Voltage/Frequency Control MCQ [Free PDF] – Objective Question Answer for Voltage/Frequency Control Quiz

11. Armature reaction is purely demagnetizing in nature due to a capacitive load in the synchronous motor.

A. True
B. False

Answer: A

Due to a capacitive load, the armature current is in opposite and 90° phase with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in the 180° phase and 90° phase with the field flux. The nature of the armature reaction is partially demagnetizing and cross magnetizing. 

 

12. Armature reaction is cross-magnetizing in nature due to a resistive load in the synchronous generator.

A. True
B. False

Answer: A

Due to a purely resistive load, armature current is in quadrature with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in quadrature with the field flux. It will try to increase the net magnetic field. The nature of the armature reaction is cross magnetizing. 

 

13. Calculate the value of the short circuit ratio if Voc=78 V, Isc=15 A with field current = 5 A.
A. 5.2
B. 4.8
C. 3.2
D. 1.8

Answer: A

The value of the short circuit ratio is the ratio of open-circuit voltage to the short circuit current with the same field current.

SCR=Voc÷Isc=78÷15=5.2. 

 

14. SCR determines the stability of the synchronous machine.

A. True
B. False

Answer: A

The value of the short circuit ratio determines the stability of the synchronous machine. It is inversely proportional to the per-unit value of reactance. The stability of the synchronous machine depends upon the maximum power transfer capability. 

 

15. Calculate the value of the short circuit ratio if the per-unit value of synchronous reactance is 1.5 Ω.

A. 0.66
B. 0.33
C. 0.17
D. 0.12

Answer: A

The value of the short circuit ratio determines the stability of the synchronous machine. It is inversely proportional to the per-unit value of reactance.

SCR=1÷Xs(p.u)=1÷1.5=.66. 

 

16. A higher value of armature reaction means a lower value of the short circuit ratio.

A. True
B. False

Answer: A

SCR is inversely proportional to the per-unit value of synchronous reactance. Armature reaction depends upon the value of the synchronous reactance. A higher value of armature reaction means a lower value of the short circuit ratio. 

 

3. The unit of SCR is Ω.

A. True
B. False

Answer: B

SCR is the ratio of the field currents required to produce open circuit voltage and short circuit current. It is a ratio. It has no unit. 

 

17. Calculate the power factor if the power angle is 12°.

A. .87
B. .47
C. .97
D. 0

Answer: C

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(12°)=.97. 

 

18. Calculate the reactive power in an 84 MΩ resistor.

A. 1.47 VAR
B. 45.6 VAR
C. 0.59 VAR
D. 0 VAR

Answer: D

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. 

 

19. Calculate the value of the resistance of a wire of length = 12 m, area = 2 m2, and Ρ = 6 Ω-m.

A. 36 Ω
B. 6 Ω
C. 26 Ω
D. 10 Ω

Answer: A

Resistance is the opposition offered by the body to the flow of the current. It is expressed in terms of Ω. R = ΡL/A = 36 Ω. 

 

20. Calculate the reactive power in a 29.6 Ω resistor.

A. 45.1 VAR
B. 41.1 VAR
C. 46 VAR
D. 0 VAR

Answer: D

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. 

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