17. A three-phase, 60-Hz, wye-connected turbine-driven alternator has three sing

Question

17. A three-phase, 60-Hz, wye-connected turbine-driven alternator has three singlephase windings. Each winding is at 8000 V and 625 A. The alternator has four
poles. Determine
a. the kilovolt-ampere rating of the alternator.
b. the kilowatt output of the alternator when it delivers the rated current to a load
with an 80% lagging power factor.
c. the line voltage.
d. the rated full-load line current.
e. the speed in r/min of the revolving field of the alternator.

18. For the turbine-driven, high-speed alternator described in question 17, answer the
following questions:
a. Why is the field, rather than the armature, the rotating member?
b. Why is a cylindrical rotating field used, rather than one with salient poles?
c. How is the dc excitation current supplied to the rotor of the rotating field
alternator?

Explanation / Answer

17. Given data 3-phase alternator having Vph=8000V

                                                              IL=IP=625A

Frequency of alternator f=60Hz , no of poles P=4

N=120f/P=1800 r.p.m

a) KVA Rating of the Alternator =3 VLIL=3 . 3. Vph.625=3.8000.625/1000=15000KVA

b) given p.f=0.8

KW=KVA x p.f=15000X0.80=12000KW

c) VL=3. Vph=1.732X8000=13856V

d) rated full load current = rated KVA / Rated voltage =(15000/8000)X10^3=1875A

e) N=1800 r.p.m

18. a) In small rating generators general convention is followed that is field windings are used as stator and armature is used as rotors similar to that in DC generators. But in industrial generators, rotating field is preferred over rotating armature because of high current generated in high rating industrial generators

b) Flux distribution is relatively poor than non-salient pole rotor, hence the generated emf waveform is not as good as cylindrical rotor

c) By using potential divider we can give required field current to get required output voltage at terminals of alternator.

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