PROBING MYSTERIES OF THE INVERTER: PART I

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To make it more clear, let's consider inductance (a reactive value created by the windings in a motor). By its very nature, inductance resists changes in the current flowing through it (it would rather the current remained steady). But as we have discovered, both the voltage and current in an AC system are changing constantly.

So, as the voltage (which is unaffected by inductance) increases across the motor winding, the current must work harder to stay in step with it. However, despite such efforts, the current can't keep up and lags behind the voltage, as shown in Fig. 6.

When you consider that the simultaneous combination of voltage and amperage makes up power, you can begin to see what problems this time distortion might present. If the voltage and current are out of phase with each other, then the power—which is calculated by multiplying the instantaneous values of voltage and amperage—will be less than you would get by multiplying the peak voltage by the peak amperage in the ideal situation.

The discrepancy between the true power (the amount of energy the load is actually using) and the apparent power (the power the load seems to be using when the voltage and current are multiplied, disregarding timing) is the power factor. For example, an induction motor with a power factor of 0.7 is really consuming only 70% of the power available to it. To distinguish between the two values, apparent power is expressed in volt-amps (VA), while true power is measured in watts. Consequently, the volt-amp rating of an appliance is always higher than its actual wattage.

Unfortunately, this phase shift also reflects back into the inverter, where it can do real damage. The situation is easily corrected, though, by placing a capacitor across the inductive load. You see, a capacitor has a reactive component which is just opposite that of an inductor: It resists changes in voltage while leaving current unaffected. By paralleling the proper amount of capacitance to the inductive load, the effects of the inductance can be cancelled, and the sine wave will once again be in phase with itself.

INVERTER INFLUENCE

Just as the nature of the load affects the inverter, so can the inverter affect some loads. There are three important cases in which this can come into play ... and we'll discuss those situations briefly here.

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