Power Electronics Solved Objective Questions: Part-6

[1] The reverse recovery time of the diode is defined as the time between the instant diode current becomes zero and the instant reverse recovery current decays to
(a) Zero
(b) 10% of the reverse peak current (IRM)
(c) 25% of (IRM)
(d) 15% of (IRM)

Ans: C

[2] The cut-in voltage and forward-voltage drop of the diode are respectively
(a) 0.7V, 0.7V
(b) 0.7V, 1V
(c) 0.7V, 0.6V
(d) 1V, 0.7V

Ans: C

[3] The softness factor for soft-recovery and fast-recovery diodes are respectively
(a) 1, >1
(b) <1, 1
(c) 1, 1
(d) 1, <1

Ans: D

[4] Reverse recovery current in a diode depends on
(a) Forward field current
(b) Storage charge
(c) Temperature
(d) PIV

Ans:A

[5] The three terminals of power MOSFET
(a) Collector, Emitter, base
(b) Drain, source, base
(c) Drain, source, gate
(d) Collector, emitter, gate

Ans: C

[6]The three terminals of IGBT
(a) Collector, emitter, base
(b) Drain, source, base
(c) Anode, cathode, gate
(d) Collector, emitter, gate

Ans:D

[7] The three terminals of MCT
(a) Anode, cathode, gate
(b) Collector, emitter, gate
(c) Drain, source, base
(d) Drain, source, gate

Ans: A

[8] Compared to Power MOSFET, the Power BJT has
(a) Lower switching losses but higher conduction loss
(b) Higher switching losses and higher conduction loss
(c) Higher switching losses but lower conduction loss
(d) Lower switching losses and lower conduction loss

Ans:C

[9] Which one of the following statement is true
(a) MOSFET has positive temperature coefficient whereas BJT has negative temperature coefficient
(b) Both MOSFET and BJT have positive temperature coefficient
(c) Both MOSFET and BJT have negative temperature coefficient
(d) MOSFET has negative temperature coefficient whereas BJT has positive temperature coefficient

Ans:A

[10] Which one of the following statement is true
(a) Both MOSFET and BJT are voltage controlled devices
(b) Both MOSFET and BJT are current controlled devices
(c) MOSFET is a voltage controlled device and BJT is current controlled device
(d) MOSFET is a current controlled device and BJT is voltage controlled device

Ans: C . MOSFET operation is controlled by gate voltage and the BJT operation is controlled by base current

[11] Practical way of getting static voltage equalization in series connected SCRs is be the use of
(a) One resistor across the string
(b) Resistors of different values across each SCR
(c) Resistors of the same value across each SCR
(d) One resistor in series with the string
OPTIONS:
1) A & D
2) A
3) C
4) B
5) None of the above options

Ans: 3

[12] For series connected SCRs, dynamic equalizing circuit consists of
(a) Resistor R and capacitor C in series but with a diode D across C
(b) Series R and D circuit but with C across R
(c) Series R and C circuit but with D across R
(d) Series C and D circuit but with R across C
OPTIONS:
1) A
2) B
3) C
4) D
5) None of the above options

Ans: 3

[13] During forward blocking of two series connected SCRs, a thyristor with
(a) High leakage impedance shares lower voltage
(b) High leakage impedance shares higher voltage
(c) Low leakage impedance shares higher voltage
(d) Low leakage impedance shares lower voltage
OPTIONS:
1) B
2) D
3) B, D
4) A, C
5) None of the above options

Ans:3

[14] Thyristors A has rated gate current of 1A and thyristor B rated gate current of 100mA
(a) A is a GTO and B is a conventional SCR
(b) B is a GTO and A is a conventional SCR
(c) A may operate as a transistor
(d) B may be operate as a transistor
OPTIONS:
1) A
2) B
3) C
4) A, C
5) B, D

Ans: 4

[15] A resistor connected across the gate and cathode of an SCR increases its
(a) dv/dt rating
(b) Holding current
(c) Noise immunity
(d) Turn-off time
OPTIONS:
1) A, C
2) A, B
3) B, C
4) A, B, C
5) D

Ans: 4

Hint for Questions 5, 6, 7:
MOSFET:

IGBT

Remember that for IGBT we can represent the terminals either Drain, Source, Gate or Collector, Emitter, Gate.


