PROBING MYSTERIES OF THE INVERTER: PART I

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Some inverters extend this concept even further by stepping in multiple currents to generate the "staircase" effect you see in Fig. 5C. The intricate switching pattern, however, requires a controller that borders on being a microcomputer, which increases the complexity and cost of the devices. In fact, as a general rule, you can assume that the closer an inverter's waveform is to sinusoidal, the more you should expect to pay for the device.

SURGE DEMANDS

Inverter design and selection is further complicated by the fact that not all AC loads react equally to alternating voltage. A utility company can compensate for this inherent problem within its sizable grid ...

but in a small, independent electrical system, the behavior of the load(s) is reflected back into the inverter.

An induction motor, for instance, requires substantially more power to get going than it does to operate at its design speed. (It actually takes six times as much juice to start up one of these motors as it does to run it!) As a matter of fact, every electrical device in your household has some surge demand, though none is as demanding as the induction motor.

A surge current can last for anywhere from a fraction of a second to almost a full minute ... and, during this time, the inverter is placed under heavy stress. Fortunately, inverters can be engineered to withstand very short periods of extreme overload ... but this sort of abuse is best handled by SCR-equipped devices.

Of course, you could just buy an inverter large enough to handle the calculated surge loads of the devices you want to run. The disadvantages to this approach, however, are twofold. First, and most obvious, a bigger inverter costs more. Second, if you'll recall our discussion of standby (or tare) power, you'll realize that the upsized inverter will be forced to operate in its inefficient, low-demand range once the load has passed surge and dropped to its conservative operating level.

POWER FACTOR

Another aspect to consider when dealing with AC electricity is the power factor of the load(s). The power factor results from a time distortion between the voltage and current components of the AC waveform, and is created during an operation with a reactive load (such as the running of an induction motor).

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