Refer to ASME Standard LOS-1M.
Generator Types. Generators are classified as either synchronous (AC) or
direct current (DC) machines. Synchronous generators are available for either 60 cycles
(usually used in U.S.A.) or 50 cycles (frequently used abroad). Direct current
generators are used for special applications requiring DC current in small quantities
and not for electric power production.
Self Ventilation. Generators, approximately 2,000 kVA and smaller, are air
cooled by drawing air through the generator by means of a shaft-mounted propeller fan.
Air Cooled. Generators, approximately 2,500 kVA to 25,000 kVA, are air cooled
with water cooling of air coolers (water-to-air heat exchangers) located either
horizontally or vertically within the generator casing. Coolers of standard design are
typically rated for 95 degrees F (35 degrees C) cooling water at a maximum pressure of
125 psig (862 kPa gage) and supplied with 5/8-inch minimum 18 Birmingham wire gage (BWG)
inhibited admiralty or 90-10 copper-nickel tubes. Design pressure of 300 psig (2068 kPa
gage) can be obtained as an alternate. Also, alternate tube materials such as aluminum-
brass, 70-30 copper-nickel, or stainless steel are available.
Hydrogen Cooled. Generators, approximately 30,000 kVA and larger, are
hydrogen cooled by means of hydrogen to air heat exchangers. The heat exchangers are
similar in location and design to those for air-cooled generators. Hydrogen pressure in
the generator casing is typically 30 psig (207 kPa gage).
Turbine Generator Control. For turbine generator control description, see
Turning Gear. In order to thermally stabilize turbine rotors and avoid rotor
warpage, the rotors of turbine generators size 12,500 kW and larger are rotated by a
motor-driven turning gear at a speed of approximately 5 rpm immediately upon taking the
turbine off the line. The rotation of the turbine generator rotor by the turning gear
is continued through a period of several hours to several days, depending on the size of
the turbine and the initial throttle temperature, until the turbine shaft is stabilized.
The turning gear and turbine generator rotor are then stopped until the turbine
generator is about to be again placed in service. Before being placed in service, the
turbine generator rotor is again stabilized by turning gear rotation for several hours
to several days, depending on the turbine size.
Turbine generators smaller than 12,500 kW are not normally supplied with a
turning gear, since the normal throttle steam temperature is such that a turning gear is
not necessary. However, should a turbine be selected for operation at higher than usual
throttle steam temperature, a turning gear would be supplied.