MCT:

Power Electronics Part- 5 Objective Questions with Answers

Power Electronics Objective Questions with Answers: Part-5

1. SCR is the solid state equivalent of
(a) Transistor
(b) Thyratron
(c) Vacuum diode
(d) Crystal diode

Answer: B
2. With gate open, if the supply voltage exceeds the breakover voltage of SCR, then SCR will conduct
(a) False
(b) True
(c) For DC
(d) For AC

Answer: B
3. The SCR is turned off when the anode current falls below
(a) Forward current rating
(b) Break over voltage
(c) Holding current
(d) Latching current

Answer: C
4. In a SCR circuit, the angle of conduction can be changed by changing
(a) Anode voltage
(b) Anode current
(c) Forward current rating
(d) Gate current

Answer: D
5. The normal way to close a SCR is by appropriate
(a) Gate current
(b) Cathode current
(c) Anode current
(d) Forward current

Answer: A
6. An SCR has ......PN junctions
(a) Two
(b) Four
(c) Three
(d) One

Answer: C
7. If gate current is increased, the anode-cathode voltage at which SCR closes is.....
(a) Increased
(b) Decreased
(c) Maximum
(d) Least

Answer: B
8. A conducting SCR can be opened by reducing ..... to zero
(a) Supply voltage
(b) Grid voltage
(c) Grid current
(d) Anode current

Answer: A
9. With gate open, a SCR can be turned-on by making supply voltage....
(a) Minimum
(b).Reverse
(c) Equal to cathode voltage
(d) Equal to breakover voltage

Answer: D
10. If firing angle in a SCR rectifier is increased, output is....
(a) Increased
(b) Maximum
(c) Decreased
(d) Unaffected

Answer: C
11. A SCR is a ...switch
(a) Two directional
(b) Unidirectional
(c) Three directional
(d) Four directional

Answer: B
12. The anode of SCR is always maintained at........potential with respect to cathode
(a) Positive
(b) Zero
(c) negative
(d) Equipotential

Answer: A
13. IF the chopper switching frequency is 200Hz and Ton time is 2ms, the duty cycle is
(a) 0.4
(b) 0.8
(c) 0.6
(d) None of the above

Answer: A
14. Speed control of induction motor can be effected by varying
(a) Flux
(b) Voltage input to stator
(c) Keeping rotor coil open
(d) None of the above

Answer: B
15. In a Dc motor if the field coil gets opened, the speed of the motor will
(a) Decrease
(b) Come to stop
(c) Increase
(d) None of the above

Answer: C

Solution Hint:
[11] SCR will conduct in one direction only. So it act as a unidirectional switch.
[13] T=1/f, D = TON/T

Objective Type Questions on Power Electronics: Part-4

Power Electronics Objective Type Questions with Answers: Part-4

[1] In a SCR,
(a) Gate current is directly proportional to forward breakover voltage
(b) As gate current is raised, forward breakover voltage reduces
(c) Gate current has to be kept on continuously for conduction
(d) Forward breakover voltage is low in the forward blocking state.

Answer: B
[2] In a SCR, at switch on, there is a sudden increase of current, then
(a) The tolerance limit of di/dt varies from 3 to 30A/us
(b) di/dt upper limit is 3A/us
(c) di/dt rating can be increased by having an inductive load
(d) di/dt rating can be increased by a resistive load
Which of the above statements are true?
(1) a, c
(2) c, d
(3) a, b, c, d
(4) a, b

Answer: 1
[3] The following PNPN device has a terminal for synchronising purpose
(a) SUS
(b) DIAC
(c) TRIAC
(d) Schmitt trigger

Answer: A
[4] In single phase full wave controlled rectifier, maximum output voltage is obtained at conduction angle....and minimum at conduction angle...
(a) 0°, 180°
(b) 180°, 0°
(c) 0°, 0°
(d) 180°, 180°

Answer: A
[5] A thyristor is triggered by a pulse train of 5kHz. The duty ratio is 0.4. If the allowable average power is 100W the maximum allowable gate drive power is
(a) 100√2 W
(b) 50 W
(c) 150 W
(d) 250 W

Answer: D
[6] The consequence of introducing inductance in the load circuit of SCR is
(a) Anode current through SCR rises slowly
(b) Anode current reached is more than without it
(c) SCR has to be derated for anode current
(d) SCR's current rating can be raised
Which of the above statements are true?
(1) a, c
(2) a, b, c, d
(3) b, d
(4) a, d

Answer: 4
[7] When a SCR is in the forward blocking state,
(a) All the 3 junctions are reverse biased
(b) The anode and cathode junctions are forward biased but the gate junction is reverse biased
(c) The anode junction is forward biased but the cathode and gate junctions are reverse biased
(d) The anode and gate junctions are forward biased but the cathode junction is reverse biased

Answer: B
[8] When we need to drive a dc shunt motor at different speeds in both 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 SCR bridge
(b) A full controlled thyristor bridge
(c) A dual converter
(d) A diode bridge

