897. If increasing the power demand on a generator that is turning at a steady rate simply increases the torque needed to keep that generator turning, why do brownouts occur?
As long as the generator continues to turn steadily, it will produce its normal voltage rise and the frequency of its alternating current won't change. When the homes powered by the generator draw more current, then the generator simply becomes more difficult to turn and the steam turbine that spins it has to exert more torque on it. But suppose that the turbine can't exert any more torque on the generator. In that case, the power company can either shut down the generator or it can reduce the strength of the generator's rotating magnet. This rotating magnet is actually an electromagnet and its strength determines the voltage rise across the generator. During a period of excessive current demand, the power company may choose to weaken the rotating electromagnet to prevent the steam turbine from becoming overloaded. When they weaken the electromagnet, the generator becomes easier to spin but it produces less voltage. The electricity leaving the generator still has the right frequency alternating current, but it voltage is somewhat lower than normal and the light bulbs it powers glow relatively dimly—a brown-out.