ECE - Power Electronics Quiz (or) MCQ

1 .

Power diode is

A)

Two terminal semiconductor device

B)

Three terminal semiconductor device

C)

Four terminal semiconductor device

D)

Five terminal semiconductor device

Correct Answer : Two terminal semiconductor device

Explanation : A power diode refers to a semiconductor device that is used to convert alternating current to direct current. It consists of two terminals, cathode and anode, the same as that of a normal diode. A power diode is nothing but a signal diode with an extra layer.

2 .

The V-I characteristics of diode lie in the

A)

First quadrant

B)

Fourth quadrant

C)

The third and second quadrant

D)

The first and third quadrant

Correct Answer : The first and third quadrant

Explanation : V- I characteristics of diode lies in the first and third quadrant. The first quadrant works in the forward region, and the third quadrant works in reverse biased mode.

3 .

The p-region has a greater concentration of __________ as compared to the n-region in a P-N junction.

A)

electrons

B)

holes

C)

phonons

D)

Both holes & electrons

Correct Answer : holes

Explanation : Holes are the majority charge carriers in p-type material.

4 .

A cycloconverter can be

A)

step up

B)

step down

C)

step down or step up

D)

None of the above

Correct Answer : step down or step up

5 .

In a 3 phase fully controlled converter the firing frequency is

A)

3 times the line frequency

B)

6 times the line frequency

C)

9 times the line frequency

D)

12 times the line frequency

Correct Answer : 6 times the line frequency

6 .

Two thyristor of same rating and same specifications

A)

will have equal turn on and turn off periods

B)

will have unequal turn on and turn off periods

C)

will have equal turn on but unequal turn off periods

D)

may have equal or unequal turn on and turn off periods

Correct Answer : may have equal or unequal turn on and turn off periods

Explanation : Turn on and turn off times of similar SCRs may not be same.

7 .

A p-type semiconductor material is doped with ____________ impurities whereas a n-type semiconductor material is doped with __________ impurities

A)

acceptor, donor

B)

donor, donor

C)

donor, acceptor

D)

acceptor, acceptor

Correct Answer : acceptor, donor

Explanation : Donor impurities denote an electron to the n-type material making it a electron majority carrier & vice-versa.

8 .

A snubber circuit is used to

A)

Limit the rate of rising in current across BJT

B)

Limit the rate of rising in voltage across BJT

C)

Limit the rate of rising in current across TRIAC

D)

Limit the rate of rising in voltage across SCR

Correct Answer : Limit the rate of rising in voltage across SCR

Explanation : Snubber circuit refers to a kind of DV/dt protection circuit of the thyristor. It is used to limit the high rate of change of voltage from cathode to anode. It is usually used to protect SCR thyristor from high DV/dt and di/dt stress.

9 .

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

A)

83.88

B)

84.52

C)

87.62

D)

89.59

Correct Answer : 84.52

Explanation :

10 .

In the forward blocking mode of a silicon controlled rectifier, the SCR is

A)

In off state

B)

In on state

C)

In natural state

D)

Forward biased state

Correct Answer : In off state

Explanation : The forward blocking mode of a silicon-controlled rectifier refers to a mode when the anode is made positive with respect to the cathode. It is also called off state.

11 .

A semiconverter feeding an RLE load operates in

A)

first and third quadrants

B)

first and second quadrants

C)

first quadrant only

D)

None of the above

Correct Answer : first quadrant only

12 .

A dc circuit breaker must use

A)

forced commutation

B)

natural commutation

C)

both natural and forced commutation

D)

either natural or forced commutation

Correct Answer : forced commutation

13 .

An RC snubber circuit is used to protect a thyristor against :

A)

failure to turn on

B)

false triggering

C)

failure to commutate

D)

switching transients

Correct Answer : switching transients

14 .

A single phase semiconverter has a freewheeling diode. If the firing angle is a and the load is purely resistive, the periods of conduction and freewheeling respectively are

A)

Ï€ - Î± and 0

B)

Ï€ - Î± and Î±

C)

Ï€ + Î± and 0

D)

Ï€ + Î± and Î±

Correct Answer : Ï€ - Î± and 0

Explanation : Freewheeling diode conducts only when load is inductive. If load is purely resistive free wheeling diode does not come into operation.

15 .

In the p & n regions of the p-n junction the _________ & the ___________ are the majority charge carriers respectively.

A)

holes, holes

B)

holes, electrons

C)

electrons, holes

D)

electrons, electrons

Correct Answer : holes, electrons

Explanation : Holes are the majority charge carriers in p-type material & vice-versa.

16 .

Which of the below mentioned statements is false regarding a p-n junction diode?

A)

Diodes are rectifying devices

B)

Diode are uncontrolled devices

C)

Diodes have three terminals

D)

Diodes are unidirectional devices

Correct Answer : Diodes have three terminals

Explanation : Diode is a two terminal device, anode & cathode are the two terminals.

17 .

Which of the following is true in case of a forward biased p-n junction diode?

A)

The negative terminal of the battery sucks electrons from the p-region

B)

The positive terminal of the battery injects electrons into the p-region

C)

The positive terminal of the battery sucks electrons from the p-region

D)

None of the above mentioned statements are true

Correct Answer : The positive terminal of the battery sucks electrons from the p-region

Explanation : The diode is forward biased, positive is connected to p & vice-versa, as such batter provides EMF to drive electrons from n-region to p-region.

18 .

Which of the following is true in case of an unbiased p-n junction diode?

A)

Diffusion does not take place

B)

Diffusion of electrons & holes goes on infinitely

C)

There is zero electrical potential across the junctions

D)

Charges establish an electric field across the junctions

Correct Answer : Charges establish an electric field across the junctions

Explanation : A potential difference is established across the junctions due to recombination of holes & electrons. This growing filed (barrier potential) stops the further diffusion.

19 .

A silicon controlled rectifier is turned on if the anode current is greater than

A)

Cathode current

B)

Anode current

C)

Trigger current

D)

Holding current

Correct Answer : Cathode current

Explanation : SCR stands for Silicon Controlled Rectifier. Once the SCR is turned on, it will remain in the same conducting state until the anode current becomes lower than the holding current.

20 .

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?

A)

5 A

B)

10 A

C)

20 A

D)

30 A

Correct Answer : 20 A

Explanation : Once the SCR gets active or in on state, the gate current has no control over it. So the value of current remains unchanged.

21 .

A silicon controlled rectifier is turned off mode when its turn off time is

A)

Greater than the circuit turn off time

B)

Less than the circuit turn off time

C)

Equal to the circuit turn off time

D)

None of the above

Correct Answer : Less than the circuit turn off time

Explanation : The term turns off means that it has changed its form from on to off state and capable of blocking the forward voltage.

22 .

A single phase full wave regulator feeds R-L load. The best gating signal is

A)

pulse train

B)

long duration pulses

C)

short duration pulses

D)

Either (A) or (B)

Correct Answer : pulse train

23 .

A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if

A)

load torque is constant

B)

load torque decreases with increase in speed

C)

load torque increases with increase in speed

D)

Either (A) or (B)

Correct Answer : load torque increases with increase in speed

24 .

A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20Â°C. If thermal resistance is 1.6Â° C/W, the junction temperature is

A)

164Â°C

B)

114Â°C

C)

94Â° C

D)

84Â° C

Correct Answer : 84Â° C

Explanation : 1.6 = T-20/40 or T=84â°C

25 .

The number of gates in an SCR is

A)

1

B)

3

C)

4

D)

6

Correct Answer : 1

26 .

A 1 - âˆ… 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

A)

0.25 V

B)

0.45 VM

C)

0.65 VM

D)

0.85 VM

Correct Answer : 0.45 VM

Explanation : We know that for a single-phase full-wave rectifier

27 .

An ideal power diode must have

A)

high reverse recovery time

B)

high ohmic junction resistance

C)

large reverse breakdown voltage

D)

low forward current carrying capacity

Correct Answer : large reverse breakdown voltage

Explanation : Large reverse breakdown voltage is desirable whereas others will increases the losses.

28 .

To make a signal diode suitable for high current & high voltage carrying applications with minimum losses, ________

A)

a lightly doped n layer is grown between the two p & n layers

B)

a heavily doped n layer is grown between the two p & n layers

C)

a lightly doped p layer is grown between the two p & n layers

D)

a heavily doped p layer is grown between the two p & n layers

Correct Answer : a lightly doped n layer is grown between the two p & n layers

Explanation : The above process simply the one used to manufacture power diodes.

29 .

The V-I Characteristics of the diode lie in the

A)

Only in the 1st quadrant

B)

1st & 2nd quadrant

C)

1st & 4th quadrant

D)

1st & 3rd quadrant

Correct Answer : 1st & 3rd quadrant

Explanation : First in the forward region & Third in the reverse biased mode.

30 .

A diode is said to be reversed biased when the

A)

anode is positive with respect to the cathode

B)

cathode is positive with respect to the anode

C)

cathode is negative with respect to the anode

D)

both cathode & anode are negative

Correct Answer : cathode is positive with respect to the anode

Explanation : K is positive w.r.t the A when the device is reversed biased.

31 .

The peak inverse current IP for a power diode is given by the expression

A)

Ip=t + di/dt

B)

Ip=t * log â¡i

C)

Ip=t * di/dt

D)

Ip=t * âˆ« t*i dt

Correct Answer : Ip=t * di/dt

Explanation : The leakage current is the reveres recovery time (t) into the rate of change of current.

32 .

A diode is said to be forward biased when the

A)

anode is positive with respect to the cathode

B)

anode is negative with respect to the anode

C)

cathode is positive with respect to the anode

D)

both cathode & anode are positive

Correct Answer : anode is positive with respect to the cathode

Explanation : A is positive w.r.t the K when the device is forward biased.

33 .

A power diode with small softness factor (S-factor) has

A)

small peak reverse current

B)

large peak reverse current

C)

large oscillatory over voltages

D)

small oscillatory over voltages

Correct Answer : large oscillatory over voltages

Explanation : Peak reverse current is independent of S-factor smaller the value of S-factor larger the oscillatory over voltage.

34 .

In order to reduce the reverse recovery time of the diodes, __________ is carried out.

A)

antimony doping

B)

adding an extra silicon layer

C)

shortening of the length of the device

D)

platinum & gold doping

Correct Answer : platinum & gold doping

Explanation : Platinum & gold doping improves the performance of the devices.

35 .

A Schottky diode can be switchd off much faster than an equivalent p-n junction diode due to its

A)

higher operating frequency

B)

no recombination of charges

C)

more compact structure

D)

None of the above

Correct Answer : no recombination of charges

Explanation : Due to the metal-silicon junction there are no stored charges, hence no reverse recovery time due to which the switching is faster.

36 .

