Can someone explain what KVA is......

[hoover]

.... and its relationship to volts/amps. If you know the wattage of an installation how do you work out the kva? Cheers in advance.

 

[slocm3105]

Is kva not simply kilowatts?

 

[wattsup]

Real power is measured in watts

Apparent power is va

to convert kw to kva / by 0.8

So suppose you require 5kw of real power, you would need a sytem to provide 6.2 kva. Mistakes are often made with generating sets, where many assume kw is the same as kva.

(assuming no motors in circuit, start current etc need special attention)

 

[Guest123]

Hi.

Essentialy W = true power

VA = apparent power

W will be equal to VA only if the load purely resistive. If not then the power factor will come into play. A safe bet is to multiply by 0.8 as wattsup said, however it could be more, could be less depending on the exact nature of the load i.e how inductive.

So VA = W/PF.

 

[ghost]

KVA is apparent power, here's an explaination I found on some bod's site

With alternating current electricity, the direction of current flow rapidly alternates between one direction and the other and the voltage rapidly alternates polarity between one direction and the other. That means that unless the polarity of the voltage reverses at the same instant that the direction of the current reverses, the direction of power flow (voltage times current) can also alternate directions.

If the effective values (RMS values) of voltage and current are multiplied together, the result, the VA (or kVA for 1000’s of VA) is called the apparent power. The VA (or kVA) is the total power that appears to be flowing, but it includes both the net power that is flowing in one direction and the amount of power that is circulating back and forth between the source and the load. The net power that is flowing from the source to the load is called the real power, watts (W) or kilowatts (kW). The power that is circulating between the source and the load is called the reactive volt-amperes, VARs or kVARs.

The relationship between the kW, kVA and kVAR is kVA squared = kW squared + kVAR squared. Another way to express this relationship is to say that the kW = kVA X power factor. The power factor (pf) is a number between +1 and -1. If pf = 1, all of the power is flowing from the source to the load as is does when the load is purely resistive and does not include any elements like capacitors and inductors that can store energy momentarily and return energy to the source. If pf is -1, all of the power is flowing from the load to the source, the roles of source and load are reversed. If pf is between +1 and -1, some of the power is circulating. Pf can be calculated as the consine of the angle between the voltage and current waveforms.

 

[tizbad]

real power is voltage x amps x power factor (cos phi)
a truly resistive (theoretical) circuit will have a power factor of one so kva is kw

most circuits have some inductance and/or capacitance. this shifts the voltage and current out of phase, giving you a power factor of less than one.
the more out of phase they are, the less power you get for the same volts and amps.

this is why inductive loads such as fluorescent lights have a capacitor attached to bring the power factor back towards one.

kva is shown as the possible power output from a generator, depending on the type of load it is feeding.

i hope this makes some sense as my brain now hurts. its been a while since i was taught this and its hard trying to explain in words.
graphs showing sine waves of the voltage and current will help, a good tutor or text book will answer this properly if you can find one

 

[jjjinx]

kaaaaa boooom!! me head exploded after reading that.... reminds me of college where I would write "errr......" alot and not much else

 

[MarcT]

remember those phasor diagrams

 

[TF and the wire]

Some good answers there but I'll try to explain it simpler. Ignore the k, that is just to avoid writing or saying thousands.

VA is simply volts X amps. EG an electric fire that takes 8 amps with 230v applied is 230x8= 1840VA or 1.84kVA

Stop here if that is all you want to know.

The above is correct in 100% pure resistive circuits, which is virtually impossible. If there is any capacitance or inductance, the current and voltage are out of step with each other by an amount dependant on how capacitive or inductive the circuit is. The angle by which they lag or lead is given it's cosine value and this is used to multiply the VA to give the power in watts.

The power in watts is V x A X cos

 

[jjjinx]

Yeah its ohms law etc..


i bought a copy of phyics for dummys today rofl

once I've finished my course i'm on I'm guna try and penitrate the walnut of a brain I have and get it to remember some of this stuff lol.

 

[Mac]

Explaining Watts and Volt Amps - Or "What the Heck is a KVA?"

 

[ghost]

Interesting article, it's all about UPS systems for computer server rooms. Some useful conversion factors to help size air con systems amongst it.

 

[Risteard]

this is why inductive loads such as fluorescent lights have a capacitor attached to bring the power factor back towards one.

