81. Calculate the active power in a 7481 H inductor.
A. 1562 W
B. 4651 W
C. 0 W
D. 4654 W
Answer: C
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.
Voltage leads the current in the case of the inductor.
82. Calculate the active power in a 56 F capacitor.
A. 6.45 W
B. 0 W
C. 15.45 W
D. 14.23 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.
Current leads the voltage in the case of the capacitor.
83. Calculate the active power in .an 18.064 H inductor.
A. 4.48 W
B. 17.89 W
C. 0 W
D. 25.45 W
Answer: C
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.
84. Calculate the active power in a 1.7 Ω resistor with a 1.8 A current flowing through it.
A. 5.5 W
B. 5.1 W
C. 5.4 W
D. 5.7 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=1.8×1.8×1.7 = 5.5 W.
85. Calculate the value of the time period if the frequency of the signal is .48 sec.
A. 2 Hz
B. 3 Hz
C. 7 Hz
D. 9 Hz
Answer: A
The time period is defined as the time after the signal repeats itself. It is expressed in second.
T = 1÷F=1÷.48=2 Hz.
86. Choose the correct in the case of V/F control.
A. Ns-Nr=constant
B. Ns*Nr=constant
C. Ns%Nr=constant
D. Ns+Nr=constant
Answer: A
In variable frequency control, Ns-Nr remains constant. V/f control is part of the synchronous speed changing technique. It is the most used technique in controlling the induction motor.
87. Calculate the minimum value of the active power in the cylindrical rotor synchronous machine? (Eb represents armature emf, Vt represents terminal voltage, δ represents rotor angle, X represents reactance)
A. Eb×Vt×sinδ÷X
B. 0
C. Eb2×Vt×sinδ÷X
D. Eb×Vt÷X
Answer: B
The real power in the cylindrical rotor machine is Eb×Vt×sinδ÷X. It is inversely proportional to the reactance. The stability of the machine is decided by the maximum power transfer capability. Its minimum value occurs for delta=0°.
88. Cylindrical pole machines are less stable than salient rotor machines.
A. True
B. False
Answer: A
Cylindrical pole machines are less stable than salient rotor machines because of the less short circuit ratio and less real power transfer capability. The air gap length in cylindrical pole machines is less compared to salient rotor machines.
89. The unit of area is m2.
A. True
B. False
Answer: A
The area is defined as the product of length and breadth. The length and breadth are expressed in the meter. The unit of area is m2.
90. Calculate the power factor if the power angle is 45°.
A. .707
B. .407
C. .608
D. 1
Answer: A
The power factor is the ratio of the real power to the apparent power. It measures the useful power contained in the total power cosΦ=cos(45°)=.707.