As compared to a p-n junction diode(of the same rating), a Schottky diode has ___________

A)

higher cut-in voltage

B)

higher operating frequency

C)

higher switching time

D)

lower reverse leakage current

Correct Answer : higher operating frequency

Explanation : Due to the metal-silicon junction there are no stored charges, hence no reverse recovery time due to which the switching is faster.

37 .

In a Schottky diode, the silcon is usually

A)

N-type

B)

P-type

C)

silicon is not used

D)

un-doped semiconductor

Correct Answer : N-type

Explanation : Usually only n-type silicon is used because the p-type has certain limitations.

38 .

Which of the below mentioned statements is false regarding Schottky diodes?

A)

Schottky diodes have a Al-Silicon junction

B)

There is no storage of charges in a Schottky diode

C)

The majority charge carriers in a Schottky diode are holes

D)

Schottky diodes can be switched off faster than a p-n junction diode of the same rating

Correct Answer : The majority charge carriers in a Schottky diode are holes

Explanation : The majority charge carriers in a Schottky diode are electrons not holes.

39 .

A Schottky diode _____

A)

has zero cut-in voltage

B)

has current flow due to holes only

C)

has large amount of storage charges

D)

has no reverse recovery time

Correct Answer : has no reverse recovery time

Explanation : Due to the metal-silicon junction there are no stored charges hence, no reverse recovery time, due to which the switching is faster.

40 .

Which of the following are/is the majority charge carriers in a Schottky diode?

A)

Holes

B)

Electrons

C)

Both holes & Electrons carry equal current

D)

None of the above

Correct Answer : Electrons

Explanation : The metal has no holes hence major(almost full) current flows due to the electrons only.

41 .

If V & I are the forward voltage & current respectively, then the power loss across the diode would be

A)

VI

B)

V/I

C)

IÂ² V

D)

VÂ² IÂ²

Correct Answer : VI

Explanation : Simply power (P) is voltage into current i.e. VI

42 .

The power loss in which of the following cases would be the maximum?

A)

When only I is maximum

B)

When both V & I are maximum

C)

When only V is maximum

D)

When both V & I are minimum

Correct Answer : When both V & I are maximum

Explanation : P=VI Hence, it would be maximum when both V and I are maximum.

43 .

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

A)

118â°

B)

128â°

C)

138â°

D)

148â°

Correct Answer : 138â°

Explaination : EMF of a solar cell = 450 V

DC current I_dc = 10 A

Resistance of solar cell, R_cell = 1 ?

We know that, Voltage across the inverter = output voltage of the solar cell,

Vout = -(450 - 10 × 1) = -440 V

For three phase controlled bridge

44 .

Which of the given device is the most suitable power device for a higher frequency (above 100 kHz) switching application

A)

BJT

B)

SCR

C)

GTO

D)

Power MOSFET

Correct Answer : Power MOSFET

Explanation : Most suitable power device for higher frequency (above 100 kHz) switching application is power MOSFET because it has lower switching losses. So, for lower frequency switching applications, BJT is used.

45 .

Which of the given device is a modern semiconductor device that combines the characteristics of MOSFET and BJT?

A)

IGBT

B)

SCR

C)

Schottky diode

D)

Microwave transistor

Correct Answer : IGBT

Explanation : IGBT stands for Insulated Gate Bipolar Transistor. It includes the best features of power MOSFETS and power transistors(BJT). Same as a MOSFET, it has low input capacitance and high input impedance. In one state, it has low resistance and high current controlling capacity like a BJT.

46 .

The thyristor is equivalent to a thyratron tube is

A)

BJT

B)

SCR

C)

GTO

D)

TRIAC

Correct Answer : SCR

Explanation : The thyristor is known as SCR because it is a silicon device used as a rectifier and that rectification can be controlled. It is made up of silicon only because the leakage current in silicon is less than that of germanium. If any device is used as a switch, it is necessary that leakage current must be minimum.

47 .

Even after the forward current reduces to zero value, a practical diode continues to conduct in the reverse direction for a while due to the

A)

resistance of the diode

B)

high junction temperature

C)

Both (A) and (B)

D)

stored charges in the depletion region

Correct Answer : stored charges in the depletion region

Explanation : Due to the stored charges during the earlier current flow, even when the current reduces to zero due to some structural properties of the device, the device takes time to sweep out the stored charges.

48 .

For a p-n junction diode, the peak inverse current & the reverse recovery time are dependent on

A)

di/dt

B)

inverse voltage

C)

forward Voltage

D)

All of the above

Correct Answer : di/dt

Explanation : It only depends upon the number stored charges which depends upon the rate of change of current.

49 .

When the p-n junction diode is forward biased, the width of the depletion region __________

A)

increases

B)

decreases

C)

remains Constant

D)

increases than Decreases

Correct Answer : decreases

Explanation : When forward biased depletion layer decreases & finally it collapses to allow the current flow.

50 .

When the p-n junction diode is reversed biased, the width of the depletion region __________

A)

increases

B)

decreases

C)

remains Constant

D)

None of the above

Correct Answer : increases

51 .

In the equilibrium state the barrier, potential across a unbiased germanium diode is __________

A)

1.7 V

B)

0.7 V

C)

0.3 V

D)

0 V

Correct Answer : 0.3 V

Explanation : Barrier potential is due to the charges that establish an electric field across the two junctions.

52 .

A power transistor is a

A)

two layer, one junction device

B)

four layer, three junction device

C)

three layer, three junction device

D)

three layer, two junction device

Correct Answer : three layer, two junction device

Explanation : It has three layers p-n-p or n-p-n forming two p-n junctions.

53 .

In a power transistor, ____ is the controlled parameter.

A)

I_C

B)

I_B

C)

V_CE

D)

V_BE

Correct Answer : I_C

Explaination : The collector current is the controlled parameter.

54 .

The MOSFET combines the areas of _______ & _________

A)

semiconductor & TTL

B)

mos technology & CMOS technology

C)

field effect & MOS technology

D)

None of the above

Correct Answer : field effect & MOS technology

Explanation : It is an enhancement of the FET devices (field effect) using MOS technology.

55 .

Which of the following terminals does not belong to the MOSFET?

A)

Gate

B)

Base

C)

Drain

D)

Source

Correct Answer : Base

Explanation : MOSFET is a three terminal device D, G & S.

56 .

Choose the correct statement

A)

MOSFET is a voltage controlled device

B)

MOSFET is a uncontrolled device

C)

MOSFET is a current controlled device

D)

MOSFET is a temperature controlled device

Correct Answer : MOSFET is a voltage controlled device

Explanation : It is a voltage controlled device.

57 .

IGBT possess

A)

low input impedance

B)

high input impedance

C)

high on-state resistance

D)

second breakdown problems

Correct Answer : high input impedance

Explanation : Like MOSFET IGBT possess high input impedance.

58 .

IGBT & BJT both posses _______

A)

high input impedance

B)

high on-state power losses

C)

low switching losses

D)

low on-state power losses

Correct Answer : low on-state power losses

Explanation : Low on state power loss is one of the best parameters of both BJT & the IGBT.

59 .

The three terminals of the IGBT are

A)

base, source & drain

B)

gate, source & drain

C)

gate, emitter & collector

D)

base, emitter & collector

Correct Answer : gate, emitter & collector

Explanation : IGBT is a three terminal device. It has a gate, a emitter & a collector.

60 .

For a transistor, the safe operating area (SOA) is a plot of

A)

Ib versus Ic

B)

Ib versus Vce

C)

Ic versus Vce

D)

Ic versus time

Correct Answer : Ic versus Vce

Explanation : For reliable operation the collector current & voltage must remain within the SOA curves.

61 .

The forward safe operating area (FSOA) pertains to the operation when

A)

the device is forward-biased

B)

the device is operated on AC

C)

the device is operated on DC

D)

the device is fired at a 50% Duty cycle

Correct Answer : the device is forward-biased

Explanation : The FSOA is for forward biased operations. The FSOA is plotted for AC as well as DC for different duty cycles. Hence, option (b) is the most appropriate choice.

62 .

The SOAs are plotted always on a _________ scale

A)

time

B)

logarithmic

C)

frequency

D)

polynomial

Correct Answer : logarithmic

Explanation : The scale is always logarithmic, irrespective of the type of device.

63 .

The SOAs provided by the manufacturers are for

A)

a particular duty cycle operation

B)

multi pulse operation & all the temperature

C)

All the conditions

D)

single pulse operation & a particular temperature

Correct Answer : single pulse operation & a particular temperature

Explanation : The manufacturer specifies the SOAs only for single pulse DC operation & a particular temperature (usually 20Degree Centigrade Scale). For actual operations, The SOA’s have to be modified using the thermal impedance charts.

64 .

For MOSFETâ€™s SOA, as the pulse width goes on increasing, the maximum voltage rating ____ & current rating ____

A)

decreases, is constant

B)

constant, decreases

C)

increases, decreases

D)

is constant, increases

Correct Answer : decreases, is constant

65 .

The SOA for an IGBT is plotted for

A)

Ig versus Ic

B)

Ic versus Vge

C)

Ic versus Vce

D)

Ig versus Vce

Correct Answer : Ic versus Vce

Explanation : It is a plot of collector current vs collector to emitter voltage.

66 .

For a BJT, find the maximum power dissipation when the device is safely operated at Vce = 90V and Ic = 0.5A

A)

30 Watts

B)

35 Watts

C)

45 Watts

D)

None of the above

Correct Answer : 45 Watts

Explanation : P=90*0.5=45Watts.

67 .

A device is operating at Ic = 4A & Vce = 50V. For the device to operate at Ic = 20A (Without damaging),

A)

voltage should be increased

B)

voltage should be reduced

C)

voltage can be kept constant

D)

current has to increased further

Correct Answer : voltage should be reduced

Explanation : For safe operation, the values should be within the limits. P = V.I – with increase in one of the values, another value should decrease.

68 .

A thyristor (SCR) is a_____

A)

P-N device

B)

P-N-P device

C)

N-P-N device

D)

P-N-P-N device

Correct Answer : P-N-P-N device

Explanation : An SCR (silicon controlled rectifier) is a four layer p-n-p-n type device.

69 .

For an SCR in the forward blocking mode (practically)

A)

leakage current flows from anode to cathode

B)

leakage current does not flow

C)

leakage current flows from gate to anode

D)

leakage current flows from cathode to anode

Correct Answer : leakage current flows from anode to cathode

Explanation : In the forward blocking mode, the gate current is zero & only the middle J2 junction is reversed biased.

70 .

With the anode positive with respect to the cathode & the gate circuit open, the SCR is said to be in the

A)

reverse blocking mode

B)

forward blocking mode

C)

reverse conduction mode

D)

forward conduction mode

Correct Answer : forward blocking mode

Explanation : The SCR is in the forward blocking mode with its top and bottom junctions forward biased and the middle junction reversed biased.

