Mansoura University.
Faculty of Engineering.
Electrical Engineering Dept.
First Semester.
Exam Date: 14 March 2021.
Allowed Time: One Hour/Part.
›
Electrical Power and Machines Program – 3rd Grade students.
Final-Term Exam of (Electrical Machines II – EE 2312) –
of pages [3] – Full Mark: [55]
RR
- Answer the following questions and assume properly any missing data.
- Choose the correct answer then fill in its corresponding circle in the answer sheet.
Points from 1 to 27 have an equal weigh of 1.7 marks.
Points 28 and 29 have an equal weigh of 2.5 marks.
Point 30 weights 4.1 marks.
Part-II
R
(1) (I) Large synchronous machines are constructed with armature winding on stator.
(II) Stationary armature winding would have reduced armature reactance.
Which of the following options is correct?
a Both I and II are true and II is a correct explanation of I.
c Both I and II are true and II is not a correct explanation of I.
b I is true but II is false.
d I is false but II is true.
(2) (I) AC armature windings are short chorded by selecting value of coil span more than the pole pitch.
(II) Short chording is done to eliminate harmonics in the induced emf.
Which of the following options is correct?
a Both I and II are true and II is a correct explanation of I.
c Both I and II are true and II is not a correct explanation of I.
b I is true but II is false.
d I is false but II is true.
(3) (I) The short-circuit ratio (SCR) of a three-phase alternator should be high.
(II) A high value of SCR will decrease the value of voltage regulation and will increase the maximum
power output.
Which of the following options is correct?
a Both I and II are true and II is a correct explanation of I.
c Both I and II are true and II is not a correct explanation of I.
b I is true but II is false.
d I is false but II is true.
(4) (I) For a three-phase alternator operating on leading PF at full-load, the terminal voltage may be
more than the no-load induced emf.
(II) At leading PF, the effect of armature reaction is demagnetizing.
Which of the following options is correct?
a Both I and II are true and II is a correct explanation of I.
c Both I and II are true and II is not a correct explanation of I.
b I is true but II is false.
d I is false but II is true.
(5) (I) In cylindrical rotor synchronous machine, the armature reaction flux φa is in phase with the
armature current Ia , but in salient pole machine φa is not in phase with Ia .
(II) The reluctances along the direction axis and the quadrature axis being different, the ratios of
the mmfs and the respective fluxes in the two cases are not equal.
Which of the following options is correct?
a Both I and II are true and II is a correct explanation of I.
c Both I and II are true and II is not a correct explanation of I.
b I is true but II is false.
d I is false but II is true.
(6) Under short-circuit condition, the power factor of an alternator is
b almost zero lagging.
a Unity.
c almost zero leading.
d 0.707 leading.
(7) The figure below shows the per-phase Open Circuit Characteristics (measured in V) and per-phase
Short Circuit Characteristics (measured in A) of a 14 kVA, 400 V, 50 Hz, 4-pole, 3-phase, delta
connected alternator, driven at 1500 rpm. The field current, If is measured in A. Readings taken
are marked as respective (x, y) coordinates in the figure. How much is the Ratio of the synchronous
impedance estimated corresponding to armature current of 10 A to that estimated corresponding to
rated voltage?
E
OCC
I
(8, 400)
b 2.0
d 2.025
a 1.5
c 2.1
o
SC
(2, 210)
(1, 110)
(4, 20)
SCC
(0, 10)
If
(0, 0)
Page 1 of 3
(8) For maximum current during slip test on a synchronous machine, the armature mmf aligns along
a d-axis.
c 45◦ to d-axis.
b q-axis.
d 45◦ to q-axis.
(9) For a full-pitched winding, the generated voltage in both coil sides is
a exactly 180◦ out of phase.
b exactly in phase.
c approximately in phase.
d approximately 180◦ out of phase.
(10) The distribution factor in a rotating electrical machine is defined as the ratio of
a arithmetic sum of coil emf to phase sum of coil emf.
b phasor sum of the emf per coil to the arithmetic sum of emf per coil.
c phasor sum of coil emfs to the arithmetic sum of coil emf.
d phasor sum of coil emf to the per phase voltage.
(11) The pitch factor in a rotating electrical machine is defined as the ratio of resultant emf of a
a full-pitched coil to that of a chorded coil.
b full-pitched coil to the phase emf.
c chorded coil to that of a full-pitched coil.
d chorded coil to the phase emf.
(12) Distributed winding and short chording employed in synchronous alternator will result in :
a Increase in emf and reduction in harmonics.
b Reduction in emf and increase in harmonics.
c Increase in both emf and harmonics.
d Reduction in both emf and harmonics.
