Mr

Bear Mug ,

......not again...

This is a good analogy, but although the tension in the rope (kva) increases (along with the hassle sideways force component), the ahead force (kw or hp) remains the same.

Bartender, gimme 5kw of beer, with at least 95% power factor.

KV I understand you were in sales for sometime? LOL!!

It's such a shame that there isn't a voting facility for great answer!

In an ac circuit, KVA represents apparent power (Pa = VI) where VI is volt-amperes and K = kilo or 1,000. KVAR is reactive power (vars or volt-ampere-reactive) and is calculated from Pq = VI sin θ where θ is the phase angle between voltage and current.

Engineers often explain and calculate the difference between kVA, kVAr [and kW] by using triangles. It is one place where those "right angle triangles", in which "the square on the hypotenuse (longest side) is equal to the sum of the squares on the other two sides" (thanks, Pythagoras!), which you probably learnt in school, earn a living. If you leave out the "k" in the preceding (which stands for kilo and means all the values are in thousands), you have VA (Volts x Amps or volt-amps, also known as Apparent Power), W (Watts = Apparent Power x Power Factor, also known as Real Power) and Var (Volts x Amps - Reactive Power). They are related by a triangle like the one below, which has a "right angle" at corner B.

In the diagram the line O to A represents the current in an AC (alternating current) circuit Ia. The line from O to V represents the voltage in the circuit, V. Note that V and Ia have an angle (marked with the symbol phi) between them. V represents the voltage measured by a voltmeter measuring at an electrical consumer's supply terminals, while Ia represents the current going into the consumer, measured by an ammeter. V multiplied by Ia is called the "apparent power" because some of the current going in stores energy in magnetic fields (such as those in motors and transformers) and electric fields (such as in the "power factor correction" capacitors of fluorescent tube lamp units) during one half cycle which is returned on the following half cycle. It is like the process of lifting bricks up a building in a crate on rope going round a pulley - you have to lift both crate and bricks up to do the job, but if you tied a bag of tools on the rope at the bottom before returning the (empty) crate you can recover the effort put into lifting the crate - the wise man makes his lift with a counterweight equal to the weight of the crate, to reduce the pull he has to apply to the rope (a bit of mechanical "power factor compensation"!). You could describe the pull to be applied to the rope by the man to move both crate and bricks as the "apparent" pull, while knowing that only part of that, the "real" pull, was moving the bricks - the difference you could call "reactive" because it stores energy in the crate, which can be got back. In the case of alternating voltage/current it is more complicated - but it turns out that if you construct a right-angled triangle with longest side in line with Ia, and next longest in line with V, like my drawing THEN the length OB, divided by length OA is the ratio of the "real" power (watts W) to the "apparent power" (Ia times V), known as the "power factor" - it is also the "cosine of the angle phi, often called "COS phi". It is also true Ir represents the reactive current and "Ir times V" is the reactive power which is stored and recycled from the load. Now I drew the triangle with Ia = 10 cm, Ib = 8 cm and Ir = 6 cm, because Ib/Ia = 0.8 ( a commonly quoted power factor) and 10 x 10 = 100 = (6 x 6) + (8 x 8) = 36 + 64 (Thanks, Pythagoras!). The "real" power is called real because it is the sum of the mechanical shaft power coming out of any motors and heat losses in resistance (including those in filament lamps, which also create a bit of light energy), insulation and magnetic materials. It is interesting to consider that, in a motor with an input of 4.35 amps at 230 volts (230 x 4.35 = 1000 VA) at 0.8 power factor, 1000 x 0.8 = 800 Watts is used in heating the air around the motor and turning its shaft while a surprising 600 VAR is being repeatedly stored and returned by its magnetic fields (Ir is 6 units in the drawing, compared to 8 units for Ib). Finally, the triangle was drawn with current "leading" voltage, but it is more common for the current to be "lagging" the voltage (with Ib on the right hand side of Ia) - hence use of terms "leading power factor" and "lagging power factor" [ lagging is often called positive (+) and leading is called negative (-), particularly when numerical VAR values are given].

It's like pushing your son on a swing.

KVA is like saying you raised 100 pounds 4 foot every second.

KW is the truth or you raised 12 pounds 1/2 foot every second.