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Basics of Alternating
Part One—Sine Waves
©1996 Richard Perez Renewable energy systems have
always been big users of direct
current (DC) electricity. PV modules make DC electricity and DC electricity is what batteries store. With the advent of modern power inverters, however, more and more RE systems are using most of their power as alternating current (ac), just like everyone who lives on the grid. Understanding the basics of alternating current is simple—it’s really just a matter of timing...
Direct current electricity is constant and consistent. Current flow is unidirectional. Voltage polarity is rigid— positive is positive and negative is negative and that’s that. If you are not familiar with direct current electricity,
then read Ben Root’s article beginning on page 64 of this issue. You will need the concepts and terminology there to understand what you will read here.
When we enter the realm of alternating current (ac) everything changes. And I mean everything—voltage, current and even the concept known as resistance in DC electricity. The static world of direct current is lost in a sea of changes.
Fortunately the world of alternating current has its own consistencies. Sound confusing? Well, it can be. Alternating current electricity is constantly changing. Current flow and voltage vary by the millisecond. But these constantly changing electric manifestations follow a regular and repeating pattern. It is in the structure of this endlessly repeating pattern that ac electricity reveals its secrets. It’s just a matter of waves and timing.
Alternating current is based on sinusoidal waves. These sinusoidal waveforms betray ac electricity’s beginnings in rotational motion. Rotary ac alternators produce power with sine wave characteristics. Before understanding ac electricity, it is first necessary to understand the sine wave and how it behaves.
A sine wave is derived from angular motion. Imagine a circle with a rotating radius, exactly like a clock’s face with only a minute hand. As the hand ticks off the time, the angle between the hand and the horizontal 9 o’clock axis changes. The height of the hand’s pointer above the horizontal axis also changes. The sine of the angle at any point is the height of the point above the horizontal axis divided by the radius of the circle.
74 Home Power #52 • April / May 1996
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