61. Calculate the active power in a 181 H inductor.
A. 2448 W
B. 1789 W
C. 4879 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.
62. Calculate the active power in a 17 ω resistor with 18 A current flowing through it.
A. 5508 W
B. 5104 W
C. 5554 W
D. 5558 W
Answer: A
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 0°.
P=I2R
=18×18×17=5508 W.
63. A three-phase slip ring induction motor is fed from the rotor side with the stator winding short-circuited. The frequency of the current flowing in the short-circuited stator is ____________
A. Slip frequency
B. Supply frequency
C. The frequency corresponding to rotor speed
D. Zero
Answer: A
The relative speed between rotor magnetic field and stator conductors is sip speed and hence the frequency of induced e.m.f is equal to slip frequency.
64. An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 720 rpm. The frequency of the rotor current of the motor in Hz is __________
A. 2
B. 4
C. 3
D. 1
Answer: A
Given a number of poles = 8.
The supply frequency is 50 Hz.
Rotor speed is 720 rpm.
Ns = 120×f÷P
=120×50÷8 = 750 rpm.
S=Ns-Nr÷Ns
= 750 – 720÷750 = .04.
F2=sf=.04×50=2 Hz.
65. Calculate the phase angle of the sinusoidal waveform z(t)=78sin(456πt+2π÷78).
A. π÷39
B. 2π÷5
C. π÷74
D. 2π÷4
Answer: A
The sinusoidal waveform is generally expressed in the form of V=Vmsin(ωt+α) where Vm represents peak value, ω represents angular frequency, and α represents a phase difference.
66. Calculate the moment of inertia of the disc having a mass of 54 kg and diameter of 91 cm.
A. 5.512 kgm2
B. 5.589 kgm2
C. 5.487 kgm2
D. 5.018 kgm2
Answer: B
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=(54)×.5×(.455)2=5.589 kgm2.
It depends upon the orientation of the rotational axis.
67. Calculate the moment of inertia of the thin spherical shell having a mass of 73 kg and diameter of 36 cm.
A. 1.56 kgm2
B. 1.47 kgm2
C. 1.38 kgm2
D. 1.48 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=(73)×.66×(.18)2=1.56 kgm2.
It depends upon the orientation of the rotational axis.
68. A 50 Hz, 4poles, a single-phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. The slip of motor in the direction of rotation & opposite direction of the motor will be respectively.
A. 0.05, 0.95
B. 0.04, 1.96
C. 0.05, 1.95
D. 0.05, 0.02
Answer: C
Synchronous speed, Ns=120×50÷4=1500 rpm.
Given a number of poles = 4.
The supply frequency is 50 Hz.
Rotor speed is 1425 rpm.
S=Ns-Nr÷Ns
=1500-1425÷1500=.05
Sb=2-s=1.95.
69. The frame of an induction motor is made of _________
A. Aluminum
B. Silicon steel
C. Cast iron
D. Stainless steel
Answer: C
The frame of an induction motor is made of cast iron. The power factor of an induction motor depends upon the air gap between the stator and rotor.
70. The slope of the V-I curve is 5°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. .3254 Ω
B. .3608 Ω
C. .3543 Ω
D. .3443 Ω
Answer: D
The slope of the V-I curve is resistance. The slope given is 5° so
R=tan(5°)=.3443 ω.
The slope of the I-V curve is reciprocal to resistance.
