11. Which of the following core has linear characteristics?
A. Air core
B. Steel core
C. CRGO core
D. Iron core
Answer: A
Air core coils have linear magnetization characteristics that are they do not saturate. The open-circuit characteristics graph is linear in the case of a synchronous machine.
12. Calculate the active power in a 1 Ω resistor with 2 A current flowing through it.
A. 2 W
B. 4 W
C. 7 W
D. 1 W
Answer: B
The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat.
The voltage and current are in the same phase in the case of the resistor so the angle between V & I is 90°.
P = I2R = 2×2×1=4 W.
13. The slope of the V-I curve is 13°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. .2544 Ω
B. .7771 Ω
C. .2308 Ω
D. .5788 Ω
Answer: C
The slope of the V-I curve is resistance.
The slope given is 13° so R=tan(13°)=.2308 Ω.
The slope of the I-V curve is reciprocal to resistance.
14. A 2-pole lap wound generator with 44 armature conductors and a flux per pole of .07 Wb has an armature current of 80 A. The developed torque is _________
A. 39.7 N-m
B. 39.2 N-m
C. 38.4 N-m
D. 37.2 N-m
Answer: B
The developed torque in the motor is Km×I. The value of the machine constant(Km) is
Eb÷ωm = Φ×Z×P÷2×3.14×A
= .07×44×2÷2×3.14×2 = .49 Vs/rad.
The developed torque is .49×80 = 39.2 N-m.
15. Calculate the active power in a .45 H inductor.
A. 0.11 W
B. 0.14 W
C. 0.15 W
D. 0 W
Answer: D
The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy.
The voltage and current are 90° in phase in the case of the inductor so the angle between V & I is 90°.
P=VIcos90° = 0 W.
16. A 10-pole, 3-phase, 60 Hz induction motor is operating at a speed of 100 rpm. The frequency of the rotor current of the motor in Hz is __________
A. 52.4
B. 54.8
C. 51.66
D. 51.77
Answer: C
Given a number of poles = 10.
Supply frequency is 60 Hz.
Rotor speed is 100 rpm.
Ns = 120×f÷P
=120×60÷10 = 720 rpm.
S=Ns-Nr÷Ns
= 720-100÷720=.86.
F2=sf=.86×60=51.66 Hz.
17. Calculate the phase angle of the sinusoidal waveform z(t)=.99sin(4578πt+78π÷78).
A. π÷3
B. 2π
C. π÷7
D. π
Answer: D
The sinusoidal waveform is generally expressed in the form of
V=Vmsin(ωt+α)
where
Vm represents peak value
Ω represents angular frequency
α represents a phase difference.
18. Calculate the moment of inertia of the disc having a mass of 4 kg and diameter of 1458 cm.
A. 106.288 kgm2
B. 104.589 kgm2
C. 105.487 kgm2
D. 107.018 kgm2
Answer: A
The moment of inertia of the disc can be calculated using the formula
I=mr2×.5.
The mass of the disc and diameter is given.
I=(4)×.5×(7.29)2=106.288 kgm2.
It depends upon the orientation of the rotational axis.
19. Calculate the moment of inertia of the thin spherical shell having a mass of 703 kg and a diameter of 376 cm.
A. 1639.89 kgm2
B. 1628.47 kgm2
C. 1678.12 kgm2
D. 1978.19 kgm2
Answer: A
The moment of inertia of the thin spherical shell can be calculated using the formula
I=mr2×.66.
The mass of the thin spherical shell and diameter is given.
I=(703)×.66×(1.88)2=1639.89 kgm2.
It depends upon the orientation of the rotational axis.
20. Calculate the value of the torque when 89 N force is applied perpendicular to a 78 m length of stick fixed at the center.
A. 6942 N-m
B. 3000 N-m
C. 1000 N-m
D. 4470 N-m
Answer: A
Torque can be calculated using the relation
T = (length of stick) × (Force applieD. = r×F×sin90).
F is given as 89 N and r is 78 m then torque is
89×78 = 6942 N-m. (the angle between F and r is 90 degrees).
