Inverters

Last updated: June 29, 2009.

One of the most significant battles of the 19th century was fought not over land or resources but to establish the type of electricity that powers our buildings.

At the very end of the 1800s, electrical pioneer Thomas Edison (1847–1931) went out of his way to demonstrate that direct current (DC) was a better way to supply electrical power than alternating current (AC), a system backed by his arch-rival Nikola Tesla (1856–1943). Edison tried all kinds of devious ways to convince people that AC was too dangerous, from electrocuting an elephant to (rather cunningly) supporting the use of AC in the electric chair for adminstering the death penalty. Even so, Tesla's system won the day and the world has pretty much run on AC power ever since.

The only trouble is, though many of our appliances are designed to work with AC, small-scale power generators often produce DC. That means if you want to run something like an AC-powered gadget from a DC car battery in a mobile home, you need a device that will convert DC to AC—an inverter, as it's called. Let's take a closer look at these gadgets and find out how they work!

Photo: A selection of electricity inverters that can be used with renewable energy generating equipment, such as solar cells and micro-wind turbines. Photo by Warren Gretz courtesy of US Department of Energy/NREL (DoE/NREL).

What's the difference between DC and AC electricity?

When science teachers explain the basic idea of electricity to us as a flow of electrons, they're usually talking about direct current (DC). We learn that the electrons work a bit like a line of ants, marching along with packets of electrical energy in the same way that ants carry leaves. That's a good enough analogy for something like a basic flashlight, where we have a circuit (an unbroken electrical loop) linking a battery, a lamp, and a switch and electrical energy is systematically transported from the battery to the lamp until all the battery's energy is depleted.

Diagram: When we think of electricity as a flow of electrons, we're usually picturing DC (direct current) in our minds.

In bigger household appliances, electricity works a different way. The power supply that comes from the outlet in your wall is based on alternating current (AC), where the electricity switches direction around 50-60 times each second (in other words, at a frequency of 50-60 Hz). It can be hard to understand how AC delivers energy when it's constantly changing its mind about where it's going! If the electrons coming out of your wall outlet get, let's say, a few millimeters down the cable then have to reverse direction and go back again, how do they ever get to the lamp on your table to make it light up?

The answer is actually quite simple. Imagine the cables running between the lamp and the wall packed full of electrons. When you flick on the switch, all the electrons filling the cable vibrate back and forth in the lamp's filament—and that rapid shuffling about converts electrical energy into heat and makes the lamp bulb glow. The electrons don't necessarily have to run in circle to transport energy: in AC, they simply "run on the spot."

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.

Photo: A typical electricity inverter. This one is made by Xantrex/Trace Engineering. Photo by Warren Gretz courtesy of US Department of Energy/NREL (DoE/NREL).

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 race cars round 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. Inverters like this often produce what's known as a square-wave output: the current is either flowing one way or the opposite way or it's instantly swapping over between the two states:

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!