## Power Electronics MCQs1) Which of the given device is the fastest switching device - MOSFET
- Triode
- BJT
- JEFT
2) A snubber circuit is used to - Limit the rate of rising in voltage across BJT
- Limit the rate of rising in voltage across SCR
- Limit the rate of rising in current across TRIAC
- Limit the rate of rising in current across BJT
3) If the maximum value of applied voltage for a full-wave center ap (M-2) rectifier is 30 V, find the magnitude of ripple voltage - 83.88
- 84.52
- 87.62
- 89.59
4) Find the displacement power factor of a single-phase fully controlled rectifier in which feeds constant DC current in a load when a magnitude of power angle is 30
Given, ∅=30 We know that, Displacement factor = cos?∅ = cos30 5) In the forward blocking mode of a silicon controlled rectifier, the SCR is - In on state
- In natural state
- Forward biased state
- In off state
6) A silicon controlled rectifier is turned on if the anode current is greater than - Trigger current
- Anode current
- Cathode current
- Holding current
7) The anode current passing through a conducting silicon-controlled rectifier is 20 A. If its gate current is made half, what will be the anode current? - 10 A
- 20 A
- 5 A
- 30 A
8) A silicon controlled rectifier is turned off mode when its turn off time is - Greater than the circuit turn off time
- Less than the circuit turn off time
- Equal to the circuit turn off time
- None of these
9) A 1 - ∅ full-wave rectifier is made by using thyristors. If the peak value of the sinusoidal input voltage is Vm and the value of the delay angle is ?/4 radian, find the average value of output voltage - 0.25 V
- M
- 0.45 VM
- 0.65 VM
- 0.85 VM
10) The TRIAC is the same as - Two SCR connected in parallel
- Two SCR connected in antiparallel
- One SCR and one thyristor connected in parallel
- Two SCR connected in series
11) A half-wave rectifier circuit using an ideal diode has an input voltage of 10 sin ?t V; find the average and RMS value of output voltage? - 3.18 V, 5V
- 3.68 V, 8V
- 4.18 V, 5V
- 4.68 V, 8V
Given; Maximum input voltage = 10 V We know that, for half wave rectifier circuit And rms value of the output voltage 12) A solar cell of 450 V supplies power to an ac source of 440 V, 50 Hz through a 3 phase fully controlled bridge converter. An inductance with a value of 10 A is connected to the dc circuit to maintain the dc current. If the resistance of the solar cell is 1 Ω, each thyristor will be reverse biased for a period of - 148
^{0} - 138
^{0} - 128
^{0} - 118
^{0}
EMF of a solar cell = 450 V DC current I Resistance of solar cell, R We know that, Voltage across the inverter = output voltage of the solar cell, V For three phase controlled bridge 13) A 1 - ∅ full wave rectifier is designed using thyristors. It the peak value of the sinusoidal input voltage Vm and the delay angle isradian, find the average value of output voltage? - 0.35 V
_{max} - 0.45 V
_{max} - 0.55 V
_{max} - 0.65 V
_{max}
14) Which of the given device is the most suitable power device for a higher frequency (above 100 kHz) switching application - SCR
- Power MOSFET
- GTO
- BJT
15) Which of the given device is a modern semiconductor device that combines the characteristics of MOSFET and BJT? - SCR
- Schottky diode
- Microwave transistor
- IGBT
16) The thyristor is equivalent to a thyratron tube is - BJT
- SCR
- TRIAC
- GTO
17) A single-phase controlled, a full-bridge converter is supplying a highly inductive DC load. The conductor is fed from a 220 V, 50Hz, ac source. Find the fundamental frequency in Hz of the voltage ripple on the dc side - 300 Hz
