Preboard Sept 2013 Set B Solution

POWERLINE REVIEW CENTER ( The Ultimate E.E. Review Center )

REGISTERED ELECTRICAL ENGINEERING Pre-Board Examination Sunday August 11, 2013 8:00 a.m. – 12:00 n.n. --------------------------------------------------------------------------------------------------------------------------------------------ELECTRICAL ENGINEERING SET - B INSTRUCTION : Select the correct answer for each of the following questions. Mark only one answer for each item by shading the box corresponding to the letter of your choice on the answer sheet provided. STRICTLY NO ERASURES ALLOWED MULTIPLE CHOICE

1. A 4-pole, 3-phase induction motor is energized from a 60-Hz supply, and is running at a load condition for which the slip is 0.03. Determine speed of the rotor rotating magnetic field with respect to stator frame, in rpm. A. 1744 rpm C. 1764 rpm B. 1714 rpm D. 1800 rpm Speed of Rotor field with respect to Stator=Ns

Ns 

120(60)  1800rpm 4

Answer is D. 2. The rotor of a 3-phase, 60-Hz, 4-pole induction motor takes 120 kW at 3 Hz. Determine the rotor speed. A. 1750 rpm C. 1710 rpm B. 1720 rpm D. 1760 rpm

f 3   0.05 f 60 120(60) 1  0.05  1710rpm N  4

S

R

R

Answer is C 3. Which of the following Electrode is not permitted for grounding? A. Aluminum electrode C. Concrete encased electrode B. Metal frame of the building structure D. Metal underground water pipes Answer is A PEC 2009 page 216 4. What force will be required to move a short circuited conductor across a magnetic field of 5000 gauss if the conductor is 100 cm long, moves at a velocity of 100 cm per second and the circuit has a resistance of 0.04 ohm. A. 1.4 lbs C. 0.112 lbs B. 2.4 lbs D. 0.36 lbs

e Blv 0.5T (1m)(1m / s)    12.5 A R R 0.04 F  BIL  0.5T 12.5 A(1m)  6.25N  0.637kg  1.4lbs

I

Answer is A 5. One farad is A. coulomb-joule B. joule per volt Answer is C

C. one coulomb per volt D. one volt per coulomb

1

6. A coil that has a resistance of 0.05 ohms is connected to a cell that develops an emf of 1.5 volts. Find the current flowing if the internal resistance of the cell is 0.01 ohm A. 2.5A C. 25A B. 20A D. 50A

I

1.5v  25 A 0.05  0.01

Answer is C 7. The total core losses for a sheet steel core are found to be 500 watts at 25 Hz. When the frequency is increased to 50 Hz and the maximum flux density is kept constant, the total core loss is 1400 watts. Find the eddy current loss at 50 Hz. A. 800 watts C. 600 watts B. 600 watts D. 300 watts For constant Bm,

CL  Khf  Kf

CL  Kh  Kf f

2

500  kh  25K 25 1400 At 50 Hz,  kh  50 K 50 Combining equations, k  0.32 Therefore, W  Kf  0.32(50)  800watts At 25 Hz,

2

2

Answer is A 8. What is the circular-mil area of a wire 0.2 inch in diameter? A. 400,000 C. 40,000 B. 4,000 D. 400

Cmil  dmil  (0.2 x1000)  40,000 2

2

Answer is C 9. A 115 volts dc motor draws a current of 200 amperes and is located 1000 ft. from the supply source. If the copper transmission wire has a diameter of 0.45 inch, what must be the voltage of the supply source? A. 135.5V C. 125.3V B. 120. V D. 132.5 V

 1000 ft  r  10.4   0.0513ohm  (450mils)  Vs  115  200(2 x0.0513)  135.5v W

2

Answer is C 10. In a parallel circuit , a resistance of 130 ohms and a capacitance of 30 microfarads are connected in parallel across a 230-volts, 50 Hertz supply. Find the angle difference between the total current and the applied voltage. A. 39.2o C. 52.8o o B. 51.8 D. 50.8o

1  106.16 ohm (2 50)30 x10 130(0  j106.16) Zp   82.23  50.8 130  (0  j106.16) Xc 

6

Answer is D

2

0

11. A series circuit of Resistance of 3000 ohms and a capacitor of 0.01 micro-farad is suddenly connected to a 100 volts battery. The capacitor has an initial stored voltage of 40 volts. At this instant, at what rate is the voltage Vc changing? A. 2x106 volts/sec C. 106 volts/sec 6 B. 3x10 volts/sec D. 4x106 volts/sec Transient voltage across C, Rate of change,

At t=0,

e  (100  40)(1  e

de  60(33333)e dt

 t / RC

)  60(1  e

33333t

)

