Single and Three Phase Generators

Single-phase ac power consists of either a single voltage or two voltages in series with exactly the same phase relationship. NEMA lists standard single-phase voltages for 60 Hz systems as 120 volts, 240 volts, or a combination of 120 and 240 volts. There are three basic types of single-phase generators:

• Single-voltage, two-lead generators. These usually have a single circuit armature with two load leads and are designed to produce a single-output voltage.

• Three-load lead, dual-voltage generators. These are similar to the two-load lead design, except that a center-tapped lead is brought out to allow a dual single-phase voltage output.

• Four-load lead reconnectable generators. These are the most versatile and commonly used single-phase generators. They have a two-circuit armature winding, with each coil group designed for 120 volt output. Low voltage is produced by connecting the coils in parallel, and high voltage is produced by connecting them in series. The series configuration allows 240 volt, two-wire service. A third output lead can be used for three-wire, 120/240 volt service.

Three-phase generator armatures are constructed so there are three separate independent phase coil groups spaced 120 degrees apart. Each phase may be composed of one or two major coil groups (single or two-circuit design). There are two basic methods of connection for three-phase armatures: wye- (or star-) connected and delta-connected.

In the wye method of armature connection, as illustrated in Figure 25-6, the finish leads of each of the three-phase coil groups are tied together to form a neutral point. This is the wye, or star, point. The start leads then become the three output load leads. Sometimes, a fourth, or neutral, lead is affixed to the neutral point and brought out for external connection.

Fig. 25-6 Wye (or Star) Connection.

The voltage generated in each phase of a wye-connected generator is termed line-to-neutral, or phase voltage. The voltage measured between any two output load leads is termed line-to-line, or phase-to-phase, and is equal phase voltage times the square root of three (1.732). A wye-connected 480 volt generator (480 volts phase-tophase) can also provide 277 volts (phase-to-neutral). Most generators are wound with wye connections to minimize voltage across winding insulation.

Alternatively, on delta-connected generators (Figure 25-7), the phase coil groups are configured in a triangle. The delta connection is made by joining the start lead of one phase coil group to the finish lead of the phase coil group to its left, and so on. An output lead is affixed to each corner of the triangle. On a three-lead delta-connected unit, any single point can be grounded.

Fig. 25-7 Delta Connection.

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Renewable Energy Eco Friendly

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