Power factor correction capacitor.

 

[SirKit Breaker]

Sounds like someone might have got one of those forms to fill in for a new or increased electricity supply, as these ask for amongst other things, how many kVA do you require. I always put 21kVA as this ensures that a 100amp fuse is fitted. Anything above about 25 kVA puts you onto 3 phase.

I'm a bit too long in the tooth for serious maths, and i've forgot most of the formulaes i sort of learnt 30 odd years ago. I'll stick with V over IR etc etc.

Cheers........Howard

 

[jean]

VA is the unit used to measure apparent power where as watt is for true power. VA is obtain by multiplying current to voltage, watt is VA X power factor. Distribution transformers is rated in KVA

 

[ian.settle1]

Is kva not simply kilowatts?

If a pure resistive circuit then yes otherwise no

 

[Des 56]

If you and the missus were at it
When you thrust,the missus does the same at the same time,result,bliss (good power factor)

If you and the girl next door were at it
When you thrust, she is little bit faster than yours,result,out of sinc ( possible dis conection even ) (poor power factor)

Power factor is trying to achieve unity ( 1 to 1 ) It may be only 0.8 or so or worse even)

volt x amps (x power factor) = true power

So the better the power factor the more the unchanging volt amps value matches the true power

 

[TF and the wire]

If you and the missus were at it
When you thrust,the missus does the same at the same time,result,bliss (good power factor)

If you and the girl next door were at it
When you thrust, she is little bit faster than yours,result,out of sinc ( possible dis conection even ) (poor power factor)

Power factor is trying to achieve unity ( 1 to 1 ) It may be only 0.8 or so or worse even)

volt x amps (x power factor) = true power

So the better the power factor the more the unchanging volt amps value matches the true power

I don't remember that at college, I also didn't realise that women were supposed to move.

 

[hoover]

Sounds like someone might have got one of those forms to fill in for a new or increased electricity supply, as these ask for amongst other things, how many kVA do you require. I always put 21kVA as this ensures that a 100amp fuse is fitted. Anything above about 25 kVA puts you onto 3 phase.

I'm a bit too long in the tooth for serious maths, and i've forgot most of the formulaes i sort of learnt 30 odd years ago. I'll stick with V over IR etc etc.

Cheers........Howard

Correct mate, i do have one of these forms to fill in. So i have a load of 83kw so i devide or multiply this by 0.8 to get my kva? Its a 3 phase supply i need so do i devide the 83kw by 230 or 415 to get my ampage? Cheers in advance.

 

[john.irvine]

Correct mate, i do have one of these forms to fill in. So i have a load of 83kw so i devide or multiply this by 0.8 to get my kva? Its a 3 phase supply i need so do i devide the 83kw by 230 or 415 to get my ampage? Cheers in advance.

(83kW x 1000) / (230 x 1.73 x 0.8) = Amps

 

[hoover]

Correct mate, i do have one of these forms to fill in. So i have a load of 83kw so i devide or multiply this by 0.8 to get my kva? Its a 3 phase supply i need so do i devide the 83kw by 230 or 415 to get my ampage? Cheers in advance.

(83kW x 1000) / (230 x 1.73 x 0.8) = Amps

Where does the 1.73 come from mate?

 

[john.irvine]

Where does the 1.73 come from mate?

Its the square root of three, it converts single phase to three.

83kW x 1000) / (230 x 0.8) = Amps single Phase
83kW x 1000) / (230 x 1.73 x 0.8) = Amps Three Phase

 

[IP4X]

Where does the 1.73 come from mate?

If you are really interested try this: Seriously...
Its also a good way to see the star/delta relationship

1. Draw a triangle with equal sides. (all angles 60 degs) call each point A,B,C
2. inside the triangle draw a line from each point to the centre of the opp side.
3. Where the three lines intersect you have now created a star point...used as a neutral, call it N
4. You have also created 6 right angle triangles with angles 60, 90 and 30 Degs.

So use basic trig to find the relationship between the relevant sides of the star/delta triangles...

So Cos(30) = (half of CA)/ BN ............Cos(x) = adjacent/hyp
0.866 = (0.5 x CA)/ BN
BN = (0.866/0.5)CA

or BN = 1.732 CA

Hope that helps.
Also remember that a power factor of 0.8 may be common and that is probably why they use it in the classroom examples but this figure will differ in every installation!
Cheers