Answer: C
[9] The frequency of the ripple present in the output voltage of the 3 phase half controlled bridge rectifier depends on the
(a) Firing angle
(b) Load inductance
(c) Load resistance
(d) Supple frequency

Answer: D
[10] A fully controlled thyristor bridge drives a DC motor. The system is capable of
(a) Motoring and braking in both directions
(b) Only motoring in both directions, no braking
(c) Motoring in forward direction and braking in reverse direction
(d) Only motoring in forward direction, no braking

Answer: D
[11] Three-phase to three-phase cycloconverter employing 18 SCRs and 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of VA rating of 36 SCR device to that of 18 SCR device is
(a) 1/2
(b) 1
(c) 2
(d) 4

Answer: C
[12] Three phase to three phase cycloconverter employing 18 SCRs are 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of power handled by 36 SCR device to that handled by 18 SCR device is
(a) 4
(b) 2
(c) 1
(d) 1/2

Answer: A
[13] The number of SCRs required for single phase to single phase cycloconverter of the mid point type and for three phase to three phase type cycloconverter are respectively
(a) 4, 6
(b) 8, 18
(c) 4, 18
(d) 4, 36

Answer: C
[14] For dynamic equalizing circuit used for series connected SCRs, the choice of C is based on
(a) Reverse recovery characteristics
(b) Turn-on characteristics
(c) Turn-off characteristics
(d) Rise-time characteristics
OPTIONS:
1) A
2) B, C
3) D
4) A, B, C, D
5) None of the above options

Answer: 1
[15] The function of connecting zener diode in an UJT circuit, used for the triggering of SCRs, is to
(a) Expedite the generation of triggering pulses
(b) Delay the generation of triggering pulses
(c) Provide a constant voltage to UJT to prevent erratic firing
(d) Provide a variable voltage to UJT as the source voltage changes.
OPTIONS:
1) B, C
2) C
3) A
4) D
5) None of the above options

Answer: 2

Solution Hint:
[4] When the conduction angle is 0, it will not suppress any part of the input, whereas during 180 degree conduction angle, it will suppress entire input waveform.
[5] PMAX >= Pavg/(f.TON)
Duty cycle D =TON/T = f.TON
PMAX >= Pavg/D =100/0.4 =250W
[7] When a SCR is in the forward blocking state, outer junctions (j1 & j3) are forward biased and middle junction(j2) is reverse biased
[15] Remember that the use of zener diode in a circuit is to provide voltage regulation ( constant voltage)

Power Electronics Multiple Choice Questions: Part 3

Multiple Choice Questions Based on Power Electronics and Drives

[1] When a line commutated converter operates in the inverter mode
(a) It draws both real and reactive power from the AC supply
(b) It delivers both real and reactive power to the AC supply
(c) It delivers real power to the AC supply
(d) It draws reactive power from the AC supply

Answer: C
[2] A chopper operating at a fixed frequency is feeding an RL load. As the duty ratio of the chopper is increased from 25% to 75%, the ripple in the load current is
(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: A
[3] To turn off a SCR, the reverse bias should be applied for a period ....... the turn-off time of the SCR
(a) Equal to 
(b) Longer than
(c) Less than
(d) Irrespective of

Answer: B
[4] In class A and class B commutation the resonating circuit has to be
(a) Over damped
(b) Critically damped
(c) Under damped
(d) Negatively damped

Answer: C
[5] In phase controlled rectification power factor (PF)
(a) Remains unaffected
(b) Improves with increase of firing angle α
(c) Deteriorates with increase of α
(d) Is unrelated to α

Answer: C
[6] Comparing with the full wave rectifier using two diodes, the four diode bridge rectifier has the dominant advantage of
(a) Higher current carrying 
(b) Lower peak inverse voltage requirement
(c) Lower ripple factor
(d) Higher efficiency

Answer: B
[7] A SCR is rated at 75A peak, 20A average. The greatest possible delay in the trigger angle if the dc is at rated value is
(a) 47.5°
(b) 30° to 45°
(c) 74.5°
(d) 137°

Answer: B
[8] The applied sine voltage to a SCR is VM=200V and R=10Ohm. If the gate trigger lags the ac supply by 120°, the average load current is
(a) 15/π A
(b) 5/π A
(c) -5/π A
(d) -15/π A

Answer: A
[9] A sine voltage of 200Vrms, 50Hz is applied to an SCR through 100ohm resistor. The firing angle is 60°. Consider no voltage drop. The output voltage in rms is
(a) 89.7 V
(b) 126.7 V
(c) 166.7 V
(d) 200√2 V

