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MCQOPTIONS
This section includes 109 Mcqs, each offering curated multiple-choice questions to sharpen your Power Electronics and Drives knowledge and support exam preparation. Choose a topic below to get started.
| 51. |
A dc source of 100 V supplies a purely inductive load 0.1 H. The controller is a thyristor in series with the source and load. The latching current specified is 100 mA. The minimum width of the gating pulse to ensure thyristor turn ON is ______ s . |
| A. | 100 |
| B. | 200 |
| C. | 500 |
| D. | 1000 |
| Answer» B. 200 | |
| 52. |
In dc-chopper, per unit ripple is maximum when duty cycle is ______ . |
| A. | 0.5 |
| B. | 5 |
| C. | 0.05 |
| D. | 1.5 |
| Answer» B. 5 | |
| 53. |
A single-phase SCR based ac regulator is feeding power to a load consisting of 5 Ω resistance and 16 mH inductance. The input supply is 230 V, 50 Hz ac. The maximum firing angle at which the voltage across the device becomes zero all throughout and the rms value of current through SCR, under this operating condition, are |
| A. | 30 and 46 A |
| B. | 30 and 23 A |
| C. | 45 and 23 A |
| D. | 45 and 32 A |
| Answer» D. 45 and 32 A | |
| 54. |
The average power delivered to an impedance (4 j3) Ω by a current 5 cos (100 t + 100) A is |
| A. | 44.2 W |
| B. | 50 W |
| C. | 62.5 W |
| D. | 125 W |
| Answer» C. 62.5 W | |
| 55. |
The turn-on time of a SCR with inductive load is 20 s. The pulse train frequency is 2.5 kHz with a mark/space ratio of 1/10, then the SCR will |
| A. | turn on |
| B. | not turn on |
| C. | turn on if inductance is removed |
| D. | turn on if pulse frequency is increased to two times. |
| Answer» B. not turn on | |
| 56. |
Which of the following does not cause permanent damage of an SCR? |
| A. | High current |
| B. | High rate of rise of current |
| C. | High temperature rise |
| D. | High rate of rise of voltage |
| Answer» B. High rate of rise of current | |
| 57. |
The Triac can be used only in |
| A. | inverter |
| B. | rectifier |
| C. | multiquadrant chopper |
| D. | cycloconverter |
| Answer» D. cycloconverter | |
| 58. |
MOS controlled thyristors have |
| A. | 1, 2 and 3 are correct |
| B. | 3, 4 and 5 are correct |
| C. | 2, 3 and 4 are correct |
| D. | 1, 3 and 5 are correct |
| Answer» B. 3, 4 and 5 are correct | |
| 59. |
A step down chopper operates from a dc voltage source V |
| A. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>s</sub> t<sub>r</sub></center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">(t<sub>r</sub> + t<sub>f</sub> + t<sub>0</sub>)</td></tr></table> |
| B. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>(V<sub>s</sub> t<sub>r</sub> + E<sub>b</sub> t<sub>f</sub>)</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">(t<sub>r</sub> + t<sub>f</sub> + t<sub>0</sub>)</td></tr></table> |
| C. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>(V<sub>s</sub> t<sub>r</sub> + E<sub>b</sub> t<sub>0</sub>)</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">(t<sub>r</sub> + t<sub>f</sub> + t<sub>0</sub>)</td></tr></table> |
| D. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>s</sub>t<sub>r</sub> + E<sub>b</sub>[ t<sub>f</sub> + t<sub>0</sub>]</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">(t<sub>r</sub> + t<sub>f</sub> + t<sub>0</sub>)</td></tr></table> |
| Answer» D. <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>s</sub>t<sub>r</sub> + E<sub>b</sub>[ t<sub>f</sub> + t<sub>0</sub>]</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">(t<sub>r</sub> + t<sub>f</sub> + t<sub>0</sub>)</td></tr></table> | |
| 60. |
The typcial ratio of latching current to holding current in a 20 A thyristor is |
| A. | 5.0 |
| B. | 2.0 |
| C. | 1.0 |
| D. | 0.5 |
| Answer» C. 1.0 | |
| 61. |
A half-controlled single-phase bridge rectifier is supplying an R-L load. It is operated at a firing angle and the load current is continuous. The fr action of cycle that the freewheeling diode conducts is |
| A. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| B. | <table><tr><td rowspan="2"><img src="https://www.indiabix.com/_files/images/data-interpretation/common/15-sym-oparen-h1.gif"></td><td rowspan="2">1 - </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"><img src="https://www.indiabix.com/_files/images/data-interpretation/common/15-sym-cparen-h1.gif"></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| C. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">2 </td></tr></table> |
| D. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| Answer» E. | |
| 62. |
An UJT used for triggering an SCR has the supply voltage VBB = 25 V. The intrinsic stand off ratio, = 0.75. The UJT will conduct when the bias voltage V |
| A. | 25 V |
| B. | 18.75 V |
| C. | 33.3 V |
| D. | 19.24 V |
| Answer» E. | |
| 63. |
VI characteristic of emitter of a UJT is |
