Preguntas y Respuestas

1.What is the color code for a 2205% resistor?

A.Red, Red, Brown, Gold

B.Orange, Orange, Black, Gold

C.Red, Red, Black, Gold

D.Red, Red, Brown, Silver

View AnswerWorkspaceReportDiscuss in Forum

2.If 60 J of energy are available for every 15 C of charge, what is the voltage?

A.4 V

B.60 V

C.15 V

D.0.25 V


3.An atom's atomic number is determined by the number of:

A.neutrons minus protons

B.protons

C.electrons

D.neutrons


4.A voltage will influence current only if the circuit is:

A.open

B.insulated

C.high resistance

D.closed


5.Which resistive component is designed to be temperature sensitive?

A.Thermistor

B.Rheostat

C.Potentiometer

D.Photoconductive cell

6.The battery symbol is:

A.

B.

C.

D.


7.What type of switch is this?

A.push button

B.SPST

C.DPDT

D.SPDT


8.Primary batteries, unlike secondary batteries, may be:

A.charged once

B.used once

C.recharged over and over

D.stored indefinitely


9.In practical applications, battery voltage:

A.is restored as soon as disconnect occurs

B.is lowered as the load increases

C.may be stored indefinitely

D.will be reduced to zero as power is drawn


10.The negative and positive charge symbols are assigned (in that order) to the:

A.proton and electron

B.electron and proton

C.atom and nucleus

D.electron and element

[Q-1] What is holding current in SCR?It is the minimum current required to hold the SCR in forward conduction state.When the forward current becomes less than holding current, SCR turns from forward conduction state to forward blocking state.[Q-2] What is latching current in SCR?It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state.[Q-3] What are the different turn on methods of SCR?Forward voltage triggeringGate Triggeringdv/dt triggeringTemperature triggeringLight triggering[Q-4] What is snubber circuit?The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR.[Q-5] What is hard switching of the thyristor?When gate current is several times higher than the required gate current, the SCR is said to be hard fired. It reduces the turn ON time and enhances the di/dt capability.[Q-6] What is firing angle?The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.[Q-7] What is meant by SOA?SOA Safe Operating Area determines the voltage and current boundary within which the Power Device can be operated without destructive failure.[Q-8] What are the main components used for isolating the Power Circuits, Power Semiconductor from the low-power circuit?Opto-Couplers,Transformers[Q-9] Name some of the current controlled (current driven) devicesSCR,GTO,GTR[Q-10] Name some of the voltage driven ( Voltage controlled) devicesIGBT,MCT,IGCT,SIT[Q-11] What is duty cycle?It is the ratio of the ON time of the chopper to total time period of the chopper.D = Ton/ [Ton+ Toff][Q-12] Can fuses with an AC voltage rating be used in a DC applications?Fuses must be rated for the voltage AC or DC in which they will be used.Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.[Q-13] What are the characteristics of ideal Opamp?Infinite open loop voltage gainInfinite input impedanceZero output impedanceInfinite BandwidthZero offset voltage[Q-14] For High voltage applications will you prefer MOSFET or IGBT? For High voltage applications we have to use IGBT. Because MOSFETs are low voltage devices. ie, Their voltage rating is lesser than IGBT. General rule is MOSFETs are suitable for applications which has breakdown voltage less than 250V. The IGBTs are suitable for applications which has breakdown voltage upto 1000V.[Q-15]For High frequencyapplications will you prefer MOSFET or IGBT? Why? For High frequencyapplications, MOSFET is the right choice of the device. Because MOSFET has low switching losses compare to that of IGBT. General rule of thumb is for low-frequency applications having frequency range upto 20kHz, we have to use IGBT. For high frequency applications having frequency range of more than 200kHz, we have to use MOSFET.

[Q-2] Which is the good choice among the 2 options mentioned below to get the 12V as an output? why?Option I: Input voltage = 48V,Option II:Input voltage = 24VAnswer:We know thatDuty cycle D = Vout/ VInputFor case ID = 12/48 = 0.25For case IID = 12/24 = 0.5So for case II, the duty cycle is more than case I.It means device is turned ON more time in the second case. while designing SMPS we have to consider the principle mentioned in Q-3.[Q-3] What is effect of having more duty cycle and less duty cycle? ( continuation of above question)If the duty cycle D is greater than 0.5, the core would not be completely demagnetized at the end of the off-period, and a DC magnetization of the core would build up, resulting in core saturation.Relate this answer the previous question.[Q-4] To design the buck converter, what are basic & essential information (parameters) we need to get from the Customer?We need the following inputs from the customer,Output Voltage VOUTInput Voltage VINOutput Current (load current) IOUTMaximum Ripple voltage allowed at the output sideEfficiency of the converter[Q-5] How to select the inductor for the buck converter?Once the topology is finalized in the process of power supply design, the next step is selecting the Inductor.We know the following basic equationVoltage across Inductor VL= L di/dtRewriting the above equation as followsL = VL.dt/diVL= Maximum voltage appears across the Inductordt= The time during which the maximum voltage appears across the inductordi = Ripple currentVLThe maximum voltage appears across the inductor when the power switch is turned on. During that time the voltage will beVL= VOUT VINdt = The time during which the maximum voltage appears across the inductor= D/FS= Duty cycle/Switching frequencydi = Ripple current= Generally it is assumed in between 10% to 30% of the load currentBy knowing all these values we can get the required inductor value for the buck converter.[Q-6] What are the various parameters we have to consider, while selecting IGBT?In IGBT datasheet, we have to focus on following parametersCollector to emitter voltage VCESCollector Current ICCollector Dissipation PCJunction Temperature Tj[Q-6] For Selecting MOSFET, what are the major parameters we have to consider in the datasheet?In MOSFET datasheet, we have to focus on following parameters:Drain to Source Voltage VDSSOn state Drain to Source Resistance RDS(ON)Reverse Recovery Current IRRReverse Recovery Charge QRRGate ChargeQGBody Diode CharacteristicsIDat ambient temperatureChannel power dissipationChannel temperature

