Multiple Choice Questions in Power Electronics - 1

  Important multiple choice questions from Power Electronics

1.      Which one of the following statements is not correct?

a)      Power MOSFETs are so constructed as to avoid punch through.

b)      In a power MOSFET, the channel length is relatively large and channel width is relatively small

c)      Power MOSFETs do not experience any minority carrier storage

d)      Power MOSFETs can be put in parallel to handle large currents

2.      Which one is most suitable power device for high frequency (>100 kHz) switching application?

a)      Power MOSFET

b)      BJT

c)      Schottky diode

d)      Microwave transistor

3.      A thyristor equivalent of thyratron tube is

a)      SCR

b)      UJT

c)      Diac

d)      Triac

4.      Turn-on of thyristor takes place when

a)      Anode to cathode voltages is positive

b)      Anode to cathode voltages is negative

c)      There is a positive current pulse at the gate

d)      The anode to cathode voltage is positive and there is a positive current pulse at the gate

5.      Suppose the anode current of a conducting SCR is 50 A. if its gate current is reduced to one-fifth, its anode current will become

a)      10 A

b)      50 A

c)      25 A

d)      Zero

6.      It is preferable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce

a)       dv/dt problem

b)      di/dt problem

c)      the size of the pulse transformer

d)      the complexity of the firing circuit

7.      if a diode is connected in antiparallel with a thyristor , then

a)      both turn-off power loss and turn-off time decreases

b)      turn-off power loss decreases but turn-off time increases

c)      turn-off power loss increases but turn-off time decreases

8.      in a commutation circuit employed to turn-off an SCR, satisfactory turn-off is obtained when

a)      circuit turn-off time < device turn-off time

b)      circuit turn-off time > device turn-off time

c)      circuit time constant  >  device turn-off time

d)      circuit time constant  <  device turn-off time

9.      An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current rating will be

a)      1500 A

b)      2,121.32 A

c)      4,242.64 A

d)      6000 A

10.  The snubber circuit is used in thyristor circuits for

a)      Triggering

b)      dv/dt protection

c)      di/dt protection

d)      phase shifting

11.  Traic cannot be used in

a)      Ac voltage regulator

b)      Cycloconverter

c)      Solid state type of switch

d)      Inverter

12.  Two thyristors A and B have rated gate current of 100 mA and 2 A respectively.

a)      B is GTO and A is conventional thyristor

b)      A is GTO and B is conventional thyristor

c)      Thyristor A may opearate as transistor

d)      None of the above

13.  In a thyristor controlled reactor, the firing angle of thyristor is to be controlled in the range of

a)      0° to 90°

b)      0° to 180°

c)      90° to 180°

d)      90° to 270°

14.  In a single phase full wave controlled bridge rectifier, minimum output voltage and maximum output voltages are obtained at which conduction angles?

a)      0°,180° respectively

b)      180°,0° respectively

c)      0°, 0° respectively

d)      180°,180° respectively

15.  A single-phase fully controlled thyristor bridge ac-dc conveter is operating at a firing angle of 25° and on overlap angle of 10° constant dc output current of 20 A. the fundamental power factor (displacement factor) at input ac mains is

a)      0.78

b)      0.827

c)      0.866

d)      0.9 

16.  A single phase fully bridge converter supplies a load drawing constant and ripple free load current. If the triggering angle is 30°, the input power factor will be

a)      0.65

b)      0.78

c)      0.85

d)      0.866

17.  In a single-phase semiconveter, with discontinuous conduction and extinction angle β<Π  free wheeling action takes place for

a)      α

b)      Π-β

c)      β-Π

d)      zero degree

18.  The advantage of using free-wheeling diode with bridge type ac/dc conveter is

a)      Regenerative breaking

b)      Reliable speed control

c)      Improved power factor

d)      Reduced cost of the system

19.  In dual converter, the circulating current

a)      Allows smooth reversal of load current, but increase the response time

b)      Allows smooth reversal of load current with improved speed of response

c)      Does not allow smooth reversal of load current, but reduces the response time

d)      Flows if there is no interconnecting inductor

20.  In a 3-phase bridge rectifier circuit for HVDC transmission, the ratio of outout dc voltage to input ac voltage is

a)      0.5

b)      1.0

c)      1.71

d)      2

21.  A three phase full coverter is supplying a purely resistive load at 300 V dc for 0° firing angle. The output voltage for 90° firing angle would be

a)      0 V

b)      40 V

c)      50 V

d)      100 V

22.  A thyristorised, three phase, fully controlled converter feeds a dc load that draws constant current. Then the input ac line current to the converter has

a)      An rms value equal to the dc load current

b)      An average value equal to the dc load current

c)      A peak value equal to the dc load current

d)      A fundamental frequency component, whose rms value is equal to the dc load current