71 .

Which terminal does not belong to the SCR?

A)

Gate

B)

Anode

C)

Base

D)

Cathode

Correct Answer : Base

Explanation : The SCR is having three terminals viz. anode, cathode and the gate.

72 .

An SCR is a______

A)

three layer, single junction device

B)

four layer, two junction device

C)

four layer, four junction device

D)

four layer, three junction device

Correct Answer : four layer, three junction device

Explanation : SCR is a four layer p-n-p-n device which forms three p-n junctions.

73 .

In the SCR structure the gate terminal is located

A)

near the anode terminal

B)

in between the anode & cathode terminal

C)

near the cathode terminal

D)

None of the above

Correct Answer : near the cathode terminal

Explanation : The gate is located near the cathode, because it allows fast turning on of the device when the gate signal is applied by forward basing the second junction.

74 .

The forward dv/dt rating of an SCR

A)

increases with increase in the junction temperature

B)

decreases with increase in the junction temperature

C)

increases with decrease in the rms value of forward anode-cathode voltage

D)

decreases with decrease in the rms value of forward anode-cathode voltage

Correct Answer : increases with increase in the junction temperature

Explanation : If the temperature is high, lesser dv/dt is required to turn on the device as the higher temperature has already excited few of the holes & electrons.

75 .

The finger voltage of an SCR is__________.

A)

maximum value of Vak to turn on the device with gate triggering

B)

minimum value of Vak to turn on the device with gate triggering

C)

minimum value of Vak to turn on the device without gate triggering

D)

maximum value of Vak to turn on the device without gate triggering

Correct Answer : minimum value of Vak to turn on the device with gate triggering

Explanation : Finger voltage is the minimum value of Vak (anode to cathode voltage) to turn on the device with gate triggering, it is to be avoided for accidental turn-on of the device.

76 .

An SCR has half cycle surge current rating of 3000 A for 50 Hz. Calculate its one-cycle surge current rating

A)

2131.32 A

B)

2121.32 A

C)

3121.32 A

D)

3131.32 A

Correct Answer : 2121.32 A

Explanation : By equating the energies involved in one cycle & subcycle
I2.T = Isb2.t
Isb = 3000 A
T = 1/50
t = T/2.

77 .

For an SCR the average & rms values of current are I/4 & I/2 respectively. Calculate the average on-state current rating (ITAV). Take maximum RMS on-state current = 35 A.

A)

24.74 A

B)

10.10 A

C)

8.78 A

D)

17.5 A

Correct Answer : 17.5 A

Explaination : Form factor = RMS/Average vaules of current.

Therefore, FF = 2

ITAV = 35/2 = 17.5.

78 .

The ampÂ²-sec rating of the SCR specifies

A)

The power dissipated by the device when fault occurs

B)

The energy dissipated by the device when fault occurs

C)

The energy that the device can absorb before the fault is cleared

D)

The energy that the device can absorb while operating in the forward blocking mode.

Correct Answer : The energy that the device can absorb before the fault is cleared

Explanation : The amp²-sec is the energy that the device can handle before the fault is cleared. Its value decides how fast the fault has to be cleared to avoid damage to the device.

79 .

The usual way to accomplish higher gate current for improved di/dt rating is by using

A)

varistors

B)

op-amps

C)

twisted cables

D)

pilot thyristors

Correct Answer : pilot thyristors

Explanation : Pilot SCR is an SCR which is fired which activates the firing circuit and fires the main SCR.

80 .

Inter-digitating of gate-cathode regions in SCR devices improves the

A)

di/dt rating

B)

IÂ²t rating

C)

dv/dt rating

D)

thermal resistance

Correct Answer : di/dt rating

Explanation : Inter-digitating is the inter-mixing of the gate-cathode area to improve the di/dt ratings. di/dt rating is improved by providing more cathode conduction area during the delay and rise time.

81 .

The dv/dt rating of SCR can be improved by using

A)

cathode-short structure

B)

gate-short structure

C)

centre gate thyristor

D)

anode-short structure

Correct Answer : cathode-short structure

Explanation : Cathode shorts are realized by overlapping metal on cathode n+ layers with a narrow p-region in between.

82 .

The total thermal resistance between junction and ambient θjA is 10°C/W. θjc is 2°C/W. θcs is 4°C/W. θsA = ?

A)

2°C/W

B)

4°C/W

C)

10°C/W

D)

16°C/W

Correct Answer : 4°C/W

Explaination : θjA = θjc+ θcs+ θsA.

83 .

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

A)

50 Hz

B)

100 Hz

C)

220 Hz

D)

300 Hz

Correct Answer : 100 Hz

Explaination : The formula to caculate the even harmonics are given as

= 2f_S= 2 × 50 = 100 Hz

84 .

A freewheeling diode is phase-controlled rectifiers.

A)

Stops rectifier operations

B)

Improves line power factor

C)

Is the reason for additional harmonics

D)

Is the reason for the sudden breakdown

Correct Answer : Improves line power factor

Explanation : Freewheeling diode is also called bypass diode or commutating diode. It is used to improve the waveform of the loaded current and power factor. It is connected across a realy, which is driven by a transistor. It enables for dissipation of energy stored in the inductance of the load in the circuit and provides a continuous flow of load current when the thyristor is blocked.

85 .

In a three-phase bridge rectifier, the maximum conduction of each thyristor is

A)

30â°

B)

60â°

C)

90â°

D)

120â°

Correct Answer : 120â°

Explanation : In a three-phase bridge rectifier, the maximum conduction of each thyristor is 120â°

86 .

In a 3-âˆ… controlled bridge rectifier, the frequency of ripple in the output voltage depends on

A)

Firing angle

B)

Power factor

C)

Supply frequency

D)

Voltage source

Correct Answer : Supply frequency

Explanation : In a three-phase controlled bridge rectifier, the ripple frequency in the output voltage depends on supply frequency. The frequency of ripple in the output voltage

Fout = total number of pulses × supply frequency

In a 3-Ø controlled bridge rectifier, the ripple frequency in the output voltage is 6 times the supply frequency.

87 .

A three-phase full converter is supplying a purely resistive load at 220 V dc for 00 firing angle, find the output voltage for 900 firing angle

A)

0 V

B)

30 V

C)

90 V

D)

120 V

Correct Answer : 0 V

Explanation : We know that,

88 .

What is the name of a converter which can work on both in 3-pulse and 6 pulse modes?

A)

Single-phase semi converter

B)

Three-phase semi converter

C)

Three-phase half wave converter

D)

Three-phase full-wave converter

Correct Answer : Three-phase semi converter

Explanation : A three-phase converter has an additional feature that it works as a 6 pulse converter for firing angle α < π/3 Power Electronics MCQand as a three pulse converter for firing angle α ≥ π/3 Power

89 .

A power transistor is a _________ device.

A)

two terminal, bipolar, voltage controlled

B)

two terminal, unipolar, current controlled

C)

three terminal, unipolar, voltage controlled

D)

three terminal, bipolar, current controlled

Correct Answer : three terminal, bipolar, current controlled

Explanation : Power transistor is simply many BJT’s connected in series parallel on a single silicon chip for power applications. It is a three terminal, bipolar, current controlled device.

90 .

A three phase full controlled converter can operate as a

A)

Converter for Î± = 0 to 60â°

B)

Converter for Î± = 0 to 90â°

C)

Converter for Î± = 0 to 120â°

D)

Converter for Î± = 0 to 180â°

Correct Answer : Converter for Î± = 0 to 180â°

Explanation : As we know, a three-phase full controlled converter operates in only two quadrants (First and Fourth).

91 .

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 = 15â° and firing angle α= 60,â°, find the rating of shunt compensator and power factor.

A)

0.5

B)

1

C)

1.5

D)

2

Correct Answer : 0.5

Explaination : V = 210

I = 90 A

α= 60â°

We know that, Shunt compensation

= VI tan alpha;

Putting the value V , I, and αin above equation we get

= 210 × 90 tan 60â°

= 72, 735 VAR = 72.8 KVAR

And power factor =

Cos α= Cos 60â° = 0.5

92 .

A chopper converts

A)

AC to DC

B)

AC to AC

C)

DC to DC

D)

DC to AC

Correct Answer : DC to DC

Explanation : Choppers refer to a static switch used to provide Variable DC voltage from a constant DC voltage source. It is a DC to DC converters. It can be either step up or step down DC to DC converter. In Step up DC converter, the output voltage is less than the input voltage. It is also known as Buck converter. In step-up, DC converter, which is also called Boost Converter, is opposite to buck converter. There is also a converter based on the combination of these two; it works in both buck and boost mode depending upon operating conditions; this type of chopper is called Buck-Boost Converter.

93 .

Which of the given regulator provide output voltage polarity reversal without the involvement of a transformer.

A)

Linear voltage regulator

B)

Buck-Boost regulator

C)

Shunt voltage regulator

D)

Series voltage regulator

Correct Answer : Buck-Boost regulator

Explanation : Buck-boost regulator provides output voltage polarity reversal without a transformer. The buck-boost converter refers to a voltage regulator used to regulate dc power supplies. A negative polarity output may be required with respect to the same terminal of the input voltage. The output voltage can be either more or less than the input voltage.

94 .

In a power transistor, the I_B vs V_BE curve is

A)

a parabolic curve

B)

resembling the diode curve

C)

a straight line Y = I_B

D)

an exponentially decaying curve

Correct Answer : resembling the diode curve

Explaination : The B-E junction of a BJT resembles a p-n junction diode, hence the curve.

95 .

The arrow on the symbol of MOSFET indicates

A)

the direction of electrons

B)

that it is a N-channel MOSFET

C)

that it is a P-channel MOSFET

D)

the direction of conventional current flow

Correct Answer : the direction of electrons

Explanation : The arrow is to indicate the direction of electrons (opposite to the direction of conventional current flow).

96 .

In the internal structure of a MOSFET, a parasitic BJT exists between the

A)

drain & gate terminals

B)

source & drain terminals

C)

source & gate terminals

D)

there is no parasitic BJT in MOSFET

Correct Answer : source & drain terminals

Explanation : Examine the internal structure of a MOSFET, notice the n-p-n structure between the drain & source. A p-channel MOSFET will have a p-n-p structure.

97 .

In the transfer characteristics of a MOSFET, the threshold voltage is the measure of the

A)

minimum voltage to turn off the device

B)

minimum voltage till which temperature is constant

C)

Both (A) and (B)

D)

minimum voltage to induce a n-channel/p-channel for conduction

Correct Answer : minimum voltage to induce a n-channel/p-channel for conduction

Explanation : It is the minimum voltage to induce a n-channel/p-channel which will allow the device to conduct electrically through its length.

98 .