Answer: B
[10] A 100VDC is applied to the inductive load through a SCR. The SCR's specified latching current is 100mA. The minimum required width of gating pulse to turn on the SCR is
(a) 100μS
(b) 100S
(c) 1mS
(d) 50μS

Answer: A
[11] A cycloconverter is a
(a) Frequency changer from higher to lower frequency with one-state conversion
(b) Frequency changer from higher to lower frequency with two-stage conversion
(c) Frequency changer from lower to higher frequency with one-stage conversion
(d) Either a or c

Answer: D
[12] The cycloconverter require natural or forced commutation as under
(a) Natural commutationin bothstep-up and step down cycloconverter
(b) Forced commutation in both step-up and step-down cycloconverter
(c) Forced commutation in step-up cycloconverter
(d) Forced commutation in step-down cycloconverter

Answer: C
[13] In synchronized UJT triggering of an SCR, voltage VC across capacitor reaches UJT threshold thrice in each half cycle so that there are three firing pulses during each half cycle. The firing angle of the SCR can be controlled
(a) Once in each half cycle
(b) Thrice in each half cycle
(c) Twice in each half cycle
(d) Four times in each half cycle
(e) None of the above 

Answer: A
[14] In a GTO, anode current begins to fall when gate current
(a) Is negative peak at time t=0
(b) Is negative peak at t = storage period tS
(c) Just begins to become negative at t = 0
(d) Is negative peak at t = (tS + fall time)

Answer: B
[15] The SCR can be turned on by
(a) Applying anode voltage at a sufficiently fast rate
(b) Applying sufficiently large anode voltage
(c) Increasing the temperature of SCR to a sufficiently large value
(d) Applying sufficiently large gate current
OPTIONS:
1) A, B
2) C, D
3) B, C
4) A, B, C, D
5) None of the above options

Answer: 4

Solution Hint:

[2] VL= V[(Ton/(Ton+Toff)] 

        =V. f .Ton 
 There are two ways to vary VL. Either by varying f or Ton. Here f is fixed and Ton is     varied. As the frequency is kept constant ripple remains constant.

[6] The PIV of diodes used in the full wave rectifier using two diodes is 2 times that of  the four diode bridge rectifier

[7] RMS Current/ Average Current = Form Factor

[8] Idc = [Vm/2πR](1+cosα)

        α = delay angle

[15] a= dv/dt triggering, b = forward voltage triggering, c= temperature triggering, d = gate triggering. So all are correct

Power Electronics Objective Type Questions: Part-2

Power Electronics Multiple Choice Questions
In Last Post we shared Power Electronics MCQ Part 1[1] During forward blocking state, the SCR has
(a) Low current, medium voltage
(b) Low current, large voltage
(c) Medium current, large voltage
(d) Large current, low voltage

Answer: B
[2] Once SCR starts conducting a forward current, its gate loses control over
(a) Anode circuit current only
(b) Anode circuit voltage only
(c) Anode circuit voltage and current
(d) Anode circuit voltage, current and time

Answer: C
[3] In a SCRs
(a) Both Latching current and holding current are associated with turn-off process
(b) Latching current is associated with turn-off process and holding current with turn on process
(c) Holding current is associated with turn-off process and Latching current with turn-on process
(d) Both Latching current and holding current are associated with turn on process

Answer: C
[4] The SCR can be termed as
(a) DC switch
(b) AC switch
(c) Square-wave switch
(d) Either A or B

Answer: A
[5] In a thyristor, the magnitude of anode current will
(a) Increase if gate current is increased
(b) Increase if gate current is decreased
(c) Decrease if gate current is decreased
(d) Not change with any variation in gate current

Answer: D
[6] Turn on time of an SCR can be reduced by using a
(a) Rectangular pulse of high amplitude and narrow width
(b) Rectangular pulse of low amplitude and wide width
(c) Triangular pulse
(d) Trapezoidal pulse

Answer: A
[7] Turn off time of an SCR in series with RL circuit can be reduced by
(a) Increasing circuit resistance R
(b) Decreasing circuit resistance R
(c) Increasing circuit inductance L
(d) Decreasing circuit inductance L
Options:
1) B, C
2) A, D
3) B, D
4) D only

Answer: 4
[8] A forward voltage can be applied to an SCR after its
(a) Anode current reduces to zero
(b) Gate recovery time
(c) Reverse recovery time
(d) Anode voltage reduces to zero

Answer: B
[9] Turn-off time of an SCR is measured from the instant
(a) Anode current becomes zero
(b) Anode voltage becomes zero
(c) Anode voltage and anode current become zero at the same time
(d) Gate current becomes zero

