1.

What is the frequency of the generated emf of a 10 pole 2 alternator driven at

720rpm?
A. 50 hz C. 60 hz
B. 55 hz : D. 25 hz

2. Calculate the line value of generated voltage of a wye—connected 60 Hz

alternator with the following data :Slots = 144; Poles = 8 ; Turns per coil =
Flux per pole = 1.8 x 10° fines; Coll span = slots 1-18 & whole-coiled winding.

A. 1276 Volts C. 2,200 Volts

B. 635 Voits : D. 1,100 Voits

3. An alternator on open circuit generates 360 Volts at 60 Hz when the field current
is 3.6 Amp. Neglecting saturation, determine the generated emf at a frequency of
30 Hz and a field current of 3.0 Amp.

A. 240 Volts : C. 180 Volts

B. 300 Volts D. 150 Volts.

4. A generator is rated 100 MW, 13.8 kV and 80 % power factor. The effective
resistance to ohmic resistance is 1.5. The ohmic resistance is obtained by
connecting two terminals to a 6 volts d.c. source. if the current 43.8 Amperes,
what is the percent resistance per phase.

A. 1.5% C. 45%

B. 6% D. 6.9%

5. A 150 kva, 1000 volts three phase wye-connected alternator has an open circuit
emf of 1000 volts. When an alternator is short circuited at the same excitation, the
armature current is 460 amperes. What is the synchronous impedance?

A. 1.26 ohms C. 2.17 ohm

. B. 2.23 ohms D. 3.23 ohms

6. A three phase, 318.75 KVA, 2300 V alternator has an armature resistance of

0.35 ohms per phase and a synchronous reactance of 1.2 ohms per phase,
Determine the voltage regulation at full load 0.6 power factor leading.
A. -4.33% C. 4.33%

B 6.12% D.-6.12%

. A 100 kva, 400 volts, wye connected salient pole synchronous generator runs at
full load and 80% power factor leading. If Xd=2Xg=1.1 ohms per phase and Rai is
negligible, calculate the voltage regulation.

A. -26.4% C. -24.6%
B. 64.2% D. 248%
8. Athree phase salient pole, 440 volts, wye connected synchronous generator
operates at a power angle of 20° while developing power of 36KW. The machine
constant are Xd=24Xq=5 ohms and Ra is negligible. Calculate the %VR.

A. 70% C.78%
B. 144% D. 54%

8. A 500 KVA, 3,600 Volt, 3-phase, 60 Hz alternator has an effective armature

resistance per phase of 0.40 ohm. Mechanical and iron losses is 10 kw, field
current at full load unity pf is 70 Ampere. The exciter voltage is constant at 120
Volts. What is its efficiency when supplying incandescent lamp at rated load?

A 934% GC 923%
B. 89.7% D. 95.0%

10. A generator at no-load operates 61.2 Hz, slope 1mHz/W. Load 1= 1200w and
load 2=900w. Find the frequency before and after adiing; the load 2.

A 60 Hz & 59 Hz C.612Hz&592Hz
B. 60 Hz & 59.11HZ D. 61.3 Hz & 59.3 Hz

11.A 250 MW, 60 Hz turbine generator set has a speed regulation of 5% based on
its
own rating, The generator frequency decreases from 80 Hz to a steady value of
59.7 Hz. Determine the increase in the turbine power output.

A 25 MW C. 50MW
B. 12.5 Mw D. 20 MW
12. Two generating units rated 250 MW and 400 MW have governor speed
regulation
of 8% and 6.4% resp. from no load fo fuit load. They are operating in parallel and
share a load of 500MVY. Determine the icad shared by each unit.

A, 190MW & 310 MWY C. 220 MW & 280 MW

B. 225MW & 275 MW D. 240 MW & 260 Mw

13.A 3¢ Y-connected , 1000 kVA, 6.6 kVolt turbo-alternator has a synchronous

reactance of 8.7 ohms per phase but negligible resistance . While it is
supplying full load at 80% power factor lagging to a large power system, the
excitation is reduced by 5 % . What is the new value of operating power factor?
A B75% : C. 77.2%

B. 93.1% D. 952 %
oul,

14.A 1000 kva, 3300 volt 80Hz, 3-phase, wye connected, wound rotor synchronous
generator has an impedance of Zs=0+j1.0. The generator is operating at full load
0.8 pf lagging to an infinite bus. Calculate the synchronizing power per phase if a
disturbance causes the power angle to swing by 1°(mech’)).

