Re: Production of commercial electricity

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.