[Q] What are the advantages of free wheeling diode in rectifier circuit?The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is improved.[Q] What is meant by commutation?The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.[Q] What are the types of commutation?Natural commutationForced commutation

[Q] What is natural commutation?The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.[Q] What is forced commutation?The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.[Q] What are the types of commutation with respect to commutation process?Voltage commutated chopperCurrent commutated chopperLoad commutated chopper[Q] What is meant by cyclo-converter?It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.[Q] What are the types of cyclo-converters?Step up cyclo-converterStep down cyclo-converter.[Q]What is step down cyclo-converter?It is the converter whose output frequency is less than the input frequency.[Q] What is step up cyclo-converter ?It is the converter whose output frequency is more than the input frequency.[Q] What does the Voltmeter in AC mode show?Is it RMS value or peak value?Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC mode it will show the RMS value only.[Q] What is the necessity to use the special machines?General purpose motors (Induction motors, synchronous motors) are neither precision speed nor precision position motors. For many automated systems require high precise speed and high precise positioning motors. In such cases special purpose motors like stepper motors, PMDC motors etc. are used.[Q] What are the control strategies of chopper?The control strategies of chopper are1. Pulse width modulation PWM (Variable TON, Constant frequency)2. Frequency modulation (Constant TONor TOFF, Variable frequency)3. Current Limit Control (CLC)[Q] What is delay angle or what is firing angle of phase controlled rectifier?The delay angle is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of thyristors isforward biased. After delay angle() these SCRs are triggered.[Q] What is Universal Motor?It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output.The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.We cannot use these motors in the industrial applications due to the low efficiency (25% -35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.[Q] Give some examples of power electronics applications in the day-to-day life?We can list a huge number of power electronics applications. Few of the applications which we can see in our daily life areUPS Uninterruptible Power SupplySMPS Switch Mode Power SupplySpeed Control of MotorsICU[Q] What is meant by PMDC?PMDC stands for Permanent Magnet DC MotorA Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure.There are three types of permanent magnets used for such motors namely;(i) Alnico Magnets(ii) Ceramic magnets(iii) Rare-earth magnetsThe major advantages are low noise, small size, high-efficiency, low manufacturing cost.

11.A voltmeter is used:

A.to measure current

B.in series with the circuit

C.in parallel with the circuit

D.to measure coulombs


12.If the current in a circuit equals 0 A, it is likely that the

A.voltage is too high

B.resistance is too low

C.circuit has a short

D.circuit is open


13.Current equals:

A.

B.coulombs time

C.

D.voltage time


14.What are the unit and symbol for current?

A.Ampere,A

B.Coulomb,I

C.Ampere,Q

D.Ampere,I


15.Which part of an atom has no electrical charge?

A.Electron

B.Neutron

C.Proton

D.All of the above

16.Which voltage source converts chemical energy to electrical energy?

A.Electrical generator

B.Battery

C.Solar cell

D.Electronic power supply


17.An example of potential energy is:

A.tea-kettle steam

B.a moving vehicle

C.the sun

D.a battery


18.If 40 C of charge flow past a point in 20 s, what is the current?

A.2 AB.0.5 A

C.20 AD.40 A


19.Batteries differ from fuel cells in that

A.a battery is a closed system

B.a battery uses hydrogen and oxygen to create electricity

C.a battery uses a polymer electrolyte membrane

D.none of the above


20.The removal of a non-neutral subatomic particle converts the atom into a:

A.charged ion

B.nucleus

C.heavier element

D.compound

21.One coulomb passing a point in one second is one:

A.ampereB.volt

C.ohmD.charge


22.In which states may matter may be found?

A.solid, liquid, or mineral

B.solid, gas, or liquid

C.mineral, gas, or liquid

D.plastic, solid, or gas


23.Material that consists of a mixture of elements is considered to be:

A.an elementB.an ion

C.a compoundD.a molecule


24.How many valence shell electrons are there in semiconductor atoms?