33333t

de  2 x10 volts / sec dt 6

Answer is A 12. A series circuit containing a resistance of 20 ohms, an inductance of 0.10 henry, and a 100  f capacitor all connected in series. The applied voltage is 240 volts at 60 cycles. Find the true power of the circuit. A. 2240 W C. 2195W B. 2514 W D. 1226 W

Z  R  j( X  X ) L

C

Z  20  j[377 x0.1 

1 ]  22.92  29.2 377(100 x10 )

0

6

 240  P I R  20  2192W  22.92  2

2

Answer is C 13. The dimension of the pole faces are 20cmx40cm. The flux density B is 5000 lines per square centimeter and the conductor moves across flux in ½ second. What is the average value of the induced pressure? A. 0.008 volt C. 8 volts B. 0.8 volt D. 0.08 volt

d 5000(20 x 40) x10 i   0.08volt dt 1/ 2 8

Answer is D 14. Unbalanced set of impedances connected in Y, to which are applied balanced line voltage of 100 volts. The impedances are given by their ohmic values Za=10, Zb=j10 and Zc=-j10. Find the line A current using ABC sequence with Vab=100   60 0 volts. A. –j10 C. -1.34+j5 B. 1.34+j5 D. j10 By Wye to delta transformation,

N 10( j10)  ( j10)( j10)  ( j10)(10)   j10 Zc  j10 N 10( j10)  ( j10)( j10)  ( j10)(10) Zca     j10 Zb j10 Vab Vca 100  60 10060 Ia  Iab  Ica       j10 A Zab Zca j10  j10 Zab 

Answer is A

3

15. A balanced three phase three wire system has a delta connected load with 50 ohms resistor, a 5 micro-farad capacitor and a 0.56 H inductor in series in each phase. Using positive phase sequence with Van=390 30 0 Vrms and  =500 rad/sec, find the magnitude of line A current. A. -1.156+j8.925A C. 8.307-j3.461A B. -7.152-j5.464A D. -1.788-j7.693A

Z  R  j( X  X ) L

C

Impedance per Phase of Delta Connected load,

Z  Z  50  j (500 x0.56  

1 )  50  j120 500 x5 x10 6

The equivalent Wye connected impedance

50  j120  16.7  j 40 3 Van 39030 Ia    1.156  j8.925 A Z ' 16.7  j 40

Z '  Z  Y

Answer is A 16. Three impedances of 3 + j4  are connected in  to a 220-V, three-phase line. Find the complex power. A. 15-j20 kVA C. 7.74+j5.8 kVA B. 17.5+j24 kVA D. 17.4+j23.2 kVA

I 

2200  44  53.13 3  j4

0

*

S   3V I   3(2200)(4453.13)  17424  j 23232VA 3

Answer is D 17. The total resistance of the series-parallel connection of three resistors is 11.67 ohms. With Rx is the series resistance, Ry=2Rz and Rz=2Rx. Find the resistance Ry. A. 10 ohms C. 5 ohms B. 20 ohms D. 30 ohms

11.67  Rx  Rx  5

RyRz 4 Rx(2 Rx)  Rx  Ry  Rz 4 Rx  2 Rx Therefore, Ry  4(5)  20ohms

Answer is B 18. Four capacitors 10, 15, 20 and 30 micro-farad to be connected series- parallel. Which arrangement will result to maximum total capacitance A. 10 F in series and other 3 in parallel C. 20 F in series and other 3 in parallel

 B. 15  F in series and other 3 in parallel C

MAX



 D. 30  F in series and other 3 in parallel

(10  15  20)30  18F 10  15  20  30

Answer is A 19. Two previously uncharged capacitors are connected in series and then charged with a 12 volt source. One capacitor is 30 micro-farad and the other is unknown. If the voltage across the first capacitor is 8 volt, find the capacitance of the unknown capacitor. A. 30 micro-farad C. 20 micro-farad B. 40 micro-farad D. 60 micro-farad

C  2

Q CV 30(8)    60F V V  V 12  8 2

2

1

T

1

1

Answer is D

4

20. A wye connected balanced load is supplied from a balance source, the line voltage Vab = 201+j54 volts and the phase b line current Ib= -1+j2.89 amperes. Find the power factor of the balanced 3-phase load. A. 56% leading C. 99.8% leading B. 56% lagging D. 99.8% lagging By phasor analysis,

V  208.1315

0

ab

Ib  3.058109

0

Ia  3.058  11 3.058 3.058 Iab  (11  30)  (41 ) 3 3 0

0

0

Therefore,  41  15  56

0

Power Factor= cos 56=56%lagging Answer is B 21. A balanced delta connected load of impedance 45+j30 ohms is connected to a three phase balanced source of with Vcn=7620  900 through a line with 1+j2 ohms per line, find the line A current using ABC sequence. A. 228.6-j304.8 A C. 378-j46 A B. 119.3-j82 A D. 150-j350 A

