Power Electronics

A.Choose the correct option from the alternatives.

1. The device which converters DC into AC is called---- i)Chopper, ii)Inverter,iii)Rectifier.
2. For HVDC applications, the preferred power device for switching is--- i)SCR,ii)MOSFET,iii)BJT.
3. For an ideal switch, the ON state voltage drop across it is--- i) zero, ii)infinity, iii) finite high value.
4. A DC chopper circuit can operate in-- i)1st quadrant only, iii)3rd quadrant only, iii) all the 4 quadrants.
5. In the process of diode based rectification, the alternating input voltage is converted into--
i) an uncontrolled alternating output voltage,ii) an uncontrolled direct output voltage,
iii) a controlled direct output voltage.
6. In a half-wave rectifier, the
i) current & voltage both are uni-directional
ii) current is always uni-directional but the voltage can be bi-directional or uni-directional
iii) current can be bi-directional or uni-directional but the voltage is always uni-directional
7. A GTO is a switch which is – i)uncontrolled, ii)semi-controlled, iii)fully controlled.
8. For a rectifier, which of the following relations is correct? RF=Ripple Factor, FF=Form Factor---
i)RF=√(FF2-1), ii)RF=√(FF2+1), iii)RF=√2.FF
9. In a dc chopper circuit, the load voltage is governed by—i)Type of connection used in the circuit, ii)duty
cycle of the circuit, iii)dc voltage applied to the circuit.
10. A cycloconverter is a device which –i)measures the frequency of ac mains, ii)converts ac into
dc,iii)converts ac of one frequency into ac of some other frequency.
11. In VVVF control, maximum torque--- i)varies with voltage, ii)varies with frequency, iii)remains approx.
constant.
12. The output power rating of a static inverter unit is—i)240kVA, ii)180kVA,iii)120kVA.
13. The cooling arrangement of static inverter unit is—i)by forced air, ii)by oil, iii)by water.
14. The output power rating of an AC coach inverter is i)180kVA, ii)15kVA, iii)25kVA.
15. The cooling arrangement of GTO based 3 phase loco converter is – i) forced oil cooled, ii) water cooled,
iii)forced air cooled.
16. The output frequency of the drive converter (ASR) to the traction motor can vary from i) 0 to 160 Hz,
ii)50Hz to 200 Hz, iii)80 Hz to 120 Hz.
17. The drive converter of 3 phase loco contains – i)2 no. GTOs, ii)4 no. GTOs, iii)6 no. GTOs

B. State if the following statements are TRUE or FALSE.

1. A MOSFET is ideally used in high voltage low frequency applications.

2. The ripple frequency of the output waveform of a bridge rectifier when fed with a 50 Hz sine wave is
25 Hz.
3. The ripple factor of a rectifier is the ratio of rms value to dc value.
4. The capacitor filter is used for relatively light loads.
5. A Zener diode is always forward biased.
6. The main advantage of a bridge inverter is that it does not use any transformer.
7. The output of a step-up chopper can be increased by varying the duty cycle of the chopper circuit.
8. A step-up chopper is a type of ac chopper circuit.
9. The duty cycle of a chopper is defined as the ratio of OFF time to the Chopping period.
10. Regenerative braking cannot be achieved with the help of a cyclo-converter.
11. The slip of a SCIM is less than zero in the regenerating phase.
12. In VVVF controlled drive, the ratio of voltage to frequency is kept constant.
13. Metal Oxide Varistors are used in the input of SI unit for protection from surges.
14. The input to the AC coach inverter is directly fed from axle driven alternator.
15. MUB is the drive converter of three phase locomotives.
16. The output voltage of drive converter of 3 phase loco is PWM sine wave.
17. In IGBT based converter, snubber circuit is not required.
18. The output frequency of auxiliary converter in 3 phase loco can be varied.
C. Fill in the blanks with the correct answer.
 1. The ripple factor of a full wave rectifier is ________.
2. An uncontrolled rectifier consists of ________ switches.
3. IGBT stands for _________.
4. MOSFET is a ____________ controlled device.
5. Light triggering of a SCR is used for electrical ____________ of triggering circuit.
6. PWM is an acronym of __________________.
7. DC four quadrant choppers are called Type __ chopper.
8. In VVVF drives, VVVF stands for _______________.
9. When a SCIM runs at supersynchronous speed, the slip becomes ____.
10. The output of a SI unit is _________ voltage and ________ frequency.
11. In the rectifier section of SI unit, a ____ controller is used to determine the firing angle.
12. The current ratio of the CT used in the input section of SI unit is ________ A.
13. ZCT1 is used to measure the input _________ current in SI unit.
14. The output voltage of SI unit for battery charging is _________.
15. The DC-DC converter used in AC coach inverters converts 110V DC to _____V DC.
16. BUR 1,2,3 are called ___________ in three phase locomotives.
17. The DC link voltage of 3 phase loco converter is __________.