A.16B.8

C.4D.2


25.When considering conventional current versus electron current flow:

A.electron current flow came first

B.protons move in conventional current flow

C.conventional current flow came first

D.the direction of current is the same in both methods

26.Which unit of charge contains 6.25 1018electrons?

A.An ampereB.A coulomb

C.A voltD.A joule


27.A switch is a device that:

A.short circuits complex circuits

B.holds a fuse

C.has double poles

D.opens or completes a current path


28.The term used to designate electrical pressure is:

A.voltageB.current

C.resistanceD.conductance


29.Which electronics material opposes the movement of free electrons?

A.ConductorB.Insulator

C.SemiconductorD.Element


30.Current is considered as the movement of:

A.electronsB.protons

C.chargeD.nuclei

31.A lead-acid battery is an example of a

A.solar cell

B.fuel cell

C.primary battery

D.secondary battery


32.The charge of one coulomb is equal to:

A.6.24 x 1018electrons

B.one ampere

C.one second

D.6.24 x 1018electrons


33.A basic electric circuit is made up of what components?

A.A load, a resistor, and a conductive path for current

B.A voltage source, a load, and a conductive path for current

C.A voltage source, a conductive path for current, and a battery

D.A conductive path for current, a battery, and a copper wire


34.An ammeter is used to measure

A.voltageB.current

C.resistanceD.All of the above


35.What is the name of the pressure that moves electrons in a closed circuit?

A.amperesB.ohms

C.voltageD.coulombs

36.What is a characteristic of a secondary cell?

A.rechargeability

B.not rechargeable

C.a dry cell

D.non-liquid


37.What are the basic building blocks that all matter is composed of?

A.electrons, neutrons, and protons

B.two protons for each neutron

C.two protons for each electron

D.electrons, neutrons, and charged ions


38.If a fluid system is compared to an electrical system, the fluid pump will correspond to a:

A.conductorB.lamp

C.batteryD.insulator


39.An element is a substance that is:

A.varied throughout the entire piece

B.used only in electronics

C.electrically charged

D.found only in its pure form


40.Current is:

A.the presence of a positive charge

B.the abundance of electrons

C.the movement of electrons

D.the repulsion of electrons

41.What do you call a diagram that shows the electrical connections of a circuit's components?

A.A schematic diagram

B.A pictorial diagram

C.A block diagram

D.An electrical diagram


42.A short circuit will have:

A.a small current flow

B.a large current flow

C.no current flow

D.some current flow


43.The basic difference between a fuse and a circuit breaker is

A.a fuse is slower

B.a fuse is reusable

C.a circuit breaker is reusable

D.a circuit breaker is more reliable


44.Which is the smallest?

A.atomB.proton

C.neutronD.electron


45.A secondary cell generates dc via chemical activity; a primary cell generates dc:

A.electricallyB.thermally

C.opticallyD.chemically

46.Which of the following statements is true?

A.Unlike charges repel and like charges attract.

B.Like charges repel and unlike charges attract.

C.Unlike charges attract and like charges attract.

D.Like charges repel and unlike charges repel.


47.If an electrical system is compared to a fluid system, the electrical current corresponds to the:

A.pressureB.pump

C.water wheelD.water flow

2.In the below figure the average load current is 15 A. The rms value of transformer secondary current is

[A].15 A

[B].10.61 A

[C].7.5 A

[D].14.14 A

1.A cycloconverter can be

A.step down

B.step up

C.step down or step up

D.none of the above


2.In the below figure the average load current is 15 A. The rms value of transformer secondary current is

A.15 A

B.10.61 A

C.7.5 A

D.14.14 A


3.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


4.Two thyristor of same rating and same specifications

A.will have equal turn on and turn off periods

B.will have equal turn on but unequal turn off periods

C.may have equal or unequal turn on and turn off periods

D.will have unequal turn on and turn off periods


5.The amount of dielectric heating is inversely proportional to frequency.

A.True

B.False

6.A semiconverter feeding an RLE load operates in

A.first quadrant only

B.first and second quadrants

C.first and fourth quadrants

D.first and third quadrants


7.The phenomenon of overlap in converter operation due to

A.source resistance

B.source inductance

C.both source resistance and source inductance

D.high value of firing angle


8.A thyristor has a maximum allowable junction temperature of 120C and the ambient temperature is 40C. If thermal resistance is 1.6 C/W, the maximum allowable internal power dissipation is

A.20 W

B.50 W

C.92 W

D.128 W


9.For a 12 pulse operation, the two 6 pulse units are fed by

A.Y - Y and Y - Y transformers

B. - and - transformers

C.Y - Y and Y - transformers

D.Y - and Y - transformers


10.In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is

A.-

B.

