Reactive power explained

Several of years ago, my then-roommate – a mechanical engineer-in-the-making – asked what the difference between active, reactive and apparent power was. Bear in mind that I was only a bachelor’s student back then. Second year or so. Obviously, I had taken my courses on circuit analysis etc. so I definitely knew all the formulae backwards and inside-out, and could calculate power factors in my sleep.

But, my roommate obviously didn’t, just as I didn’t know about material strength calculations. Thus, I couldn’t just spout some equations with complex conjugates and be done with it. I had to come up with an actual explanation. Or even better, an analogy.

Needless to say, it was quite difficult. Indeed, being able to explain something to a complete outsider is often said to be the true definition of an expert. True or not, I like this classification.

Anyway, back then I was only a student, so definitely not an expert. So, I mumbled something incoherent and hid in my room. Ouch.

But! A couple of weeks later I was able to come up with a nice explanation! So here goes, the concepts of active and reactive power explained to mechanical engineers:

Imagine you are pumping water with a hand-operated pump, with a very, very heavy lever. Something like this, but much uglier. The water you actually manage to pump out, that’s the active power. But, any time you push the heavy lever up, you have to overcome both gravity and the very inertia of the lever. Obviously, work is being done, but no water comes out because of that. It’s accumulated on the potential and kinetic energy of the lever, instead.

Then you reach the top, and it’s time for the lever to come down again. The pull of gravity may be so strong, the lever so heavy, that you actually have to actively slow it down, in order for it to not crash down and break the pump (and your toes). Again, work is being done. But, as any mechanical engineer understands, this time it’s the gravity doing the work, giving you back the energy you stored on the potential of the lever mere seconds ago. (And which you then pretty much waste, thanks to the nigh-completely non-regenerative nature of the human muscle system, save for the elasticity of your tendons.)

All in all, only a portion of your work does what you intend – pumps out some water from the well or whatever. That would be the active power. But, some of your work is accumulated on the increasing potential energy of the lever, only to be received back completely just seconds later. This is the reactive power. Your combined efforts in total – the amount of sweat, grunts and hunger generated – would then correspond to the apparent power.

And exactly the same thing happens on AC power systems. Power is all the time flowing back and forth. The mean amount that goes from the power supply to the system and (hopefully) does something useful (or is simply lost as heat) is the active power. By contrast, the power that flows back from the system to the supply a half-cycle later is the reactive power.

And that, my dear friends, is how you explain reactive power to a mechanical engineer.

And confuse him even further, most likely.

Reactive power explained