Van  7620  30 Van 7620  30 Ia    150  j 350 A Zan  Zline (45  j30) / 3  (1  j 2) 0

By phasor,

0

Answer is D 22. A three phase, 60 Hz, 500 kV transmission line is 300 km long. The line inductance is 0.97 mH per km and its capacitance is 0.0115 micro-farad per km per phase. The receiving end rated load is 800MW, 0.8 power factor lagging at 500 kV. Determine the SIL of the line. Assume a lossless line. A. 1000MW C. 650 MW B. 860 MW D. 500 MW For lossless line, R=0 3

L 0.97 x10 Zc    290.4ohms C 0.0115 x10 kV 500 SIL    860.8MW Zc 290.4 6

2

2

Answer is B 23. Two 50 kVA transformer are V-connected. They supply a balanced 3-phase load of 100 kVA at a line voltage of 220 volts and a lagging power factor of 80%. Find the overload kVA of the open delta bank. A. 50 kVA C. 7.7 kVA B. 5.4 kVA D. 13.4 kVA Overload KVA of Open Delta Bank=Load kVA- Capacity of open delta Bank = 100-1.732(50)=13.4 kVA Answer is D 24. What is the IEEE standard device number for distance relay A. 51 C. 21 B. 52 D. 87 Answer is C

5

25. Which of the following heating method is based on transformer principle? A. Resistance heating C. Eddy current heating B. Induction heating D. Dielectric heating Answer is B 26. What is the device number for differential protective relay A. 89 C. 64 B. 87 D. 81 Answer is B 27. Transmission line consists of two No. 0 copper wires 0.528 ohm per mile and is 5 miles long. How much power is wasted in the line when 75 amperes are flowing? A. 29.7 kW C. 59.3 kW B. 14.85 kW D. 43.05 kW

Line loss  I (2 Rw)  75 (2 x0.528 x5)  29,700watts 2

2

Answer is A 28. Three 12kW electric Range are supplied by a feeder, What is the demand load of the feeder? A. 11 kW C. 12 kW B. 14 kW D. 18 kW Answer is B PEC 2009 page 108 29. Receptacle outlet shall be located not more than ____ mm above countertop of kitchen. A. 300 C. 250 B. 500 D. 600 Answer is B PEC 2009 page86 30. The magnetism that remains in a magnet after magnetizing force has been removed is called A. Permeability C. Induction B. Residual D. Saturation Answer is B 31. What is the minimum length and size of grounding electrode conductor in concrete encased electrode. A. 5000mm, 22sqmm C. 6000mm, 22 sqmm B. 5000mm, 30 sqmm D. 6000mm, 30 sqmm Answer is C PEC 2009 page 215 32. A wattmeter is installed in a balanced 3-phase system with current coil at line A while voltage coil across B and C. The wattmeter will measure A. total power C. real power B. active power D. reactive power Answer is D 33. What is the rating of disconnecting means of a hermetic motor having 30 amp. rated current A. 75A C. 34.5A B. 52.5A D. 37.5A Disconnecting means=115%(FLA)=1.15(30)=34.5A Answer is C

34. What is the total impedance of a 200 ohms resistance in parallel with a 200 ohms capacitive reactance? A. 100-j100 ohms C. 141.4-j141.4 ohms B. 200-j200 ohms D. 70.7-j70.7 ohms

Zp 

200(0  j 200)  100  j100 200  (0  j 200)

Answer is A 35. A 115 kV, 60 Hz, three phase transmission line is 45 km long. The resistance per phase is 6 ohm and the inductive reactance per phase is 12.33 ohm. Calculate the transmission efficiency

6

when the line is supplying a three phase load of 220 MVA at lagging power factor of 0.8 at 115 kV. A. 88.9% C. 90.6% B. 91.6% D. 90.4% 3

Sr 220 x10 I    1104.5 A 3Vr 3(115) Pr 220 x10 (0.8)    88.9% Pr Line Loss 220 x10 (0.8)  3(1104.5) (6) R

6

6

2

Answer is A 36. A 120 volts dc source is supplying three resistors in series. The first has 50 ohm resistance, second has 45 volts drop and third dissipates 25 watts power. What is the resistance R2? A. 240 ohms C. 120 ohms B. 100 ohms D. 90 ohms

P  P1  P2  P3 120I  50I  45I  25 I  0.5 A 45V R2   90ohms 0.5 A T

2

Answer is D 37. Where does 2400 volts – 34,500 volts belong? A. high voltage C. low voltage B. Extra high voltage D. Medium voltage Answer is D 38. A magnetic flux of 1000 lines cuts across a coil of 800 turns in 2 induced in the coil? A. 4000 volts C. 400 volts B. 2000 volts D. 800 volts

 s. What is the voltage

d  1000 x10  eN  800   4000volts dt  2 x10  8

6

Answer is A 39. An electric stove draws 7.5 A from a 120V dc source. What is the maximum value of an alternating current which will produce heat at the same rate? A. 15A C. 7.5A B. 13A D. 10.6A For the same Power,

