1. 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°.
2. 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.
3. 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.
4. 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.
5. Calculate the reactive power in a 451.26 Ω resistor.
A. 1 VAR
B. .6 VAR
C. 0 VAR
D. .9 VAR
Answer: C
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°.
Q=VIsin0° = 0 VAR.
6. What is the unit of displacement?
A. m/s
B. m
C. atm/m
D. Volt
Answer: A
Displacement is the difference between the final and initial points. It is a vector quantity. It is expressed in the meter. It is not a tensor quantity. It has direction.
7. Calculate the reactive power in a 725.45 Ω resistor.
A. 122.1 VAR
B. 261.1 VAR
C. 0 VAR
D. 199.7 VAR
Answer: C
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°.
Q = VIsin0° = 0 VAR.
8. The slope of the I-V curve is 180°. Calculate the value of resistance.
A. inf Ω
B. .81 Ω
C. 45 Ω
D. 41.2 Ω
Answer: A
The slope of the I-V curve is the reciprocal of the resistance.
The slope given is 180° so R=1÷tan(180°)=inf Ω.
The slope of the V-I curve is resistance. It behaves as an open circuit.
9. Calculate the value of the duty cycle if the system is on for 1 sec and off for inf sec.
A. 0
B. .89
C. .148
D. .46
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.
D = Ton÷Ttotal=1÷inf = 0.
10. The phase difference between voltage and current in the purely inductive coil.
A. 20°
B. 90°
C. 40°
D. 88°
Answer: B
In the case of a purely inductive coil, the voltage leads the current by 90°, or the current lags the voltage by 90°. The phase difference between voltage and current is 90°.