The output characteristics of a MOSFET, is a plot of

A)

Id as a function of Vgs with Vds as a parameter

B)

Ig as a function of Vgs with Vds as a parameter

C)

Id as a function of Vds with Vgs as a parameter

D)

Ig as a function of Vds with Vgs as a parameter

Correct Answer : Id as a function of Vds with Vgs as a parameter

Explanation : It is Id vs Vds which are plotted for different values of Vgs (gate to source voltage).

99 .

In IGBT, the nâ» layer above the pâº layer is called as the

A)

body layer

B)

injection layer

C)

drift layer

D)

collector Layer

Correct Answer : drift layer

Explanation : It is called as the drift layer because its thickness determines the voltage blocking capabilities of the device.

100 .

The voltage blocking capability of the IGBT is determined by the

A)

drift layer

B)

body layer

C)

injection layer

D)

metal used for the contacts

Correct Answer : drift layer

Explanation : The drift layer which is a n– layer determines the voltage blocking capabilities.

101 .

The structure of the IGBT is a

A)

P-N-P structure connected by a MOS gate

B)

P-N-P-N structure connected by a MOS gate

C)

N-N-P-P structure connected by a MOS gate

D)

N-P-N-P structure connected by a MOS gate

Correct Answer : P-N-P-N structure connected by a MOS gate

Explanation : The IGBT is a semiconductor device with four alternating layers (P-N-P-N) that are controlled by a metal-oxide-semiconductor (MOS) gate structure without regenerative action.

102 .

The major drawback of the first generation IGBTs was that, they had

A)

sluggish operation

B)

latch-up problems

C)

noise & secondary breakdown problems

D)

latch-up & secondary breakdown problems

Correct Answer : latch-up & secondary breakdown problems

Explanation : The earlier IGBT’s had latch-up problems (device cannot turn off even after the gate signal is removed), and secondary breakdown problems (in which a localized hotspot in the device goes into thermal runaway and burns the device out at high currents).

103 .

The SCS (Silicon Controlled Switch) is a

A)

two terminal device

B)

three terminal device

C)

four terminal device

D)

five terminal device

Correct Answer : four terminal device

Explanation : The SCS is a four terminal device A,K,KG & AG.

104 .

The SCS is a four layer, four terminal thyristor. Can be turned on by

A)

the anode gate

B)

the cathode gate

C)

either of the gates

D)

gating both the gates together

Correct Answer : either of the gates

Explanation : The SCS has two gates, anode-gate and cathode-gate. Either of the gates could be used to turn on the device.

105 .

The DIAC can be represented by

A)

two diodes in anti-parallel

B)

two SCRs in parallel

C)

two diodes in parallel

D)

two SCRs in anti-parallel

Correct Answer : two diodes in anti-parallel

Explanation : The DIAC is nothing but a bi-directional diode.

106 .

The sink to ambient thermal resistance of SCR θ_sA

A)

depends on the length of the device

B)

is independent on thyristor configuration

C)

depends on the flatness of the device

D)

depends on the current carrying capabilities

Correct Answer : is independent on thyristor configuration

Explaination : It does not depend on any of the device configurations.

107 .

In the ___________ type of mounting the SCR is pressed between two heat sinks

A)

stud-mounting

B)

press-pak mounting

C)

bolt-down mounting

D)

cross-fit mounting

Correct Answer : press-pak mounting

Explanation : In the press-pak type the device is pressed or clamped between two heat-sinks & external pressure is applied from both the sides.

108 .

For a power transistor, if the base current IB is increased keeping V_CE constant, then

A)

I_C increases

B)

I_C decreases

C)

I_C remains constant

D)

None of the above

Correct Answer : I_C increases

Explaination : Ic is directly proportional to Ic.

109 .

The forward current gain α is given by

A)

I_C/I_B

B)

I_C/I_E

C)

I_E/I_C

D)

I_E/I_B

Correct Answer : I_C/I_E

Explaination : Collector current by emitter current is the current gain, its value is close to one but never greater than.

110 .

The value of β is given by the expression

A)

I_C/I_B

B)

I_C/I_E

C)

I_E/I_C

D)

I_E/I_B

Correct Answer : I_C/I_B

Explaination : Collector current by the base current is beta, its value is in the range 50 to 300.

111 .

An ideal diode has _________ &__________

A)

high switching losses, high reverse voltage drop

B)

some forward voltage drop, some reverse recovery time

C)

no reverse recovery time, high leakage current

D)

no forward voltage drop, negligible reverse recovery time

Correct Answer : no forward voltage drop, negligible reverse recovery time

Explanation : An ideal diode has no losses and negligible reverse recovery time.

112 .

A circuit is so formed such that the source-R-C-diode-switch are in series. Consider the initial voltage across the C to be zero. The diode is so connected that it is forward biased when the switch is closed. When the switch is closed,

A)

the voltage and current both remain constant

B)

the current will decay exponentially & the voltage will increase exponentially

C)

the current will fall to zero & the voltage both will decay exponentially

D)

the current will increase exponentially & the voltage will increase exponentially

Correct Answer : the current will decay exponentially & the voltage will increase exponentially

Explaination : Instant switch is closed, the current is maximum Vs/R than starts to reduce, whereas voltage starts to increase from 0 to Vs (Capacitor Charging).

113 .

The time constant of a series RC circuit (Ï„) is given by

A)

RC

B)

C/R

C)

R/C

D)

1/RC

Correct Answer : RC

Explanation : For a series RC circuit, τ = RC.

114 .

A circuit is so formed such that source-R-L-diode-switch are all in series. Consider the initial current in L to be zero. The diode is so connected that it is forward biased when switch is closed.
When the switch is closed,

A)

the voltage and the current both remain constant

B)

the current will fall to zero & the voltage both will decay exponentially

C)

the current will increase exponentially & the voltage will decay exponentially

D)

the current will decay exponentially & the voltage will increase exponentially

Correct Answer : the current will increase exponentially & the voltage will decay exponentially

Explaination : Instant switch is closed, the current is minimum zero than starts to increase till it reaches a constant value Vs/R, whereas voltage starts to reduce from Vs to 0 (Inductor Charging).

115 .

The time constant of a series RL circuit (Ï„) is given by

A)

RC

B)

R/L

C)

L/R

D)

1/RL

Correct Answer : R/L

Explanation : The time constant τ for a series RL circuit is R/L.

116 .

In the process of diode based rectification, the alternating input voltage is converted into

A)

a controlled direct output voltage

B)

a controlled alternating output voltage

C)

an uncontrolled direct output voltage

D)

an uncontrolled alternating output voltage

Correct Answer : an uncontrolled direct output voltage

Explanation : Rectification is AC to DC. In DIODE biased rectification, control is not possible.

117 .

In a half-wave rectifier, the

A)

current & voltage both are bi-directional

B)

current & voltage both are uni-directional

C)

current is always uni-directional but the voltage can be bi-directional or uni-directional

D)

current can be bi-directional or uni-directional but the voltage is always uni-directional

Correct Answer : current is always uni-directional but the voltage can be bi-directional or uni-directional

Explanation : Current is always in one direction only, but voltage can be bi-directional in case of an L load.

118 .

For a single phase half wave rectifier, with R load, the diode is reversed biased from Ï‰t =

A)

0 to Ï€, Ï€ to 2Ï€

B)

0 to Ï€, 2Ï€ to 2Ï€/3

C)

Ï€ to 2Ï€, 2Ï€ to 2Ï€/3

D)

Ï€ to 2Ï€, 2Ï€/3 to 3Ï€

Correct Answer : Ï€ to 2Ï€, 2Ï€/3 to 3Ï€

Explanation : Diode will be reversed biased in the negative half cycles.

119 .

Find the average value of output current for a 1-phase HW diode rectifier with R load, having RMS output current = 100A.

A)

200/RÏ€ A

B)

200/Râˆš2 A

C)

100/Râˆš2 A

D)

200R A

Correct Answer : 200/RÏ€ A

Explanation : I(rms) = Vm/2R
Therfore, Vm = 200R
I(avg) = Vm/πR = 200R/πR.

120 .

A single-phase full wave mid-point type diode rectifier requires __________ number of diodes whereas bridge type requires _________

A)

1,2

B)

2,4

C)

3,2

D)

4,8

Correct Answer : 2,4

Explanation : A bridge type requires 4 diodes which are connected in a bridge, and the mid-point has 2 diodes.

121 .

In a 1-phase full wave bridge rectifier with M-2 type of connection has secondary side voltage Vs = Vm sin ωt, with R load & ideal diodes.
The expression for the average value of the output voltage can be given by

A)

Vm/π

B)

Vm/√2

C)

2Vm/π

D)

2Vm/√2

Correct Answer : 2Vm/π

Explaination : The voltage waveform is a pulsating voltage with peak value Vm & symmetrical about π.
Vo = (1/π) ∫π Vm sinωt d(ωt)

122 .

In a three-phase half wave rectifier usually, the primary side of the transformer is delta connected because

A)

it has no neutral connection

B)

it provides a path for the triplen harmonics

C)

we can get greater output voltage

D)

it provides better temperature stability

Correct Answer : it provides a path for the triplen harmonics

Explanation : The delta connected winding help circulating and eliminating the triplen (3rd order) harmonics.

123 .

A step-down delta-star transformer, with per-phase turns ration of 5, is fed from a 3-phase, 1100 V, 50 Hz source. The secondary of this transformer through a 3-pulse type rectifier feeds a R load of 10 â„¦. Find the maximum value of the load current (phase).

A)

1 x 11 A

B)

1 x 22 A

C)

√2 x 11 A

D)

√2 x 22 A

Correct Answer : √2 x 22 A

Explaination : Vph = 1100/5 = 220 V (Transformer ratio = 5)
Vmp = √2 x 220 V
Imp = Vmp/R.

124 .

The average value of the output voltage, in a 3-phase half wave diode rectifier with Vml as the maximum line voltage value, is given by the expression

A)

Vml/3Ï€

B)

2Vml/3Ï€

C)

3Vml/2Ï€

D)

3Vml

Correct Answer : 3Vml/2Ï€

Explanation : The average value can be obtained by
3 x [ 1/2π x Vml sin ωt d(ωt) ]
The integration runs from π/6 to 5π/6 as the diode is conducting for 120 degrees each.

125 .

In practice the output from the diode rectifier has

A)

AC component only

B)

DC component only

C)

AC + DC component

D)

None of the above

Correct Answer : AC + DC component

Explanation : The output contents along with the DC components the AC harmonics which does no useful work & reduces the efficiency.

126 .

Choose the correct statement

A)

The AC component in the output of rectifier does the useful work

B)

The AC component in the output of rectifier causes ohmic losses

C)

The AC component in the output of rectifier does not affect the operation

D)

The AC component in the output of rectifier increases the efficiency of the system

Correct Answer : The AC component in the output of rectifier causes ohmic losses

Explaination : A rectifier is used to convert AC to DC. Lower the AC (Non-DC) components in the output lower are the ohmic losses.

