PROBING THE MYSTERIES OF THE INVERTER: PART II

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The VA ratings of many appliances are included on their labels ... but power factors are seldom listed. And, without such correction figures, you have no way of knowing exactly how much wattage the unit will require. The easiest way through this dilemma would be to take the load rating at face value. In other words, just assume that the VA rating is the same as the wattage rating. (It isn't, of course. The wattage will always be something less than the VA.)

However, when you're considering the voltamp listing on an inverter, you can't simply take it at face value. An inverter rated at 500 VA won't produce 500 watts ... not even close to it. As a rule, though, you can assume that the manufacturer included a power factor of 0.7 in the unit's figures. And using that number, you can see that a 500-VA inverter is good for only 350 watts! But keep in mind that some inverters do have ratings as high as 0.85 .'. . which would provide significantly more "juice" than would be indicated by our guesstimated power factor of 0.7.

And what would happen if we used the "face value" method of matching inverters and loads? Well, there's a good chance that the power factors would be close enough for the arrangement to work, but there's no guarantee. If you have any doubt, always go with a slightly "oversized" inverter.

OTHER CONSIDERATIONS

For systems that require less than 1,000 watts of power, an inexpensive transistorized inverter will probably fill the bill. Furthermore, such units are available at most auto or department stores ... and you should be able to pick up one of about 350-watt capacity for a little more than $100.

Unfortunately, these small inverters have a limited scope of application. For instance, most of them generate a square waveform, which won't be compatible with all your equipment. Square-wave distortion runs counter to the needs of inductive-load mech- anisms, such as refrigerator motors, and will cause the equipment to run hotter than normal ... sometimes with catastrophic results!

The solution is to use a sine wave inverter-or one of the modified square wave types-for those critical loads. Of course, since it takes more circuitry (and therefore expense) to generate a sine wave than to produce a square wave, such inverters are more costly. For that reason, most sine wave inverters are rated at a couple of KW or more ... since the expense of the added circuitry becomes a smaller part of the whole cost in a larger unit.

Another figure to keep in mind when choosing an inverter is surge power capacity. An induction motor, for instance, will draw up to six times its rated power when first started! This overload is generally short lived, and many inverters are designed to cope with it ... but make sure yours will handle the demand before you buy it.

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