KVA Again:

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rattus

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No one has explained why we use VA/KVA ratings instead of simply using the voltage and current ratings.

In a flash of genius I realized that many devices such as motors and transformers can be wired to operate at different voltages and currents. The KVA rating applies for any and all wiring options, and this greatly simplifies load calculations.

Opinions?
 
Re: KVA Again:

Hello rattus,
I had the similar type of question in mind. It is subjected as "TRANSFORMER vs MOTOR" in "electrical calculations and engineering "topic.I do not know how to link it.
 
Re: KVA Again:

Usually in power engineering, its the Watts or KVA that we want. The voltage gets stepped up and down (as does the current) at points in a system, but the power remains more constant (ignoring losses).

Also, the end user wants KVA or KW to turn their motors. The voltage and current doesn't really tell them that without some calculating.

It's also easier to use a single number (say 480KW) instead of two numbers (1000A and 480V).

P.S. Have you heard of "Per Unit" values? A 1MW system may have a base power of 1MW and 13.9KV. Then at the generator, you might have a voltage of .5PU, and a power of 1PU (per unit). After the first transformer, due to losses, you might get .98PU of power. The voltage would stay .5PU, because the per unit value of voltage changes on the secondary side of the transformer.

Anyhow, that's often how power systems are described.

Steve
 
Re: KVA Again:

lquadros,

I believe this is the link to the thread you are talking about.

On the Post a Reply page, where we write our contributions to a thread, there is the link: What is UBB Code? that explains how to make a link, and other useful tricks.
 
Re: KVA Again:

Originally posted by rattus: No one has explained why we use VA/KVA ratings instead of simply using the voltage and current ratings.
Click to expand...
I think that is because no one has asked that particular question (at least, not that I recall). I'll hold off on issuing an opinion for now, until I find something I can disagree with. :)
 
Re: KVA Again:

If it is transformer, it is usually rated in VA because transformer sees the total (I) and the voltage (V) regardless of what type of loads are being fed to it. I hope my understanding is correct to satisfy your question.
 
Re: KVA Again:

Actually both items mentioned, motors and transformers, also have maximum current and voltage ratings, as well as "power" ratings.

Technically, the Watt, like horsepower, is not strictly an electrical value. It is the rate of energy (or work) flow; i.e., Joules/Second.

"VA" on the other hand is strictly a "convention" to help keep the math simpler in many general practical applications. That is, we COULD compute everything based on vector/scalar math using only Volts and Amps (and vector notation as necessary), but it doesn't usually yield any greater PRACTICAL results.
 
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