|MadSci Network: Engineering|
As you say, the electricity flow is from the fixed portion of the generator that surrounds the rotating magnets. The fixed part is called the "stator" and the central rotating magnetic field device that is turned by the turbine is called the "rotor." The frequency output of the generator is controlled by the turbine's speed, or how fast it rotates the magnets. A DC generator is connected to the opposite end of the generator from the turbine. It turns at the same speed as the turbine and is designed to produce more than enough DC power to excite the magnetic poles of the main generator. The voltage output of the generator is controlled by controlling the DC output of this "exciter." It provides it's own DC supply to its rotor by using a trick known as "residual magnetism." When starting a large Turbine-Generator set, the turbine is accelerated to operating speed very slowly to allow it to warm up and expand accordingly. As this is occurring, the exciter is turning faster and faster. The slight amount of magnetism that is a residual in the iron in the exciter's rotor is enough to start it producing a small DC voltage, and as the speed increases, the exciter voltage increases which then increases it's own rotor voltage until automatic controls begin to limit and control that voltage - the exciter excitation voltage, so to speak. Once the generator is brought "up to speed" and connected "on-line." The exciter control and the turbine throttles are used to regulate the output in the desired manner.
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