POWER ELECTRONICS PART ONE

Instructions: Click the answer button to see the correct answer.

The typical value of SCR for modern alternator is
1. 1.5.
2. 1.0.
3. 0.5.
4. 1.2.
Answerc
An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be
1. 4242.64 A.
2. 2121.32 A.
3. 6000 A.
4. 1500 A.
Answerb
An SCR is considered to be a semi controlled device because
1. it can be turned OFF but not ON with a gate pulse.
2. it can be turned ON but not OFF with a gate pulse.
3. it conducts only during one half cycle of an alternating current wave.
4. it can be turned ON only during one half cycle of an AC.
Answerb
A
single phase one pulse controlled circuit has a resistance R and
counter emf E load 400 sin(314 t) as the source voltage. For a load
counter emf of 200 V, the range of firing angle control is
1. 30° to 150°.
2. 60° to 180°.
3. 30° to 180°.
4. 60° to 120°.
Answera
A single phase full bridge inverter can operated in load commutation mode in case load consist of
1. RLC overdamped.
2. RLC underdamped.
3. RL.
4. RLC critically damped.
Answerb
A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is
1. 0.27.
2. 0.32.
3. 0.45.
4. 0.26
Answerd
Which statement is true for latching current ?
1. It is related to turn on process of the device.
2. It is related to turn off process of the device.
3. all of these
4. It is related to conduction process of device.
Answera
If holding current of a thyristor is 2 mA then latching current should be
1. 0.004 A.
2. 0.009 A.
3. 0.002 A.
4. 0.01 A.
Answera
In reverse blocking mode of a thyristor
1. junction J2 is in reverse bias and J1, J3 is in forward bias.
2. junction J3 is in forward bias and J1, J2 in reverse bias.
3. junction J1 and J2 is in forward bias and J3 is in reverse bias.
4. junction J1, J3 is in reverse bias and J2 is in forward bias.
Answerd
In forward blocking mode of a thyristor
1. Junction J1, J3 is in reverse bias and J2 is in forward bias.
2. junction J2 is in reverse bias and J1, J3 is in forward bias.
3. junction J3 is in forward bias and J1, J2 is in reverse bias.
4. Junction J1 and J2 is in forward bias and J3 is in reverse bias.
Answerb
Leakage current flows through the thyristor in
1. forward blocking mode.
2. reverse blocking mode.
3. forward conduction mode.
4. both forward and reverse blocking mode.
Answerd
Which triggering is the most reliable?
1. Thermal triggering.
2. Forward voltage triggering.
3. Gate triggering.
4. dV / dt triggering.
Answerc
Light triggering mainly used in
1. low – voltage direct current transmission.
2. high voltage direct current transmission.
3. all of these.
4. medium voltage direct current transmission.
Answerb
In a P1N1P2N2 thyristor which layer is less doped ?
1. P1
2. N1
3. P2
4. N2
Answerb
Let of a thyristor Vc1, Vc2, Vc3 are forward break over voltage for gate current Ig1, Ig2, Ig3 respectively. Then
1. Vc1 > Vc2 > Vc3 when Ig1 > Ig2 > Ig3.
2. All of these
3. Vc1 = Vc2 = Vc3 any value of Ig.
4. Vc1 > Vc2 > Vc3 when Ig1 < Ig2 < Ig3.
Answerd
SCR will be turned off when anode current is
1. both < latching current but greater than holding current and gate signal is 0 & less than holding current.
2. < latching current but greater than holding current and gate signal is present.
3. less than holding current.
4. < latching current but greater than holding current and gate signal is 0.
Answera
After proper turn on of thyristor
1. none of these
2. gate signal is always present.
3. gate signal must be removed.
4. gate signal should present but can be removed.
Answerc
Gate circuit or triggering circuit of a thyristor is
1. magnetic circuit.
2. lower power circuit.
3. high power circuit.
4. may be low power or high power circuit.
Answerb
The latching current of SCR is 20 mA. Its holding current will be
1. 40 mA.
2. 60 mA.
3. 10 mA.
4. 23 mA.
Answerc
Thyristor can be protected from over voltages by using
1. voltage clamping device.
2. heat sink.
3. snubber circuit.
4. fuse.
Answera
Example of a voltage clamping device
1. aluminium block.
2. metal oxide varistor.
3. snubber circuit.
4. fast acting fuse.
Answerb
Under over voltage condition impedance offered by the voltage clamping device is
1. low.
2. infinity.
3. moderate.
4. high.
Answera
Under normal operating condition voltage clamping device offers impedance of
1. low value.
2. moderate value.
3. zero value.
4. high value.
Answerd
What is used to protect the SCR from over current ?
1. CB and fuse.
2. Heat sink.
3. Voltage clamping device.
4. Snubber circuit.
Answera
CB used for over current protection of thyristor operates when the fault current is
1. of long period.
