Inverter

What is an inverter?

One of Tesla's legacies (and that of his business partner George Westinghouse, boss of the Westinghouse Electrical Company) is that most of the appliances we have in our homes are specifically designed to run from AC power. Appliances that need DC but have to take power from AC outlets need an extra piece of equipment called a rectifier, typically built from electronic components called diodes, to convert from AC to DC.

Inverter

An inverter does the opposite job and it's quite easy to understand the essence of how it works. Suppose you have a battery in a flashlight and the switch is closed so DC flows around the circuit, always in the same direction, like a race car around a track. Now what if you take the battery out and turn it around. Assuming it fits the other way, it'll almost certainly still power the flashlight and you won't notice any difference in the light you get—but the electric current will actually be flowing the opposite way. Suppose you had lightning-fast hands and were deft enough to keep reversing the battery 50–60 times a second. You'd then be a kind of mechanical inverter, turning the battery's DC power into AC at a frequency of 50–60 hertz.

Of course the kind of inverters you buy in electrical stores don't work quite this way, though some are indeed mechanical: they use electromagnetic switches that flick on and off at high speed to reverse the current direction. These kind of sudden power reversals are quite brutal for some forms of electrical equipment. In normal AC power, the current gradually swaps from one direction to the other in a sine-wave pattern, like this:

electronic inverters can be used to produce this kind of smoothly varying AC output from a DC input. They use electronic components called inductors and capacitors to make the output current rise and fall more gradually than the abrupt, on/off-switching square wave output you get with a basic inverter.

Inverters can also be used with transformers to change a certain DC input voltage into a completely different AC output voltage (either higher or lower) but the output power must always be less than the input power: it follows from the conservation of energy that an inverter and transformer can't give out more power than they take in and some energy is bound to be lost as heat as electricity flows through the various electrical and electronic components. In practice, the efficiency of an inverter is often over 90 percent, though basic physics tells us some energy—however little—is always being wasted somewhere!

How does an inverter work?

Inverter

We've just had a very basic overview of inverters—and now let's go over it again in a little bit more detail.

Imagine you're a DC battery and someone taps you on the shoulder and asks you to produce AC instead. How would you do it? If all the current you produce flows out in one direction, what about adding a simple switch to your output lead? Switching your current on and off, very rapidly, would give pulses of direct current—which would do at least half the job. To make proper AC, you'd need a switch that allowed you to reverse the current completely and do it about 50‐60 times every second. Visualize yourself as a human battery swapping your contacts back and forth over 3000 times a minute. That's some neat finger work you'd need!

In essence, an old-fashioned mechanical inverter boils down to a switching unit connected to an electricity transformer. If you've studied our article on transformers, you'll know that they're electromagnetic devices that change low-voltage AC to high-voltage AC, or vice-versa, using two coils of wire (called the primary and secondary) wound around a common iron core. In a mechanical inverter, either an electric motor or some other kind of automated switching mechanism flips the incoming direct current back and forth in the primary, simply by reversing the contacts, and that produces alternating current in the secondary—so it's not so very different from the imaginary inverter I sketched out above. The switching device works a bit like the one in an electric doorbell. When the power is connected, it magnetizes the switch, pulling it open and switching it off very briefly. A spring pulls the switch back into position, turning it on again and repeating the process—over and over again.