A. 133.6 KW C. 145.2 KW
B. 125.4 KW ~D. 168.9 KW

15.A 50 MVA, 30 KV 3-phase synchronous generator has synchronous impedance of

0+9 Q/phase. The generator is delivering rated power at 80% pf lagging at the
rated terminal voltage to an infinite bus. With the same excitation, what is the
steady state maximum power the machine can deliver before losing synchronism?
A 126MW C. 136 MW
B. 108MW D. 256MW
AC Machines (Alternator) Supplementary Problems

1. In case of turbo alternators the rotor is usually made of

A. cast iron C. forged steel

BE. laminated stainless steel Co D. manganese steel

2. What kind of rotor is most suitable for turbo-alternators which are designed ta
run at high speed
A. salient pole type C. eitherAorB
B. non-salient pole type D. none of these

“3. In an alternator one of the advantages of distributing the winding is to

A. improve the voltage waveform C. reduce noise
B. none of the above D. save on copper

4. In large capacity ac generator, the moving g pait is

A. Brushes C. Poles
B. Armature - D. Frame

5. Hydrogen is used in large alternators mainly to

A. reduce distortion of waveform C. cool the machine
2. strengthen the magnetic field D. reduce the eddy current losses

6. The power factor of an alternator is determined by its

fead C. excitation
-B. speed D. voltage rating
7. A 4 pole, 1500 rpm alternator will generate emfat ___ frequency
A. 20 Hz C. 50 Hz
8. 60 Hz D. 40 Hz

8. Calculate the siectrical displacement between adjacent slots of a 16 pole

alternator with 144 slots. .
A. 30° C. 60°
B. 20° D. 186°

9. The winding of a 3-phase, 6 pole, 60 cycle alternator has five-sixths pitch.

What is the chording angle?
A. 30° C. 150° ~
B. 180° D. 0°

10. The induced emf in each coil side of a certain alternator winding is calculated
as 10 volts, if the measured voltage across the coil is 18. 32 volts, the winding
is short by how many electrical degrees? vy wi
A. 30° C. 60°
B. 50° - Db. 20°
“11.AB6 pole, three phase AC generator has 72 siots, the coil span is 12 slots.
What is the pitch factor ?
A. 1.00 © C. 0985
B. 0.939 D. 0.966

12. Determine the distribution factor for a 3- phase winding in which there are 18 .
slots per pole.
A. 0.968 C. 0.958
B. 0.857 : D. 0.956 -

13.A 3-$, 60 hz, 10-pole alternator has 120 slots and 4 turns in every coil. The coil
pitch is short by 2 slots. The winding distribution factor of the alternator is ......
A. 0.925 C. 0.987
8. 0.967 D. 0.957

14_A three phase alternator has three armature coils each rated 1200 voits and
120 amperes. What is the kva rating of thig unit? :
A. 288 C. 432
B. 2494 ‘D. 144

15. Calculate generated emf per phase of a 10 pole, 3-phase, 60 Hz alternator with
5 slot per pole per phase and 4 conductors per slots in two layers. The winding
is short by 3 siots & the flux per pole is 0.12 Weber

A. 2808 Volts C. 720 Volis

B. 1000 Volts } ] ~ DD. 3200 volts

16.A 100 MVA, 13.8 kVolt, 3 Y-cennected alternator will have a per phase
nominal impedance of ..
A. 7.5 ohms C. 1.8 ohms
B. 2.9 ohms : D. 3.5 ohms

17.A generator is rated 100 MW, 13.8 kV and 90 % power factor. The effective
resistance to ohmic resistance is 1.5. The ohmic resistance is obtained by
connecting two terminals to a d.c.source. The current & voltage are 87.6
Amperes and 5 Volts respectively . What is the percent resistance?

A 3 C. 42
B. 2 D. 1

18. A wye-connected alternator was tested for its effective resistance. The ratio of
the effective resistance to ohmic resistance was previously determine to be
1.35. A 12-V battery was connected across two terminals and the ammeter
read 120 amperes. What is the per phase effective resistance of the alternator?
A. 0.135 ohms C. 0.0675 ohms
B. 0.117 ohms D. 0.0234 ohms

19. Which of the following relay come into operation in the event of failure of prime-
mover connected to the generator?
A. Buchholz relay - C. Differential relay
B. Reverse power relay D. Distance relay
20. Dirt accumutation in generator can cause all of the following except .
A. low power factor C. poor voltage regulation
B. flashover - D. overheating

21.A generator is rated 100 MW, 13.8 kV and 80 % power factor. The effective
resistance to ohmic resistance is 1.5. The ohmic resistance is obtained by
connecting two terminals to a d.c. source. The current & voltage are 87.6
Amperes and 6 Volts respectively . What is the resistance per phase ?

