81. What will happen if we increase the air gap in the induction motor?

A. Power factor will reduce
B. Power factor will increase
C. Reduction in harmonics
D. Speed will increase

Answer: A

If we increase the air gap in the induction motor, the machine will draw more magnetizing current to maintain flux in the air gap. The power factor of the machine will reduce.

82. The ratio of the rotor copper loss and air gap power is _______

A. s
B. 1-s
C. 1+s
D. 0

Answer: A

The ratio of the rotor copper loss and air gap power is s. The air gap power in the induction motor is P_{g} and the rotor copper loss is s×P_{g}. The ratio is s.

83. Magnetizing current in the induction motor is ______

A. more than the transformer
B. equal as a transformer
C. less than the transformer
D. 0

Answer: A

The magnetizing current in the induction motor is (23-25)% of the full load current. The magnetizing current in the transformer is 3-5% of the full load.

84. Single phase induction motor is self-starting.

A. True
B. False

Answer: B

The single-phase induction motor is not self-starting because of the equal and opposite forward & backward magnetic field. The net torque developed is zero.

85. What is the speed of the 6^{th} order time harmonics in the induction motor?

A. N_{s}
B. N_{s}/6
C. 6×N_{s}
D. 0

Answer: C

The time harmonics are present in the supply voltage. The speed of the 6^{th} order time harmonics in the induction motor is N_{s}/6.

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86. What is the speed of the 11th-order harmonics in the induction motor?

A. N_{s}
B. N_{s}/11
C. 2×N_{s}
D. N_{r}

Answer: B

The speed of the 6k±1 is N_{s}÷6k±1. The speed of the 11^{th} order time harmonics in the induction motor is N_{s}/11.

87. A 3 – phase, 4-pole IM is supplied from a 40 Hz source. Calculate rotor frequency when the rotor runs 100 rpm.

A. 35 Hz
B. 36.6 Hz
C. 9 Hz
D. 8 Hz

Answer: B

The slip of the induction motor can be calculated using the relation s = N_{s}-N_{r}÷ N_{s}. N_{s}=120F÷P=1200 rpm. The value of the slip is .91. The rotor frequency is 36.6 Hz.

88. Calculate the three-phase power using the following data: V_{p}=23 V,

I_{p}=23 A, ∅=60.
A. 793.5 W
B. 790 W
C. 792 W
D. 791 W

Answer: A

Power input to a 3-Φ induction motor is 3V_{p}I_{p}cosΦ. The power remains the same for Delta and star connections.

P=3×23×23×co60°=793.5 W.

89. What is the condition for the maximum power transfer theorem in DC circuits?

A. R_{L}=2R_{th}
B. R_{L}≠R_{th}
C. R_{L}≫ R_{th}
D. R_{L}=R_{th}

Answer: D

The condition for the maximum power transfer theorem is load should be variable and R_{L}=R_{th}. During maximum power transfer conditions voltage across load becomes half of Thevenin voltage and efficiency becomes 50 %.

90. Instantaneous power in the 3-φ system is constant.

A. True
B. False

Answer: A

The 3-φ system is more economical than the 2-∅ system because of no vibrations in the power waveform. The instantaneous power in the 3-∅ system is constant.