UNIVERSITY OF SAINT LOUISTuguegarao City, Cagayan

ELECTRICAL ENGINEERING DEPARTMENT

EE REVIEW 2: Pre-final Examination

INSTRUCTION: Select the correct answer for each of the following questions. Mark only one answer for each item by encircling the letter of your choice.

1. Which of the following alloys is NOT a linear resistor.A. Manganin C. thyriteB. Advance D. constantan

2. A No. 12 copper wire common to house wiring installations has a cross-sectional area of 0.00513 sq. in. Calculate the electron velocity when the current is 15 Amp.

A. 0.0112 in./sec C. 1.0012 in./secB. 0.0221 in./sec D. 0.0212 in./sec

3. The maximum value of the pulsating rectified current is 6Amp. Calculate the current that the dc ammeter would register if the supply comes from a full-wave rectifier.

A. 1.91Amp C. 4.24AmpB. 3.82Amp D. 2.12Amp

4. A 120-V battery, having an internal resistance of 0.5ohm, is connected through line resistances of 9.5 ohms to a variable load resistor. For what load resistance will the latter consume maximum power.

A. 0.5 ohm C. 9.5 ohmsB. 2 ohms D. 10 ohms

5. Permanent magnets are known as…A. Magnesia C. perminvarB. Lodestone D. alnico

6. Who discovered the relationship between electrical and magnetic phenomena in 1820

A. Hans Christian Oersted C. Michael FaradayB. William Maxwell D. William Gilberts

7. Calculate the flux density at the surface of a very long No. 0000 conductor with diameter of 0.460 inch that carries a current of 2500A.

A. 345 gauss C. 532 gaussB. 1497 gauss D. 855 gauss

8. Calculate the force (in dynes) exerted upon a conductor 6 in. long if it carries a current of 180A and is placed at right angles to a field whose flux density is 48000lines per sq inch.

A. 20.4 x 105 C. 50.4 x 105

B. 23.8 x 105 D. 40.6 x 105

9. A 10 MVA, 10kV, 3Φ, 50Hz, 1500rpm alternator in parallel with others at its capacity. The moment of inertia of the rotor is 2 x 105 kg-m2 and the synchronous reactance of the machine is 40%. What is the natural time period of oscillation.

A. 5 sec C. 2 secB. 6sec D. 4 sec

10. A 1000 kVA, 3,3kV, 60Hz, 4 pole, 3 phase star connected, wound rotor alternator has a synchronous reactance of 1 ohm per phase and negligible armature resistance. The generator is operating at full load, 0.80 p.f. lagging on an infinite bus. If a disturbance causes the power angle to swing by 1 degree (mechanical), what is the synchronizing power per phase.

A. 133.6kW C. 148.5kW

B. 90.8kW D. none of these11. A 1000 kVA, 3,3kV, 60Hz, 4 pole, 3 phase star connected, wound rotor

alternator has a synchronous reactance of 1 ohm per phase and negligible armature resistance. The generator is operating at full load, 0.80 p.f. lagging on an infinite bus. If a disturbance causes the power angle to swing by 1 degree (mechanical), what is the synchronizing torque.

A. 708.9N-m C. 508N-mB. 367.6N-m D. none of these

12. A 100MVA, 2 pole, 60Hz alternator has a moment of inertia of 50 x 103 kg-m2. What is the energy stored in the rotor at rated speed.

A. 5533MJ C. 3553MJB. 3355MJ D. 5335MJ

13. A squirrel cage motor develops a maximum torque at 2.25 times its full load torque and has a stand still reactance which is four times its resistance per phase. Determine the slip when working on full load.

A. 0.0586 C. 0.0865B. 0.0685 D. 0.075

14. A 3,300V, 24 pole, 50hz, 3-phase, star connected induction motor has a slip ring rotor resistance of 0.016 ohm and stand still reactance of 0.265 ohm per phase. Calculate the ratio of full-load torque to maximum torque if full-load torque is obtained at 247rpm.

A. 0.298 C. 0.288B. 0.375 D. 0.382

15. If the surge impedance of a long transmission line is 266.1 ohm, wavelength of 5000km, and the sending and receiving end values are maintained at 765kV, what is the theoretical steady-state stability limit.

