and a watt?

9/17/2007 2:51:55 PM

My guess is that a volt-amp is like a peak rating, in that the V-A number can't necessarily provide that as usable power. Where watt is what you can actually dissipate continuously over a resistive load.

9/17/2007 2:53:47 PM

it is because of the power factor, whether it is a capacitive load or an inductive load, you will get a different number of watts

9/17/2007 2:57:25 PM

VA can drive anything (inductive motor, large cap bank)

Watts are only "real" power and are related to VA's by the "power factor" which is the ratio of resistive and reactive components. a 100% pf will mean that it's a purely resistive load and no energy is stored in the system.

9/17/2007 10:03:36 PM

if you're looking at ups's a good rule of thumb is get one with 30% more VA than the wattage of your psu + whatever else you plug into it

9/17/2007 11:43:15 PM

haha, no, i was in class today learning about my jet, and noticed a couple of components were rated in volt-amps. i've seen it before and always wondered why they didn't just call it a watt.

9/17/2007 11:50:10 PM

Real Power (watts) is consumed by resistive parts of loads. Light bulbs, being puresly resistive, will consume only real power.

Induction motors have loads that have both resistive and inductive characteristics. These inductive characteristics are going to consume Reactive Power (vars).

The resultant of the two powers is called apparent power (volt-amp). Power factor is the ratio between real power and apparent power.

9/18/2007 7:53:56 AM

V-A includes the reactive power. Complicated idea but this best way to describe it is if you have two sinusoids one representing your voltage and one representing your current. The reactive power is related to the time difference between the peaks of the current and the voltage (technically its a phase difference).

This reactive power is not capable of performing any [b]Real[b] work but must be taken into account for various electrical calculations. The result in practical terms is the greater the difference between the V-A power of a circuit and the Real power (in watts) is a larger inefficiency.

Electrical companies often employ the use of large capacitors in the power distribution grids in order to maximize the ratio of Real Power to Apparent Power b.c most loads on the grid tend to be Inductive (contributing a positive angle) to the apparent power spectrum.

9/18/2007 8:29:30 PM

Reactive loads (capacitive or inductive) cause the current to lead or lag the voltage by a different phase amount than a purely Resistive load.

this phase shift causes product to be complex rather than purely real.

9/19/2007 10:40:32 AM

Duke did you fall asleep in ECE 331??????

9/19/2007 10:56:35 AM

who didnt

9/19/2007 11:14:04 AM

This thread makes me feel like a fucking idiot.

9/19/2007 11:22:30 AM

i used to know this, once upon a time

9/19/2007 11:45:24 AM

Quote :

"Electrical companies often employ the use of large capacitors in the power distribution grids in order to maximize the ratio of Real Power to Apparent Power b.c most loads on the grid tend to be Inductive (contributing a positive angle) to the apparent power spectrum."

They can use static capacitor banks, etc, but can also use actively controlled power factor correction devices such as VAR compensators and series synchronous condensers. My dissertation and research is the development of a statistical model of a large power grid (using various types of sensors all over the area) to develop an intelligent control system to manipulate these devices for faster performance and maintaining grid security and stability.

9/19/2007 1:58:33 PM

Quote :

"This thread makes me feel like a fucking idiot."

9/19/2007 2:11:28 PM

yeah i neglected taking ECE 305

9/19/2007 2:19:37 PM

ECE 305/451/453 FTW

Grainger would be proud of this thread.

9/19/2007 2:23:39 PM

grainger would be proud of you, and maybe HUR.

probably not so much for the rest of the folks.

9/20/2007 12:29:36 AM

I was confusing VA with VAR (and some other stuff probably too...)

9/20/2007 1:31:16 AM