Content area
Full text
Understanding an electrical principle that is too often taken as a given
WHY CAN CAPACITORS "CORRECT" power factor on circuits involving motors? We know that in inductive apparatus, such as an a-c motor, current lags voltage. Capacitors draw current that leads the voltage-therefore, they offset the lag.
But why does current lead voltage in a capacitor? Back when we were learning basic electricity, a common explanation went like this: When the circuit is first closed, current must flow into the capacitor to build up a charge on the plates before voltage can build up on those plates. Hence, current must lead voltage.
That explanation leaves much to be desired. More often, "fundamentals" training begins with lead-lag relationships as "givens," and only the consequences of those relationships are explained in detail.
What, then, is the real reason we say that, in a "perfect" capacitor, the current not only leads the voltage in time phase, but leads by exactly 90 degrees? Why does current lead, and why not some other angle?
What is capacitance?
Our answer must begin with a review of what we mean by "capacitance." Whether or not we choose the "electron theory" of...