| A. | similar to CE with linear and saturation region. |
| B. | similar to FET with a linear and pinch off region. |
| C. | similar to tunnel diods in some respects. |
| D. | linear between the peak point and valley point. |
| Answer» D. linear between the peak point and valley point. | |
| 64. |
For a sinusoidal input voltage, V |
| A. | m + n |
| B. | m n |
| C. | mn |
| D. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>n</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">m + n</td></tr></table> |
| Answer» E. | |
| 65. |
The average on-state current for an SCR is 20 A for a resistive load. If an inductance at 5 mH is included in the load, then average on-state current would be |
| A. | more than 20 A |
| B. | less than 20 A |
| C. | 15 A |
| D. | 20 A |
| Answer» B. less than 20 A | |
| 66. |
A SCR fires at a minimum current of 80 mA and 65 C. The minimum current required to fire at 25 C is |
| A. | > 80 mA |
| B. | equal to 80 mA |
| C. | < 80 mA |
| D. | half of 80 mA |
| Answer» E. | |
| 67. |
When the SCR conducts, the forward voltage drop |
| A. | is 0.7 V |
| B. | is 1 to 1.5 V |
| C. | increases slightly with load current |
| D. | remains constant with load current |
| E. | Options (B , C) |
| Answer» F. | |
| 68. |
At a room temperature of 30 C, minimum voltage and current required to fire a SCR is |
| A. | 3 V, 40 mA |
| B. | 0.6 V, 40 mA |
| C. | no limit |
| D. | 3 V, 100 mA |
| Answer» B. 0.6 V, 40 mA | |
| 69. |
For thyristors, pulse triggering is preferred to dctriggering because |
| A. | gate dissipation is low |
| B. | pulse system is simpler |
| C. | triggering signal is required for short duration |
| D. | all of these |
| Answer» E. | |
| 70. |
When two identical SCRs are placed back to back in series, with a load and if each is fired at 90 , a dc-volt meter across the load read |
| A. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>2</center></td><td rowspan="2"> peak voltage</td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| B. | zero |
| C. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>2</center></td><td rowspan="2"> peak voltage</td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| D. | none of these |
| Answer» C. <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>2</center></td><td rowspan="2"> peak voltage</td></tr><tr><td style="text-align: center;"> </td></tr></table> | |
| 71. |
The main reason for connecting a pulse transformer at the output stage of a thyristor triggering circuit is to |
| A. | amplify the power of the triggering pulse |
| B. | provide electrical isolation |
| C. | reduce the turn on time of the thyristor |
| D. | avoid spurious triggering of the thyristor due to noise |
| Answer» C. reduce the turn on time of the thyristor | |
| 72. |
If a diode is connected in anti-parallel 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 |
| D. | none of these |
| Answer» C. turn-off power loss increases, but turn-off time decreases | |
| 73. |
The applied sine voltage to SCR is V |
| A. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>10</center></td><td rowspan="2">A</td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| B. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>5</center></td><td rowspan="2">A</td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| C. | <table><tr><td rowspan="2">-</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>5</center></td><td rowspan="2">A</td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| D. | <table><tr><td rowspan="2">-</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>15</center></td><td rowspan="2">A</td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| Answer» C. <table><tr><td rowspan="2">-</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>5</center></td><td rowspan="2">A</td></tr><tr><td style="text-align: center;"> </td></tr></table> | |
| 74. |
A step down chopper is operated in the continuous conduction mode in steady state with a constant duty ratio D. If V |
| A. | D |
| B. | 1 D |
| C. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">1 - D</td></tr></table> |
| D. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>D</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">1 - D</td></tr></table> |
| Answer» B. 1 D | |
| 75. |
The function of connecting zener diode in an UJT circuit, used for triggering SCR is to |
| A. | expedite the generation of triggering pulses. |
| B. | delay the generation of triggering pulses. |
| C. | provide a constant voltage to VJT to prevent erratic firing. |
| D. | provide a variable voltage to VJT as the source voltage changes. |
| Answer» D. provide a variable voltage to VJT as the source voltage changes. | |
| 76. |
In the given circuit, if the resistance R is varied from 0 to , then |
| A. | phase remains constant. |
| B. | phase varies from 0 to 90 |
