Probing the Mysteries of Power Inverters: Part I

Thanks to power inverters, generating your own electricity doesn't mean you have to give up the convenience of AC living.

| March/April 1983

The progress made in developing alternative energy sources over the last decade has demonstrated that independent power systems (those using other than fossil fuels) are not only possible but are also practical. In fact, a wide variety of generating equipment is now available to allow individuals to take advantage of just about any renewable source of energy.

For a number of reasons, however, most of these systems produce only direct current (DC), and often do so only at low voltages. Nonetheless, it's generally agreed that the greatest potential use for alternative energy in the future will be to serve alternating current (AC) loads because those exist in the vast majority of modern homes. Conventional refrigerators, most televisions, and all induction motors (which comprise 97% of the U.S. supply of electric motors) simply won't work on direct current. Furthermore, although there are DC-compatible appliances, they're typically quite expensive.

Of course, one way out of this dilemma would be to convert direct current to alternating current, and one of the most effective ways to accomplish this is through the use of electronic inverters. In this article, I'm going to tell you about the different sorts of power inverters that are available and their applications. In a future article I'll get into the details of putting one of these devices to work.

AC Power

To understand how inversion is accomplished, you should first comprehend how AC power differs from DC. Compare the voltage components of both types of power and you'll see that in alternating current the voltage is constantly reversed. The polarity (positive or negative) switches rhythmically in the form of a sine wave. On the other hand, the polarity of direct current is constant; that is, the positive lead is forever positive (even though the voltage may vary).

The basic idea of an inverter, therefore, is to periodically change the polarity of the DC source. Of course, there is more than one way to approach the problem, but semiconductors (such as transistors) — which accomplish the metamorphosis by switching currents through a transformer — are the most common solution.

Inside the Inverter

Figure 2 is a simplified diagram of a typical inverter circuit. The workhorses are transistors — solid-state switches that can be changed from "off" to "on" simply by applying a small voltage to their control elements. To take advantage of this property, the emitter leads from a pair of transistors are connected to opposite ends of a center-tapped transformer. The collector lead from each transistor is then wired to the positive leg of a DC power source—a storage battery, for example—and the center tap from the transformer is returned to the negative lead of the power source.

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