Questions

1. How many types of rectifiers are there? Draw a single phase full wave diode rectifier
with resistive load and associated input and output waveform.
2. Draw the 4 quadrant DC chopper circuit type E and briefly describe the sequence of
switching in different quadrants.
3. What are the requirements of a good inverter? Name the different methods of triggering
of SCR.
4. Write short note on VVVF control drive.
5. Draw a schematic diagram of SI unit labeling its various parts.
6. What are the advantages of IGBT over GTO?
 TRS/Loco/3-phase

1. The MEMOTEL initiates buzzer at – i)105% overspeed,ii)110% overspeed, iii)115%

overspeed.
2. The capacity of each compressor in 3 phase loco is – i)1500 LPM, ii)1750 LPM,
iii)2000LPM.
3. The type of traction motors used in WAG-9 loco is i)4601AZ, ii) 6 FRA 6068, iii) HS
15250A.
4. Priority 1 fault message is overwritten by priority 2 fault message.(True/False)
5. The harmonic filter choke used in 3 phase loco is assembled within the transformer tank.
(True/False)
6. In the traction control system MICAS S2, MICAS stands for

7. The train bus administrator processor is called .

8. The assembly level language FUPLA, used for vehicle level software, is the short of

9. What are the advantages of AC drive over DC drive?

Advantage of AC drive
i) Less unsprung mass saves track geometry.
ii) Induction Motors are 20% more energy-efficient.
iii) Higher adhesion is possible with AC Drives.
iv) Wheel-slip arrest is easily possible in AC Drives.
v) Unity p.f. and Regenerative Braking is possible.
vi) Electrical Braking down to standstill is possible.
vii) Const. Power is possible beyond rated speed.
viii) Desired T-S Characteristics of AC Motor can be obtained by GTO/IGBT based VVVF
Drive.

Disadvantage of DC drive
i) DC series motor requires frequent periodical maintenance due to commutator &
brushes.
ii) These additional parts affect reliability.
iii) Commutator puts a limit on maximum speed & operating voltage.
iv) DC series motor is prone to spin.
v) DC series motor has less power to weight ratio, hence limits the loco-HP for a given axle-
load.

10. Briefly state the advanced technological features used in 3 phase locomotives.

a. Digital electronics based real time traction control system to obtain precise control over
the tractive effort and speed.
b. Fast acting wheel slip/ slide control system.
c. On board fault diagnostics systems to eliminate elaborate trouble shooting by engine
crew and also to help maintenance staff to trace faults.
d. Automatic isolation of faulty equipment/ subsystems.
e. Simulation mode of operation to facilitate a complete functional testing of the
locomotive without raising pantograph.
f. An exclusive harmonic filter circuit to reduce harmonics in the OHE generated by loco
current.
g. Static auxiliary converter, to supply auxiliary 3 phase motors. The auxiliary converter,
depending on the traction load, operates at an optimal frequency to minimise power
consumption by auxiliaries.
h. Fire detection and alarm system for the machine room.
i. Electronic brake system for precise and fast control of braking effort, blending between
electrical brake and pneumatic brake on the locomotives.
j. Use of inertial filters and pressurized machine room to prevent entry of dust into
sensitive equipments in the machine room.
11. Draw a schematic diagram of cooling arrangement in 3 phase locomotive.

12. Draw a schematic overview of power circuit of 3 phase locomotive.