127 .

A four-quadrant chopper can not be operated as

A)

Thyristor

B)

Inverter

C)

One quadrant chopper

D)

Cycloconverter

Correct Answer : Cycloconverter

Explanation : A cycloconverter is a kind of AC to AC converter based on thyristors that converts AC at one frequency to ac of another frequency without the involvement of a dc source. It is used primarily to increase or decrease the frequency of the output voltage(s) with respect to the frequency of the input voltage(s), without using any AC-DC nor DC-AC converters during the process. The supply side of the cycloconverter is considered as the input, while the load side is considered the output of it.

128 .

How many switches are used to build a three-phase to three-phase cycloconverter?

A)

10

B)

14

C)

18

D)

24

Correct Answer : 18

Explanation : 18 switches are required to construct a three-phase to three-phase cycloconverter. Three phases of a three-phase cycloconverter need three sets of three-phase half-wave circuits, and each circuit needs 6 thyristors. So the total number of thyristors required is 18.

129 .

A three-phase to three-phase cyclo converter requires

A)

48 SCRs for 3 pulse device

B)

36 SCRs for 6 pulse device

C)

24 SCRs for 3 pulse device

D)

24 SCRs for 6 pulse device

Correct Answer : 36 SCRs for 6 pulse device

Explanation : A three-phase to three-phase cyclo converter requires 36 SCRs for 6 pulse devices.

130 .

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

A)

20 V

B)

220 V

C)

240 V

D)

290 V

Correct Answer : 290 V

Explaination : Vout(rms)= 240 V

m = 3 (for 3 phase)

We know that the fumdamental value of rms output voltage is given by

131 .

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

A)

188 V

B)

178 V

C)

168 V

D)

158 V

Correct Answer : 188 V

Explaination : Vs = 60

We know that the fundamental component of the output voltage of a single-phase half-bridge inverter is given as

132 .

An L filter is connected ________

A)

in parallel

B)

in series

C)

in both series and parallel

D)

None of the above

Correct Answer : in series

Explanation : Inductor (L) has a very important property that the current through it cannot change rapidly. We can make use of this property by connecting it in series.

133 .

In case of an L filter connected with a rectifier in series with the load, it offers ________ impedance to ac whereas _______ resistance to dc respectively.

A)

high, low

B)

low, high

C)

low, low

D)

high, high

Correct Answer : high, low

Explanation : It offers high impedance to AC such as the AC ripples do not pass through the load.

134 .

In case of a C filter, the AC is not allowed to pass to the load by

A)

offering it high impedance

B)

offering it low impedance

C)

open circuiting the AC component

D)

short circuiting the AC component

Correct Answer : short circuiting the AC component

Explanation : AC ripples are not allowed to pass, by S.C the AC ripples as the C is always connected in parallel with the load.

135 .

In a single pulse semi-converter using two SCRs, the triggering circuit must produce

A)

one firing pulse in each cycle

B)

two firing pulses in each half cycle

C)

one firing pulse in each half cycle

D)

three firing pulses in each cycle

Correct Answer : one firing pulse in each half cycle

Explanation : A single phase semi-converter has only two SCRs & two diodes. Hence, only two pulses are required in each cycle, one in each half.

136 .

In a 3-phase full converter using six SCRs, gating circuit must provide

A)

one firing pulse every 30Â°

B)

one firing pulse every 60Â°

C)

one firing pulse every 90Â°

D)

three firing pulses per cycle

Correct Answer : one firing pulse every 60Â°

Explanation : 60° x 6(devices) = 360°.

137 .

In the complete firing circuit, the driver circuit consists of

A)

gate leads & power supply

B)

pulse generator & power supply

C)

pulse detector & pulse amplifier

D)

pulse amplifier & pulse transformer

Correct Answer : pulse amplifier & pulse transformer

Explanation : The driver circuit consists of a pulse amplifier to increase the magnitude of the gate pulse to a sufficient value. The pulse transformer then provides pulses to individual SCRs.

138 .

The magnitude of gate voltage and gate current for triggering an SCR is

A)

inversely proportional to the temperature

B)

directly proportional to the temperature

C)

inversely proportional to the anode current requirement

D)

directly proportional to the anode current requirement

Correct Answer : inversely proportional to the temperature

Explanation : Higher the temperature lesser will be the gate current required as the temperature must have already excited some of the atoms.

139 .

Find the amplitude of the gate current pulse, when the gate-cathode curve is given by the relation Vg = [(1+10) x Ig] The peak gate drive power is 5 Watts.

A)

1.359 A

B)

1.659 A

C)

359mA

D)

659mA

Correct Answer : 659mA

Explanation : (1+10 Ig).Ig = 5 Watts
Ig = 0.59 A.

140 .

If the RC firing circuit used for firing an SCR is to be used to fire a TRIAC then

A)

the capacitor should be removed

B)

the diode should be replaced by a diac

C)

the diode should be replaced by a bjt

D)

the diode should be shorted using a resistor

Correct Answer : the diode should be replaced by a diac

Explanation : The TRIAC is a bidirectional SCR, hence it will need gating in both the directions. This can be achieved by replacing the diode by a DIAC (bidirectional diode).

141 .

In the thyristor gating circuit, the supply to the pulse amplifier is provided by the

A)

zcd

B)

control signal generator

C)

isolation transformer

D)

synchronizing transformer

Correct Answer : isolation transformer

Explanation : Isolation transformer provides the supply to the amplifier and also provides the necessary isolation for the load and triggering circuit.

142 .

In the thyristor gating circuit, the ZCD is used to

A)

amplify the voltage

B)

produce a train of pulses

C)

used to step-down the voltage

D)

convert AC input the ramp voltage

Correct Answer : convert AC input the ramp voltage

Explanation : It is used to convert the AC synchronizing input voltage into ramp voltage & synchronizes it with the zero crossing of the AC supply.

143 .

The decaying factor in the wave shape of the output pulses from the pulse transformer is its

A)

inductance

B)

resistance

C)

capacitance

D)

transformer ratio

Correct Answer : inductance

Explanation : L is the decaying factor in the waveform which emerges from the PT.

144 .

The improved version of the UJT oscillator triggering circuit is the

A)

rc triggering

B)

ramp triggering

C)

cosine-pulse triggering

D)

ramp & pedal triggering

Correct Answer : ramp & pedal triggering

Explanation : The ramp & pedal triggering is the improved version of the UJT oscillator triggering circuit.

145 .

The thyristor turn-off requires that the anode current

A)

falls below the latching current

B)

falls below the holding current

C)

rises above the holding current

D)

rises above the latching current

Correct Answer : falls below the holding current

Explanation : For effective turn-off of the SCR the anode current must fall below the holding current value.

146 .

In case of class A type commutation or load commutation with low value of R load the

A)

L is connected across R

B)

L-C is connected across R

C)

L-C is connected in series with R

D)

L is connected in series with R

Correct Answer : L-C is connected in series with R

Explanation : In case of Class A commutation the requirement is that the circuit should be an under-damped RLC circuit.

147 .

The class A commutation or load commutation is possible in case of

A)

dc circuits only

B)

ac circuits only

C)

Both DC and AC circuits

D)

None of the above

Correct Answer : dc circuits only

Explanation : The nature of the circuit should be such that when energized from the source, current must a a natural tendency to decay to zero for load commutation to occur in a SCR circuit.

148 .

An SCR is connected in series with L = 5 mH and C = 20 Î¼F. Find the resonant frequency of the circuit.

A)

2400 rad/s

B)

2569 rad/s

C)

3162 rad/s

D)

7889 rad/s

Correct Answer : 3162 rad/s

Explanation : ω = 1/√LC.

149 .

The type of commutation when the load is commutated by transferring its load current to another incoming thyristor is

A)

class A or load commutation

B)

class B or resonant commutation

C)

class D or impulse commutation

D)

class C or complementary commutation

Correct Answer : class C or complementary commutation

Explanation : In the Class C type commutation also called as complementary commutation the load is commutated by transferring the current th another device.

150 .

_________ commutation technique is commonly employed in series inverters.

A)

line

B)

load

C)

forced

D)

external-pulse

Correct Answer : load

Explanation : Load commuation is used in inverter in which L and C are connected in series with the load or C in parallel with the load such that overall load circuit is under damped.

151 .

The type of commutation in which the pulse to turn off the SCR is obtained by separate voltage source is

A)

class B commutation

B)

class C commutation

C)

class D commutation

D)

class E commutation

Correct Answer : class E commutation

Explanation : In class E commutation, another voltage source is used. It is also called as external pulse commutation.

152 .

A voltage source Vs = Vm sinωt is connected in series with a resistance R and an SCR. At some firing angle delay of α a positive gate pulse is applied to the SCR which turns on the SCR. Considering ideal conditions, at the instant α the voltage at the resistor terminals Vo

A)

falls to zero

B)

falls to –Vm sin α

C)

rises to Vm sin α

D)

rises to Vm sin ωt

Correct Answer : rises to Vm sin α

Explaination : At the firing angle, the SCR turns on and thus the resistor or load R is connected to the source.

153 .

The average output voltage is maximum when SCR is triggered at Ï‰t =

A)

0

B)

Ï€

C)

Ï€/2

D)

Ï€/4

Correct Answer : 0

Explanation : The sooner the conduction starts higher the average power. Though practically a device cannot be triggered at exactly zero degrees.

154 .

For a single phase thyristor circuit with R load & firing angle Î±, the conduction angle can be given by

A)

Î±

B)

Ï€-Î±

C)

Ï€+Î±

D)

2Ï€+Î±

Correct Answer : Ï€-Î±

Explanation : SCR is fired at α, hence it conducts from α till it is commutated by the reversal of the AC supply i.e at π radian

155 .

In a single phase half-wave thyristor circuit with R load & Vs=Vm sinÏ‰t, the maximum value of the load current can be given by

A)

Vs/R

B)

Vs/2

C)

Vm/2

D)

2Vm/R

Correct Answer : Vm/2

Explanation : Vm is the peak value of the load as well as supply voltage. I = Vm/R.

156 .

Find the expression for the average value of the output voltage for the below given circuit. Consider the load current to be continuous, firing angle = α, transformer ration 1:1 and Vs = Vm sinωt.

A)

(2Vm/π)cosα

B)

(Vm/π)cosα

C)

(Vm/π)(1+cosα)

D)

(2Vm/π)(1+cosα)

Correct Answer : (2Vm/π)cosα

Explaination : As the load current is continues, the voltage is positive from α to π and negative from π to π+α and so on. Thus,
Vavg = 1/Period ∫ Vm sinâ¡ωt d(ωt)
Where period = π
And the integral runs from α to α+π.

157 .