A. 2794MW C. 9752MWB. 5974MW D. 6354MW

16. The ABCD parameters of a given long transmission line is as follows:A=D= 0.9313cis 0.209° B= 97cis87.2°ohm C = 1.37 x 10-3 cis 90.06°Determine the theoretical maximum power delivered if the sending and receiving end voltages are maintained at 765kV.

A. 2794MW C. 3798MWB. 5738MW D. 5974MW

17. What is the unit of reluctanceA. Mho C. henryB. Unit is unnamed D. reciprocal henry

18. Each of the pole faces of an electromagnet has an area of 6 sq. in. If the exciting winding creates a flux density of 50,000 lines per square inch in the air gap between magnet poles and a smooth block of iron, calculate the tractive effort exerted upon the latter.

A. 417 lb C. 198lbB. 274lb D. 287lb

19. A 1250 turn coil is uniformly wound over the non-magnetic core of a toroid whose average diameter is 4 in., and cross sectional area is 0.45sq.in. Calculate the inductance of the toroid.

A. 0.00178H C. 0.00238HB. 0.00718H D. 0.00287H

20. How much magnetic-field energy is stored in each of the poles of a generator if 8 x 105 maxwells are produced by a current of 2.6A in a 2200-turn coil.

A. 17.1 J C. 16.9JB. 5.7J D. 22.8J

21. “In all cases of electromagnetic induction, the current set up by an induced voltage tends to create flux whose direction opposes any change in the existing flux”. This is know as____.

A. Faraday’s Law C. Maxwell’s Law

B. Tesla’s Law D. Lenz’ Law22. Two units, connected in parallel are energized by an ac source. If the

component currents (rms) are 12 and 16A, respectively and the corresponding sinusoidal waves are out of phase by 60 elec degrees, calculate the rms value of the total current.

A. 28A C. 24.4AB. 19.8A D. none of these

23. A series circuit consisting of a 0.0795-henry inductor and 177μF capacitor is connected to a 120-volt variable-frequency source. At what frequency will the circuit take a leading current of 4A.

A. 60Hz C. 58HzB. 82Hz D. 22 Hz

24. A 50μF is connected in series with an impedance coil whose resistance and inductance are 12.25 ohms and 0.1 H, respectively. Calculate the Q factor of the circuit.

A. 3.66 C. 100B. 81.8 D. 62.2

25. A 480-V, 60Hz, 50Hp, three phase induction motor is drawing 60A at 0.85 pf. lagging. The stator copper losses are 2kW, and the rotor copper losses are 700W. The friction and windage losses are 600W, the core losses are 1800W, and the stray losses are negligible. What is the air-gap power.

A. 42.4kW C. 40.4kWB. 38.6kW D. 40.6kW

26. How many Coulumbs are there in one Faraday.A. 10 C. 96500B. 14650 D. none of these

27. A circuit contains an inductance of 0.1062mH, a capacitance of 106pF, and a resistance in series. At resonance the impedance equals 10 ohms. Determine the bandwidth at resonance.

A. 1.5MHz C. 15MHzB. 15KHz D. 150MHz

28. The core of a 60 Hz transformer has a net cross section of 20 sq in., and themaximum flux density in the core is 60,000 lines per sq in. There are 700 turns in the primary and 70 turns at the secondary. What is the voltage at the primary?

A. 2630 C. 2330 B. 2230 D. 2430

29. A 5 kva 60 Hz single phase transformer has a full load efficiency of 96% and an

iron loss of 40 watts under rated condition. If the transformer is connected as an

autotransformer operating from a 240 volts mains and delivering 5 kw at unity power factor to a 120 volts circuit, calculate the percent efficiency.

A. 95.6 C. 98.7B. 95.8 D. 98.4

30. Two single-phase transformers each rated 75kVA are connected in open-delta to

serve a three-phase load of 120kW at 0.80 pf lagging. To prevent overloading of

transformers, determine the size of the capacitor in KVAR.A. 40 C. 50B. 60 D. 90

31. Meralco installed three-single phase transformers, each rated 50kVA, 2400V

primary and 277V secondary, 4% impedance, have their primaries connected in delta and their secondaries in wye. Solve for the fault current delivered if a phase to phase fault occurred at the secondary terminals.

