AC Generation

Anunderstanding of how an AC generator develops an AC output will help us analyzethe AC power generation process.

Theelementary AC generator (Figure 1) consists of a conductor, or loop of wire ina magnetic field that is produced by an electromagnet. The two ends of the loopare connected to slip rings, and they are in contact with two brushes. When theloop rotates it cuts magnetic lines of force, first in one direction and thenthe other.

Developmentof a Sine-Wave Output

Figure1 Simple AC Generator

Atthe instant the loop is in the vertical position (Figure 2, 0o), the coil sidesare moving parallel to the field and do not cut magnetic lines of force. Inthis instant, there is no voltage induced in the loop. As the coil rotates in acounter-clockwise direction, the coil sides will cut the magnetic lines offorce in opposite directions. The direction of the induced voltages depends onthe direction of movement of the coil. The induced voltages add in series,making slip ring X (Figure 1) positive (+) and slip ring Y (Figure 1) negative(-). The potential across resistor R will cause a current to flow from Y to Xthrough the resistor. This current will increase until it reaches a maximumvalue when the coil is horizontal to the magnetic lines of force (Figure 2,90o). The horizontal coil is moving perpendicular to the field and is cuttingthe greatest number of magnetic lines of force. As the coil continues to turn,the voltage and current induced decrease until they reach zero, where the coilis again in the vertical position (Figure 2, 180o). In the other halfrevolution, an equal voltage is produced except that the polarity is reversed(Figure 2, 270o, 360o). The current flow through R is now from X to Y (Figure1).

Figure2 Developing a Sine-Wave Voltage

Theperiodic reversal of polarity results in the generation of a voltage, as shownin Figure 2. The rotation of the coil through 360° results in an AC sine waveoutput.

Summary

• Asimple generator consists of a conductor loop turning in a magnetic field,cutting across the magnetic lines of force.

•The sine wave output is the result of one side of the generator loop cuttinglinesof force. In the first half turn of rotation this produces a positivecurrent and in the second half of rotation produces a negative current. Thiscompletes one cycle of AC generation.