EE August 15, 2021 Flashcards
1
Q
- A washing machine is, in effect, an R-L circuit. If the machine takes 311 W and 4.5 A from a 155-V source when operating normally. Find its power factor. *
- 75 lagging
- 65 lagging
- 60 lagging
- 70 lagging
A
0.60 lagging
2
Q
- Approximately how far away must an isolated positive point charge of 1 x 10^(–8) C be in order for it to produce an electric potential of 100 V? The charge is in free space with e = 8.85 x 10^(–12) C^2/N*m^2. *
- 6 m
- 2 m
- 90 m
- 3 m
A
0.90 m
3
Q
- Three single-phase transformers rated 25 kVA each are connected in delta-delta to supply a balanced load of 75 kVA. One transformer is removed for repair, what is the kVA of each remaining transformer? *
57.7
50
43.3
34.3
A
43.3
4
Q
- The per phase line loss in a 40 km long transmission line is NOT to exceed 60 kW while it is delivering 100 A per phase. If the resistivity of the conductor material is 1.72 x 10-8 ohm-meter, determine the required conductor diameter. *
- 2071 cm
- 1721 cm
- 2083 cm
- 2204 cm
A
1.2083 cm
5
Q
11. The dc armature winding resistance between terminals of a 750 kVA, 4400 volt, three-phase alternator is 0.9 ohm. Calculate the copper loss in the winding at full-load. * 13,075 W 16,343 W 10,800 W 26,149 W
A
16,343 W
6
Q
- A hermetic motor compressor has a rated load current of 25 A. What is the allowable rating for branch disconnecting means? *
31.25 A
28.75 A
30 A
37.5 A
A
28.75 A
7
Q
- Three impedances Z₁ = 2.2 + j2.2 Ω, Z₂ = 10 + j10 Ω and Z₃ = −j5 Ω are connected in series. The voltage across Z₁ is 26.59 − j4.69 V. What is the total voltage? *
93 + j62.4 V
93 − j62.4 V
112 − j51 V
A
112 − j51 V
8
Q
39. If the current in the armature of a dc series motor is reduced by 50%, the torque of the motor will be reduced by \_\_\_\_\_ % of the previous value. * 25 50 33 75
A
75
9
Q
- Find the power factor of a balanced three-phase load in an ABC circuit in which one line voltage is Vac = 440∠−90° v- and one-line current to the load is Ib = 2∠−90° A. *
0.5 lagging
0.5 leading
0 lagging
0 leading
A
0 leading
10
Q
- The following data were obtained when a short-circuit test was performed upon a 100-kVA 2,400/240-volt distribution transformer: Esc = 72 volts; Isc = 41.6 A, Psc = 1,180 watts. What is the equivalent reactance as referred to the secondary? *
- 0173 Ω
- 73 Ω
- 59 Ω
- 0159 Ω
A
0.0159 Ω
11
Q
- What is the capacitance per kilometer of two parallel wires 7 mm in diameter placed 30 cm apart from each other in air? *
625 x 10⁻⁹ F/km
- 5 x 10⁻⁹ F/km
- 25 x 10⁻¹² F/km
- 25 x 10⁻⁹ F/km
A
6.25 x 10⁻⁹ F/km
12
Q
Two generators G₁ and G₂ are rated 15 MVA, Z equals 0.10, 11 kV and 10 MVA, Z equals 0.10, 11 kV, respectively. The generators are connected to a transformer of 15 MVA, 11 per 66 kV and 6 percent impedance. Calculate the fault current in per unit when three-phase fault occurs on the high voltage side of the transformer. *
- 5
- 9
- 2
- 33
A
8.33
13
Q
A balanced three-phase three-wire system has a delta connected load with a 50 ohm resistor, a 5 microfarad capacitor and a 0.56 H inductor in series in each phase. If Van = 390 v rms with 30 deg angle and ω = 500 rad/sec, find the line current to phase c. * 8.308 − j3.462 A −8.308 + j3.462 A −7.152 − j5.464 A −1.158 − j8.925
A
−7.152 − j5.464 A
14
Q
A 3ϕ transmission line has two aluminum conductors in bundle. The self GMD of each conductor is 8.90 mm and the distance between the conductors and phases is 40 cm and 9 m, respectively. What is the inductance of the line per phase? * 1 μH/m 2 μH/m 3 μH/m 4 μH/m
A
1 μH/m
15
Q
A three-phase transformer is rated 25 MVA, 69/13.2 kV. It has a series Z = 7%. What is the new per unit impedance if the system study requires 100 MVA base and 67 kV base? *
- 4%
- 7%
- 8 %
- 9%
A
29.7%
