ABSTRACT
Alternating Current Generators All magnets have “north and south” poles, each of opposite magnetic orientation. This is true of every magnet, and is an unavoidable law of nature. As these poles alternately sweep by a wire loop, i.e., the generator, they create current flows in opposite directions: One pulls the current, the other pushes it, and the series of pulses created are a “push-pull” flow, or alternating current (AC). One complete set of push pull, two pulses of opposite direction, is called a cycle. The diagram in Figure 6.1 shows the basic concept behind a rotating AC generator. Actual generators are much more refined versions of the one shown. The wire, stator winding, often has many loops, arranged in a non-circular shape engineered with extreme care, to have just the right thickness, just the right number of turns in each loop, just the right length and depth to its shape, etc., for optimum performance. Similarly, the rotor is seldom a simple bar magnet, as shown, but something closer to a round cylinder, optimized for peak performance. Sometimes the rotor is made of wire loops, and a magnet is used for the outer portion. In others, both rotor and outer portion are wire loops, since a wire loop, if fed electricity, creates a magnetic field similar to a bar magnet. Regardless, the basic concept is as illustrated. Most generators have more than one set of magnets. Imagine a second bar magnet in Figure 6.1, perpendicular to the first, creating a rotor with an “X” cross-section. The generator would now produce twice as many pulses per revolution. Some generators have dozens of magnets. Multiple magnetic poles mean the rotor need not spin at a very high speed to produce the required number of pulses per second: A generator with four magnets in its rotor, for example, needs turn only 900 or 750 RPM to produce the 3,600 or 3,000 cycles
per minute, respectively. This permits the rotor to be lighter weight and more efficient. Much the same result can be accomplished through innovative and complicated ways of arranging many wire loops in the stator. Higher speed requires more strength, which means heavier parts throughout. A typical large “central station” generator, capable of producing 600 MW of AC electric power (enough to meet all the residential, commercial, and industrial needs of a city of 200,000 people), is actually not very large if one thinks of the amount of resulting power, as illustrated in Figure 6.2. The largest electric generators are perhaps 50 feet long and 15 feet wide, with the rotor inside being four to ten feet in diameter, spinning at 1,200 to 2,000 RPM. Generators in large hydro plants are similar in concept, but quite different in design. They have a vertical rather than horizontal shaft, are wider than they are high, with dozens of magnetic poles inside a rotor that is perhaps 40 feet across, and turn much more slowly, at only about 100 RPM.
