21. 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.
22. 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.
23. 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.
24. 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.
25. 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.
26. 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.
27. 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.
28. 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.
