Speed Control of Induction Motor by V/F Method MCQ [Free PDF] – Objective Question Answer for Speed Control of Induction Motor by V/F Method Quiz

91. 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. 

 

92. 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. 

 

93. 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. 

 

94. 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. 

 

95. 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. 

 

96. 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°. 

 

97. 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. 

 

98. 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. 

 

99. 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. 

 

100. 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. 

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