, that's what the Slang Term is referring to - the way Reactive Power "Scuttles" between the Reactive Load and the Supply (typically the Secondary Windings of a Transformer).

Same Power does not perform any usable output work at the Load Device, but does result in Circuit Conductors doubling as low Power Space Heaters!!!
[Linked Image]

In addition to the extremely inefficient "Conduit Heater Properties" of the Conductors, due to the "higher than should be" Total Line Current carried in them
{per the total Apparent Power (VA) Package, for both the True Power (Wattage) and the Reactive Power (VARs)...},
one may also receive a "Special Meter", + a "Special Monthly Billing Rate" from the Utility Company, allowing someone to become a very generous donator of $$$ to the PoCo, for bouncing VARs between given loads and the PoCo Transformer.
(Detent Meter and penalties for bouncing too much Reactive Power off the PoCo's Transformer Secondary Windings)

Trying to describe this in a funny way, yet not succeeding.

Scott35
Posted By: WFO Re: Watt/KVA - 02/14/07 02:37 AM
Not to wander too far off topic, but what are you seeing for power factor requirements? Our new standards require a P.F. of 97%.
Posted By: ScubaDan Re: Watt/KVA - 02/14/07 05:56 AM
Power factor values are device specific and can not be averaged.

In My Professional Opinion:
Using an average power factor is as insane as finding 2.4 kids in your neighbor's house. It is just plain wrong and will always be wrong.
Posted By: SteveFehr Re: Watt/KVA - 02/14/07 12:17 PM
Since nobody has really explained it yet, and this could potentially be a difficult concept if you've never seen if before, this is why power factor happens:

Straight resistor circuits are simple. Volts = Amps x Ohms. When you put in more voltage, more current flows. If you look a a sinusoidal plot of 60Hz AC power over time, the Voltage and Ampere curves will be perfectly aligned.

Everyone has probably turned a coil of wire around a nail and created an electromagnet- that's a good example of inductance, where flowing current creates a magnetic field. In AC circuits, that field is constantly being built up and drained away, and that stores and releases energy. Capacitors behave in a similar way. So, inductors and capacitors store energy and release it, but not in phase with the voltage! Current flow through an inductor will peak before the voltage peaks. Current flow through a capacitor will peak after the voltage peaks. In both cases, all the energy stored in the inductor or capacitor is released later in the cycle- leading to a condition where you're not really adding or subtracting any power overall, but there is a lot more current flowing!

In the real world, nothing is ideal. Inductors and capacitors all have some resistance. A straight piece of wire acts as an inductor (that's how clamp ammeters work). And there is always some capacitance between "hot" wires and the ground. So even a very simple circuit will have some power factor that's not exactly 1.0


In our circuits, motors and transformers often have large inductive components, which usually leads to "leading" power factor. Now, as luck has it, inductors and capacitors are diametrically opposed- current stored in a capacitor while it's being released by an inductor. If you match the two together, they cancel out perfectly. So, devices with poor power factor usually can be "corrected" with capacitors.

[This message has been edited by SteveFehr (edited 02-14-2007).]
Posted By: WFO Re: Watt/KVA - 02/15/07 01:09 AM
I guess I didn't phrase that properly.
I work for a utility. Our regulatory commission requires us to maintain a 97% power factor at our substations, which we do by switching capacitor banks in and out as the loads change.


My question was...What are the utilities requiring their customers to maintain in terms of a minimum power factor before they are penalized on their bill?
Posted By: SteveFehr Re: Watt/KVA - 02/15/07 12:01 PM
Isn't that done at the poco more to maximize line efficiency and reduce line loss? At the distribution level, it's often more economical just to oversize the copper a bit and live with a lousy PF. I have many sites with PF as low as 0.8 and have never heard anyone complaining about it.
Posted By: ghost307 Re: Watt/KVA - 02/15/07 02:08 PM
ComEd has no requirements (which may be part of their problems); but I've seen 0.89 to 0.92 for other utilities.
FYI; China changes their rate structure for every 0.01 change in power factor...and it's VERY punitive. If your power factor gets below 0.8, they cut your power off!
Posted By: rat4spd Re: Watt/KVA - 02/16/07 02:43 AM
Our power factor is whatever it is as long as the POCO is carrying less than 1 MVAR. When it drifts over that, the tap changer in our feed transformer changes supplied voltage which sheds VARS back to us.

Thus, voltage, in this instance changes our power factor, albeit indirectly, as we co-generate.
Posted By: moti7 Re: Watt/KVA - 03/21/07 02:17 PM
hello !
there are two wayes to calculate the PF.

the original definition is that the PF is the angle between the current through and the voltage on a consumer (electrical machine-not DC - , lamp atc...) - cos(fi).

the second way is to calculate the power factor by measuring the active power(P) and the reactive power [Q] (or the imaginary power S = V * I ) and use the formula PF = P/S. or by measuring the related enrgies.

this way of computing the PF used in calculating the consumer PF (at the main board ).

if there are harmonics in the sysyem then PF not equel cos(fi).

for en individual electrical equipment it is the same pf=cos(fi)

yours,
moti bohadana
Posted By: Trumpy Re: Watt/KVA - 03/22/07 05:58 AM
Gidday there Moti!.
Welcome to the group mate. wink
© ECN Electrical Forums