What is reactive power? I am trying to understand what is reactive power. I have read that it has a relation with voltage, that is has a relation with the creation of a magnetic field in a motor, that it is coming and going between consumption and generators. But what exactly is the physical meaning of reactive power? 
 A: 
But what exactly is the physical meaning of reactive power?

Essentially, reactive power is the component of power that has zero time average.
For example, consider a load consisting of a resistance $R$ in parallel with an inductance $L$ driven by a source with voltage $v_S(t) = V_S\cos \omega t$
Clearly, the source current is
$$i_S(t) = \frac{V_S}{R}\cos \omega t + \frac{V_S}{\omega L}\sin\omega t$$
Thus, the instantaneous power delivered by the source is
$$p_S(t) = v_S\cdot i_S = \frac{(V_S)^2}{R}\cos^2 \omega t + \frac{(V_S)^2}{\omega L}\sin \omega t\cos \omega t$$
Carefully note that the first term is never negative which is to say that the flow of energy described by this term is always from source to load or never from load to source.

However, the second term is positive over half of a cycle and negative the remaining half of a cycle.  That is, this term describes energy that flows back and forth, in equal measure, between the source and load.
Take the time average of the power over a period:
$$\langle p_S\rangle = \frac{\omega}{\pi}\int_0^{\frac{\omega}{\pi}} p_S(\tau)\:\mathrm{d}\tau = \frac{1}{2}\frac{(V_S)^2}{R}$$
and see that only the first term has a non-zero time average; the second term does not contribute to the time average energy flow.
In the context of phasor analysis, the real power (the real part of the complex power) is equal to the time average of the instantaneous power.
The reactive power (the imaginary part of the complex power) is (proportional to) the amplitude of the second term in the instantaneous power.
And this is the physical meaning of reactive power; it is a measure of the energy flow back and forth between source and load.
A: It means that the current an voltage are not exactly in phase with one another as they would be for a resistor.
If the phase difference between the voltage and the current is $\delta$ then the power dissipated is in a component is $V_{\text{rms}} I_{\text{rms}} \cos \delta$.  This is called the read or active power and is measured in watts.
The reactive power is $V_{\text{rms}} I_{\text{rms}} \sin\delta$ and it is measured in volt-ampere reactive or var.
It is the maximum power absorbed or given out by reactive circuit elements.
So for your motor the coil would have inductance and resistance (and possibly capacitance).
The resistive part would dissipate energy as heat whereas the reactive part would store and then give back energy.
If $\delta = 90^\circ$ then the current and voltage are such that every quarter of a cycle the reactive component is absorbing power (the magnetic field in an inductor is increasing or the electric field in a capacitor is increasing) and then for the next quarter cycle the the reactive component is giving out energy (magnetic/electric field decreasing).
A: I'm an electrical engineer also at odds with how has reactive power being described or rather thought about, and this is because utilities don't like it. This is because reactive power brings losses to transmission lines, but you know what? It's innevitable, because of two reasons:


*

*To correct one of your concepts, reactive power (KVAR) is a natural component of electrical power (KVA), not of voltage. The simplest way I put it is: KVA = KW + KVAR And then is a matter of how much of each, which brings the other point.

*Many loads need it, in fact every magnetic load will use it. So, unless there are devices that supply reactive power at the load, the utility generators provide it...naturally. Along with whatever undesirable effects.
A: Another relative term..."work", which for electricity usually means some measure of heat. Saying that reactive power doesn't perform real work, seems to be incorrect. Of course, for an iron heating element, the reactive component is nearly zero (it's there, just contributes almost nothing), but the fan...it needs this reactive component, and so...it's doing something.
Magnetic systems, as in...a maglev train system is pushing up and forward tons of weight...at very high speeds; isn't that work? Maybe there's not much heat (well, transformers and cables will get hot), but there is certainly a big force.
I think it's important to understand that some concepts were influenced by whatever knowledge was at the time; new knowledge should modify them. "Work" means something is moving, something is being done, which uses energy or power. It's not just the real (watts) component or the imaginary (reactive) component, they're both contained in the electrical energy form, and the nature of the load determines how much of each component will be used.
A: Reactive power is basically the pulsating power whose time average for a complete cycle is zero. This power does not perform any real work. Here real work means that heater will not produce heat using this power or fan will not rotate merely by reactive power.
For physical significance, just take the example of a coolie carrying your language. In terms of physics he is not performing any work but still we pay them. The same analogy can be applied for reactive power.
If voltage and current are in quadrature as is the case with pure inductor / capacitor, the real power consumed will be zero. But in case of pure resistance, as the current is along the voltage, it will consume real power and hence heating effect will be produced.  
Source: Active, Reactive and Apparent Power - Electrical Concepts