- 220 Hz
- 100 Hz
- 50 Hz
= 2f 18) A single-phase fully controlled thyristor bridge ac dc converter is operating at a firing angle is 30 - 0.968
- 0.766
- 0.163
- 0.586
Given; α = 30 µ = 20 We know that the fundamental power factor or displacement factor at input ac mains. 19) Find the average output of a semiconverter connected to a 220 V, 50 Hz power supply, and firing angle is - 178.52
- 248.05
- 148.55
- 198.49
Given; Firing angle α = We know that the average output voltage of a semiconductor is given as 20) A freewheeling diode is phase-controlled rectifiers. - Stops rectifier operations
- Improves line power factor
- Is the reason for additional harmonics
- Is the reason for the sudden breakdown
21) In a three-phase bridge rectifier, the maximum conduction of each thyristor is - 120
^{0} - 90
^{0} - 30
^{0} - 60
^{0}
22) In a 3-∅ controlled bridge rectifier, the frequency of ripple in the output voltage depends on - Power factor
- Supply frequency
- Voltage source
- Firing angle
F In a 3-Ø controlled bridge rectifier, the ripple frequency in the output voltage is 6 times the supply frequency. 23) A three-phase full converter is supplying a purely resistive load at 220 V dc for 0 - 30 V
- 0 V
- 90 V
- 120 V
24) What is the name of a converter which can work on both in 3-pulse and 6 pulse modes? - Three-phase full-wave converter
- Three-phase half wave converter
- Three-phase semi converter
- Single-phase semi converter
25) A three phase full controlled converter can operate as a - Converter for α = 0 to 120
^{0} - Converter for α = 0 to 90
^{0} - Converter for α = 0 to 180
^{0} - Converter for α = 0 to 60
^{0}
26) A 3 phase, 6 pulse SCR converter is connected to a voltage source of 220 V, 50 Hz, AC mains, and controlling a dc drive with terminal voltage 210 V and rated current value is 90 A. If the commutation angle µ = 15 - 1
- 0.5
- 2
- 1.5
Given; V = 210 I = 90 A α= 60 We know that, Shunt compensation = VI tan alpha; Putting the value V , I, and αin above equation we get = 210 × 90 tan 60 = 72, 735 VAR = 72.8 KVAR And power factor = Cos α= Cos 60 27) Which of the given regulator provide output voltage polarity reversal without the involvement of a transformer. - Linear voltage regulator
- Shunt voltage regulator
- Buck-Boost regulator
- Series voltage regulator
28) A chopper converts - AC to DC
- AC to AC
- DC to AC
- DC to DC
29) A chopper circuit works on TRC control mode at a frequency of 4 kHz of a 220 V dc supply. For 180 V output voltage, find the conduction and blocking periods of a thyristor in each cycle. - 0.209 ms, 0.234 ms
- 0.404 ms, 0.055 ms
- 0.204 ms, 0.045 ms
- 0.704 ms, 0.897 ms
Given; V V f = 4 kHz = 4 × 10 We know that, 30) A four-quadrant chopper can not be operated as - Thyristor
- Cycloconverter
- One quadrant chopper
- Inverter
31) If a step-up chopper has source voltage V and duty cycle α, find the chopper's output voltage.
Given; Source voltage= V Duty cycle = α We know that The average value of output voltage of the step-up chopper is given as Where V is the voltage source, and α is the duty cycle. 32) How many switches are used to build a three-phase to three-phase cycloconverter? - 10
- 14
- 18
- 24
33) A three-phase to three-phase cyclo converter requires - 24 SCRs for 6 pulse device
- 36 SCRs for 6 pulse device
- 48 SCRs for 3 pulse device
- 24 SCRs for 3 pulse device
34) In a three-phase to single phase cycloconverter using a 6 pulse bridge circuit, if the input voltage is 220 V per phase, find the value of the fundamental RMS value of the output voltage?