A fully controlled converter uses

A)

diodes only

B)

thyristors only

C)

both diodes and thyristors

D)

None of the above

Correct Answer : thyristors only

Explanation : Fully controlled implys that all the elements are “fully controlled” hence, it uses SCRs only except the FD.

158 .

A single phase full-converter using R load is a _________ quadrant converter and that using an RL load without FD is a __________ quadrant converter

A)

two, two

B)

two, one

C)

one, one

D)

one, two

Correct Answer : one, two

Explanation : In R load both V and I are positive, in RL load the voltage can be negative but current is always positive.

159 .

In a B-2 type full controlled bridge converter

A)

two SCRs conduct at a time

B)

one SCR conducts at a time

C)

four SCRs conduct at a time

D)

three SCRs conduct at a time

Correct Answer : two SCRs conduct at a time

Explanation : B-2 is the bridge type controller, in which 2 devices conduct at a time. One acting as the current supplying path and other acts as a return path.

160 .

A single phase full controlled bridge converter (B-2) uses

A)

6 SCRs

B)

4 SCRs and 2 diodes

C)

4 SCRs

D)

None of the above

Correct Answer : 4 SCRs

Explanation : 4 SCR’s are connected in a bridge fashion.

161 .

A semi-converter with RLE load and a freewheeling diode has discontinuous load current with firing angle α and extinction angle β. If β < π and Vm sinβ < E then

A)

the freewheeling period is absent

B)

the conduction period is absent

C)

the converter operates as a line commutated inverter

D)

None of the above

Correct Answer : the freewheeling period is absent

Explaination : As the extinction angle is less than 90 degrees, the freewheeling period is absent.

162 .

A semi-converter with RLE load and a freewheeling diode has discontinuous load current with firing angle α and extinction angle β. The freewheeling period is

A)

π>ωt<β

B)

π>ωt<β

C)

π<ωt>β

D)

π>ωt>β

Correct Answer : π>ωt<β

Explaination : The freewheeling period is from π to the extinction angle.

163 .

A three-phase, three-pulse, M-3 type controlled converter uses ____________ number of SCRs.

A)

1

B)

2

C)

3

D)

4

Correct Answer : 3

Explanation : It uses three SCRs with a three-phase transformer. M-3 type 3-pulse converters are not practically used.

164 .

A three-phase, three-pulse, M-3 type controlled converter has firing angle for one of the SCRs set as 15°. This SCR would start conducting at

A)

0°

B)

15°

C)

30°

D)

45°

Correct Answer : 45°

Explaination : In a three phase controller, the actually conduction starts at 30° + α. Hence, ωt = 30+15 = 45°.

165 .

A three-phase three pulse type controlled converter is constructed using 3 SCR devices. The circuit is supplying an R load with α < 30°. As such, each SCR device would conduct for

A)

60° each cycle

B)

120° each cycle

C)

180° each cycle

D)

360° each cycle

Correct Answer : 120° each cycle

Explaination : Each conduct for 120° per cycle is the firing angle is less than 30°. 120 x 3 = 360°.

166 .

A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find, the value of average output voltage and average current rating of SCR for a firing angle of 30°.

A)

135 V, 12 A

B)

135 V, 36 A

C)

234 V, 12 A

D)

234 V, 36 A

Correct Answer : 234 V, 12 A

Explaination : Vo = (3√3/2π) x Vmp x cosα = 233.874 V.
Ia = Io/3 = 12 A.

167 .

A three-phase three-pulse converter would operate as a line commutated inverter when

A)

90° > α

B)

30° < α < 60°

C)

90° < α <180°

D)

It can never operate as a line commutated inverter

Correct Answer : 90° < α <180°

Explaination : The output voltage is proportional to cosine of the firing angle α. If α goes above 90° then the output voltage is negative, meaning that it is operating as an inverter.

168 .

In a 3 phase M-6 controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.

A)

2Ï€/6 radians

B)

3Ï€/2 radians

C)

30 degrees

D)

120 degrees

Correct Answer : 2Ï€/6 radians

Explanation : Each SCR conducts for 60° or 2π/6 radians. 6 x 60 = 360°.

169 .

In a 3 phase, 12-pulse controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.

A)

Ï€/6 radians

B)

3Ï€/2 radians

C)

12 degrees

D)

60 degrees

Correct Answer : Ï€/6 radians

Explanation : Each SCR conducts for 30° or π/6 radians. 12 x 30 = 360°.

170 .

What is the relationship between DF, CDF and PF?

A)

PF = DF/CDF

B)

PF = CDF x DF

C)

PF = CDF/DF

D)

PF = CDF = DF

Correct Answer : PF = CDF x DF

Explanation : PF (power factor) = (distortion factor) x (current distortion factor).

171 .

The commutation period when both incoming and outgoing SCRs are conducting due to source inductance is called as the

A)

distorting angle

B)

conduction delay

C)

one on one period

D)

overlap period

Correct Answer : overlap period

Explanation : Due to source inductance, SCRs cannot start and stop conducting immediately, hence a time occurs when both incoming and outgoing SCRs are conducting together. This delay is called as commutation angle or overlap period (μ).

172 .

In a p-pulse converter, each SCR conducts for (per cycle)

A)

p radians

B)

p degrees

C)

2Ï€/p radians

D)

p/2Ï€ radians

Correct Answer : 2Ï€/p radians

Explanation : In a p-pulse controlled converter, each device conducts for p radians. p-pulse converter would have p devices. Hence, 2π/p x p = 2π = 360°.

173 .

Which of the below mentioned converter can operate in both 3-pulse and 6-pulse modes?

A)

3-phase full converter

B)

3-phase semi-converter

C)

3-phase half wave controller

D)

None of the above

Correct Answer : 3-phase semi-converter

Explanation : Semi-converters can operate in 6-pulse modes gating the SCRs at appropriate intervals.

174 .

A M-6 controlled converter or 6-pulse half-wave controlled converter is obtained by using a transformer having

A)

a double delta connected primary winding

B)

a double delta connected secondary winding

C)

a double star connected secondary winding

D)

6-windings on both primary and secondary sides

Correct Answer : a double star connected secondary winding

Explanation : M-6 requires a transformer having a delta connected primary and a double star connected secondary such that 6 SCRs are connected to it on the secondary side.

175 .

Dual converters provide

A)

four quadrant operation

B)

three quadrant operation

C)

two quadrant operation

D)

None of the above

Correct Answer : four quadrant operation

Explanation : Dual converters provide four quadrent operation, which means voltage can be positive or negative and so can be the current. Hence, AC-DC, DC-AC any converter configuration can be used.

176 .

A dual converters has

A)

two half converters in series

B)

two full converters in series

C)

two half converters in anti-parallel

D)

two full converters in anti-parallel

Correct Answer : two full converters in anti-parallel

Explanation : Dual converters have two full converters connected in anti-parallel which provides a four quadrant operation.

177 .

The major advantage of using dual converters is that

A)

it has better pf

B)

it is cheaply available

C)

its operating frequency is very high

D)

no mechanical switch is required to change the mode of operation

Correct Answer : no mechanical switch is required to change the mode of operation

Explanation : No mechanical arrangement is required to change from inverter to converter and converter to inverter, which was required in earlier methods.

178 .

In non-circulating current mode dual converters, the circulating current is avoided by

A)

adding an extra SCR

B)

operating only one converter

C)

maintaining Î±1 + Î±2 = 180Â°

D)

connecting a series reactor

Correct Answer : operating only one converter

Explanation : Reactor is added in circulating current mode not in non-circulating mode. The circulating current is avoided by using only one of the converters.

179 .

For a single-phase dual converter, with converters C1 and C2 connected in anti-parallel, which relation among the following is true to keep the average voltages from C1 and C2 equal? C1 and C2 have firing angles α1 and α2 respectively.

A)

α1 = α2

B)

α1 + α2 = 180°

C)

α1 + α2 = 360°

D)

None of the above

Correct Answer : α1 + α2 = 180°

Explaination : By maintaining α1 + α2 = 180°, one converter can operate as converter and another as an inverter hence, the average output voltages are equal . This can be proved as follows
Vom cos α1 = Vom cos α2
Vom = 2Vm/π . . (Vm for both the converters is same)
cos α1 = cos α2
cos α1 = cos (180 – α2)
α1 + α2 = 180°.

180 .

A single full converter alone can given a

A)

two quadrant operation

B)

three quadrant operation

C)

four quadrant operation

D)

None of the above

Correct Answer : two quadrant operation

Explanation : A single full converter alone gives two quadrent operation, hence for all four quadrant operation two full converter circuits are connected in anti-parallel.

181 .

PWM switching is used in voltage source inverters for the purpose of

A)

Controlling input power

B)

Controlling input voltage

C)

Controlling output current

D)

Controlling output harmonics and output voltage.

Correct Answer : Controlling output harmonics and output voltage.

Explanation : PWM stands for Pulse Width Modulation. It is used in the voltage source inverter to control the output voltage and output harmonics.

It is a method of controlling the average power of a signal over a continuous range by switching it between on and off states.

It is a method of generating amplitude modulation. By increasing the number of pulses per half cycle, the order of harmonic frequency increases so that the filter's size is reduced.

182 .

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.

A)

Speed negative and toque positive

B)

Speed positive and toque negative

C)

Speend positive and toque positive

D)

Speed negative and toque negative

Correct Answer : Speend positive and toque positive

Explanation : 3-∅ semiconverter drive only in a single quadrant; therefore, the moto can operate only in the first quadrant, which has positive speed and positive toque.

183 .

The armature current of a dc motor fed from a thyristor power converter consists of ripples. The ripple in the armature affects

A)

Speed of the motor

B)

Commutation of the motor

C)

Toque of the motor

D)

The efficiency of the motor

Correct Answer : Commutation of the motor

Explanation : Ripple frequency refers to a frequency of the residual AC voltage. After that, it has been rectified to DC in a power supply. Because of higher ripple content in the converter output motor, heating and commutating problems arise. It works on high DC voltage. For a half-wave rectifier, the value of ripple frequency i the same as the AC frequency, and for a full-wave, the value of ripple frequency is twice the original AC frequency.

184 .

A motor armature is supplied through a phase control silicon controlled rectifiers receives a smoother voltage shape at

A)

Constant motor speed

B)

Lower motor speed

C)

Higher motor speed

D)

None of the above

Correct Answer : Higher motor speed

Explanation : We know that the voltage is directly proportional to the speed. Therefore smoother voltage shape is achieved at a higher speed.

185 .

The torque produced by a single-phase induction motor fed by an ac voltage controller for speed control due to

A)

A fundamental component of current only

B)

A fundamental component of harmonics only

C)

Fundamental component and even harmonics

D)

A fundamental component of harmonics as well as current

Correct Answer : A fundamental component of current only

Explanation : Single-phase induction motors work on single-phase AC supply. It has two windings; Main windings and auxiliary winding. For speed control of a single-phase induction motor by an ac voltage controller, and only a fundamental component of current is required to develop a toque. The harmonics in the motor current may cause power losses resulting in heating of the motor.