(13) A 90-slot stator has a three-phase, six-pole, whole coiled winding, each coil of which spans from slots
1 to slot 14. The pitch factor is
a 0.978
b 0.84
c 0.960
d 0.859
R
Data for points 14 and 15 is given as:
A 4-pole, star-connected, 50 Hz, synchronous generator has 48 slots in which a double layer winding
is housed. Each coil has 10 turns and is short pitched by 36◦ elec. The fundamental flux per pole is
0.025 Wb.
(14) The line-to-line induced emf (in volts) is approximately:
a 808.
b 888.
c 1400.
d 1538.
(15) The fifth harmonic component of phase emf (in volts) is:
a 0.
b 269.
c 281.
d 808.
R
Data for points 16 to 18 is given as:
A 5000 kVA, 6600 V, 3-phase, star-connected alternator has a dc resistance of 0.05 Ω/phase. Opencircuit and full-load ZPF experimental results are given as:
Field current, A
Open-circuit terminal voltage, V
Full-load ZPF terminal voltage, V
32
50
75
100
140
3100
4900
6600
7500
8300
0
1850
4250
5800
7000
(16) By using the ZPF method, the voltage regulation for a load of 500 A, unity power factor is around:
b 6%.
d 15.2%.
a 2%.
c 11%.
(17) By using the ZPF method, the voltage regulation for a load of 500 A, 0.9 leading power factor is
around:
b -6.48%.
d -15.89%.
a -2%.
c -11.27%.
(18) By using the ZPF method, the voltage regulation for a load of 500 A, 0.71 lagging power factor is
around:
a 9.12%.
b 12.56%.
c 15.22%.
d 17.56%.
(19) A three-phase alternator has negligible stator resistance. A short-circuit test is conducted on this
alternator. At a particular speed a field current of If is required to drive the rated armature current.
If the speed of the alternator is reduced to half, the field current required to maintain rated armature
current would be equal to:
b 32 If .
d 12 If .
a 2If .
c If .
Page 2 of 3
(20) A stand alone engine driven synchronous alternator is feeding a partly inductive load. A capacitor
is now connected across the load to completely nullify the inductive current. For the alternator
operating condition:
a the field current and fuel input have to be reduced.
b the field current and fuel input have to be increased.
c the field current has to be increased and fuel input left unaltered.
d the field current has to be reduced and fuel input left unaltered.
(21) If the value of SCR in an alternator is low, then the air gap and voltage drop across synchronous
reactance will be
b High, low.
d High, high.
a Low, high.
c Low, low.
(22) Two alternators A and B are sharing a resistive load equally. Now if the excitation of alternator A
is increased:
a alternator A will become lagging and alternator B will become leading.
b alternator A will become leading and alternator B will become lagging.
c both alternators will continue to operate on unity power factor.
d both alternators will operate on lagging power factor.
(23) The speed regulation of a synchronous motor is always:
a 1%.
b 0.5%.
c zero.
d positive.
(24) Under which of the following conditions hunting of synchronous motor is likely to occur?
a Periodic variation of load.
b Over-excitation.
c Over-loading for long periods.
d Small and constant load
(25) A synchronous motor can operate at:
a lagging power factor only.
c unity power factor only.
b leading power factor only.
d lagging, leading and unity power factors.
(26) A synchronous motor draws minimum current at a given load when its power factor is
a zero.
b lagging.
c leading.
d unity.
(27) A three-phase synchronous motor connected to an infinite bus bar is running at full load and unity
PF. If its shaft load is reduced by half, with field current held constant, its new PF will be
a unity.
c lagging.
b leading.
d dependent on machine parameters.
(28) A three-phase, salient pole synchronous motor is connected to an infinite bus. It is operated at
no load with normal excitation. The field excitation of the motor is first reduced to zero and then
increased in reverse direction gradually. Then the armature current:
a First decreases and then increases steeply.
b Increases continuously.
c First increases and then decreases steeply.
d Remains constant.
(29) A salient-pole synchronous motor running at synchronous speed suddenly losses its excitation. Then
it will
b run at a lower speed.
a continue to run at synchronous speed with reduced output.
d not run.
c continue to run at synchronous speed with same output.
(30) Two alternators are running in parallel supply lighting load of 2500 kW and a motor load of 5000
kW at 0.707 PF. One machine is loaded to 4000 kW at a PF of 0.8 lagging. The kW output and PF.
of the other machine will be:
a 3500, 0.87 lagging.
b 3500, 0.67 lagging.
c 3500, 0.95 lagging.
d 3500, unity.
R
Please refer to Part - I. (Points from 31 to the end)
Good Luck,
Dr. Abdelhady Ghanem
Page 3 of 3