Given; Total number of pulses (m) = 6 Input phase voltage (V We know that the fumdamental value of rms output voltage is given by 35) A three-phase cycloconverter is used to find the variable frequency single-phase ac output. If the single-phase ac load is 240 V, 50 A at a power factor of 0.8 lagging, find the RMS value of input voltage per phase - 220 V
- 240 V
- 290 V
- 20 V
Given, V m = 3 (for 3 phase) We know that the fumdamental value of rms output voltage is given by 36) A single-phase, half-bridge inverter has an input voltage of 60 VDC. If the inverter is feeding a load of 3.6 ohms, find the RMS output voltage at a fundamental frequency - 188 V
- 168 V
- 158 V
- 178 V
Given, Vs = 60 We know that the fundamental component of the output voltage of a single-phase half-bridge inverter is given as 37) PWM switching is used in voltage source inverters for the purpose of - Controlling output current
- Controlling input voltage
- Controlling input power
- Controlling output harmonics and output voltage.
38) A single-phase full-bridge voltage source inverter (VSI) is fed from a 240 V dc source. If a pulse of 60 - 128 v
- 148 V
- 108 V
- 168 V
Given pulse duration (d) = 60 Vs = 240 V We know that pulse width is given as 2d = 60 D = 30 So the RMS value of the fundamental component of the output voltage is given as 39) An electric motor, developing a starting torque of 18 Nm, starts with a load toque of 9 Nm on its shaft. If the acceleration at the initial point is 3 rad/sec - 3 Kg - m
^{2} - 9 Kg - m
^{2} - 27 Kg - m
^{2} - 81 Kg - m
^{2}
Given, Starting toque Test of the motor = 18 Nm Load toque T Accelarating toque Ta = Tst- T Accelaration α =3 rad/sec We know thar moment of inertia is given as 40) If a 3 phase semi converter is feeding by a separately excited dc motor at constant field current, the motor can operate in the given condition. - Speed positive and toque negative
- Speend positive and toque positive
- Speed negative and toque positive
- Speed negative and toque negative
41) A 240 V, 1200 rpm. 30 A separately excited dc motor has an armature resistance of 3 ohms. A step-down chopper controls the dc motor with a frequency of 1 kHz. The input dc voltage given to the chopper is 280 V. If the duty cycle of the chopper for the motor to operate at a speed of 700 rpm, find the rated toque - 0.233
- 0.338
- 0.633
- 0.951
Given, Rated speed (N Voltage across the armature (V Armature current (I Armature resistance (R Rated speed (N We know that, The rated speed of 1200 rpm voltage across the motor armature, V E = 240 - 30 × 3 = 240 - 90 = 150 V At 700 rpm, At rated toque, armature current will also be rated that is 30 A because the current is directly proportional to the toque So the applied voltage V = 87.5 + 30 × 3 = 177.5 V So the duty ratio, 42) A single-phase half-controlled rectifier is driving a separately excited dc motor. The back emf constant of the motor is 0.30 V/rpm. The armature current's value is 8 A without any ripple, and the armature resistance is 3 ohm. The converter is working from a 240 V power supply of a single phase ac source with a firing angle of 60 - 120. 6 rpm
- 168. 7 rpm
- 190. 2 rpm
- 240. 8 rpm
Given, The back emf constant of the motor E Armature current (I Armature resistance (R V Firing angle α = 60 We know that, The average output voltage of a single phase half controlled rectifier is given as 43) The armature current of a dc motor fed from a thyristor power converter consists of ripples. The ripple in the armature affects - Commutation of the motor
- Speed of the motor
- Toque of the motor
- The efficiency of the motor
44) A motor armature is supplied through a phase control silicon controlled rectifiers receives a smoother voltage shape at - Lower motor speed
- Constant motor speed
- Higher motor speed
- None of these
45) The torque produced by a single-phase induction motor fed by an ac voltage controller for speed control due to - A fundamental component of harmonics as well as current