186 .

In the ___________ type of chopper, two stage conversions takes place.

A)

AC-DC

B)

AC link

C)

DC link

D)

None of the above

Correct Answer : AC link

Explanation : In AC link chopper, DC is converter to AC than stepped up/down than again AC to DC conversation takes place.

187 .

Choppers converter

A)

AC to AC

B)

AC to DC

C)

DC to DC

D)

DC to AC

Correct Answer : DC to DC

Explanation : Choppers are used to step up or step down DC voltage/current levels. Hence, they are DC to DC converters.

188 .

A chopper may be thought as a

A)

Diode rectifier

B)

Inverter with DC input

C)

DC equivalent of an AC transformer

D)

DC equivalent of an induction motor

Correct Answer : DC equivalent of an AC transformer

Explanation : It is a DC equivalent of an AC transformer because it behaves in the similar manner i.e. converting fixed DC to variable DC.

189 .

The load voltage of a chopper can be controlled by varying the

A)

duty cycle

B)

firing angle

C)

extinction angle

D)

reactor position

Correct Answer : duty cycle

Explanation : The output voltage can be changed by changing the duty cycle (Ton/T).

190 .

If T is the time period for a chopper circuit and Î± is its duty cycle, then the chopping frequency is

A)

Î±/Toff

B)

Ton/Î±

C)

Toff/Î±

D)

Î±/Ton

Correct Answer : Î±/Ton

Explanation : α = Ton/T
T = Ton/ α
f = 1/T = α/Ton.

191 .

In case of TRC (Time Ratio Control), _________ is varied

A)

duty cycle

B)

firing angle

C)

supply frequency

D)

supply voltage magnitude

Correct Answer : duty cycle

Explanation : In case of TRC, α (Duty cycle) is varied to change the average value of output voltage.

192 .

In case of variable frequency system __________ is varied

A)

T

B)

Ton

C)

Toff

D)

Supply frequency

Correct Answer : T

193 .

In case of a constant frequency system, Ton = (1/4)T. If the chopping frequency 2 kHz, find the value of Toff.

A)

(3/8) Î¼s

B)

(3/8) ms

C)

(1/8) ms

D)

(1/8) Î¼s

Correct Answer : (3/8) ms

Explanation : Ton = (1/4) T
T = 1/2kHz = 0.5 ms
Ton = T/4
Therefore, Toff = T – Ton = T – T/4 = 3T/4 = 3/8 msec.

194 .

The control strategy in which on and off time is guided by the pervious set of values of a certain parameter is called as

A)

time ratio control

B)

pulse width modulation

C)

current limit control

D)

constant frequency scheme

Correct Answer : current limit control

Explanation : In current ratio control, the on and off time is guided by the pervious set of value of load current.

195 .

In the current limit control method, when the load current reaches a predefined lower value, then

A)

the chopper is switched on

B)

load voltage goes to zero

C)

the chopper is switched off

D)

the source voltage is removed

Correct Answer : the chopper is switched on

Explanation : In CLC, when the current reaches the upper limit the chopper is switched off and current starts to decay. When current reaches a predefined lower limit, the chopper is again switched on and current starts to rise, the process is thus again repeated.

196 .

Type C chopper consists of

A)

one diode and one switch

B)

one diode and three switches

C)

two diodes and two switches

D)

three diodes and two switches

Correct Answer : two diodes and two switches

Explanation : Type C consists of, two diodes and two switches. One diode switch pair is used for operation in a particular quadrant.

197 .

In a type C chopper, if only one switch is operated

A)

the chopper wonâ€™t work

B)

only one quadrant operation will be obtained

C)

two quadrant operation can be obtained

D)

None of the above

Correct Answer : only one quadrant operation will be obtained

Explanation : Only one quadrant operation will be operated if only one switch is used. It can be either 1st and 2nd quadrant depending on which switch is operated.

198 .

A type C chopper consists of __________ diodes and _________ switches in anti-parallel.

A)

2, 2

B)

3, 3

C)

3, 4

D)

4, 4

Correct Answer : 4, 4

Explanation : Type C consists of 4 diodes and 4 switches arranged in anti-parallel. The switches and diodes are always numbered occurring to the quadrant in which they operate.

199 .

A type C chopper can operate in

A)

Ist and IInd quadrants

B)

IInd and IIIrd quadrants

C)

Ist, IInd and IIIrd quadrants

D)

all the four quadrants

Correct Answer : all the four quadrants

Explanation : Type C can operate in all the four quadrants by controlling appropriate switches and reversing the emf in the load circuit.

200 .

What is the expression for load voltage when the chopper is operated in the second quadrant?

A)

0

B)

E

C)

Vs

D)

E + Ldi/dt

Correct Answer : E + Ldi/dt

Explanation : In the second quadrant, the chopper acts as a step up chopper and current flows from the load to source. With CH2 on, negative current flows through L, CH2, CH3 and CH4.

201 .

The load emf E must be reversed for

A)

third quadrant operation

B)

fourth quadrant operation

C)

both third and fourth quadrant operation

D)

first and second quadrant operation

Correct Answer : both third and fourth quadrant operation

Explanation : The load E must be reversed for both third and fourth quadrant operation for proper current flow.

202 .

The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as

A)

voltage commutation

B)

forced commutation

C)

capacitor commutation

D)

current commutation

Correct Answer : voltage commutation

Explanation : It is called as voltage commutation, which is a type of forced commutation.

203 .

In case of current commutation of SCR

A)

a diode is connected in series with the main SCR

B)

a diode is connected in parallel with the main SCR

C)

a diode is connected in anti-parallel with the main SCR

D)

None of the above

Correct Answer : a diode is connected in anti-parallel with the main SCR

Explanation : The voltage drop across the anti parallel connected SCR reverse biases the main SCR.

204 .

In a voltage commutated SCR, the main SCR is commutated by

A)

sending a negative pulse to the main SCR

B)

triggering the auxiliary SCR

C)

removing the supply Vs

D)

None of the above

Correct Answer : triggering the auxiliary SCR

Explanation : The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1.

205 .

Which type of commutation circuit does not work on no load?

A)

Current commutation

B)

Voltage commutation

C)

Both voltage and current commutation

D)

None of the above

Correct Answer : Voltage commutation

Explanation : This is because the capacitor won’t be able to charge on no load.

206 .

What is the expression for the value of the capacitor in the voltage commutation circuit? Let tc be the circuit turn off time and Io be the constant load current.

A)

Vs/Io

B)

Io x (dv/dt)

C)

(tc x Io)/Vs

D)

None of the above

Correct Answer : (tc x Io)/Vs

Explaination : Capacitor current = C dv/dt
For constant load current Io, the above expression can be
Io = C x (Vs/tc) or C = (tc x Io)/Vs.

207 .

Inverters converts

A)

dc power to ac power

B)

ac power to ac power

C)

ac power to dc power

D)

dc power to dc power

Correct Answer : dc power to ac power

Explanation : Inverter is a dc to ac converter.

208 .

Line-commutated inverters have

A)

DC on both supply and load side

B)

AC on the supply side and DC on the load side

C)

AC on both supply and load side

D)

DC on the supply side and AC on the load side

Correct Answer : AC on both supply and load side

Explanation : Line-commuted inverters are actually phase-controlled converters operated in the inverting mode. They cannot act as standalone inverters as they require a AC supply. It is to be noted that “line commutated inverter” is not the conventional inverter. The conventional inverters are forced or load commutated inverters.

209 .

In a VSI (Voltage source inverter)

A)

the IGBTs are fired at 0 degrees.

B)

the internal impedance of the AC source is negligible

C)

the internal impedance of the DC source is very very high

D)

the internal impedance of the DC source is negligible

Correct Answer : the internal impedance of the DC source is negligible

Explanation : A VSI is the one in which the internal impedance of the source is negligible. It has a stiff DC source at its input.

210 .

________ based inverters do not require self-commutation.

A)

SCR

B)

GTO

C)

IGBT

D)

PMOSFET

Correct Answer : SCR

Explanation : All the devices can be turned off by their gate/base singles expect SCR. SCRs require external commutation circuits.

211 .

Single phase half bridge inverters requires

A)

two wire ac supply

B)

three wire dc supply

C)

two wire dc supply

D)

three wire ac supply

Correct Answer : three wire dc supply

Explanation : They require two voltage sources Vs/2 and Vs/2.

212 .

In a single-phase half wave inverter ________ SCR(s) are/is gated at a time.

A)

one

B)

two

C)

three

D)

None of the above

Correct Answer : one

Explanation : Only one SCR is gated at a time, gating the both of them will short the supply.

213 .

In voltage fed thyristor inverters __________ commutation is required.

A)

self

B)

load

C)

forced

D)

any commutation technique can be used

Correct Answer : forced

Explanation : In VSI, the input voltage source Vs keeps the SCRs always forward biased. Hence, forced commutation becomes essential.

214 .

The McMurray circuit is a

A)

commutation circuit

B)

self commutated VSI

C)

force commutated VSI

D)

None of the above

Correct Answer : force commutated VSI

Explanation : McMurray is a popularyly used forced commutated VSI circuit.

215 .

Forced commutation requires

A)

a precharged inductor

B)

an overdamped RLC load

C)

a very high frequency ac source

D)

a precharged capacitor

Correct Answer : a precharged capacitor

Explanation : Forced commutation requires a pre charged capacitor of correct polarity to turn-off the earlier conducting SCR.

216 .

Which elements are not present in the original McMurray inverter but are present in the modified McMurray inverters?

A)

One auxiliary SCR and one auxiliary diode

B)

Two auxiliary diodes and the damping resistor

C)

Two auxiliary thyristors and the di/dt inductor

D)

None of the above

Correct Answer : Two auxiliary diodes and the damping resistor

Explanation : In the original inverter circuit given by McMurray the elements DA1, DA2 and the damping resistor Rd were not present.

217 .

The number of diodes, SCRs and other components in full-bridge inverter McMurray inverter is ____________ of those in half-bridge McMurray inverter.

A)

same

B)

double

C)

three times

D)

None of the above

Correct Answer : double

Explaination : The modified McMurray full-bridge requires twice the components that required in the half-wave type.

218 .

The single-phase McMurray-Bedford type bridge inverter is a/an

A)

complementary-commutated inverter

B)

auxiliary-commutated inverter

C)

supplementary-commutated inverter

D)

None of the above

Correct Answer : complementary-commutated inverter

Explanation : If one SCR gets turned on, the other conducting SCR gets turned off. This type of commutation is called as complementary commutation.

219 .