 Im ax  75 ( R)    ( R) 2   Im ax  10.6volts 2

2

Answer is D 40. Find the instantaneous voltage at 45 o in a wave whose peak value is 175V. A. 123.7 volts C. 175 volts B. 247.5 volts D. 87.5 volts

e  175 sin 45  123.7volts 0

Answer is A

7

41. A shunt generator has a field resistance of 50 ohms in series with a rheostat. When the terminal voltage of the generator is 110V, the field current is 2A. How much resistance is cut in on the shunt field rheostat? A. 15 ohms C. 10 ohms B. 7 ohms D. 5 ohms

110v 50  R R  5 ohms

2

Answer is D 42. General lighting load for store shall be calculated as ____ VA per sqm. A. 16 C. 28 B. 8 D. 24 Answer is D 43. A permanent joining of metal parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safety any current likely to be imposed. A. Counterpoise C. Interlink B. Air Terminal D. Bonding Answer is D 44. Ground ring shall be buried at a depth below the earth’s surface not less than ___mm. A. 500mm C. 750mm B. 1000mm D. 600mm Answer is C PEC 2009 page 217 45. Determine the minimum ampacity of feeder for group of single phase motors with the following full load currents 28A, 40A and 50A at 230V. A. 118A C. 193A B. 130A D. 205A

Min. Ampacity  I  25%( HRM ) T

 (28  40  50)  25%(50)  130.5 A Answer is B 46. Two batteries, 12 volts with internal resistance of 1.3 ohm and 13.8 volts with internal resistance of 1.8 ohms are connected in parallel with load resistance of 5 ohms. What is the value of current supplied by the 12 volts battery? A. 1.32 A C. 1.51 A B. 2.22 A D. 0.71 A By Millman’s Theorem:

12 13.8  1 . 3 1.8  11.08volts V 1 1 1   1.3 1.8 5 Current delivered by 12 volts battery

I  1

12  11.08  0.71A 1.3

Answer is D 47. A synchronous generator having a subtransient reactance of 0.15 pu and operating 5% above its rated voltage supplies a synchronous motor having 0.2 p.u. subtransient reactance. The motor is connected to the generator by a transmission line and a transformer of total reactance 0.305 p.u. A sudden three phase short circuit occurs at the generator terminals. Determine the per unit subtransient fault current.

8

A. –j9.179 pu B. –j9.279 pu

C. –j9.379 pu D. –j9.079 pu

(0.305  0.2)(0.15)  0.115648 p.u. (0.305  0.2  0.15) E 1.05 Ipu    9.079 pu Xth 0.115648 Xth 

Answer is D 48. A lamp of 80 cd is placed 4cm from an object. If the lamp is to be replaced by a 40 cd, what must be the distance of the object to receive same illumination? A. 2.0 cm C. 8 cm B. 2.83 cm D. 5.66 cm

E

I d

2

I I  d d 1 2

2 2

1

2

80 40  4 d

d  2.83cm

2

2

2

2

Answer is A 49. If a coil of 150 turns is linked with a flux of 0.01 Weber when carrying a current of 10 amperes. If this current uniformly reversed in 0.01 second, calculate the induced electromagnetic force. A. 75 volts C. 150 volts B. 600 volts D. 300 volts

N 150(0.01)   0.15H I 10 dI  10  (10)  e  L  0.15   300volts dt 0 . 01   d  0.01  (0.01)   150  300volts Or e  N  dt 0 . 01   L

50. A square coil of 10 cm side and with 100 turns is rotated at a uniform speed of 500 rpm about an axis at right angle to a uniform field of 0.5 weber per sqm. Calculate the instantaneous value of induced emf when the plane of the coil is at 45 o with the field direction. A. 18.51 volts C. 19.52 volts B. 26.21 volts D. 33.33 volts Instantaneous voltage

d  sin t   N cos(t )  N sin(t  90) dt 500  50   2N '  2     rad / sec 60   3 When   t  90)  45 eN

m

m

m

0

e

50  (100turns )(0.5 x0.1x0.1) sin 45  18.5volts 3 0

Answer is A 51. Fuse on the motor is used to I. Short circuit Protection II. Overload Protection

9

A. l only C. Both I or II B. II only D. Either I or II Answer is A 52. A series motor, having an armature resistance of 0.1  and a field resistance of 0.15  , takes 48 A at 230 V and 720 rpm. The load of the motor is reduced so that the motor now takes 32A. Find the percentage change in torque. A. 55.6% C. 66.7% B. 46.8% D. 26.8% In DC Motor,