- A fundamental component of harmonics only
- A fundamental component of current only
- Fundamental component and even harmonics
46) A resistance of 4 ohms is connected in the rotor circuit, and a resistance of 8 ohms is additionally connected during OFF periods of a chopper. The OFF period of the chopper is 6 ms. Find the average resistance in the rotor circuit for the chopper frequency off 200 Hz. - 20.6 ohm
- 15.5 ohm
- 25.9 ohm
- 1.8 ohm
Given, Normally a 4-ohm resistor is connected to the rotor circuit during off periods; r = 8 ohm is additional T f = 200 Hz We know that frequency is reciprocal of a period So, 47) When a self-controlled synchronous motor fed from a variable frequency inverter - The speed of the stator decides stator speed
- Stability problems occur
- The frequency of the stator decides the rotor speed
- The frequency of the rotor decides the rotor speed
As we know, The synchronous motor always runs at synchronous speed Where, f = frequency p = number of poles. So the rotor speed is decided by the stator frequency supply. 48) A 3-∅ fully controlled thyristor bridge converter is used as a line commutated inverter to feed 60 kW With the power 450 V dc to a three-phase, 430 V (line), 50 Hz ac mains. If the dc-link current is constant, find the RMS current of the thyristor. - 54.68 A
- 76.98 A
- 66.08 A
- 16.88 A
Given, DC link current I DC link voltage V P = 60×10 So, the power supllied to the motor is given as P = V 49) A cycloconverter fed induction motor drive is widely used for which one of the following? - Compressor drive
- Machine tool drive
- Papermill drive
- Cement mill drive
50) Which of the given configurations is used for both regenerative and motoring breaking? - First quadrant chopper
- Fourth quadrant chopper
- Third quadrant chopper
- Two quadrant chopper
51) A dc chopper is used in the regenerative braking mode of a dc series motor. The dc supply is 400 V, and the duty cycle is 60%. The average value of armature current is 80 A. If it is considered as continuous and ripply free, find the value of power feedback to the supply? - 28200 W
- 19200 W
- 21240 W
- 19220 W
: Given, V Duty cycle α = 0.6 Armature current I We know that for regenerative mode, chopper os worked as a step-up chopper Therefore, = 600(1 - 0.6) = 240 V Now the power supplier back to the supply = V 52) Power diode is - Two terminal semiconductor device
- Three terminal semiconductor device
- Four terminal semiconductor device
- None of these
53) The V-I characteristics of diode lie in the - First quadrant
- Fourth quadrant
- The third and second quadrant
- The first and third quadrant
54) Find a triple frequency of a six-phase half-wave rectifier for 220 V 50 Hz input - 150 Hz
- 300 Hz
- 50 Hz
- 600 Hz
In a six-phase half-wave rectifier, usually six diodes are used. Since the supply frequency is 50 z, therefor the ripple frequency is six-time that of the power supply = 50 × 6 = 300 Hz 55) If a firing angle α of a single phase fully controlled rectifier feeding a constant DC current into the load is 60 - 0
- 0.5
- 1
- 1.5
SO, The displacement power factor is given as = Cos? ? is 60 Therefore, Cos60 56) A forward-biased PN junction acts as a/an - Thyristor
- Closed switch
- Amplifier
- Chopper
57) For a specific transistor, if the value of beta is equal to 400 and the Base current is 8mA, Find the value of Emitter current? - 4.308
- 3.208 A
- 7.808 A
- 9.276 A
Given, Current Gain β = 400 Base current I The dc current gain β 58) The two-transistor model of an SCR obtained by - Bisecting the SCRs diagonally
- Bisecting the SCRs top and bottom two layers
- Bisecting the SCRs only bottom two layers
- Bisecting of SCRs horizontally
59) In an SCR, the gate cathode characteristics have a slope of 160. If the gate power dissipation is 0.8 watt, find the value of the gate current? - 110 mA
- 220mA
- 71 mA
- 31 mA
Given, Slope = 160 We know that Power is defined as the product of voltage and current Therefore, P = V Slope V V 160 I I Next TopicNumber System MCQ |