| C. | magnitude is constant and phase varies from 0 to 180 |
| D. | <table><tr><td rowspan="2">phase of the circuit is given by 2 tan<sup>-1</sup> </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> CR</td></tr></table> |
| Answer» E. | |
| 77. |
A single phase ac regulator fed from 50 Hz system supplies a load having resistance and inductance of 4.0 ohms and 12.73 mH respectively. The control range of firing angle is |
| A. | 0 < < 180 |
| B. | 45 < < 180 |
| C. | 90 < < 180 |
| D. | 0 < < 45 |
| Answer» C. 90 < < 180 | |
| 78. |
It is required to drive a dc shunt motor at different speeds in both the directions (forward and reverse) and also to brake it in both the directions. Which one of the following would you use? |
| A. | A half-controlled thyristor bridge |
| B. | A full-controlled thyristor bridge |
| C. | A dual converter |
| D. | A diode bridge |
| Answer» D. A diode bridge | |
| 79. |
The frequency of ripple in the output voltage of a three-phase half controlled bridge rectifier depends on |
| A. | firing angle |
| B. | lead inductance |
| C. | load resistance |
| D. | supply frequency |
| Answer» E. | |
| 80. |
A single-phase AC regulator with an inductance load has following details : |
| A. | 0 < < |
| B. | <table><tr><td rowspan="2"></td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| C. | <table><tr><td rowspan="2">0 <</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| D. | <table><tr><td rowspan="2"> < <</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| Answer» C. <table><tr><td rowspan="2">0 <</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> </center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;">2</td></tr></table> | |
| 81. |
In a 3-phase rectifier circuit, thyristor number 1, 2 and 3 are connected respectively to R, Y and B phases of the star-connected transformer secondary. When the current is being commutated from thyristor No. 1 to No. 2, the effect of the transformer leakage and the ac system inductance will be such that it will |
| A. | prolong the conduction in No. 1 and delay the turn-on of No. 2 correspondingly |
| B. | stop the conduction in No. 1 at the scheduled time, but delay the turn-on of No. 2 |
| C. | produce conduction in both No. 1 and No. 2 in parallel for an overlapping period through a transient |
| D. | double the voltage output through a commutation transient |
| Answer» D. double the voltage output through a commutation transient | |
| 82. |
A 3-phase converter feeds a pure resistive load at a firing angle of = 60 . The average value of current flowing in the load is 10 A. If a very large inductance is connected in the load circuit, then |
| A. | average value of current will remain as 10 A |
| B. | average value of current will become greater than 10 A |
| C. | average value of current will become less than 10 A |
| D. | trend of variation of current cannot be predicted unless the exact value of the inductance connected is known |
| Answer» B. average value of current will become greater than 10 A | |
| 83. |
Analysis of voltage waveform of a single phase bridge converter shows that it cont ains x% of 6t h harmonic. The 6th harmonic content of the voltage waveform of a 3-phase bridge converter would be |
| A. | less than x% due to an increase in the number of pulses |
| B. | equal to x%, the same as that of the singlephase converter |
| C. | greater than x% due to changes in the input and output voltages of the converter |
| D. | difficult to predict as the analysis of converters is not governed by any generalised theory |
| Answer» D. difficult to predict as the analysis of converters is not governed by any generalised theory | |
| 84. |
A single-phase half-wave controlled rectifier has 400 sin 314 t as the input voltage and R as the load. For the firing angle of 60 for the SCR, the average output voltage is |
| A. | <table><tr><td rowspan="2"> </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>400</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| B. | <table><tr><td rowspan="2"> </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>300</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| C. | <table><tr><td rowspan="2"> </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>240</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| D. | <table><tr><td rowspan="2"> </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>200</center></td><td rowspan="2"> </td></tr><tr><td style="text-align: center;"> </td></tr></table> |
| Answer» E. | |
| 85. |
With reference to the output waveform given in the figure below, output of the converter will be free from 5th harmonic, when |
| A. | = 72 |
| B. | = 36 |
| C. | = 150 |
| D. | = 120 |
| Answer» B. = 36 | |
| 86. |
A thyristorised, three phase, fully controlled converter feeds a dc load that draws a constant current. Then 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 |