T  kIa

T  kIa T1  T 2 Ia  Ia 48  32 %Change in Torque=    55.6% T1 Ia 48 2

For Series Motor,

2

2

1

2

2

2

2

2

1

Answer is A 53. What is the power absorbed by the impedance 3986+j2980 ohms if the voltage is 200 Vrms. A. 4.8 watts C. 27.8 watts B. 6.44 watts D. 54.5 watts

200   P I R  (3986)  6.44watts 3986  2980   2

2

2

2

Answer is B 54. The total resistance of the parallel connection of three resistors is 2.1819 ohms. When Rx=3Ry and Rz=2Ry. Find the total resistance when three resistors are in series. A. 40 ohms C. 12 ohms B. 24 ohms D. 16 ohms

1 1 1 1    2.1819 3Ry Ry 2 Ry Ry  4 Rx  3(4)  12 Rz  2(4)  8 When connected in series.

Rt  12  4  8  24 ohms

Answer is B 55. A wave radiated by a source in air is incident upon a soil surface where upon a part of the wave is transmitted in the soil medium. If the wavelength of the wave is 60 cm to 30 cm. What is the soil permittivity? Assume soil to be a very loss less medium. A. 6 C. 4 B. 16 D. 5 See Electromag Lecture Answer is C 56. A transformer has a turn ratio of 5. If a 100 ohms resistor is connected across the secondary, what is the resistance referred to primary? A. 2500 ohms C. 6000 ohms B. 4000 ohms D. 5000 ohms

Re p  a Rp  5 (100)  2500 ohms 2

2

57. A transformer rated 1-kVA, 240/120, 60 Hz. Because of an emergency, the transformer has to be used on a 50 Hz system. If the flux density in the transformer core is to be kept the same at 60 Hz and 240 volts, what voltage should be applied at 50 Hz? A. 200 V C. 240 V

10

B. 300 V

D. 288 V

E f

 f '  50  E '  E    240   200volts  60   f  Answer is A 58. In wye system, the phase current is ____ (of) line current. A. less than C. 1.732 B. equal to

D. 0.577

Answer is B 59. A 230 volt shunt motor, having armature resistance of 0.05 ohm and a field resistance of 75 ohms, draws a line current of 7 A while running light at 1120 rpm. The line current at given load is 46 amperes. If the field circuit resistance of the motor is increased to 100 ohms, Determine the motor speed at given load. A. 1480 rpm C. 1110 rpm B. 1680 rpm D. 980 rpm

E  Vt  Ia( Ra)  kN

  If  3.067 A 230  3.933(0.05)  k 1120 When I=46A, Ia=46-230/100=43.7A   If  2.3 A When I=7A, Ia=7-230/75=3.933A

1

1

2

  2

2.3   0.75 3.067 1

1

230  43.7(0.05)  k (0.75 ) N 1

2

Combining Equations;

N  1480rpm 2

Answer is A

60. A battery is in parallel with two equal resistive loads supplies 16 watts. How much power the battery will deliver if another resistance of equal value will be added to the circuit? A. 32W C. 48W B. 24W D. 16W Pt=8+8+8=24watts Answer is B 61. The color used to indicate grounding conductor larger than 14 sqmm are ___, ___ and ___. I. Yellow II White III. Gray IV. Any color with three white stripe except green. A. I. II. III C. I, II IV B. II, III, IV D. I,II, III, IV Answer is B 62. A 10 km transmission line has an impedance of 0.25 +j0.43 per km. The line supplies a 316.8kW 3-phase load at 3.3kV, 0.8 power factor lagging. What is the sending end power? A. 320.4kW C. 360 kW B. 324 kW D. 352.8 kW

 316.8  P  316.8kw  3  (0.25 x10)  352.8kW 3 ( 3 . 3 )( 0 . 8 )   2

R

Answer is D

11

63. If P = number of fair of poles, f = the frequency in the Hz, N = speed in rpm, the relationship between the three for synchronous alternators is given as A f=PN/120 Hz C. f=PN/60 Hz B. P=fN/120 poles D. N=fP/120 rpm Answer is A 64. Month of March is the fire prevention month, what is the Electrical Safety month? A. February C. April B. May D. June Answer is B 65. Three resistors R1=3 ohms, R2=6 ohms and R3=9 ohms are connected in parallel across 12 volts dc source. What value of resistance must be connected in series with 9 ohms resistance so that the total current is 7 Amperes? A. 3 ohms C. 9 ohms B. 4 ohms D. 6 ohms

1 1 1 1    12 3 6 9  R 7 R  3 ohms Answer is A 66. A single phase induction motor is rated 5 hp, 220 V, 75%pf and 85% efficiency. What is its rated current? A. 26.6 Amp C. 22.6 Amp B. 16.95 Amp D. 15.36 Amp

