PROBING THE MYSTERIES OF THE INVERTER: PART II

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To begin with, inverters cannot be connected in parallel to increase their output ... as is often done with DC power sources. Unless the outputs are completely synchronized, one inverter will feed back into the other, thereby destroying one or both. Very few inverters on the market today provide a synchronization signal for parallel operation. (The exception to this rule is the motor/generator inverter, which is self-synchronizing.)

The most obvious way to enlarge a DC-to-AC system is to replace the existing inverter with a larger one. There are two drawbacks to this approach, however. First, you'd have to pay quite a bit for the new, larger inverter. And second, during periods when your overall load is small, the big unit would operate inefficiently.

You can avoid both of these problems when upgrading a system, though, by using several small inverters ... each tackling a specific task. Let's say, for example, that you originally intended your system to power an assortment of hand tools in the workshop. But now you've gotten a freezer that could run off your present inverter but would leave no power for the workshop.

Instead of replacing the present inverter with a larger one, it would be better to add another inverter of the same rating to the system. One would then be used to operate the hand tools and the other could accommodate the freezer. In this way, you'd have power for both uses, at a good overall efficiency, and with a relatively low cash outlay.

Let me emphasize, however, that each inverter must have completely separate AC outlet lines to the particular loads. In no way can the outlets ever be mixed! (The DC inputs,

on the other hand, can be paralleled, as shown in Fig. 3.)

SAFETY

When working with high-voltage AC systems, one becomes acutely aware of the safety hazards involved. All states have established or adopted guidelines for wiring homes and buildings with AC power. And these codes were implemented for only one reason: to insure the well-being of the occupants. So, even though you may not be forced to abide by these regulations in your particular application, keep in mind that they make good sense and bear looking into.

In short, though, never run AC power through wires that were designed to handle only low voltage . . . never overload your circuits (provide a separate line for each large load) ... and always connect the ground returns. In other words, use common sense!

An AC inverter also places large loads on the DC side of the system, since it has the capacity to supply loads which require a great deal of overall wattage. When you're working with the large currents commonly found on the input side of an inverter, for instance, heat buildup problems can arise. Localized resistance, particularly at junction points, can create hot spots ... and therefore fire hazards. What's more, a poor bond between a battery cable and an inverter screw terminal can generate enormous amounts of heat ... not to mention power loss.

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