C.+

D.- 2

11.An SCR is triggered at 40 in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80, the average anode current is likely to be

A.50 A

B.25 A

C.less than 50 A but more than 25 A

D.less than 25 A


12.The characteristics of a non-linear resistance isi=kv4. Ifibecomes 100 times,vbecomes

A.about 100 times

B.about 10 times

C.about 3 times

D.about twice


13.In a step down chopper using pulse width modulation, Ton= 3 x 10-3and Toff= 1 x 10-3s. The chopping frequency is

A.333.33B.250

C.500D.1000


14.Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.

A.TrueB.False


15.Assertion (A):Inverters and choppers use fast switching thyristorsReason (R):Fast switching SCR has low turn off time.

A.Both A and R are correct and R is correct explanation of A

B.Both A and R correct but R is not correct explanation of A

C.A is correct but R is wrong

D.A is wrong but R is correct

16.The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by

A.forward characteristics of thyristors

B.reverse recovery characteristics of thyristors

C.both forward and reverse recovery characteristics of thyristors

D.none of the above


17.A chopper

A.converts constant voltage dc into ac and then into variable voltage dc

B.converts constant voltage dc into variable voltage dc directly

C.converts ac of one frequency into ac of another frequency

D.converts ac to dc


18.Assertion (A):If the duty cycle of chopper is low the current would not become discontinuousReason (R):If the time constant L/R of the load in chopper is low the current may become discontinuous.






19.In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally

A.equal

B.different

C.equal or different

D.different but sometimes equal


20.An electric heater is controlled by thyristors through on-off control. If= 0.4, the heating is

A.16% maximum

B.40% of maximum

C.60% of maximum

D.84% of maximum

21.A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is Isand rms value of load current in Irms. Then

A.Is= Irmsirrespective of the value of firing angle

B.Is= 0.5 Irmsirrespective of the value of firing angle

C.Is= Irmsif firing angle is less than 30

D.Is= Irmsif firing angle is more than 30


22.The terminals of a power BJT are called

A.emitter, base, collector

B.emitter, base, drain

C.source, base, drain

D.source, base, collector


23.In single phase half wave regulator, the average current over one full cycle

A.is always positive

B.may be positive or negative

C.may be negative

D.is always negative


24.The dynamic equalising circuit consists of a series combination of capacitor C and resistor RCacross each thyristor. This resistance RC(along with parallel connected diode)

A.limits charging current

B.limits discharging current

C.limits both charging and discharging currents

D.none of the above


25.A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current

A.are similar

B.are not similar

C.may be similar or dissimilar

D.are similar only if firing angle is zero

26.Current derating may be necessary when thyristors are operating in

A.series

B.parallel

C.series or parallel

D.none of the above


27.If duty cycle isand the load is a resistance R, the rms value of output voltage is

A.V

B.V

C.2V

D.


28.For a BJT= 0.98, then =

A.0.02B.0.5

C.4.9D.48


29.Assertion (A):Power transistor has lower switching time than SCRReason (R):Power transistor has greater switching time than SCR






30.A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is

A.4B.5

C.6D.8

31.In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be

A.400 V

B.800 V

C.200 V

D.2002V


32.A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When= 0, speed is 500 rpm. If= 45, the speed is likely to be

A.about 350 rpm

B.about 250 rpm

C.about 175 rpm

D.about 125 rpm


33.In a multiphase chopper, all choppers operate together.

A.TrueB.False


34.The terminals of a power MOSFET are called

A.emitter, base, collector

B.source, gate, drain

C.source, base, drain

D.emitter, gate, drain


35.Assertion (A):The surge current which an SCR can withstand is much higher than rms on state current.Reason (R):The duration of surge current is very small.





36.A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is. The period of conduction of SCR and free wheeling diode respectively are

A.< t>ton

D.tonis about twicetoff


40.Assertion (A):The gate current of an SCR is always in the form of pulses.Reason (R):The gate losses, in pulse triggering, are low.





41.UJT can be used for

A.firing an SCR

B.turning off an SCR

C.both firing and turning off an SCR

D.none of the above


42.In a series inverter supplying a load resistance R, the commutating elements L and C should be such that

A.

B.

C.

D.


43.In a full wave converter (M - 2 connection) feeding an RLE load, the peak thyristor current is 15 A. For fully controlled bridge circuit and same load current, the peak thyristor current will be

A.30 A

B.152A

C.15 A

D.


44.Induction heating systems require low frequency inverters.

A.TrueB.False


45.A single phase half wave controlled rectifier circuit is feeding onRLload. The circuit has a freewheeling diode. The input voltage isv= Vmsin t. If I0is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is

A.

B.

C.

D.

46.HVDC transmission systems generally use

A.12 pulse converters

B.6 pulse converters

C.3 pulse converters

D.either 12 or 6 pulse converters


47.A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling diode across the load and firing angle is. During one cycle of input voltage, the total duration of conduction of freewheeling diode is

A.

