21. During short circuit condition, the voltage is equal to ________
A. 2 V
B. 0 V
C. 1 V
D. 3 V
Answer: B
During the short circuit condition, the voltage is equal to zero volts. According to Ohm’s law, V=IR=0 V. The value of the current can be positive, negative, or zero.
22. When 5 A current flows into the positive terminal of voltage source 6 V. Calculate the power delivered by the source.
A. -30 W
B. 36 W
C. -43 W
D. 50 W
Answer: A
When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered is
-5×6=-30 W.
23. When 10 A current leaves the positive terminal of resistance 5Ω. Calculate the power absorbed by the resistance.
A. -308 W
B. 500 W
C. -63 W
D. 60 W
Answer: B
When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. The power delivered is
100×5=500 W.
24. Calculate the equivalent inductance when two inductors are connected in a series of values 4 H and 9 H.
A. 13 H
B. 10 H
C. 11 H
D. 12 H
Answer: A
When two inductors are connected in series their equivalent inductance is equal to the sum of the individual inductances.
Leq=L1+L2=4+9=13 H.
25. What is the speed of the 6th order time harmonics in the induction motor?
A. Ns
B. Ns/6
C. 6×Ns
D. 0
Answer: C
The time harmonics are present in the supply voltage. The speed of the 6th order time harmonics in the induction motor is Ns/6.
26. What is the speed of the 11th-order harmonics in the induction motor?
A. Ns
B. Ns/11
C. 2×Ns
D. Nr
Answer: B
The speed of the 6k±1 is Ns÷6k±1. The speed of the 11th order time harmonics in the induction motor is Ns/11.
27. A 3 – phase, 4-pole IM is supplied from a 40 Hz source. Calculate rotor frequency when the rotor runs 100 rpm.
A. 35 Hz
B. 36.6 Hz
C. 9 Hz
D. 8 Hz
Answer: B
The slip of the induction motor can be calculated using the relation s = Ns-Nr÷ Ns. Ns=120F÷P=1200 rpm. The value of the slip is .91. The rotor frequency is 36.6 Hz.
28. Calculate the three-phase power using the following data: Vp=23 V,
Ip=23 A, ∅=60.
A. 793.5 W
B. 790 W
C. 792 W
D. 791 W
Answer: A
Power input to a 3-Φ induction motor is 3VpIpcosΦ. The power remains the same for Delta and star connections.
P=3×23×23×co60°=793.5 W.
29. What is the condition for the maximum power transfer theorem in DC circuits?
A. RL=2Rth
B. RL≠Rth
C. RL≫ Rth
D. RL=Rth
Answer: D
The condition for the maximum power transfer theorem is load should be variable and RL=Rth. During maximum power transfer conditions voltage across load becomes half of Thevenin voltage and efficiency becomes 50 %.
30. Instantaneous power in the 3-φ system is constant.
A. True
B. False
Answer: A
The 3-φ system is more economical than the 2-∅ system because of no vibrations in the power waveform. The instantaneous power in the 3-∅ system is constant.
