71. The slope of the V-I curve is 90°. Calculate the value of resistance.
A. 1.732 Ω
B. 1.608 Ω
C. 1.543 Ω
D. 1.648 Ω
Answer: A
The slope of the V-I curve is resistance.
The slope given is 90° so R=tan(90°)=infinite Ω.
It behaves as an open circuit.
72. In a DC shunt motor, the electromagnetic torque developed is proportional to ______
A. Ia
B. Ia2
C. Ia3
D. Ia5
Answer: A
In a DC shunt motor, the electromagnetic torque developed is equal to KmΦIa.
In a DC shunt, the motor field windings are connected separately and excited by a constant DC voltage.
T = KmΦIa α Ia.
73. What is the formula for 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. Eb×Vt2×sinδ÷X
C. Eb2×Vt×sinδ÷X
D. Eb×Vt×sinδ÷X2
Answer: A
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.
72. Salient pole machines are more stable than cylindrical rotor machines.
A. True
B. False
Answer: A
Salient pole machines are more stable than cylindrical rotor machines because of the high short circuit ratio and more real power transfer capability. The air gap length in salient pole machines is more as compared to cylindrical rotor machines.
73. The unit of reactive power is VAR.
A. True
B. False
Answer: A
Reactive power is useless power in the case of electric circuits. It is the energy trapped that keeps on oscillating between inductive and capacitive elements.
It plays a vital role in generating flux in electrical machines. It is expressed in Volt Ampere reactive.
74. Calculate the power factor during the resonance condition.
A. .58
B. .42
C. .65
D. 1
Answer: D
During the resonance condition, the reactive power generated by the capacitor is completely absorbed by the inductor.
Only active power flows in the circuit. Net reactive power is equal to zero and cosΦ=1.
75. Calculate the reactive power in a 5 Ω resistor.
A. 7 VAR
B. 0 VAR
C. 2 VAR
D. 1 VAR
Answer: B
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 90°.
Q = VIsin0 = 0 VAR.
76. What is the unit of the apparent or complex power?
A. VAR
B. VA
C. ohm
D. Volt
Answer: B
The apparent power in AC circuits is VI×. It is expressed in volt-amperes (VA. It consists of both active and reactive power. It is the vector sum of the real power and reactive power.
77. Calculate the value of the frequency of the DC supply.
A. 0 Hz
B. 50 Hz
C. 20 Hz
D. 10 Hz
Answer: A
The frequency is defined as the number of oscillations per second. It is reciprocal to the time period. DC supply magnitude is constant. It does not change with time so the frequency of the DC supply is 0 Hz.
78. The slope of the V-I curve is 0°. Calculate the value of resistance. The graph is parallel to the x-axis.
A. 1 Ω
B. 1.8 Ω
C. 0 Ω
D. 2.2 Ω
Answer: C
The slope of the V-I curve is resistance. The slope given is 0° so
R=tan(0°)=0 Ω.
The slope of the V-I curve is resistance. It behaves as a short circuit.
79. Calculate the value of the duty cycle if the system is on for 5 sec and off for 10 sec.
A. .333
B. .444
C. .201
D. .642
Answer: A
The duty cycle is Ton÷Ttotal. It is the ratio of the time for which the system is active and the time taken by the signal to complete one cycle.