I

HPx746 / 5HPx746 / 0.85   26.6 A Vxpf 220 x0.75

67. What is the speed of a 6 poles induction motor, if the percentage slip is 2.5%? A. 1755 rpm C. 1462.5 rpm B. 1170 rpm D. 731.25 rpm Take f=60Hz, Ns=120(60)/6=1200rpm

N  1200(1  0.025)  1170rpm R

68. Three-phase generators G1, 100 MVA 33kv 12 % reactance and G 2 50 MVA 33 kv 10% reactance in parallel supply motor loads M1 30 MVA 30 KV 20%X, M2 20 MVA 30 KV 15%X and M3 50 MVA 30 KV 20%X in parallel through Transformers T 1 and T2 are rated at 100MVA and 33/110kV, and each has a reactance of 0.08 per unit and transmission line of 60 ohms reactance. Assuming 100 MVA and 33 kV are used as base values, Determine the per unit fault current for a three phase fault at the motor busbars. A. 9.23 pu C. 7.98 pu B. 7.83 pu D. 7.35 pu At bases : MVAbase=100 and KVbase=33

12

Xg  0.12 pu 1

 100  Xg  0.1   0.2 pu 50   Xt  Xt  0.08 pu 2

1

2

60  0.496 pu 110 100  30   100  Xm  0.2     0.55 pu 33 30     30   100  Xm  0.15     0.62 pu  33   20   30   100  Xm  0.2     0.33 pu 33 50    Xline 

2

2

1

2

2

2

3

Parallel Equivalent of generators, transformers and line is

Xp  0.08  0.08  0.496  Thevenin’s Reactance:

Ipu 

0.12 x0.2  0.731 pu 0.12  0.2

1 1 1 1 1      7.83 pu Xth 0.55 0.62 0.33 0.731

Answer is B

69. A 20 micro-farad capacitor is charge to 12 volts battery. What is the stored energy in the capacitor? A. 1.44 mJ C. 24 mJ B. 2.88 mJ D. 12 mJ

Wc  (1/ 2)CV  0.5(20 x10 )(12)  1.44mJ 2

6

2

Answer is A 70. In dwelling units, at least one receptacle outlet for hallways more than _____meters? A. 4 C. 1.5 B. 2 D. 3 Answer is D 71. A transformer has induced emf of 220 V at 60 Hz, what is the induced emf when the frequency is 50Hz? A. 264 V C. 220 V B. 183.3 V D. 152.8 V

 f '  50  E '  E    220   183.3volts  60   f  72. A three phase induction motor is rated 50 hp, 220 volts, 1745 rpm and 78% pf. What is the percent slip? A. 2.6 C. 2.5 B. 3.06 D. 5.36

13

%Slip 

1800  1745 x100%  3.06% 1800

73. Three inductors, 20, 20 and 40mH are connected in parallel, determine the equivalent inductance. A. 4 mH C. 0.125 mH B. 8 mH D. 80 mH

1 1 1 1    L 20 20 40

L  8mH T

T

74. Resistances X, Y and Z are connected in series. The voltage across X &Y is 21 volts, across Y & Z is 24 volts and across X & Z is 27 volts. Find the value of resistance Y when the current is 1 ampere. A. 18 C. 9 B. 15 D. 12

X  Y  21 Y  Z  24 X  Z  27

Solving X , Y , Z X  12 Y 9 Z  15 Answer is C

75. The lighting load for an office building expressed in terms of volt-ampere per square meter must be at least_________. A. 16 C. 18 B. 24 D. 28 Answer is D 76. In the left hand rule, thumbs always represent A. Voltage C. Current B. Direction of force on conductor D. Magnetic field Answer is B 77. Two impedances Z1=10+jX and Z2=R+j0, are connected in parallel. Determine X so that the total circuit impedance is 8+j4 ohms. A. 20 ohms C. 24 ohms B. 10 ohms D. 40 ohms

1 1 1   8  j 4 10  jX R 8 4 10 1 X j   j 80 80 10  X R 10  X 2

2

2

Equating imaginary components

4 X  80 10  X X  10 ohms 2

2

Answer is D

14

2

78. A coil has resistance of 100 ohms at 90 oC. At 100oC, its resistance is 101 ohms. What is the temperature coefficient of wire at 90 oC? A. 0.01 /oC C. 0.1 /oC B. 0.001 /oC D. 0.0001 /oC

101  1001   (100  90)   0.001 Answer is B

79. Convert 10 inches diameter to circular mil A. 10,000 B. 100,000

C. 100,000,000 D. 1,000,000

Cmil  (10,000mil)  100,000,000 2

Answer is C 80. A single 60Hz power line 5m horizontal spacing and a telephone line with 2m spacing are parallel to each other. The telephone line is symmetrically positioned directly 3m below phase b. The power line carries an rms current of 226 A. Find the magnitude of voltage per km induced in the telephone line. A. 8 V C. 3 V B. 6 V D. 5 V