A. 0.0513 ohm C. 0.0685 chm

B. 0.342 ohm D. 0.0617 ohm

22 A generator is rated 100 MW at 22 kv, and 90% power factor. The effective ac
resistance to ohmic resistance is 1.35. The resistance is obtain by connecting
two terminals to a 12 volts dc source. If the current is 120 Amperes, What is
the percent effective resistance per phase ? oo

CAL 1.5% C. 45%

B. 6% : D. 6.5%

23 A dc resistance test is conducted ona 3-phase 200 kva, 240 volts, delta
connected generator. When the two terminals are connected from 6 volts dc
source, the ammeter inserted in the line reads 150 amperes. The effective
resistance to ohmic resistance is 1.25. What is the percent resistance?

A 75% C. 26%
B. 87% So D. 46%

24. Find the synchronous impedance of a 3-phase synchronous generator in which

"a given field current produces an armature current of 200 amperes on short
circuit and a generated emf of 1050 volts on cpen circuit.
A. 1.72 ohms C. 9.1 ohms
B. 5.25 ohms 0D. 3.0chms

25.if in a 3-phase alternator, field current of 50 A produces a full load current of

200 A on short circuit and 1730 V on open circuit, then its synchronous

impedance is ........ ohm. :

A 8686 C. 4
B. 5 D. 346

26. A 100 MW, 13,800 volts 90% power factor synchronous generator has an
ohmic resistance of 0.1 ohm and synchronous reactance of 0.42 ohm is
operating at rated condition. Find the angle between the voltages.

-A. 9.55 deg C. 6.72 deg.

B. 7.45 deg. D. 3.25 deg.

27.A single phase generator with fult load current of 450 amperes at 2400 voits
has 0.08 ohms resistance and synchronous reactance is 2.8 ohms. Determine
the voltage regulation in percent at 75% power factor lagging.
A 55 C.64

B. 41 D. 75
28.A 3-phase, 1000 kva, 2,300 volts alternator is short circuited and open
circuited and is operated at rated speed to produce rated current. With the
short circuit removed and the same excitation, the voltage between stator
terminals is 860 volts. The effective ac resistance between stator terminals is
1.2 ohms.. Determine the percent voltage regulation at 0.9 power factor lagging
A. 29 Cc. 32
B. 15 ~~ D. 26.7

29.A 25 kva 220 volt three phase alternator delivers rated kva at 0.84 power
factor. The effective ac resistance between the armature winding terminals is
0.18. The field takes 9.3 amperes at 115 voits. If the friction and windage loss
is 460 watts and the core loss is 610 watts. What is the hp delivered by the
prime mover?

A 3286 . C. 2815
B. 31.54 D. 33.23

30. Out of the following conditions, the one which does not have to be met by
alternators working parallel is
- A. the machine must have equal rating
B. terminal voltage of each machine must be the same
C. the machines must have the same phase rotation
D. the machine must operate at the same frequency

31.A generator is being synchronized to a large system. The actual system

voltage and frequency are 13.7 kv and 80 Hz respectively. The generator
* voltage and frequency are 13.9 kv and 80 Hz respectively when the generator
is switched to the system. Choose which one happens.
A. Generator delivers both MW and MVAR C. Generator delivers MVAR
B. Generator delivers MW- D. Generator takes in MW

32.1t is never advisable to connect a stationary alternator to live bus- bars because
a It is likely to run as synchronous motor
. It will get short circuited
& It will decrease bus bar voltage though momentarily
D. twill disturb generated emf of other alternator connected in parallel

33.An alternator is operating with 100 Amp field current. If the field current is
increased tc 125 Amp with the same electrical load on the machines, it will _
A. be less likely to go out of synchronism
B. be more likely to go out of synchronism
C. operates at new power factor angle
D. overheat

34.1n a large alternator dampers

A. reduce frequency fluctuations C. increase stability
B. reduce voltage fluctuation D. none of the above
35. Two 1500 kva ac generators running in parallel supplies the total load of 2,250
kW at 0.90 pf. lagging . One machine is loaded to 1000 kW. at 0. 85 p.f.
lagging. What is the kva & pf. of the other machine?

