11. 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.
12. 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.
13. 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.
14. 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.
15. Iron losses that occur in the machine are variable losses.
A. True
B. False
Answer: B
Iron losses consist of eddy current and hysteresis losses. These are constant losses. Iron losses are independent of the load current. The copper losses are variable losses.
16. Which of the following are the classes of insulating material in electrical machines?
A. Γ, A, E, F, H, C
B. Γ, E, B, F, H, C
C. A, E, B, F, H, C
D. Γ, A, E, F, C
Answer: A
Depending upon the temperature limits, insulating materials are classified into seven classes Γ, A, E, B, F, H, and C. The insulation has the lowest temperature limit.
17. Cooling air travels 10 m distance in 5 sec from point A to point B. Calculate the velocity of the cooling air.
A. 2 m/s
B. 4 m/s
C. 6 m/s
D. 10 m/s
Answer: A
The velocity of the cooling air can be calculated using the ratio of displacement and time. The value of the displacement is 10 m.
The value of the velocity is 10÷5=2 sec.
18. Calculate the heating time constant of the machine if the thermal capacity (C. of the machine is 24 watts/℃ and heat dissipation constant value (D. is 3 watts/℃.
A. 8 sec
B. 7 sec
C. 6 sec
D. 9 sec
Answer: A
The heating time constant of the machine is the ratio of the thermal capacity of the machine and the heat dissipation constant value.
Τ = c÷D=24÷3 = 8 sec.
19. Inadequate rating of the equipment can ___________
A. Increase the speed
B. Decrease the speed
C. Damage the winding
D. Increase the efficiency
Answer: C
The inadequate rating of the equipment can damage the winding and stops the machine’s operation. The windings can burn out.
20. What is the heating time constant of the machine?
A. The ratio of the thermal capacity and heat dissipation constant value
B. The ratio of the thermal capacity and heat dissipation constant value
C. The ratio of the thermal capacity and heat dissipation constant value
D. The ratio of the thermal capacity and heat dissipation constant value
Answer: A
The heating time constant of the machine is the ratio of the thermal capacity of the machine and the heat dissipation constant value. Τ = c÷D. It has a unit of second.
