71. Which test is used to determine the conditions where electric motors are being used?
A. Thermography
B. Geography
C. Seismography
D. Anthropology
Answer: A
Thermography is used to detect heat patterns. It uses an Infrared rays camera. This test is used to determine the conditions where electric motors are being used.
72. What is the range of size of the motor (kW) for efficiency nearly equals 90%?
A. 15-150
B. 0-2
C. 3-15
D. 150-250
Answer: A
As the size of the motor increases its speed decreases. The range of size of the motor (kW) for efficiency nearly equals 90 % is 15-150. The efficiency increases as the size increases.
73. Calculate the heat loss in the electric motor for time intervals of 0-5 sec using the following data: Ia=2 A, Ra=.8Ω.
A. 16 J-sec
B. 14 J-sec
C. 12 J-sec
D. 10 J-sec
Answer: A
The heat loss can be calculated using the Joule law of the heating effect. The value of the heat loss is
Ia2×Ra×t=4×4=16 J-sec.
74. 1 Btu/h is equal to ___________
A. .641
B. .334
C. .293
D. .417
Answer: C
One British thermal unit is equal to .293 Watt. 1 Btu/h is the amount of energy required to increase the temperature of one pound of water by 1℉.
75. Overloading can be prevented using _______
A. Over-current protection
B. Speed protection
C. Over frequency protection
D. Oversize protection
Answer: A
Overloading can be prevented using over-current protection. It will detect the over-current and interrupt the supply. IDMT overcurrent relay can be used for protection purposes.
76. Full form of IDMT is _________
A. Inverse definite minimum time
B. Inverter definite minimum time
C. Inverter definite maximum time
D. Insert definite minimum time
Answer: A
IDMT stands for Inverse definite minimum time over-current relay. It has both the characteristics of inverse over-current and definite time over-current relay.
77. The most common cause of motor failure is _________
A. Overloading
B. Low resistance
C. Contamination
D. Over-Heating
Answer: B
The most common cause of motor failure is low resistance. Overheating, and corrosion can damage the insulation of the windings which leads to low resistance.
78. The Full form of THD is _________
A. Total harmonic diverter
B. Total harmonic digital
C. Total harmonic distortion
D. Total harmonic discrete
Answer: C
THD stands for total harmonic distortion. It measures the harmonic distortion in the signal. According to IEEE-1992, only 5% of THD is valid.
79. Calculate the compensator rating required for a sec(Φ)=1.
A. 0.64 P.U
B. 0.21 P.U
C. 0.57 P.U
D. 0 P.U
Answer: D
The compensator rating can be calculated using the relation
QP.U=√1-cos2(∅)=0.
This per unit value VAR compensator is required to improve the power factor of the system.
80. Calculate the value of the Voltage ripple factor if the Form factor is 1.1.
A. 45.8 %
B. 46.2 %
C. 47.5 %
D. 49.9 %
Answer: A
The voltage ripple factor measures the harmonics on the DC side of the converter. The value of the voltage ripple factor is
√F.F2-1=45.8 %. The form factor is Vr.m.s÷Vavg.
81. Calculate the value of THD if the distortion factor is .8.
A. 76 %
B. 75 %
C. 74 %
D. 73 %
Answer: B
THD measures the harmonics distortion in the signal. The value of THD is
√(1÷D.2-1=75 %.
The value of the distortion factor is Ior.m.s÷IR.m.s.
82. What is the Full form of VRF?
A. Voltage ripple factor
B. Voltage revert factor
C. Volume ripple factor
D. Volume revert factor
Answer: A
The voltage ripple factor measures the harmonics on the DC side of the converter. The value of the voltage ripple factor is √F.F2-1.
The form factor is Vr.m.s÷Vavg.
83. Second lowest order harmonic present in 3-Φ fully controlled rectifier is __________
A. 5th
B. 7th
C. 3rd
D. 2nd
Answer: B
Only 6k±1 order harmonics are present in 3-Φ fully controlled rectifiers. The second-lowest order harmonic is the 7th harmonic for k=1. Third-order harmonics are absent in 3-Φ fully controlled rectifier.
84. Electric motors are placed in __________ to protect them from dust and contamination.
A. Enclosures
B. Open environment
C. Moisturizing environment
D. Hot environment
Answer: A
Electric motors are placed in enclosures to protect them from dust and contamination. There are seven most common types of enclosures. For example- Open Drip Proof.