Answer: A
[10] In an SCR, anode current flows over a narrow region near the gate during
(a) Delay time td
(b) Rise time tr and spread time tp
(c) td and tp
(d) td and tr

Answer: D
[11] Gate characteristic of a thyristor
(a) Is straight line passing through origin
(b) Is of the type Vg = a + bIg
(c) Is a curve between Vg and Ig
(d) Has a spread between two curves of Vg-Ig

Answer: D

[12] Surge current rating of an SCR specified the maximum
(a) Repetitive current with sine wave
(b) Non-repetitive current with rectangular wave
(c) Non-repetitive current with sine wave
(d) Repetitive current with rectangular wave

Answer: C
[13] The di/dt rating of an SCR is specified for its
(a) Decaying anode current
(b) Decaying gate current
(c) Rising gate current
(d) Rising anode current

Answer: D
[14] For an SCR, dv/dt protection is achieved through the use of
(a) RL in series with SCR
(b) RC across
(c) L in series with
(d) RC in series with

Answer: B
[15] For an SCR, di/dt protection is achieved through the use of
(a) R in series with SCR
(b) RL in series with SCR
(c) L in series with SCR
(d) L across SCR

Answer: C

Solution Hint:
[4] SCR will allow current flow in one direction only. So the ideal SCR is similar to DC switch
[7] By the decreasing the Inductor L value, the di/dt is increasing. ie, fast turn off process
[14] Snubber circuit ( RC circuit across device) is used for dv/dt protection
[15] Inductor will oppose sudden change in current (di/dt). For inductor, series resistance is not required. But in snubber circuit, R is mandatory for dissipating the stored energy in the capacitor.

Power Electronics Objective Type Questions with Answers Part-1

Power Electronics Objective Questions and Answers

[Q-1] When an inductance is inserted in the load circuit of SCR

(a) The turn on time of SCR is increased
(b) Output voltage is reduced for the same firing angle
(c) Conduction continues even after reversal of phase of input voltage
(d) A free wheeling diode is connected in such circuits
Which of the above statements are true?
(1) a, d
(2) b, c, d
(3) a, b, c,d
(4) c, d
Answer: 3

[Q-2] Snubber circuit is used to limit the rate of

(a) Rise of current

(b) Conduction period

(c) Rise of voltage across SCR

(d) None of the above

Answer:C

[Q-3] The provision of a free wheeling diode across an inductive load is

(a) To restore conduction angle on phase

(b) To avoid negative reversal voltage drop

(c) To reduce the PRV

(d) None of the above

Answer:A

[Q-4] While working in series operation, equalising circuits are added across each SCR to provide uniform

(a) Current distribution

(b) Firing of SCRs

(c) Voltage distribution

(d) None of the above

Answer:C

[Q-5] When the SCR conducts, the forward voltage drop

(a) Is 0.7V

(b) Is 1 to 1.5V

(c) Increases slightly with load current

(d) Remains constant with load current

Which of the above statements are true?

(1) a only

(2) b, c

(3) d only

(4) a, c

Answer: 2

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[Q-6] The latching current of a SCR is 18mA. Its holding current will be

(a) 6mA

(b) 18mA

(c) 54mA

(d) 12mA

Answer:A

Exp: Latching Current = 3 x Holding Current

[Q-7] The turn off time is longer than turn on time because

(a) The anode and cathode junctions get reverse biased while gate junction is still forward biased

(b) There is flow of reverse current

(c) The gate pulse has been removed

(d) The forward break over voltage is high.

Which of the above statements are true?

(1) b, c

(2) c, d

(3) a, b

(4) a, b, c, d

Answer: 3

[Q-8] The thyristor will turn on faster with

(a) Pulse signal applied to the gate terminal of the SCR

(b) Continuous signal applied to the gate terminal of the SCR

(c) Both are same

(d) Pulse signal but with minimum duration

Which of the above statements are true?

(1) a only

(2) b only

(3) a, d

(4) none of the above

Answer: 3

[Q-9] In an SCR,

(a) The holding current is less than latching current

(b) The holding current is greater than latching current

(c) The two currents are equal

(d) The latching current is about 3 times the holding current

Which of the above statements are true?

(1) a only

(2) d only

(3) a, c, d

(4) a, d

(5) b only

Answer: 4

[Q-10] When a positive voltage is applied to the gate of a reverse biased SCR

(a) It injects more electrons into junction J1

(b) It increases reverse leakage current into anode

(c) Heating of junction is unaffected

(d) Failure of junctions occurs due to thermal runaway

Which of the above statements are true?

(1) b only

(2) a, b ,d

(3) b, d

(4) d only

Answer: 3

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