# Electric Drives Questions and Answers – Solid-State Switching Circuits – Three-Phase Energy Recovery Systems

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This set of Electric Drives Questions and Answers for Entrance exams focuses on “Solid-State Switching Circuits – Three-Phase Energy Recovery Systems”.

1. Rectifiers are used to convert DC power into variable AC power.
a) True
b) False

Explanation: Inverters are used to convert DC power into variable AC power. The variable AC power means variable frequency and variable voltage level.

2. Short circuit problem is severe in case of current source inverter.
a) True
b) False

Explanation: Short-circuit problem is severe in case of voltage source inverter. The output voltage of VSI is fixed but the output current depends upon the load. When Z=0, I=V÷Z=∞.

3. Full form of KERS is ___________
a) The kinetic energy recovery system
b) The kinetic ever recovery system
c) Kin ever recovery system
d) The kinetic energy recovery store

Explanation: Full form of KERS is the Kinetic energy recovery system. The systems recover the kinetic energy from the heat generated by the car’s braking.
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4. Full form of IGBT is ___________
a) Insulated gate bipolar transistor
b) Insulated gate bipolar transducer
c) Insulator gate bidirectional transducer
d) Insulated gate bidirectional transistor

Explanation: Full form of IGBT is Insulated gate bipolar transistor. It is a 3-terminal device, bipolar in nature. It can be used as a switch.

5. Flywheel is analogous to ___________
a) Chemical battery
b) Mechanical battery
c) Nuclear battery
d) Electrostatic battery

Explanation: A flywheel is a heavy wheel that stores a high amount of rotational kinetic energy. It requires a lot of force to spin the wheel.
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6. Energy is stored in Flywheel in the form of __________
a) Kinetic energy
b) Electrical energy
c) Solar energy
d) Wind energy

Explanation: Energy is stored in Flywheel in the form of kinetic energy. It stores the rotational kinetic energy.

7. Calculate the coefficient of fluctuation of speed using the data: Vmax(Fluctuation)=25 m/sec, Vmean=12 m/sec.
a) 2.08
b) 2.16
c) 2.34
d) 2.74

Explanation: The coefficient of fluctuation of speed is the ratio of the maximum fluctuation speed and mean speed. C.F = Vmax(Fluctuation) ÷ Vmean = 25÷12 = 2.08.

8. Calculate the maximum fluctuation of speed using the data: Vmax=23 m/sec, Vmin=13 m/sec.
a) 12
b) 10
c) 14
d) 15

Explanation: The maximum fluctuation of speed is the difference between the maximum speed and minimum speed. Maximum fluctuation of speed= Vmax – Vmin = 23 – 13 = 10.

9. Calculate Kinetic energy of the object of mass = 7 Kg moving in a straight line with a velocity of 12 m/s.
a) 504 J
b) 502 J
c) 503 J
d) 509 J

Explanation: The Kinetic energy of the object of mass = 7 Kg moving in a straight line with a velocity of 12 m/s is .5×m×v2 = .5×7×12×12 = 504 J.

10. Calculate the rotational kinetic energy of the object using the data: I=5 Kg-m2, ω=25 rad/sec.
a) 1.46 KJ
b) 1.56 KJ
c) 2.58 KJ
d) 3.15 KJ

Explanation: The Kinetic energy of the object having a moment of inertia=5 kg-m2 rotating with an angular velocity equals to 25 rad/sec is .5×I×ω2 = .5×5×25×25 = 1.56 KJ.

11. Calculate the line voltage for 3-∅ 120° VSI if the value of DC supply is 96 V.
a) 68.25 V
b) 67.87 V
c) 66.25 V
d) 62.14 V

Explanation: The line voltage for 3-∅ 120° VSI is (VDC÷√2)=.707×96=67.87 V. In 3-phase 120° VSI each phase conducts for 120° and only three switches from different phases conduct at a single time.

12. Calculate the phase voltage for 3-∅ 120° VSI if the value of DC supply is 78 V.
a) 30.45 V
b) 15.18 V
c) 31.84 V
d) 48.15 V

Explanation: The line voltage for 3-∅ 120° VSI is (VDC÷√6)=31.84 V. In 3-phase 120° VSI each phase conducts for 120° and only three switches from different phases conduct at a single time.

13. Calculate the r.m.s value of thyristor current in 3-Φ Full wave converter for the load current=2 A and α=59°.
a) 1.15 A
b) 2.55 A
c) 2.07 A
d) 2.89 A

Explanation: The r.m.s value of thyristor current in 3-Φ Full wave converter is Io√1÷3. It is the r.m.s value of the thyristor current. I(r.m.s) = Io√1÷3=1.15 A.

14. Calculate the average value of thyristor current in 3-Φ Full wave converter for the load current=729 A and α=3°.
a) 254 A
b) 285 A
c) 247 A
d) 243 A

Explanation: The average value of thyristor current in 3-Φ Full wave converter is Io÷3. It is the average value of the thyristor current. I(r.m.s) = Io÷3=243 A.

15. 1600 V rated 8 SCRs are connected in series. The operation voltage of the string is 4400. Calculate the string efficiency.
a) 34.37 %
b) 30.15 %
c) 38.48 %
d) 35.84 %

Explanation: The string efficiency can be calculated using the formula
operation voltage ÷ (Number of SCRs×Rated voltage) = 4400÷(1600×8) = 34.37 %.

Sanfoundry Global Education & Learning Series – Electric Drives.

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