# Power Electronics Expected MCQ PDF 2 For Coal India Limited (CIL) MT Exam 2017

1)   An ideal diode is
a. Unidirectional
b. Bidirectional
c. Fixed voltage polarity
d. Only (a) and (c)

2)   Bidirectional semiconductor device is
a. Diode
b. BJT
c. SCR
d. TRIAC

3)   With gate open, the maximum anode current at which SCR is turned off from ON condition is called
a. breakdown voltage
b. peak reverse voltage
c. holding current
d. latching current

4)   The curve
between V and I of SCR when anode is positive w.r.t cathode and when
anode is negative w.r.t cathode are known as

a. both as forward characteristics
b. both as reverse characteristics
c. former as forward characteristics and later as reverse characteristics
d. former as reverse characteristics and later as forward characteristics

 ANSWER: former as forward characteristics and later as reverse characteristics
5)   Form factor of a rectifier is the ratio of
a. Root mean square value of voltage and current to its peak value
b. Root mean square value of voltage and current to its average value
c. Average value of current and voltage to its root mean square value
d. Peak value of current and voltage to its root mean square value

 ANSWER: Root mean square value of voltage and current to its average value
6)   Ripple factor is the ratio of
a. Rms value of the ac component of load voltage to the dc voltage
b. Average value of the ac component of load voltage to the peak value of voltage
c. Average value of the dc component of load voltage to the ac voltage
d. Peak value of the dc component of load voltage to the ac voltage

 ANSWER: Rms value of the ac component of load voltage to the dc voltage
7)   Transformer utilization factor is a measure of the merit of a rectifier circuit. It is the ratio of
a. AC input power to the transformer volt – amp rating required by secondary
b. AC input power to the transformer volt – amp rating required by primary
c. DC output power to the transformer volt – amp rating required by secondary
d. DC output power to the transformer volt – amp rating required by primary

 ANSWER: DC output power to the transformer volt – amp rating required by secondary
8)   In a 3 phase
half wave rectifier, when firing angle is less than 90 degree, then the
average dc output voltage becomes

a. Positive
b. Negative
c. Zero
d. None of these

9)   If the firing angle becomes negative, then the rectifier begins to work as
a. A rectifier
b. An inverter
c. A chopper
d. A regulator

10)   The Graetz bridge makes excellent use of
a. Current transformer
b. Potential transformer
c. Power transformer
d. SCR

11)   DIAC and TRIAC both are semiconductors devices and conduct in
a. DIAC conducts in forward direction and TRIAC conducts in reverse direction
b. Both conducts in forward direction
c. Both conducts in reverse direction
d. Both conducts in either direction

 ANSWER: Both conducts in either direction
12)   A TRIAC can be turned on with
a. Positive voltage at the gate terminal
b. Negative voltage at the gate terminal
c. Either (a) or (b)
d. None of these

13)   The phase angle of gate signal in TRIAC can be shifted by using
a. A capacitor
b. A variable resistor
c. An inductor
d. Only (a) and (b)

14)   The ac voltage controller can be used for
a. Lighting and heating control
b. On – line transformer tap changing
c. Soft starting
d. All of these

15)   An RC snubber network used in BJT
a. Divert the collector current during turn – off
b. Improves the reverse bias safe operating area
c. Dissipates a fair amount of switching power
d. All of these

16)   A MOSFET, for its conduction uses
a. Only minority carriers
b. Only majority carriers
c. Both minority and majority carriers
d. None of these

17)   MOSFET stands for
a. Metal – oxide semiconductor field effect transistor
b. Molybdenum – oxide semiconductor field effect transistor
c. Metal – oxide silicon field effect transistor
d. Metal – oxide semiconductor field effect transmitter

 ANSWER: Metal – oxide semiconductor field effect transistor
18)   In a push – pull converter, the filter capacitor can be obtained as
a. Cmin = V / ( Vr L f2 )
b. Cmin= ( 1 – D ) V / ( Vr L f2 )
c. Cmin= ( 1 – 2 D ) V / 32 ( Vr L f2 )
d. Cmin= ( 1 – 2 D ) V / 42 ( Vr L f2 )

 ANSWER: Cmin= ( 1 – 2 D ) V / 32 ( Vr L f2 )
19)   In a flyback converter, the inductor of the buck-boost converter has been replaced by a
a. Flyback capacitor
b. Flyback resistor
c. Flyback transformer
d. Flyback transistor

