Why build a dam to produce electricity? Dams must be constructed because the force of flowing water rarely is powerful enough or consistent enough to generate large amounts of reliable electrical power. Building a dam guarantees that enough water force will be readily available to maintain a steady stream of power moving from the plant into the national power grid.
Only dams that can back up lakes several hundred feet deep are able to produce enough electricity to worth building.
It is common for the lakes created by large scale hydroelectric producing dams to reach beyond 500 feet deep. As a rule, deeper lakes are capable of producing greater amounts of electricity. Many dam projects on smaller rivers that initially were proposed to become hydroelectric plants were converted to be used for recreation and flood control because the lake basin was not deep enough to be practical for power production.
High pressure is created at the bottom of deep lakes.
In the same way that divers have to be careful about going too deeply because of the external pressures caused by large amounts of water overhead, hydroelectric dams rely on this pressure to generate electricity. Giant tubes are built into the dam and lake basin to allow the pressurized water to flow with tremendous force to drive enormous generators. By controlling the amount of water allowed to reach the turbines that spin the generators, the quantity of electricity produced can be adjusted.
The generators have shafts that extend into the giant tubes.
At the end of these shafts is an actuator much like is found in small water pumps. Instead of pumping water, the water spins the rotor to drive the generator. To make this easy to visualize, image the old fashioned grist mills with the water wheel. As the water poured over the wheel, a shaft went through the wall of the mill and turned the gears to rotate the mill stone along a track to grind the grain in flour or meal. In a hydroelectric dam, the water pours through the tubes and spins the wheel on a vertical shaft. The shaft goes up through the center of the generator and turns an armature within a magnetic field. This spinning process creates electrical current in the wiring of the armature. The electricity then flows from the generator into wires that convey it transformers.
Power in the national grid is extremely high voltage.
These transformers pump up the voltage into the hundred thousand range. Once in the power grid, the electricity can flow for hundreds or even thousands of miles to step down transformers. These transformers lower the voltage down to thousands of volts to enter various city grids. In the local grids the power is further reduced through more transformers to meet the needs of commercial and residential customers. By the time it hits your house, you will receive a normal 220 volt electricity with 200 ampere service.