2. of short duration.
3. neither
4. both
Answera
When
a large surge current of very short duration flows through a thyristor
then which one of the following device will operate to protect the
thyristor ?
1. Fast acting current limiting device (FACL fuse).
2. Voltage clamping device.
3. Snubber circuit.
4. CB.
Answera
How can we protect SCR from thermal conditions ?
1. Using CB and fuse.
2. Use of snubber circuit.
3. Using equalizing circuit.
4. Using heat sink.
Answerd
Which of the following is used in heat sink?
1. Carbon.
2. Aluminium.
3. Silver.
4. Iron.
Answerb
Materials used in heat sink should have
1. large surface area.
2. All of these.
3. high thermal conductivity.
4. high melting point.
Answerb
What is used to protect a thyristor from high di / dt conditions?
1. Inductor.
2. Snubber circuit.
3. Voltage clamping device.
4. Fuse.
Answera
Which of the following is used in SCR to protect from high dV / dt?
1. Snubber circuit.
2. Fuse.
3. Equalizing circuit.
4. Circuit breaker.
Answera
What happen due to high di / dt?
1. None of these.
2. Insulation failure.
3. Local hot spot.
4. Breakdown of junction.
Answerc
What may happen high dV / dt?
1. Breakdown of J2 junction.
2. Anyone of these.
3. Both
4. Unwanted turn ON.
Answerc
Why resistor is used in Snubber circuit ?
1. all of these
2. To minimize the discharging current.
3. To minimize the loss
4. To minimize the charging current.
Answerb
Snubber circuit is used with SCR
1. in parallel.
2. in series.
3. either series or parallel.
4. anti parallel.
Answera
Delay time is defined by the interval when
1. all of these.
2. anode current reaches 10 % from forward leakage current.
3. anode voltage drops from 100 % to 90 % of its actual value.
4. gate current increases from 90 % to 100 % of its final value.
Answera
Rise time is defined by the interval when
1. both
2. anode voltage drops from 90 % to 10 % of its initial value.
3. none of these
4. anode current rises 10 % to 90 % of its final value.
Answera
Spread time is defined as the interval during which
1. anode current rises from 90 % to its final value.
2. none of these
3. anode current rises from 10 % to 90 % of its final value.
4. anode voltage drops from 10 % of its initial value to zero.
Answerd
Maximum power loss occurs during
1. rise time.
2. spread time.
3. all.
4. delay time.
Answera
The typical time of rising time lies between
1. 40 – 60 µs.
2. 1 – 4 µs.
3. 10 – 20 µs.
4. 90 – 100 µs.
Answerb
During which time maximum conduction spreading take place in the thyristor during turn ON?
1. Same for every case.
2. Spread time.
3. Delay time.
4. Rise time.
Answerb
During reverse recovery time
1. charge carrier of junction J3 is swept out.
2. charge carrier of junction J1&J3 is swept out.
3. charge carrier of junction J1 is swept out.
4. charge carrier of junction J2 recombined.
Answerb
During gate recovery time
1. charge carriers of J3 junction is removed.
2. charge carrier of J1 junction removed.
3. charge carriers of J2 junction is swept out.
4. charge carriers of J2 junction recombined.
Answerd
Typical range of thyristor turn OFF time is
1. 3 – 10 µs.
2. 3 – 50 µs.
3. 3 – 100 µs.
4. 3 – 500 µs.
Answerc
Which statement is true ?
1. All of these.
2. Reverse recovery time ( trr ) > gate recovery time (tgr).
3. Device turn OFF time ( tq ) > reverse recover time (trr).
4. Circuit turn OFF time > device turn OFF time ( tq ).
Answera
ON state voltage drop across SCR lie between the range
1. 0 – 0.5 V.
2. 1.5 – 2 V.
3. 1 – 1.5 V.
4. 0.5 – 1 V.
Answerc
SCRs are used in series to meet
1. high voltage demand
2. low current demand.
3. high current demand.
4. low voltage demand.
Answera
By which one of the following we can measure the reliability of a string?
1. Factor of safety.
2. String efficient.
3. Derating factor.
4. Reliability factor.
Answerc
A
thyristor string is made of a no. of SCR connected in series and
parallel. The string have volume and current of 11 KV and 4 KA. The
voltage and current rating of available SCRs are 1800 V and 1000 A. For a
string efficiency of 90 % let the number of SCRs in series and parallel
are a and b respectively. Then the value of a and b will be
1. 6, 4.
2. 4, 6.
3. 5, 7.
4. 7, 5.
Answerd
60
thyrsistors are connected in series and parallel to form a 10 KV and
5.5 KA switch. Each thyristor is rated for 1.2 KV, 1 KA. The no. of
parallel path are 6. The efficiency of the switch is
1. 83.3 %.
2. 91.6 %.
3. 76.3 %.
4. 90.9 %
Answerc

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POWER ELECTRONICS PART ONE

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