B.2

C.4

D.0.5


48.During forward blocking state of SCR, the voltage and current respectively are

A.high and high

B.low and low

C.high and low

D.low and high


49.In a 3 phase full converter the average load current is 150 A. The average thyristor current is

A.150 A

B.75A

C.50 A

D.25 A


50.The two modes of operation of three phase bridge inverters are known as

A.60 mode and 120 mode

B.90 mode and 120 mode

C.120 mode and 180 mode

D.120 mode and 240 mode

1.In a dual converter both converters work as rectifiers.

A.TrueB.False


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

A.short duration pulses

B.long duration pulses

C.pulse train

D.either (a) or (b)


3.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 increases with increase in speed

C.load torque decreases with increase in speed

D.either (a) or (b)


4.A cycloconverter cannot give a frequency higher than input frequency.

A.TrueB.False


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

A.114C

B.164C

C.94 C

D.84 C

6.In a single phase dual converter,1, and2are firing angles of the two converters. Then

A.1-2=

B.1+2=

C.1-2=

D.1+2=


7.The number of gates in an SCR is

A.1B.3

C.4D.6


8.The waveshape of output voltage of half bridge inverter is

A.sinusoidal

B.square

C.triangular

D.either (a) or (b)


9.A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/fis constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be

A.about 1350 rpm

B.about 720 rpm

C.about 360 rpm

D.either (b) or (c)


10.The gate current required to turn on an SCR is

A.few amperes

B.a few mA

C.almost equal to anode current

D.about 50% of anode current

11.A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage isv= Vmsin t. The thyristor will conduct

A.from 0 to 180

B.whenv> 50 V

C.from 0 to 90

D.Whenv< 50 V


12.The symbol in figure is for

A.DIAC

B.P-N junction diode

C.Schottky diode

D.None of the above


13.In a 3 phase semiconverter, the firing frequency is

A.equal to line frequency

B.3 times the line frequency

C.6 times the line frequency

D.12 times the line frequency


14.Present day HVDC converters are all

A.3 pulse converters

B.6 pulse converters

C.12 pulse converters

D.either 6 pulse or 12 converters


15.A single phase full converter gives maximum and minimum output voltage at firing angles of

A.0 and 180 respectively

B.180 and 0 respectively

C.0 and 90 respectively

D.90 and 0 respectively

16.A thyristor has its gate current zero. To start conduction the forward voltage must exceed

A.about 0.5 V

B.forward breakover voltage

C.on state voltage

D.peak working off state forward voltage


17.Assertion (A):Snubber circuit has an RC series circuit in parallel with SCRReason (R):Voltage across a capacitor can not change instantaneously.






18.In dielectric heating, the thyristor circuits consist of

A.rectifier-chopper combination

B.controlled rectifier

C.ac regulator

D.rectifier-inverter combination


19.On load tap changer using thyristors can have a maximum of two taps.

A.TrueB.False


20.Assertion (A):A cycloconverter may be line commutated or forced commutatedReason (R):A step up cycloconverter requires forced commutation.





21.In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is

A.200 V

B.2(200) V

C.2(400) V

D.800 V


22.Assertion (A):A GTO circuit is more compact than SCR circuitReason (R):A GTO can be turned off by negative gate pulse.






23.In a single phase full wave ac regulator, the firing angles in the two half cycles

A.are always equal

B.are sometimes equal

C.are never equal

D.may be equal or unequal


24.In a single phase full bridge converter (B - 2 connection), the input voltage isv= Vmsin t. The output dc voltage for resistive load

A.

B.

C.

D.


25.As regards single phase dual converter

A.the change in operation from one quadrant to the other is faster in non-circulating current mode

B.the change in operation from one quadrant to the other is faster in circulating current mode

C.the time response to change in operation from one quadrant to other is the same in both the modes

D.the time response to change in operation from one quadrant to the other may be faster in any of the two modes

26.To protect an SCR against highdi/dtwe connect

A.a capacitance in series with SCR

B.a capacitance in parallel with SCR

C.an inductance in parallel with SCR

D.an inductance in series with SCR


27.In a parallel inverter

A.the commutating capacitor C is across the primary of transformer and the load is across the secondary

B.the commutating capacitor and load are connected across the secondary of transformer

C.the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer

D.none of the above


28.A silicon controlled switch has :

A.2 leads

B.3 leads

C.4 leads

D.5 leads


29.A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is

A.20 mH

B.about 31 mH

C.about 51 mH

D.about 92 mH


30.A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is

A.Vm

B.0.5 Vm

C.zero

D.0.5 Vm- voltage drop across inductance

31.In a semi converter feeding a resistive load, the firing angle is. Each thyristor conducts for

A.radians

B.(-) radians

C.(+) radians

D.(- 0.5) radians


32.Between which points does UJT exhibit negative resistance characteristics

A.between peak and valley points

B.before peak point

C.after peak point

D.after valley point


33.A thyristor has maximum allowable junction temperature of 110 C and the ambient temperature is 40C, If the maximum internal power dissipation is 50 W, the thermal resistance is

A.2 C/W

B.1.4 C/W

C.0.7C/W

D.2.2 C/W


34.A single phase electric heater uses phase controlled thyristors. The firing angle is 90. The degree of heating is

A.25% of maximum

B.50% of maximum

C.75% of maximum

D.none of the above


35.Which of these commutation methods is used in AC-DC converters?

