1. Calculate the active power in a 168.12 H inductor.
a) 65 W
b) 0 W
c) 68 W
d) 64 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 maximum value of slip when rotor resistance is 2 Ω and rotor reactance is 3 Ω.
a) 0.66
b) 0.33
c) 0.44
d) 0.99
Explanation: The maximum torque occurs when the slip value is equal to R2÷X2. Maximum torque is also known as breakdown torque, stalling torque and pull-out torque. The maximum value of slip is R2÷X2=2/3=.66.
3. Calculate the total heat dissipated in a rotor resistor of 21 Ω when .81 A current flows through it.
a) 13.77 W
b) 12.56
c) 16.78 W
d) 13.98 W
Explanation: The rotor 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=.81×.81×21=13.77 W.
4. Calculate the active power in an 8.12 F capacitor.
a) 89 W
b) 41 W
c) 0 W
d) 48 W
Explanation: The capacitor 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 capacitor so the angle between V & I is 90°. P=VIcos90° = 0 W.
5. Calculate the value of new slip using the given data: V1=12 V, S1=.1, V2=5.
a) 0.576
b) 0.247
c) 0.487
d) 0.987
Explanation: This question is based on the concept of stator voltage control method. The load torque remains constant. S1V12=S2V22=constant. S2=.576.
6. R.M.S value of the periodic square waveform of amplitude 20 V is _______
a) 20 V
b) 18 V
c) 17 V
d) 13 V
Explanation: R.M.S value of the periodic square waveform is Vm and r.m.s value of the trapezoidal waveform is Vm÷3½. The peak value of the periodic square waveform is Vm. Vm=20 V.
7. Calculate the time period of the waveform y(t)=sin(.1πt)+cos(.2πt).
a) 20 sec
b) 30 sec
c) 40 sec
d) 10 sec
Explanation: The fundamental time period of the sine and cosine wave is 2π. The time period of y(t) is L.C.M {20,10}=20 sec. The time period is independent of phase shifting and time shifting.
8. The slope of the V-I curve is 56.489°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) 1.5 Ω
b) 1.6 Ω
c) 2.5 Ω
d) 1.4 Ω
Explanation: The slope of the V-I curve is resistance. The slope given is 56.489° so R=tan(56.489°)=1.5 Ω. The slope of the I-V curve is reciprocal of resistance.
9. Calculate the value of inductive reactance if F=50 Hz and L=12 H.
a) 3768 Ω
b) 2578 Ω
c) 2477 Ω
d) 2456 Ω
Explanation: Inductive reactance can be calculated using the relation XL=2×3.14×f×L. The value of inductive reactance is XL=2×3.14×50×12=3768 Ω.
10. Calculate the value of capacitive reactance if F=60 Hz and C=14 H.
a) 189.5 mΩ
b) 252.4 mΩ
c) 244.5 mΩ
d) 244.8 mΩ
Explanation: Capacitive reactance can be calculated using the relation Xc=1÷2×3.14×f×L. The value of capacitive reactance is Xc=1÷2×3.14×50×12=189.5 mΩ.