| Answer» D. a fundamental frequency component, whose rms value is equal to the dc load current | |
| 87. |
A three phase semiconverter feeds the armature of a separately excited dc motor, supplying a non-zero torque. For steady state operation, the motor armature current is found to drop to zero at certain instances of time. At such instances, the voltage assumes a value which is |
| A. | equal to the instantaneous value of the ac phase voltage |
| B. | equal to the instantaneous value of the motor back emf |
| C. | arbitrary |
| D. | zero |
| Answer» C. arbitrary | |
| 88. |
Resonant converters are basically used to |
| A. | generate large peaky voltage. |
| B. | reduce the switching losses. |
| C. | eliminate harmonics. |
| D. | convert a square wave into a sine wave. |
| Answer» C. eliminate harmonics. | |
| 89. |
A three phase semiconverter feeds the armature of a separately excited dc motor, supplying a nonzero torque. For steady state operation, the motor armature current is found to drop to zero at certain instances of time. At such instances, the voltage assumes a value that is |
| A. | equal to the instantaneous value of the ac phase volatge |
| B. | equal to the instantaneous value of the motor back emf |
| C. | arbitrary |
| D. | zero |
| Answer» C. arbitrary | |
| 90. |
In a dual converter, the circulating current |
| A. | allows smooth reversal of load current, but increases the response time |
| B. | does not allow smooth reversal of load current, but reduces the response time |
| C. | allows smooth reversal of load current with improved speed of response |
| D. | flows only if there is no inter connecting inductor |
| Answer» D. flows only if there is no inter connecting inductor | |
| 91. |
In a 3-phase controlled bridge rectifier, with an increase of overlap angle, the output dc voltage |
| A. | decreases |
| B. | increases |
| C. | does not change |
| D. | depends upon load inductance |
| Answer» B. increases | |
| 92. |
A single phase diode bridge rectifier supplies a highly inductive load. The load current can be assumed to be ripple free. The ac supply side current waveform will be |
| A. | sinusoidal |
| B. | constant dc |
| C. | square |
| D. | triangular |
| Answer» D. triangular | |
| 93. |
In a thyristor dc chopper, which type of commutation results in best performance ? |
| A. | Voltage commutation |
| B. | Current commutation |
| C. | Load commutation |
| D. | Supply commutation |
| Answer» D. Supply commutation | |
| 94. |
A chopper operating at a fixed frequency is feeding an R-L load. As duty ratio of the chopper is increased from 25% to 75%, the ripple in the load current |
| A. | remains constant |
| B. | decreases, reaches a minimum at 50% duty ratio and then increases |
| C. | increases, reaches a maximum at 50% duty ratio and then decreases |
| D. | keeps on increasing as the duty ratio is increased |
| Answer» B. decreases, reaches a minimum at 50% duty ratio and then increases | |
| 95. |
In type A chopper source voltage is 100 V, d.c. ON period = 100 s, OFF-period = 150 s and load RLE consists of R = 2Ω , L = 5 mH, E = 10 V. |
| A. | 40 V, 5 A |
| B. | 66.66 V 28.33 A |
| C. | 60V, 25 A |
| D. | 40V, 20 A |
| Answer» B. 66.66 V 28.33 A | |
| 96. |
A voltage commutated chopper has the following parameters : |
| A. | 1000 s, 200 A |
| B. | 700 s, 382.8 A |
| C. | 700 s, 282.8 A |
| D. | 1000 s, 382.8 A |
| Answer» C. 700 s, 282.8 A | |
| 97. |
A dc to dc transistor chopper supplied from a fixed voltage dc source feeds a fixed resistive-inductive load and a free-wheeling diode. The chopper operates at 1 kHz and 50% duty cycle. Without changing the value of the average dc current through the load, if it is desired to reduce the ripple content of load current, the control action needed will |
| A. | increase chopper frequency keeping the duty cycle constant |
| B. | increases chopper frequency and duty cycle in equal ratio |
| C. | decrease only the chopper frequency |
| D. | decrease only the duty cycle |
| Answer» B. increases chopper frequency and duty cycle in equal ratio | |
| 98. |
Turn-on time of SCR can be reduced by using a |
| A. | rectangular pulse of high amplit ude and narrow width |
| B. | rectangular pulse of low amplitude and wide width |
| C. | triangular pulse |
| D. | trapezoidal pulse |
| Answer» B. rectangular pulse of low amplitude and wide width | |
| 99. |
Which semiconductor power device out of the following is not a current triggered device? |
| A. | Thyristor |
| B. | G.T.O. |
| C. | Triac |
| D. | MOSFET |
| Answer» E. | |
| 100. |
A switched mode power supply operating at 20 kHz to 100 kHz range uses which of the following as the main switching element? |
| A. | Thyristor |
| B. | MOSFET |
| C. | Triac |
| D. | UJT |
| Answer» C. Triac | |