The McMurray-Bedford half-bridge inverter requires

A)

2 SCRs, 2 diodes, 2 capacitors and 2 inductors

B)

2 SCRs, 4 diodes, 2 capacitors and 1 inductor

C)

4 SCRs, 2 diodes, 2 capacitors and 2 inductors

D)

4 SCRs, 4 diodes, 2 capacitors and 2 inductors

Correct Answer : 2 SCRs, 2 diodes, 2 capacitors and 2 inductors

Explanation : McMurray-Bedford type requires less number of SCRs and diodes as compared to the McMurray type, however the number of capacitors and inductors required is the same.

220 .

The 120Â° mode of operation of a three phase bridge inverter requires ___________ number of steps.

A)

2

B)

4

C)

6

D)

8

Correct Answer : 6

Explanation : Like the 180 mode, the 120° mode also requires six steps, each of 60° duration.

221 .

In case of the 120Â° mode of operation, __________ devices conduct at a time.

A)

2

B)

3

C)

4

D)

None of the above

Correct Answer : 2

Explanation : Unlike the 180° mode, in the 120° mode one 2 devices are conducting at a time as each conduct for 120°.

222 .

Safe commutation can be achieved in case of the ____________ operating mode.

A)

120Â°

B)

180Â°

C)

360Â°

D)

None of the above

Correct Answer : 120Â°

Explanation : In the 120 mode of operation, there is a 60° interval between turning off of T1 and turning on of T4 (or any two SCRs belonging to the same leg), hence SCRs are commutated safely.

223 .

The peak value of the line voltage in case of 120Â° mode of operation of a three-phase bridge inverter is

A)

Vs/2

B)

Vs

C)

Vs/âˆš2

D)

3Vs/2

Correct Answer : Vs

Explanation : The peak value for 120° mode is Vs. The line voltage waveform is a sine wave with a peak value of Vs ( = supply voltage).

224 .

Which of the following is used as a harmonic reduction technique in inverters?

A)

Amplitude modulation

B)

Cycloconverter control

C)

Transformer connection

D)

Series connection of two inverters

Correct Answer : Transformer connection

Explanation : Two or more inverters are connected together by means of a transformer to get the net output voltage with reduced harmonic content.

225 .

For harmonic reduction by transformer connection, the output voltages from the two inverters must be

A)

similar and in-phase with each other

B)

dissimilar but in-phase with each other

C)

dissimilar and phase shifted from each other

D)

similar but phase shifted from each other

Correct Answer : similar but phase shifted from each other

Explanation : The essential condition of this scheme is that the output voltages from the two inverters must be similar but phase shifted from each other.

226 .

The output voltage obtained by connecting two inverters through a transformer is a

A)

sine wave

B)

square wave

C)

quasi-square wave

D)

None of the above

Correct Answer : quasi-square wave

Explanation : The net output voltage is the addition of the two inverter voltages, as they are phase shifted from each other, the waveform obtained is a quasi-square wave.

227 .

Pulses of different widths and heights are superimposed in case of __________ harmonic reduction technique.

A)

stepped-wave inverter

B)

pulse width modulation

C)

transformer connection

D)

None of the above

Correct Answer : stepped-wave inverter

Explanation : In the steeped wave inverters pulses of different widths and heights are superimposed to produce a resultant stepped wave with reduced harmonic content.

228 .

In case of stepped wave inverters,

A)

both the transformers have 1:1 turns ratio

B)

both the transformers have 1:2 turns ratio

C)

both the transformers have different transformer ratio

D)

None of the above

Correct Answer : both the transformers have different transformer ratio

Explanation : The two transformers have different turns ratio, hence, different voltages levels are obtained at the secondary’s which are the added to get the net output voltage with reduced harmonic content.

229 .

The waveform obtained by __________ is more near to a sinusoidal wave

A)

Stepped wave inverters

B)

PWM inverters

C)

parallel connected inverters

D)

None of the above

Correct Answer : PWM inverters

Explanation : The stepped wave output, is more nearer to a sine wave. More the number of steps, closer is the wave to a sinusoidal wave.

230 .

In three-level modulation

A)

the output voltage is zero in the first half cycle.

B)

the output voltage either zero or positive in the first half cycle.

C)

the output voltage either zero or negative in the first half cycle.

D)

the output voltage either zero, positive or negative in the first half cycle.

Correct Answer : the output voltage either zero, positive or negative in the first half cycle.

Explanation : In stepped wave inverters, one of the inverters is so gated as to obtain a three level modulation.

231 .

A cycloconverter fed induction motor drive is widely used for which one of the following?

A)

Machine tool drive

B)

Papermill drive

C)

Cement mill drive

D)

Compressor drive

Correct Answer : Machine tool drive

Explanation : Cycloconverters are mainly used where precise speed control is needed. It means cycloconverter is not suitable for cement mill drive as well as compressor drive. Papermill requires constant speed drive. So cycloconverter fed induction motor drive is widely used for machine tool drive.

232 .

Which of the given configurations is used for both regenerative and motoring breaking?

A)

First quadrant chopper

B)

Two quadrant chopper

C)

Third quadrant chopper

D)

Fourth quadrant chopper

Correct Answer : Two quadrant chopper

Explanation : Two quadrant chopper is used for both regenerative and motoring breaking.

233 .

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?

A)

19200 W

B)

21240 W

C)

19220 W

D)

28200 W

Correct Answer : 19220 W

Explaination : Vout = 400 V

Duty cycle α = 0.6

Armature current IS = 80 A

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_S I_S = 240 × 80 = 19200 W

234 .

AC voltage controllers convert

A)

fixed ac to fixed dc

B)

variable ac to fixed ac

C)

fixed ac to variable ac

D)

variable ac to variable dc

Correct Answer : fixed ac to variable ac

Explanation : Voltage controllers convert the fixed ac voltage to variable ac by changing the values of the firing angle.

235 .

In AC voltage controllers the

A)

variable dc with fixed frequency is obtained

B)

variable ac with variable frequency is obtained

C)

variable ac with fixed frequency is obtained

D)

variable dc with variable frequency is obtained

Correct Answer : variable ac with fixed frequency is obtained

Explanation : Voltage controllers convert the fixed ac voltage to variable ac by changing the values of the firing angle. The available ac obtained has the same fixed frequency as the input ac.

236 .

Earlier then the semiconductor technology, ___________ devices were used for voltage control applications.

A)

vacuum tubes

B)

cycloconverters

C)

induction machine

D)

tap changing transformer

Correct Answer : tap changing transformer

Explanation : A tap changing transformer can give variable ac from fixed ac without a change in frequency.

237 .

In the principle of phase control

A)

control cannot be achieved

B)

the load is on for some cycles and off for some cycles

C)

control is achieved by adjusting the number of on off cycles

D)

control is achieved by adjusting the firing angle of the devices

Correct Answer : control is achieved by adjusting the firing angle of the devices

Explanation : Switching devices is so operated that the load gets connected to ac source for a part of each half cycle

238 .

A single-phase half wave voltage controller consists of

A)

one SCR is parallel with one diode

B)

one SCR is anti parallel with one diode

C)

two SCRs in parallel

D)

two SCRs in anti parallel

Correct Answer : one SCR is anti parallel with one diode

Explanation : As it is half wave, it consists of one SCR ( the control element) in anti parallel with one diode.

239 .

Sequence control of ac voltage controllers is employed for the improvement of _________

A)

system power factor

B)

commutation

C)

input frequency

D)

output frequency

Correct Answer : system power factor

Explanation : It is used to improve the power factor at both the input and output side.

240 .

A two stage sequence control is

A)

two SCRs in anti parallel

B)

two voltage controllers in series

C)

two voltage controllers in parallel

D)

a voltage controller having two voltage level

Correct Answer : two voltage controllers in parallel

Explanation : Sequence control of ac voltage controller means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage.

241 .

A single-phase sinusoidal voltage controller has

A)

n primary and n secondary windings

B)

(n-1) primary and n secondary windings

C)

one primary and (n-1) secondary windings

D)

one primary and n secondary windings

Correct Answer : one primary and (n-1) secondary windings

Explanation : A sinusoidal voltage controller is used to obtain continuous voltage control over wide range with low harmonic content.

242 .

A single-phase two stage sequence controller is designed to work on 230 V supply, and upper and lower current ratings must be 20 A and 21 A respectively. Find the transformer rating.

A)

230 VA

B)

4600 VA

C)

9200 VA

D)

9430 VA

Correct Answer : 9430 VA

Explanation : Transformer rating is Vs(I1 + I2).

243 .

A cycloconverter is a _________

A)

one stage voltage converter

B)

one stage frequency converter

C)

one stage power converter

D)

None of the above

Correct Answer : one stage frequency converter

Explanation : A cycloconverter converters input power at one frequency to output power at a different frequency with one-stage conversion.

244 .

Applications of cycloconverters include

A)

induction heating

B)

static VAr compensation

C)

speed control of ac drives

D)

All of the above

Correct Answer : induction heating

Explanation : Speed of induction machines can be controlled by controlling the input frequency. In induction heating, eddy current is proportional to the square of input frequency.

245 .

In three phase cycloconverters, the reduction factor is given by

A)

input frequency/output frequency

B)

(input frequency/output frequency) â»Â¹

C)

(input frequency/output frequency) â»Â½

D)

(input frequency/output frequency) Â½

Correct Answer : (input frequency/output frequency) â»Â¹

246 .

In a three phase half-wave cycloconverter ________

A)

only inversion action takes place

B)

only converting action takes place

C)

both inverting and converting action takes place

D)

None of the above

Correct Answer : both inverting and converting action takes place

Explanation : During the conversion process, the current flows in both the directions, hence both inverting and converting action takes place.

247 .

SMPS is used for

A)

obtaining controlled dc power supply

B)

storage of dc power

C)

switch from one source to another

D)

obtaining controlled ac power supply

Correct Answer : obtaining controlled dc power supply

Explanation : SMPS (Switching mode power supply) is used for obtaining controlled dc power supply.

248 .

SPMS are based on the ________ principle.

A)

MOSFET

B)

Chopper

C)

Phase control

D)

Integral control

Correct Answer : Chopper

Explanation : SMPS (Switching mode power supply) are based on the chopper principle. The output dc voltage is controlled by varying the duty cycle of the chopper circuit.

249 .

HVDC transmission has ___________ as compared to HVAC transmission.

A)

higher corona loss

B)

smaller transformer size

C)

smaller power transfer capabilities

D)

smaller conductor size

Correct Answer : smaller conductor size

Explanation : The conductor size is smaller as there is no sink effect, and the whole conductor is utilized for transmitting power.

250 .

Static UPS requires ______

A)

only rectifier

B)

only inverter

C)

both inverter and rectifier

D)

None of the above

Correct Answer : both inverter and rectifier

Explanation : Rectifier to converter the dc from the battery to ac. Inverter to charge the battery from mains.