A.Class A

B.Class C

C.Class D

D.Class F

36.When thyristors are connected in series and parallel, it may be necessary to have :

A.current derating

B.voltage derating

C.both current and voltage derating

D.none of the above


37.A single phase half wave regulator uses

A.2 diodes and 2 thyristors

B.2 thyristors

C.1 diode and 1 thyristor

D.2 diodes


38.A Shockley diode can be used

A.in place of SCR

B.as a trigger switch for SCR

C.in place of UJT

D.both in place of SCR and as a trigger switch for SCR


39.An SCR has a turn on time of 4 s. The gate pulse should have a duration of about

A.2 s

B.5 s

C.20 s

D.50 s


40.In a multiphase chopper, the different choppers operate in

A.series and all simultaneously

B.parallel and one at a time

C.parallel and simultaneously

D.either (b) or (c)

41.In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing angle is

A.30B.60

C.90D.120


42.A high frequency series inverter consists of

A.a number of series inverters in series

B.a number of series inverters in parallel

C.a number of parallel inverters in parallel

D.a number of parallel inverters in series


43.In an SCR, the anode current consists of

A.only holes

B.only electrons

C.both electrons and holes

D.either electrons or holes


44.In the below figure, the average value of load current is 15 A. The load is highly inductive. The peak value of thyristor current is

A.15 A

B.152A

C.

D.30 A


45.A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load

A.rectification efficiency will be more than 0.6 and TUF will be more than 0.35

B.rectification efficiency will be more than 0.6 but TUF will be less than 0.35

C.rectification efficiency will be less than 0.6 and TUF will be less than 0.35

D.rectification efficiency will be less than 0.6 but TUF will be more than 0.35

46.Assertion (A):A highdv/dtmay cause triggering of a thyristor.Reason (R):A highdv/dtresults in charging current in the thyristor.






47.Assertion (A):When an SCR is conducting, the voltage drop across is about 1 V.Reason (R):When an SCR is conducting, the junction temperature is about 200C.






48.When the core of a saturable reactor is saturated, the inductance is high.

A.TrueB.False


49.In controlled heating circuits, thyristors act as

A.rectifier

B.inverter

C.ac regulator

D.chopper


50.In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current

A.are always similar

B.can never be similar

C.may be similar or dissimilar

D.are similar only if= 0

1.A chopper cannot be line commutated.

A.TrueB.False


2.Natural commutation can be used in :

A.DC circuits only

B.AC circuits only

C.both AC and DC circuits

D.none of the above


3.In a thyristor the gate lead is connected to thenlayer near theplayer to which anode is connected.

A.TrueB.False


4.A single phase half wave rectifier circuit has a free wheeling diode. The free wheeling diode will conduct only if

A.load is purely resistive

B.load is purely inductive

C.load is combination of R and L

D.load is purely inductive or combination of R and L


5.McMurray Bedford full bridge inverter uses

A.natural commutation

B.auxiliary commutation

C.complementary commutation

D.any of the above

6.Which converter is used in HVDC transmission?

A.3 pulse

B.6 pulse

C.12 pulse

D.24 pulse


7.Assertion (A):The turn on time of SCR is about 1 to 4 ms.Reason (R):The turn off time of SCR is about 10 to 300 ms.






8.An n pulse rectifier is fed by a source having an inductance L. If load current is I0, the voltage drop due to overlap is

A.

B.

C.

D.


9.Which chopper circuit operates in all the 4 quadrants?

A.AB.C

C.DD.E


10.A single phase cycloconverter feeds a resistance heating load. The heating power is provided by

A.only the fundamental component of output wave

B.only higher harmonics in the output wave

C.both fundamental and higher harmonics in the output wave

D.either (a) or (b)

11.A 3 phase fully controlled bridge converter is fed by a 3 phase system having phase voltagev= Vmsin t. The firing angle is. The dc output voltage is

A.

B.

C.

D.


12.Which of these commutation methods uses an external pulse for commutation?

A.Class A

B.Class C

C.Class E

D.Class F


13.In a single phase full wave regulator feeding an R-L load, the power factor angle of load is . The range of control of firing angleis

A.0

B.