A. 1750 & 0.8 lagging C. 1335 & 0.94 lagging

‘B. 470 & 0.96 leading D. 1250 &.0.78 leading

36. Two-3¢ alternators operating in parallel delivers power at a line potential of

2200 Volts to an inductive load of 150 kW at 80% p.f. if the armature currents
of the two alternators be equal one he operating at unity p.f. How much power
does each alternator supply to the load ?

A. 117.2 kW for unity p.f. & 32.8 kW

B. 112.7 kW for unity p.f. & 38.2 KW
C. both 75 kW one at unity pf.

D. 100 kW for unity & 50 kW

37.The driving power from the prime mover driving an alternator is lost but the
alternator remain connected to the supply (i.e. its power circuit breaker failed to
trip} and its excitation is on. The aiternator wili-.....
A. behaves as an induction motor
B. stop & get burnt
C. behaves as synchronous motor but will rotate in reverse or opposite
direction.
D. behaves as synchronous motor and rotate at same direction .

38. Two identical 2000 kw alternator operates in parallel. The governor of the first
machine is such that the frequency drops uniformly from 50 Hz at no-load to
48 Hz at full-load. The corresponding uniform speed drop of the second
machine is 50 Hz to 47.5 Hz, What is the maximum load that can be delivered
without overioading either machine ?

A. 3600 kW C. 6300 kw
B. 4000 kW . D. 3800 kW

39.A wye-connected turbo alternator having s synchronous reactance of 0.5 and

negligible resistance is connected to a large power system having a busbar
voltage of 13.8 kv supplying a load of 15000 kva at 0.8 tagging power factor. If
the steam supply is cut-off, what current wili the machine carry assuming
negligible losses?

A. 256.55 A C. 52562 A
B. 384.26 A D. 627.55 A

40. A generator is being synchronized to a large system. The actual system

voltage and frequency are 13.8 kv and 60 Hz respectively. The generator
voltage and frequency are 13.8 kv and 60.1 Hz respectively when the
generator is switched to the system. Choose which one happens.

A. Generator delivers both MW and MVAR C. Generator delivers MVAR

B. Generator delivers MW D. Generator takes in MW

41. Increasing the speed of alternator, what will happen?

A. Decrease MW C. Increase MVAR
B. Increase frequency D. Decrease MVAR
42.For a 3-phase winding with 6 slots per pole per phase and with coil span of 15
slot , the value of pitch factor is
A. 0.851 Co CGC. 0.951
B. 0.966 D. 1.05

43. Determine the distribution factor for a 3- phase winding in which there are 5
slots per pole per phase.

A. 0.966 ~C. 0959

B. 0.957 D. 0.952
44 Damper or “Amortisseur” windings are used in alternators to .__
A. achieve synchronism C. reduced windage losses
B. prevent hunting D. start up
"45.A three phase wye connected synchronous machine has a stator with 2000
_ turns per phase. What rotor flux would be required to produce of voltage of §
kV line to line.
A. 6.5mWb C. 54mWwb
B. 8.4mWb : D. 7.8 mWb

48.An ac generator 1500 kva capacity has a full load efficiency of 91% at 100%
power factor and 86% efficiency at 80% power factor lagging. What engine
drive in hp is required if the genset is to supply entirely incandescent lamp?

A. 2125 C. 1980
B. 2525 : D. 2210

47. Synchronous impedance test is taken on a 3-phase generator. Under short

-circuit condition, the currents in the three lines are 26.2 A, 23.7 A, and 27.4
A. What current should be used for the test ?
A 283A : C. 246A
B. 257A D. 254A

48. The power factor of an alternator is 75 %. The operator is ordered to increase

‘the power factor to 80 %. What shail he do 7?
A. increase the voltage C. operate the governor
B. increase the excitation DD. decrease the excitation

49.A 75 kva single phase, 240 volts, 80 Hz alternator has an effective armature
resistance of 0.02 ohm per phase and an armature leakage reactance of 0.08
ohm. Calculate the induced emf when supplying rated load at unity pf.
A. 245.5 volts C. 249.2 volts
B. 250.4 volts ~~ D. 247.5 volts
50.A 50 MVA, 13.8 kVolt, 39, Y-connected alternator will have a per phase
nominal impedance of ......
A. 7.5chms ~~ C. 3.80hms
B. 2.9 ohms D. 15 ohms