23.  Which one of the following is the most suitable device for dc-to-dc conveter?

a)      BJT

b)      GTO

c)      MOSFET

d)      Thyristor

24.  A four quadrant chopper cannot be operated as

a)      One quadrant chopper

b)      Cycloconverter

c)      Inverter

d)      Bidirectional rectifier

25.  If an ac chopper is feeding an inductive load, the firing pulse to the SCR

a)      May have a width equal to turn ON time of the SCR

b)      Should be a series of pulses of short duration

c)      Should be a single pulse of long duration

d)      Should be train of pulses of duration equal to the conduction period of the SCR

26.  Ac regulators are widely used in

a)      Traction drives

b)      Fan drives

c)      Synchronous motor drives

d)      Slip power recovery scheme of slip-ring induction motor

27.  A single phase ac voltage controller is controlling current in a purely inductive load. If the firing angle of the SCR is α, what will be the conduction angle of the SCR?

a)      Π

b)      Π-α

c)      (2Π-α)

d)      2Π

28.  The quality of output ac voltage of a cycloconverter is improved with

a)      Increase in output voltage at reduced frequency

b)      Increase in output voltage at increased frequency

c)      decrease in output voltage at reduced frequency

d)      decrease in output voltage at increased frequency

29.  how many switches are used to construct a three-phase to three-phase cycloconverter?

a)      3

b)      6

c)      12

d)      18

30.  Compared to a single phase half-bridge inverter, the output power of a single phase full-bridge inverter is higher by a factor of

a)      12

b)      8

c)      4

d)      2

31.  The output voltage waveform of a three-phase square-wave inverter contains

a)      Only even harmonics

b)      Both odd and even harmonics

c)      Only odd harmonics

d)      Only triple harmonics

32.  In a single-pulse modulation of PWM inverters if pulse width is 72° then

a)      Third harmonic will be eliminated

b)      fifth harmonic will be eliminated

c)      seventh harmonic will be eliminated

d)      none of these

33.  in case of voltage source inverter, freewheeling diode can be avoided for the load of

a)      inductive nature

b)      capacitive natures

c)      resistive nature

d)      back emf nature

34.  In a three phase voltage source inverter operating in square wave mode, the output line voltage is free from

a)      3^rd harmonic

b)      7^th harmonic

c)      11^th harmonic

d)      13^th harmonic

35.  The armature current of a dc motor fed from a thyristor power convertor contains ripple. The ripple in the armature current affects

a)      Commutating capability of the motor

b)      Overload capacity of the motor

c)      Torque capability of the motor

d)      Control ability of the speed of the motor

36.  Which of the following configurations is used for both motoring and generative braking?

a)      First quadrant chopper

b)      Second quadrant chopper

c)      Two quadrant chopper

d)      Four quadrant chopper 

37.  A three-phase fully-controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power 420 V dc to a three phase, 415 V(line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor

a)      119.05 A

b)      79.37 A

c)      68.73 A

d)      39.68 A

38.  In a self-controlled synchronous motor fed from a variable frequency inverter

a)      The rotor poles invariably have damper windings

b)      There are stability problems

c)      The speed of the rotor decides stator frequency

d)      The frequency of the stator decides the rotor speed

39.  Which stage of the power supply uses a zener as the main component?

a)      Rectifier

b)      Voltage divider

c)      Regulator

d)      Filter

Answers

1)a       2)a       3)a       4)d       5)b       6)c      7)b       8)b       9)b       10)b    11)c     12)b      13)b        14)b     15)a     16)b     17)d     18)c    19)b     20)a     21)a     22)c     23)b     24)b     25)d     26)b    27)b     28)b     29)d     30)d     31)c     32)b     33)a     34)a     35)a     36)c     37)c     38)d     39)c