Steam electric generator

An alternating-current (ac) synchronous generator driven by a steam turbine for 50- or 60-Hz electrical generating systems.

The synchronous generator is a relatively simple machine made of two basic parts: a stator (stationary) and a rotor (rotating). The stator consists of a cylindrical steel frame. Inside the frame, a cylindrical iron core made of thin insulated laminations is mounted on a support system. The iron core has equally spaced axial slots on its inside diameter, and wound within the core slots is a stator winding. The stator winding copper is electrically insulated from the core. The rotor consists of a forged solid steel shaft. Wound into axial slots on the outside diameter of the shaft is a copper rotor winding that is held in the slots with wedges. Retaining rings support the winding at the rotor body ends. The rotor winding, commonly called the field, is electrically insulated from the shaft and is arranged in pole pairs (always an even number) to form the magnetic field which produces the flux. The rotor shaft (supported by bearings) is coupled to a steam turbine, and rotates inside the stator core.

The stator winding (armature) is connected to the ac electrical transmission system through the bushings and output terminals. The rotor winding (field) is connected to the generator's excitation system. The excitation system provides the direct-current (dc) field power to the rotor winding via carbon brushes riding on a rotating collector ring mounted on the generator rotor. The synchronous generator's output voltage amplitude and frequency must remain constant for proper operation of electrical load devices. During operation, the excitation system's voltage regulator monitors the generator's output voltage and current. The voltage regulator controls the rotor winding dc voltage to maintain a constant generator stator output ac voltage, while allowing the stator current to vary with changes in load. Field windings typically operate at voltages between 125 and 575 V dc. The synchronous generator's output frequency is directly proportional to the speed of the rotor, and the speed of the generator rotor is held constant by a speed governor system associated with the steam turbine.

Synchronous generators range in size from a few kilovoltamperes to 1,650,000 kVA. 60-Hz steam-driven synchronous generators operate at speeds of either 3600 or 1800 rpm; for 50-Hz synchronous generators these speeds would be 3000 or 1500 rpm. These two- and four-pole generators are called cylindrical rotor units. For comparison, water (hydro)-driven and air-driven synchronous generators operate at lower speeds, some as low as 62 rpm (116 poles). The stator output voltage of large (generally greater than 100,000 kVA) units ranges 13,800–27,000 V.

There are five sources of heat loss in a synchronous generator: stator winding resistance, rotor winding resistance, core, windage and friction, and stray losses. Removing the heat associated with these losses is the major challenge to the machine designer. The cooling requirements for the stator windings, rotor windings, and core increase proportionally to the cube of the machine size. The early synchronous generators were air-cooled. Later, air-to-water coolers were required to remove the heat.