21. Calculate mark to space ratio if the system is on for 20 sec and off for 3 sec.
A. 6.66
B. 4.48
C. 8.2
D. 5.6
Answer: A
Mark to space ratio is Ton÷Toff. It is the ratio of the time for which the system is active and the time for which is inactive.
M = Ton÷Toff=20÷3=6.66.
22. Calculate the average power dissipated if the value of the duty cycle is .5 and Pmax=12 W.
A. 6 W
B. 5 W
C. 4 W
D. 3 W
Answer: A
The average power dissipated can be calculated using the relation
Pavg=D×Pmax.
The value of the average power is .5×12=6 W.
23. Full form of PWM.
A. Pulse width modulation
B. Pulse run modulation
C. Power width modulation
D. Pulse wisdom mode
Answer: A
PWM stands for pulse width modulation. This technique is used to alter the voltage level of the output of the inverter. We can control the voltage level using the PWM technique.
24. What is the correct relationship between the phase voltage and line voltage in Delta connection with the positive sequence?
A. VL=Vp
B. VL=√3Vp
C. VL=√2Vp
D. IL=√2Ip
Answer: A
In the delta connection line voltage is the same as the phase voltage. The line current is √3 times the phase current and leads by 30°.
25. What is the relationship between line and phase current in Star’s connection with the positive sequence?
A. VL=Vp
B. VL=√9Vp
C. VL=√2Vp
D. IL=Ip
Answer: D
In the star connection line current is the same as the phase current. The line voltage is √3 times the phase voltage and leads by 30°.
26. Choose the correct relationship in delta connection with the positive sequence.
A. IL=Ip
B. VL=√3Vp
C. VL=Vp
D. IL=√3Ip
Answer: D
In the delta connection line voltage is the same as the phase voltage. The line current is √3 times the phase current and leads by 30°.
27. Choose the correct relationship in Star connection with the positive sequence.
A. VL=Vp
B. VL=√3Vp
C. VL=√2Vp
D. IL=√3Ip
Answer: B
In the star connection line current is the same as the phase current. The line voltage is √3 times the phase voltage and leads by 30°.
28. Calculate the 3-∅ power developed in the IM connected in star using the following data: Vp=7 V, IL=7 A, cosΦ=.6.
A. 88.5 W
B. 88.4 W
C. 88.8 W
D. 88.2 W
Answer: D
The power developed in the IM is √3VLILcos∅.
The power is independent of time and frequency.
In star connection, VL=√3Vp=12.12 V
P=√3VLILcos∅=3×20×5×.8=88.2 W.
29. Hooke’s law is applicable within the elastic limit.
A. True
B. False
Answer: A
Hooke’s law is only applicable within the elastic limit. It states that within the elastic limit the strain is directly proportional to the stress.
30. Calculate the load stress if the value of force is 20 N and the area of the cross-section is 2 m2.
A. 10 N/m2
B. 20 N/m2
C. 30 N/m2
D. 40 N/m2
Answer: A
The stress is defined as the ratio of the internal restoring force and the area of the cross-section.