Da 2  (5  1)  3  6.708m 2

2

Da1  (5  1)  3  5m 2

2

   Da 2   6.708  V     2 x10 ( Ia) Ln  377 2 x 10 ( 226 ) Ln      Da1   5    V  5 x10 V / m  5 V / km 7

7

12

3

81. What is the average output of a wind turbine in kW having a blade diameter of 60 ft if the wind velocity ranges from 10 to 30 mi/hr. A. 232.42 C. 124.2 B. 122.4 D. 224.2

P  2.46 x10 D V P min  2.46 x10 (60) (10)  9kW 3

2

3

3

2

3

P max  2.46 x10 (60) (30)  239kW Pave  124kW 3

2

3

Answer is C 82. What is the kVAR of capacitor to be connected across the load of 500 kW, 0.707 lagging power factor to improve the power factor to 95% A. 337.6 C. 364.6 B. 335.6 D. 556.4

Qc  P(tan  tan  ' )  500(tan 45  tan18.2)  335.6kVAR

83. A dc series motor is connected to a constant-torque load, which may be considered to require a constant electromagnetic torque regardless of motor speed. Neglect the voltage drop s due to armature and series-field resistance, the armature reaction and effect of saturation, by what percent is the motor speed changed when the voltage is reduced from 230 volts to 200 volts? A. 13% C. 16% B. 21% D. 32%

15

Vt  Ia( Ra  Rs )  kN Neglect VD Vt  N %Change in speed 

V1  V 2 230  200 x100  x100%  13% V1 230

84. What is the voltage across capacitor for a series RLC circuit with 20 ohm resistance, inductance of 100mH and 50 micro-farad capacitance. The series circuit is supplied at 230 volts, 60 Hertz source. A. -209.4+j272.8 C. 294.66-j383.92 B. 209.4+j272.8 D. -294.66+j383.92

V 2300 I   9.122837.5 A Zt 20  (37.7  53.05) Vc  I ( jXc)  9.122837.5 (0  j 53.05)  294.66  j 383.92volts 0

0

0

85. A parallel plate capacitor has plates 10cmx20cm are 0.15 mm apart and dielectric with relative permittivity of 5.0. Find the capacitance between parallel plates. A. 7.2 nF C. 6.8 nF B. 5.9 nF D. 6.5 nF

C

 A 0

r

12

8.85 x10 (5)(0.1mx0.2m)   5.9 x10 Farad 0.15 x10 m 9

3

t

86. A 120 volts shunt motor has rated current of 75 amperes, armature resistance of 0.2 ohm and shunt field resistance of 40 ohms and a brush drop of 2 volts. What resistance must be inserted with the armature so that the starting torque is 150% of the rated torque. A. 0.53 ohm C. 0.57 ohm B. 0.84 ohm D. 0.89 ohm

120  72 A 40 Vt  Vbc 120  2 Rst   Ra   0.2  0.89 ohm Iast 1.5(72)

Ia  I  If  75 

Answer is D 87. What is the derating factor of 4-6 conductors in raceway A. 42% C. 55% B. 80% D. 70% Answer is B 88. A 20 kVA, 220 volt, wye connected, 3-phase, salient pole synchronous generator supplies rated load at 0.707 lagging power factor. The reactance per phase are Xd=2Xq=4 ohms and resistance is negligible. Determine the percent voltage regulation. A. 100% C. 144% B. 600% D. 10% At lagging Power Factor:

tan  

IXq cos V  IXq sin 

16

0

52.5(2) cos 45 tan   127  52.5(2) sin 45   20.25 E  V cos   IXd sin(   ) E  127 cos 20.25  52.5(4) sin( 20.25  45)  309.9volts 309.9  127 %VR  x100%  144% 127 0

0

0

89. Resistors R1 and R2 are connected in parallel across 18 volts battery, the resulting current is 6 Ampere through R1. When these two resistors are connected in series with the same battery the potential difference across R2 is 12 volts. Find the resistance R2 in ohms. A. 6 C. 4 B. 3 D. 12

18v  3ohms 6A When series connected R1 

18  12  2A 3 12 R 2   6 ohms 2 I

90. A 3-phase, 8-pole, 60 Hz, wye connected, salient pole generator has 96 slots, with 4 conductor per slot connected in series in each phase. The coil pitch is 10 slots. If the maximum value of the air gap flux is 60mWb, determine the kVA rating of the machine if each phase is capable of carrying 650 amperes. A. 1846 kVA C. 1066 kVA B. 616 kVA D. 1256 kVA

