Download All the faculty requested to use Question Style - Calibri and Size

EXAMINATION BRANCH
QUESTION BANK
Name of the branch: EEE
Year & Sem: 3-1
Name of the subject: Electrical Machines-III
Name of the faculty, Designation & Mobile number: Dr. C. S. Reddy, Asst. Proff, 9492124042
Date of Examination:
Academic Year: 2016-17
NOTE: All the faculty requested to use Question Style - Calibri and Size - 12
UNIT-I
S.N0
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QUESTION
Discuss in detail the methods for suppressing harmonics in generated emf
of a 3-Ø alternator.
Deduce the relation between the number of poles, the frequency and the
speed of the synchronous generator
What are the causes of harmonics in the voltage waveform of an
alternator?
What is an armature reaction? Explain its effect on terminal voltage of an
alternator at unity power factor load.
APPEARED IN
(R05,R07,
R09, R13)
MARKS
Assigned
R07
5
5
5
5
In brief, derive an expression for the winding factor of an alternator
Derive EMF equation and describe how the introduced emf in armature
winding is affected by (a) form factor (b) pitch factor (c) distribution
factor
Discuss about the determination of synchronous reactance of an
alternator
5
Draw and describe the load characteristics of synchronous generator
5
What is the difference between integral slot and fractional slot windings
With phasor diagram, discuss about the leakage reactance of
synchronous generator
(a) Why Alternators are rated in KVA?
(b) Explain about voltage regulation of an Alternator.
Describe the constructional features of cylindrical rotor synchronous
generators and the function of each part.
Distinguish between integral slot and fractional slot winding and their
merits and demerits.
A 50 Hz alternator has a flux of 0.1 Wb/pole, Sinusoidally distributed.
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2+3=5
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Calculate the r.m.s value of the e.m.f generated in one turn of the
winding, which spans ¾ of a pole pitch.
15
Derive the expressions for distribution and pitch factor.
5
UNIT-II
S.N0
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QUESTION
Explain ZPF method of determining the regulation of alternator
A 100 KVA, 3000 V, 50 Hz 3-phase star-connected alternator has effective
armature resistance of 0.2 Ω. The field current of 40A produces shortcircuit current of 200A and an open circuit e.m.f of 1040 (line value).
Calculate the full load voltage regulation at 0.8 p.f. lagging and 0.8 p.f.
leading.
Describe how, open-circuit and short-circuit tests are conducted on a
synchronous machine
Discuss in brief, how voltage regulation can be computed by MMF
method
Explain the experimental method of determination of synchronous
reactance and impedance.
Discuss in brief, how voltage regulation can be computed by Direct
loading method.
A 3-phase, star connected alternator is rated at 1600 KVA, 13,500V. The
armature effective resistance and synchronous reactance are 1.5Ω and
30Ω respectively per phase. Calculate the percentage regulation for a
load of 1280 KW at p.f of (i)0.8 lead (b)0.8 lag.
A 3-phase, star connected alternator supplies a load of 1000 kW at a
power factor of 0.8 lagging with a terminal voltage of 11 kV. Its armature
resistance is 0.4Ω/phase while synchronous reactance is 3Ω/phase.
Calculate the line value of e.m.f generated and regulation at this load.
A 3-phase, star connected, 1000 kVA, 11000 V alternator has rated
current of 52.5 A. The A.C. resistance of the winding per phase is 0.45Ω.
The test results are given below.
O.C test: Field current=12.5 A, Voltage between lines=422V
S.C test : Field current=12.5 A, line current =52.5 a
Determine the full load voltage regulation of the alternator at (i)0.8 lead
(b)0.8 lag power factor
A 3-phase, star connected alternator gives 575 v on open circuit when
field current is 10 A. For same field current, a full load current of 75 A is
observed on short circuit. The armature resistance between the terminals
is 2.16Ω. on 0.8 leading p.f offull load condition, induced e.m.f per phase
is 6100 v. Find its rated terminal voltage per phase between the lines.
APPEARED IN
(R05,R07,
R09, R13)
MARKS
Assigned
R07
5
R05
5
5
R07
5
5
5
5
5
5
11
12
13
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15
Also calculate kVA rating of the alternator.
A 1000 kVA, 4600 V, star connected alternator is short circuited when
operating at rated speed. The field excitation is adjusted to give 150% of
rated armature current. The short circuit is then removed and with the
same field current and speed the open circuit voltage between stator line
terminals is found to be 1744 V. the effective resistance of armature is 1Ω
per phase. Find at full load and 0.8 p.f lag, the regulation using
synchronous impedance method.
A 3-phase, 50 Hz, 8-pole alternator has a star connected winding with 120
slots and 8 conductors per slot. The flux per pole is 0.05Wb, sinusoidally
distributed. Determine the phase and line voltages.
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What is armature reaction? And what is synchronous reactance?
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How do you calculate synchronous impedance experimentally?
Explain merits and demerits of E.M.F and M.M.F methods. Explain what
are the assumptions made in each case.
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