Skin effect increases with the increase in frequency. The value of skin depth decreases with the increase in frequency.
22. If induction motor air gap power is 11 KW and mechanically developed power is 2 KW, then rotor ohmic loss will be _________ KW.
A. 8
B. 10
C. 9
D. 11
Answer: C
Rotor ohmic losses are due to the resistance of armature windings. Net input power to the rotor is equal to the sum of rotor ohmic losses and mechanically developed power. Rotor ohmic losses = Air gap power – Mechanical developed power = 11-2 = 9 KW.
23. If induction motor rotor power is 26 KW and gross developed power is 21 KW, then rotor ohmic loss will be _________ KW.
A. 5
B. 6
C. 7
D. 8
Answer: A
Rotor ohmic losses are due to the resistance of armature windings. Net input power to the rotor is equal to the sum of rotor ohmic losses and mechanically developed power.
Rotor ohmic losses=Air gap power-Mechanical developed power
=26-21=5 KW.
24. Calculate the active power in 14 μH inductors if the THD value is 58.1%.
A. 5 W
B. 9 W
C. 4 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.
25. Calculate the power factor using the data: P=15 W, S=25 VA.
A. 0.5
B. 0.9
C. 0.4
D. 0.6
Answer: D
The power factor is the ratio of active power and apparent power. The value of the power factor is
P÷S=15÷25=.6.
26. Calculate the active power in a 56 μF capacitor if the VRF value is 5.1%.
A. 6 W
B. 0 W
C. 9 W
D. 2 W
Answer: B
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 the case of the capacitor so the angle between V & I is 90°.
P = VIcos90° = 0 W.
27. Calculate the apparent power using the data: P=3 W, Q=4 VAR.
A. 5 VA
B. 6 VA
C. 9 VA
D. 2 VA
Answer: A
The apparent power is the product of the voltage and current conjugate phasor. The value of apparent power is (9+16).5 = 5 VA.
28. Calculate the value of impedance using the data: R=2 Ω, XL=4 Ω.
A. 5.5 Ω
B. 5.4 Ω
C. 2.5 Ω
D. 4.4 Ω
Answer: D
The value of impedance is
{(R)2+(XL)2).5 = (4+16).5 = 4.4 Ω.
It is the total impedance offered by an AC circuit. It is expressed in Ω.
29. Which type of grounding is used to suppress the capacitive effect?
A. Solid grounding
B. Reactance grounding
C. Resistance grounding
D. Peterson coil
Answer: D
Peterson coil or resonance grounding is used to suppress the capacitive effect. The frequency in resonance grounding is ω2 = 1÷(3LC)
30. Electric trains are of two types.
A. True
B. False
Answer: A
Mainline trains and suburban trains are the two types of trains. They run on fixed rails. They are powered by electricity.