20)   Advantages of Cuk converter is / are
a. Large number of reactive component
b. Low stress on switch
c. Low stress on capacitor
d. None of these

21)   The ac output voltage waveform of VSI and AC output current waveform of CSI respectively is composed of
a. High dv / dt, low di / dt
b. Low dv / dt, low di / dt
c. Low dv / dt, high di / dt
d. High dv / dt, high di / dt

 ANSWER: High dv / dt, high di / dt
22)   If energy is taken from the AC side of the inverter and sends it back into the DC side, then it is known as
a. Motoring mode operation
b. Braking mode operation
c. Regenerative mode operation
d. None of these

23)   A combination of synchronized leading edge and trailing edge modulation has also been used to control a
a. Boost single – phase power factor converter
b. A buck dc – dc converter to reduce ripple in the intermediate dc bus capacitor
c. Both (a) and (b)
d. None f these

24)   Regulator sampling PWM is usually used in
a. High power inverters
b. Rectifiers
c. Low power inverters
d. Only (a) and (b)

25)   In a 3 phase
VSI SPWM to use a single carrier signal and preserve the features of
PWM technique, the normalized carrier frequency should be

a. Multiple of two
b. Odd multiple of three
c. Odd multiple of five
d. Odd multiple of seven

26)   In square wave operation mode of 3 phase VSI, the VSI
a. Can control the load voltage
b. Cannot control the load voltage
c. Cannot control the load voltage except by means of dc link voltage
d. Cannot control the load voltage except by means of dc link current

27)   For commutation in three phase PAC, normally balanced three phase voltages VR, VY and VB are connected to the three legs of the converter via
a. Three inductances
b. Three capacitances
c. Three resistance
d. Three transistors

28)   If all the SCR’s of 3 phase PAC is replaced by diodes, they would be triggered
a. 120 degree after the zero crossing of the corresponding line voltages
b. 60 degree after the zero crossing of the corresponding line voltages
c. 120 degree before the zero crossing of the corresponding line voltages
d. 60 degree before the zero crossing of the corresponding line voltages

 ANSWER: 60 degree after the zero crossing of the corresponding line voltages
29)   The ability
of an electronic system to function properly in its intended
electromagnetic environment and should not be a source of pollution to
that electromagnetic environment is known as

a. Susceptibility
b. Emission
c. Interference
d. Electromagnetic compatibility

30)   The opposite of susceptibility is
a. Immunity
b. Emission
c. Interference
d. Electromagnetic compatibility

31)   The output current in PWM DC – DC converters is equal to
a. Average value of the output inductor current
b. Product of an average inductor current and a function of duty ratio
c. Either (a) or (b)
d. None of these

32)   A step – down choppers can be used in
a. Electric traction
b. Electric vehicles
c. Machine tools
d. All of these

33)   In a lossless inverter, the average power absorbed in one period by the load must be
a. Equal to the average power supplied by the dc source
b. Greater than the average power supplied by the dc source
c. Lesser than the average power supplied by the dc source
d. Equal to the average power supplied by the ac source

 ANSWER: Equal to the average power supplied by the dc source
34)   In a full
bridge VSI, in order to avoid the short circuit across the DC bus and
the undefined AC output voltage condition, the modulating technique
should ensure that

a. Top switch of each leg is on at any instant
b. Bottom switch of each leg is on at any instant
c. Either (a) or (b)
d. None of these

35)   In the SPWM, the modulating signal is
a. Square
b. Sinusoidal
c. Triangular
d. Saw – tooth

36)   The transfer function of PWM is generally developed in
a. Time domain
b. Frequency domain
c. Either (a) or (b)
d. None of these

37)   In constant frequency PWM, at perturbation the amplitude of the sinusoidal component is a
a. Linear function
b. Non linear function
c. Constant function
d. None of these

38)   In variable frequency, PWM gain
a. Phase lead helps to increase the phase margin of the control loop
b. Phase lag helps to increase the phase margin of the control loop
c. Phase lead helps to decrease the phase margin of the control loop
d. Phase lag helps to decrease the phase margin of the control loop

 ANSWER: Phase lead helps to increase the phase margin of the control loop
39)   A carrier based PWM technique in CSI is composed of
a. A switching pulse generator and a shorting pulse generator
b. A shorting pulse distributor
c. A switching and shorting pulse combination
d. All of these