E  4.44KpKdfN   10   180    P   12   0.966 Kp  sin    sin   2  2     0.5 Kd   0.95766 4 sin 7.5 turn 96slot N  2 x  64 slot 3 phase E  4.44(0.966)(0.95766)(60)(0.06)64  946.35V 3EI 3(946.35)(650) KVA    1845 1000 1000 0

0

0

91. Calculate the voltage induced in the armature winding of a 4-pole, wave-wound, dc machine having 728 active conductors and running at 1800rpm. The flux per pole is 30mWb. A. 1310.4 V C. 720.4 V B. 468.2 V D. 236.4 V

17

E

PNZ 4(0.03)(1800)(728)   1310.4volts 60a 60(2 x1)

92. The flux distribution in the air-gap of a 60 Hz, 4 pole, salient pole machine is sinusoidal, having an amplitude of 0.6 T. Calculate the rms value of the voltage induced in a 150 turn coil on the armature, if the axial length of the armature and inner diameter are both 100mm. A. 120 volts C. 240 volts B. 160 volts D. 300 volts

  Bm(Pr ojected area)  BmxlxD E  4.44(1)(1)(60)(0.6 x0.1x0.1)(150)  239.8volts

Since Salient pole,

93. A 6-pole, wave wound, dc generator armature has a 720 active conductors. The flux per pole is 20.35 mWb. What speed must the generator operate to induce 630 volts in the armature? A. 860 rpm C. 758 rpm B. 665 rpm D. 956 rpm

PNZ 60a 6(0.02035)( N )(720) 630  60(2 x1) N  860rpm E

94. A 75-kVA, 230-V/115-V, 60-Hz transformer was tested with the following results: Short-circuit test: 9.5 V, 326 A, 1200 W Open-circuit test: 115 V, 16.3 A, 750 W Determine the voltage regulation at full load 0.8 power factor lagging. A. 5.5% C. 6.5% B. 4.5% D. 3.6%

FLCL 1200 x100  x100%  1.6% Srated 75000 Vsc 9.5 %Z  x100%  x100%  4.13% Vprated 230 %R 

% X  4.13  1.6  3.807% %VR  % R cos  % X sin   1.6(0.8)  3.807(0.6)  3.6% 2

2

95. The force between two long parallel conductors is 147N/m. The conductor spacing is 10cm. If one conductor carries twice the current in other conductor. Calculate the current in each conductor. A. 1000 A, 2000A C. 6,062 A, 12,124 A B. 1608 A, 3216A D. 3031 A, 6,062 A

I I  F  2 x10  N / m d    I x2I  147  2 x10   0 . 1 m   I  6,062 A 7

1

7

2

1

1

1

i  2(6062)  12,124 A 2

18

96. A series circuit consists of a 30-microfarad capacitance and a resistance of 50 ohms connected across a 110-volt, 60-hertz supply. Calculate the voltage drop across capacitor. A. 95.75V C. 55.56 V B. 93.3V D. 98.38 V

1  88.4ohms 377(30 X 10 ) 110 I  1.083 A 50  88.4 Vc  IXc  1.083(88.4)  95.75 A Xc 

6

2

2

97. A three phase wye connected synchronous motor, rated 15 hp, 400 volts, has full load efficiency of 90%. The synchronous reactance is 3 ohms per phase. If the motor operates at full load and 0.8 leading power factor. Determine the power angle. A. -14.7o C. -12.7o o B. -13.7 D. -11.7o

HPx746 / 15HPx746 / 0.9   22.43 A 3Vxpf 3( 400) x0.8 400 E  V  I ( Zs )  0  22.43  36.87 (0  j3)  276.6  11.2 3 I

0

98. An induction motor, while driving a load, takes 350 kW at 0.707 power factor lagging. An over excited synchronous motor is then connected in parallel with the induction motor, taking 150 kW in power. If the overall power factor (of the two motors combined) becomes 0.9 lagging, calculate the kVA rating of the synchronous motor. A. 184.75 kVA C. 245.2 kVA B. 185.4 kVA D. 178.5 kVA

P tan   P tan  P P 350 tan 45  150 tan  tan 25.84  350  150   35.7 tan  ' 

L

SM

L

SM

SM

0

SM

0

SM

KVA  SM

150  184.75 cos 35.7

Answer is A 99. The law that induces emf and current always opposes the cause producing them was discovered by A. Lenz C. Faraday B. Maxwell D. Ohm Answer is A 100. Calculate the rotor resistance per phase of a 6 pole, 60Hz, 3-phase induction motor if the motor develops a maximum torque of 300N-m at 780 rpm. The rotor leakage reactance is 3 ohm per phase. A. 1.05 ohm C. 1.02 ohm B. 0.05 ohm D. 1.07 ohm

19

0

1200  780  0.35 1200 R Smt  X Smt 

2

2

R 0.35 R  1.05ohm

0.35 

2

2

Answer is A

20