1. Calculate the active power in a .154 H inductor.
a) 22 W
b) 14 W
c) 45 W
d) 0 W
Explanation: 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 case of the inductor so the angle between V & I is 90°. P = VIcos90° = 0 W.
2. Calculate the time period of the waveform z(t)=24sin(πt+π÷4)+4sin(2πt+π÷6).
a) 2 sec
b) 3 sec
c) 4 sec
d) 1 sec
Explanation: The fundamental time period of the sine wave is 2π. The time period of z(t) is L.C.M {2,1}=2 sec. The time period is independent of phase shifting and time shifting.
3. Calculate the total heat dissipated in a resistor of 50 Ω when 1.4 A current flows through it.
a) 98 W
b) 92 W
c) 91 W
d) 93 W
Explanation: 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 case of the resistor so the angle between V & I is 90°. P=I2R=1.4×1.4×50=98 W.
4. Calculate mark to space ratio if the system is on for 26.3 sec and off for 24.2 sec.
a) 1.086
b) 1.042
c) 1.214
d) 1.876
Explanation: Mark to space is Ton÷Toff. It is the ratio of time for which the system is active and the time for which is inactive. M = Ton÷Toff=26.3÷24.2=1.086.
5. A particular current is made up of two components: a 10 A and a sine wave of peak value 14.14 A. The average value of current is _________
a) Zero
b) 24.14 A
c) 10 A
d) 14.14 A
Explanation: Average value of DC current is 10 A. Average value of AC current is 0 A as it is alternating in nature. The average value of current is 10+0 = 10 A
6. A 38-pole, 3-phase, 80 Hz induction motor is operating at a speed of 12 rpm. The frequency of the rotor current of the motor in Hz is __________
a) 75.2
b) 76.1
c) 79.2
d) 79.6
Explanation: Given a number of poles = 8. Supply frequency is 50 Hz. Rotor speed is 720 rpm. Ns=120×f÷P=120×80÷38 = 252.63 rpm. S=Ns-Nr÷Ns = 252.63-12÷252.63 = .952. F2 = sf = .952×80 = 76.1 Hz.
7. Calculate the phase angle of the sinusoidal waveform i(t)=sin(.6πt+π÷.88).
a) 100π÷88
b) 100π÷8
c) 100π÷8
d) π÷88
Explanation: 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.
8. Calculate the moment of inertia of the satellite having a mass of 79 kg and diameter of 83 cm.
a) 13.65 kgm2
b) 13.60 kgm2
c) 12.67 kgm2
d) 13.82 kgm2
Explanation: The moment of inertia of the satellite can be calculated using the formula I=mr2. The mass of the satellite and diameter is given. I=(79)×(.415)2=13.60 kgm2. It depends upon the orientation of the rotational axis
9. A particular voltage is made up of two components: a 5 A and a cosine wave of peak value 7.8 A. The average value of voltage is _________
a) Zero
b) 35.14 A
c) 78 A
d) 5 A
Explanation: Average value of DC voltage is 5 A. Average value of AC current is 0 A as it is alternating in nature. The average value of current is 5+0 = 5 A.
10. Armature reaction is demagnetizing in nature due to a purely capacitive load in the synchronous generator.
a) True
b) False
Explanation: Due to a purely capacitive load, armature current is in phase with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in phase with the field flux. It will try to increase the net magnetic field.