C.0 (- )

D. (+ )


14.A separately excited dc motor is run from a dc source through a chopper. At 600 rpm and rated torque, the duty cycle of chopper is 0.8. At 600 rpm and half the rated torque, the duty cycle is likely to be

A.0.9B.0.8

C.0.75D.0.6


15.A relaxation oscillator uses

A.tunnel diode

B.UJT

C.both tunnel diode and UJT

D.PIN diode

16.In a 3 phase bridge inverter with 180 mode of operation the numbers of thyristors conducting at one time are

A.1B.2

C.3D.4


17.A single phase full converter can operate in

A.4 quadrants

B.3 quadrants

C.2 quadrants

D.1 quadrant


18.In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle

A.is zero

B.is 60

C.is more than 60

D.is more than 90


19.In the circuit of figure, the load current when thyristor is off

A.is constant

B.increases exponentially

C.decreases exponentially

D.zero


20.A full bridge inverter is fed by a battery of 30 V. The rms value of output voltage is

A.15 V

B.152V

C.30 V

D.302V

21.In brushless excitation system of modern alternators, the main exciter is

A.dc generator

B.ac generator

C.either dc or ac generator

D.induction generator


22.The two-transistor model of a thyristor consists of two transistors :

A.onen-p-nand otherp-n-p

B.bothp-n-p

C.bothn-p-n

D.onen-p-nand other UJT


23.In a thyristor, the forward breakover voltage

A.is constant

B.may be constant or may depend on gate current

C.decreases as gate current is increased

D.increases as gate current is increased


24.The gate lead, in a thyristor is welded to :

A.player to which anode is connected

B.nlayer nearest to the anode terminal

C.player nearest to the cathode terminal

D.outsidenlayer


25.In a 3 phase full converter, the firing angle is less than 60. The instantaneous output voltage

A.will have positive part only

B.will have both positive and negative parts

C.may have both positive and negative parts

D.will have negative part if load is inductive

26.In a stud mounted SCR, the stud is

A.in thermal contact with heat sink

B.insulated from heat sink

C.in electrical and thermal contact with heat sink

D.in either electrical or thermal contact with heat sink


27.In pulse width modulation of chopper

A.T is kept constant and Tonis varied

B.Tonis kept constant T is varied

C.both T and Tonare varied

D.either T or Tonis varied


28.In a full bridge inverter, the waveshape of load current depends on

A.relative values of load parameters R, L and C

B.relative values of load parameters R and L

C.relative values of load parameters L and C

D.gating frequency


29.The characteristics of which device has peak point and valley point

A.SCR

B.PUT

C.SCS

D.Shockley diode


30.In a step down chopper using time ratio control, the input voltage is V and duty cycle is. The load is purely resistive having resistance R. The average and rms load currents are

A.

B.

C.

D.

31.In a thyristors arei=kva. The value ofais

A.about 0.5

B.about 2

C.about 5

D.about 20


32.In an ordinary thyristor

A.turn on and turn off times are equal

B.turn on time is more than turn off time

C.turn off time is more than turn on time

D.turn on time may be equal, less than or more than turn off time


33.Thyristors connected in series need :

A.static equalizing circuit

B.dynamic equalizing circuit

C.both static and dynamic equalizing circuits

D.none of the above


34.The symbol in figure is for

A.SUS

B.SBS

C.SCS

D.None of the above


35.If 4 quadrant operation is required we need

A.dual converter

B.full converter

C.semi converter

D.none of the above

36.A single phase half wave converter is feeding a resistive load. Output dc power is Pdcand total power delivered to load is Pacthen

A.Pdc= Pacfor all values of firing angles

B.Pdc= Pacif firing angle is less than 180

C.Pdc< Pacfor all values of firing angles

D.Pdc< Pacif firing angle is less than 180


37.Which of the following devices exhibit negative resistance characteristics?

A.UJT

B.PUT

C.Both UJT and PUT

D.UJT, PUT and SBS


38.In integral cycle control of ac regulators, the period of integral cycle control,i.e., (m+n) T should be

A.less than mechanical or thermal time constant of the system

B.more than the mechanical or thermal time constant of the system

C.more or less than the mechanical or thermal time constant

D.very high


39.All other parameters remaining the same, a 3 phase bridge inverter with 180 mode of operation

A.has the same peak thyristor current as in 120 mode of operation

B.has higher peak thyristor current than in 120 mode of operation

C.has lower peak thyristor current than in 120 mode of operation

D.any of the above


40.As compared to BJT, MOSFET has

A.lower switching losses and higher conduction losses

B.higher switching losses and higher conduction losses

C.higher switching losses and lower conduction losses

D.lower switching losses and lower conduction losses

41.If a step down chopper is fed by a dc voltage V with Tonand Toffare on and off periods thenis

A.average output voltage

B.rms output voltage

C.average or rms output voltage depending on load

D.neither of the above


42.A voltage VBBis applied across the terminals of a UJT. The emitter voltage at peak point is

A.hVBB

B.(h + 1) VBB

C.hVBB+ VD

D.VD


43.A single phase bridge type cycloconverter uses eight thyristors.

A.TrueB.False


44.In the circuit of figure, the waveshape of load current when thyristor conducting is

A.constant

B.increasing exponentially

C.decreasing exponentially

D.either (b) or (c)


45.In the basic series inverter circuit, the source supplies load

A.only in positive half cycle

B.both in positive and negative half cycles

C.only in part of positive half cycle

D.in positive half cycle and part of negative half cycle

46.A current source inverter has simple commutation circuit.

