148 Electrical Power Systems Technology
ing field through the slip ring/brush assembly, while 13.8 kilovolts AC is
induced into the stationary conductors.
Commercial power systems use many three-phase alternators connect-
ed in parallel to supply their regional load requirements. Normally, indus-
trial loads represent the largest portion of the load on our power systems.
The residential (home) load is somewhat less. Because of the vast load that
has to be met by the power systems, three-phase generators have high
power ratings. Nameplate data for a typical commercial three-phase alter-
nator are shown in Figure 6-7. The nameplate of a generator specifies the
manufacturer’s rated values for the machine. It is usually a metal plate
that is placed in a visible position on the generator frame. The following
information would typically be listed on the nameplate:
TURBINE GENERATOR
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STEAM TURBINE No. 128917
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Rating 66000 kW 3600 RPM 21 Stages
Steam: Pressure 1250 PSIG Temp 950 F Exhaust Pressure 1.5” HG. ABS.
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GENERATOR
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No. 8287069 Hydrogen Cooled Rating Capability Capability
Type ATB 2 Poles 60 Cycles Gas Pressure 30 PSIG 15 PSIG 0.5 PSIG
3 PH. Y Connected For 3800 Volts KVA 88235 81176 70588
Excitation 250 Volts Kilowatts 75000 69000 60000
Temp Rise Guaranteed Not To Exceed Armature Amp 3691 3396 2953
45 C On Armature By Detector Field Amp 721 683 626
74 C On Field By Resistance Power Factor 0.85 0.85 0.85
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Figure 6-7. Nameplate data for a commercial three-phase alternator: (A) Power
rating (in kilowatts), (B) Voltage rating (in volts), (C) Current rating (in am-
peres), (D) Temperature rise (in degrees Centigrade)
Generation of Three-phase Voltage
The basic construction of a three-phase AC generator is shown in Figure
6-8, with its resulting output waveform given in Figure 6-10. Note that
three-phase generators must have at least six stationary poles, or two poles
for each phase. The three-phase generator shown in the drawing is a rotat-
ing-field type generator. The magnetic field is developed electromagneti-
cally by a DC voltage. The DC voltage is applied from an external power