A. 4512A C. 3908AB. 2998A D. 2256A

32. A current transformer is rated 400/5 A., the ratio factor is four at 30°C. This is to

be installed in an enclosure where the highest ambient temperature might be 55°C. Find the allowable maximum primary current of the current transformer.

A. 1180A C. 1600AB. 1100A D. 565A

33. Calculate the RMS Hp required of an open type electric motor which operates at

15HP load for 8 minutes, 10HP load for 6 minutes, 20HP for 6 minutes, and the motor is at rest for 12 minutes, after which the cycle is repeated.

A. 26hp C. 14hpB. 23.5hp D. 17. 45hp

34. What is the negative sequence component of the line currents if one of the in-phase impedances of its delta connected load is removed. The delta load with impedance of 10cis0 ohm per phase is supplied from a 220V, 60Hz, 3-phase source.

A. 20cis0908 A C. 0B. 34.64cis30 A D. 34.64cis0

35. An induction motor of 30Hp, 220V, 3-phase draws 450% of the rated current with the rated voltage and delivers during the starting period 130% of the normal torque. The full load efficiency and full load pf of the motor is 80 and 70%, respectively. If an autotransformer is used as starting unit and the starting torque of the load is only 50% of the rated torque of the motor, find the starting current of the line.A. 104.88A C. 586.78AB. 471.96A D. 292.71A

36. The reciprocal of the power factor of a coil is also called its…A. figure of merit C. reactive factorB. amplitude factor D. incidence factor

37. The transformer is designed to carry 30 times its rated current for 1 second. The length of time that a current of 20 times the rated current can be allowed to flow is…A. 0.67s C. 2 sB. 1.5s D. 2.25 s

38. An alternating current and a direct current flows simultaneously in the same conductor. If the effective value of the alternating current is 5A and the direct current is 10A. What will an AC ammeter read when connected in the circuit.A. 5A C. 15AB. 11A D. 7.5A

39. For maximum power transfer, source impedance must be…A. must be very high compared to the load impedanceB. must be very small as compared to the load impedanceC. must be complex conjugate of load impedanceD. must be equal to load impedance

40. One conductor of a three-phase line is open. The current flowing to the Δ-connected load through line A is 20A. With the current in line A as reference and assuming that line C is open, what is the negative sequence current of phase A.A. 10 + j 5.78 C. -10 + j 5.78B. 10 - j 5.78 D. j11.55

41. A 10-ohm resistor is connected in parallel to an impedance Z=3+j4 ohms. Find the circuit power facto.A. 0.600 lagging C. 0.609laggingB. 0.866 lagging D. 0.809 lagging

42. The net energy required for a certain country in 2008 was approximately 3 x 106 GWH. What is the equivalent of this energy in quad.A. 12.42 B. 102.4 C. 10.24 D. 124.2

43. Which of the following is the best conductor of electricity.A. polyurethane C. graphiteB. china clay D. porcelain

44. Two identical coupled coils have an equivalent inductance of 40mH when connected in series aiding and 17.5mH in series opposing. What is the value of the mutual inductance.A. 0.5625Mh C. 5.625mHB. 56.25Mh D. 22.5Mh

45. The voltage across a 15H inductor is given by v = 28.5 + 6t1/2 V. Find the current in the inductor at 2.5s if the initial current is 15A.A. 16.5 B. 20.2 C. 21.7 D. 18.4

46. In a balanced three-phase system, the Y-connected impedances are 10cis 30 deg angle. If Vbc is equal to 416cis240 deg V. Determine Icn.A. 12 + j20.8 A C. 12-j20.8 AB. 20.8-j12 A D. 20.8+j12 A

47. A 4-pole, 3-phase synchronous motor runs at full-load speed of 1200rpm. If the load is reduced to one-half of rated, what is its speed.A. 1200rpm C. 600rpmB. 1800rpm D. 2400rpm

48. An LC circuit resonates at 2,000kHz and has a Q of 100. Find the upper cut off frequency.A. 2,020kHz C. 2,010kHzB. 2100kHz D. 2,050kHz

49. Brushes are always placed alongA. magnetic neutral axis C. perpendicular to MNAB. geometric neutral axis D. perpendicular to GNA

50. Which of the following has the highest value of relative permeabilityA. iron C. Mu-metalB. supermalloy D. 4%Si- Fe