A.TrueB.False


47.In load commutated chopper, the commutating element

A.is capacitor

B.is capacitor and inductor

C.is capacitor, inductor and auxiliary thyristor

D.any of the above depending on load


48.In a half bridge inverter each thyristor is gated at a frequencyf. The output frequency is

A.2f

B.f

C.0.5f

D.0.25f


49.Assertion (A):A transistor has lower voltage drop during conduction as compared to thyristorReason (R):Transistors are manufactured in very high voltage and current ratings.






50.Assertion (A):Typical value of thermal resistance from source to sink of an SCR is about 0.3 C/WReason (R):Heat sinks of thyristors are generally made of aluminium.





1.Multiphase choppers are used in

A.low power circuits

B.high power circuit

C.both low and high power circuits

D.medium power circuits


2.In a single phase converter using a centre tapped transformer and with a highly inductive load, the firing angles is. Each thyristor conducts for a period

A.

B.

C.-

D.


3.In a parallel inverter

A.each thyristor is turned on twice during each cycle

B.each thyristor is turned on once during each cycle

C.each thyristor is turned on either once or twice in each load cycle

D.each thyristor may be turned on upto four times in each load cycle


4.A step up chopper can give an output voltage

A.higher than input voltage

B.lower than input voltage

C.both higher and lower input voltage

D.all of the above


5.The specified maximum junction temperature of thyristors is about

A.50CB.80C

C.120CD.200C

6.A bridge type single phase cycloconverter changes the frequencyftof/3. Then one half wave of output contains

A.3 full waves of input

B.3 half waves of input

C.6 full waves of input

D.6 half waves of input


7.A half bridge inverter

A.needs a 3 wire dc supply

B.may use a two wire or 3 wire dc supply

C.has higher output voltage if fed from 3 wire dc supply

D.has higher efficiency if fed from 3 wire dc supply


8.A 3 phase series inverter has

A.12 thyristors

B.6 thyristors

C.3 thyristors

D.3 or 6 thyristors


9.In figure, thyristor Th is in off state. When thyristor is turned on, the peak thyristor current can be

A.5 A

B.10 A

C.15 A

D.20 A


10.HVDC systems are preferable for only long distances.

A.TrueB.False

11.Surge current rating of an SCR is

A.the maximum repetitive surge current in which the SCR can withstand

B.the maximum non-repetitive surge current in which the SCR can withstand

C.the maximum repetitive or non repetitive surge current in which the SCR can withstand

D.the maximum surge current in which the SCR can withstand if both surge current and steady current exist together


12.Which of the following devices has metal-silicon junction?

A.General purpose power diode

B.Schottky diode

C.SCR

D.MOSFET


13.In a single phase full wave converter (M-2 connection) feeding on R-L load the input voltage isv= Vmsin t. The expression for rms output voltage is

A.

B.

C.

D.


14.A dc chopper is feeding an RLE load. The condition for maximum ripple is

A.= 1

B.= 0.8

C.= 0.5

D.< 0.5


15.In a series inverter

A.the commutating elements L and C are in parallel

B.the commutating elements L and C are in series with load

C.the commutating L and C may be in series or parallel with the load

D.none of the above

16.When a power diode is conducting, the voltage drop across it is about

A.I V

B.5 V

C.20 V

D.50 V


17.In thermal ohm's law, temperature and power loss are analogous to

A.V and I respectively of ohm's law

B.I and V respectively of ohm's law

C.R and V respectively of ohm's law

D.I and R respectively of ohm's law


18.A string of series connected thyristors needs equalising circuit to

A.ensure proper firing

B.ensure proper current division

C.ensure proper voltage division

D.none of the above


19.A surge voltage

A.has very high magnitude and very long duration

B.has very high magnitude and very small duration

C.has very high magnitude and very small duration and can be positive or negative

D.can be only positive


20.The minimum gate current which can turn on SCR is called :

A.trigger current

B.holding current

C.junction current

D.breakover current

21.The total number of leads in SUS, SBS and SCS respectively are

A.3, 3, 4,

B.3, 3, 3

C.2, 3, 3

D.3, 3, 5


22.Which of these devices can conduct in both directions?

A.SCRB.SUS

C.SBSD.UJT


23.Assertion (A):A half wave controlled rectifier has poorer efficiency and higher ripple factor than a full wave controlled rectifier.Reason (R):The use of a freewheeling diode in half wave controlled rectifier circuit improves the waveform of load current and circuit power factor.






24.A 3 phase dual converter consists of

A.two 3 phase semiconverters connected in parallel

B.two 3 phase semiconverters connected in antiparallel

C.two 3 phase full converters connected in antiparallel

D.two 3 phase full converters connected in parallel


25.Which inverter circuit uses coupled inductors?

A.Modified McMurray full bridge inverter

B.McMurray Bedford half bridge inverter

C.Modified McMurray Bedford half bridge inverter

D.Both (b) and (c)

Preguntas y Respuestas

Documents

Transcript of Preguntas y Respuestas