1.1.2 Synchronous Generators

Although induction generators do exist, particularly in wind turbine generators, they are relatively rare compared to synchronous generators. Virtually all generators used by electrical
utilities are of the synchronous type. In synchronous generators, DC current flows through the rotor (field) winding, which creates a magnetic field from the rotor. At the same time, the rotor is spun by a steam turbine (using fossil or nuclear fuel), gas turbine, diesel engine or, a hydroelectric turbine. The spinning DC field from the rotor induces current to flow in the stator (armature) winding. As for motors, the following types of synchronous generators are determined by the design of the rotor, which is primarily a function of the speed of the driving
turbine.

Round Rotor Generators (Figure 1.2). Also known as cylindrical rotor machines, round
rotors are most common in high-speed machines, that is, machines in which the rotor revolves
at about 1000 rpm or more. Where the electrical system operates at 60 Hz, the rotor
speed is usually either 1800 rpm or 3600 rpm. The relatively smooth surface of the rotor reduces “windage” losses, that is, the energy lost to moving the air (or other gas) around in the air gap between the rotor and the stator—the fan effect. This loss can be substantial at high speeds in the presence of protuberances from the rotor surface. The smooth cylindrical shape also lends itself to a more robust structure under the high centrifugal forces that occur in high-speed machines. Round rotor generators, sometimes called “turbogenerators,” are usually driven by steam turbines or gas turbines (jet engines). Turbogenerators using round rotors have been made in excess of 1500 MW. (1000 MW is a typical load for a city of 500,000
people in an industrialized country).

Figure 1.2. Phototgraph of a small round rotor. The retaining rings are at each end of the rotor body

Such a machine may be 10 m in length and about 5 m in
diameter, with a rotor on the order of 1.5 m in diameter. Such large generators almost always have a horizontally mounted rotor and are hydrogen-cooled (see Section 1.1.3).

Salient Pole Generators (Figure 1.3). Salient pole rotors usually have individual magnetic field poles that are mounted on a rim, with the rim in turn fastened to the rotor shaft by a “spider”—a set of spokes. Since the magnetic field poles protrude from the rim with spaces between the poles, the salient pole rotor creates considerable air turbulence in the air gap between the rotor and the stator as the rotor rotates, resulting in a relatively high windage loss.
However, since the rotational speed is usually significantly less than 1000 rpm, the loss is
considered moderate. Salient pole machines typically are used with hydraulic turbines, which have a relatively low rpm (the higher the penstock, i.e., the larger the fall of the water, the faster the speed). To generate 50 or 60 Hz current in the stator, a large number of field poles are needed (recall that the generated AC frequency is the number of pole pairs times the rotor speed in revolutions per second). Fifty pole pairs are not uncommon on a hydrogenerator,
compared to one or two pole pairs on a turbogenerator. Such a large number of pole pairs requires
a large rotor diameter in order to mount all the poles. Hydrogenerators have been
made up to about 800 MW. The rotor in a large hydrogenerator is almost always vertically
mounted, and may be more than 10 m in diameter.

Pump/Storage Generator. This is a special type of salient pole machine. It is used to
pump water into an upper reservoir during times of low electricity demand. Then, at times of high demand for electricity, the water is allowed to flow from the upper reservoir to the lower reservoir, where the machine operates in reverse as a generator.

chine from the pump to generate mode is commonly accomplished by changing the connections on the machine’s stator winding to reverse rotor direction. In a few cases, the pitch of the hydraulic turbine blades is changed. In the pump motor mode, the rotor can come up to speed by using a SCI-type winding on the rotor (referred to as an amortisseur or damper winding), resulting in a large inrush current, or by using a “pony” motor. If the former is used, the machine is often energized by an inverter-fed drive (IFD) that gradually increases the rotor speed by slowly increasing the AC frequency to the stator. Since the speed is typically less than a few hundred rpm, the rotor is of the salient pole type. Pump storage units have been made up to 500 MW.