PROBING MYSTERIES OF THE INVERTER: PART I

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INSIDE THE INVERTER

Fig. 2 is a simplified diagram of a typical inverter circuit. The workhorses are transis tors ... 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.

If you're a little hazy about this, read on ... I think the picture will become clear once the sequence of events is described. First, a signal is applied to the left transistor that "tells" it to allow current to pass. The flow of electrons proceeds from the battery, through the transformer, into the transistor, and back around to the battery ... counter-clockwise as indicated by the arrow. The transformer primary winding delivers the energy to the output winding, and current moves in a clockwise direction through the load as shown.

After about eight-thousandths of a second, the first transistor is turned off and the other (on the right) is switched on. As shown in Fig. 3, the current again flows from the battery, through the transformer, into a transistor (but this time it's the one on the right), and back to the battery. However, the current is now traveling in the opposite direction (clockwise) in the transformer windings. This means that the electron movement in the load portion of the circuit is counterclockwise.

As you can see, then, this switching mimics an AC current pattern, and when it performs a backward and forward flow 60 times per second, you get the rough equivalent of 60-hertz electricity. (What's more, the transformer can be designed to raise voltage to the desired level.)

EFFICIENCY

Although transistors are employed in many small inverters, they're not really very efficient when used as switching devices. Consequently, for inverters larger than one kilowatt (KW), a component called a silicon-controlled rectifier (SCR) is substituted in the same basic circuit. Units using SCR's are available with power capabilities of 10 KW or more.

Unfortunately, even an SCR absorbs a portion of the power that it's controlling, and losses in the transformer core and from wire resistance add to the toll. All in all, you can expect an inverter to supply about 9007b of the input power to the load ... but only under ideal conditions.

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