POWER ELECTRONICS PART ONE
Instructions: Click the answer button to see the correct answer.
- The typical value of SCR for modern alternator is
- 1.5.
- 1.0.
- 0.5.
- 1.2.
Answerc
- An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be
- 4242.64 A.
- 2121.32 A.
- 6000 A.
- 1500 A.
Answerb
- An SCR is considered to be a semi controlled device because
- it can be turned OFF but not ON with a gate pulse.
- it can be turned ON but not OFF with a gate pulse.
- it conducts only during one half cycle of an alternating current wave.
- 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- 30° to 150°.
- 60° to 180°.
- 30° to 180°.
- 60° to 120°.
Answera
- A single phase full bridge inverter can operated in load commutation mode in case load consist of
- RLC overdamped.
- RLC underdamped.
- RL.
- RLC critically damped.
Answerb
- A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is
- 0.27.
- 0.32.
- 0.45.
- 0.26
Answerd
- Which statement is true for latching current ?
- It is related to turn on process of the device.
- It is related to turn off process of the device.
- all of these
- It is related to conduction process of device.
Answera
- If holding current of a thyristor is 2 mA then latching current should be
- 0.004 A.
- 0.009 A.
- 0.002 A.
- 0.01 A.
Answera
- In reverse blocking mode of a thyristor
- junction J2 is in reverse bias and J1, J3 is in forward bias.
- junction J3 is in forward bias and J1, J2 in reverse bias.
- junction J1 and J2 is in forward bias and J3 is in reverse bias.
- junction J1, J3 is in reverse bias and J2 is in forward bias.
Answerd
- In forward blocking mode of a thyristor
- Junction J1, J3 is in reverse bias and J2 is in forward bias.
- junction J2 is in reverse bias and J1, J3 is in forward bias.
- junction J3 is in forward bias and J1, J2 is in reverse bias.
- Junction J1 and J2 is in forward bias and J3 is in reverse bias.
Answerb
- Leakage current flows through the thyristor in
- forward blocking mode.
- reverse blocking mode.
- forward conduction mode.
- both forward and reverse blocking mode.
Answerd
- Which triggering is the most reliable?
- Thermal triggering.
- Forward voltage triggering.
- Gate triggering.
- dV / dt triggering.
Answerc
- Light triggering mainly used in
- low – voltage direct current transmission.
- high voltage direct current transmission.
- all of these.
- medium voltage direct current transmission.
Answerb
- In a P1N1P2N2 thyristor which layer is less doped ?
- P1
- N1
- P2
- N2
Answerb
- Let of a thyristor Vc1, Vc2, Vc3 are forward break over voltage for gate current Ig1, Ig2, Ig3 respectively. Then
- Vc1 > Vc2 > Vc3 when Ig1 > Ig2 > Ig3.
- All of these
- Vc1 = Vc2 = Vc3 any value of Ig.
- Vc1 > Vc2 > Vc3 when Ig1 < Ig2 < Ig3.
Answerd
- SCR will be turned off when anode current is
- both < latching current but greater than holding current and gate signal is 0 & less than holding current.
- < latching current but greater than holding current and gate signal is present.
- less than holding current.
- < latching current but greater than holding current and gate signal is 0.
Answera
- After proper turn on of thyristor
- none of these
- gate signal is always present.
- gate signal must be removed.
- gate signal should present but can be removed.
Answerc
- Gate circuit or triggering circuit of a thyristor is
- magnetic circuit.
- lower power circuit.
- high power circuit.
- may be low power or high power circuit.
Answerb
- The latching current of SCR is 20 mA. Its holding current will be
- 40 mA.
- 60 mA.
- 10 mA.
- 23 mA.
Answerc
- Thyristor can be protected from over voltages by using
- voltage clamping device.
- heat sink.
- snubber circuit.
- fuse.
Answera
- Example of a voltage clamping device
- aluminium block.
- metal oxide varistor.
- snubber circuit.
- fast acting fuse.
Answerb
- Under over voltage condition impedance offered by the voltage clamping device is
- low.
- infinity.
- moderate.
- high.
Answera
- Under normal operating condition voltage clamping device offers impedance of
- low value.
- moderate value.
- zero value.
- high value.
Answerd
- What is used to protect the SCR from over current ?
- CB and fuse.
- Heat sink.
- Voltage clamping device.
- Snubber circuit.
Answera
- CB used for over current protection of thyristor operates when the fault current is
- of long period.
- of short duration.
- neither
- 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 ?- Fast acting current limiting device (FACL fuse).
- Voltage clamping device.
- Snubber circuit.
- CB.
Answera
- How can we protect SCR from thermal conditions ?
- Using CB and fuse.
- Use of snubber circuit.
- Using equalizing circuit.
- Using heat sink.
Answerd
- Which of the following is used in heat sink?
- Carbon.
- Aluminium.
- Silver.
- Iron.
Answerb
- Materials used in heat sink should have
- large surface area.
- All of these.
- high thermal conductivity.
- high melting point.
Answerb
- What is used to protect a thyristor from high di / dt conditions?
- Inductor.
- Snubber circuit.
- Voltage clamping device.
- Fuse.
Answera
- Which of the following is used in SCR to protect from high dV / dt?
- Snubber circuit.
- Fuse.
- Equalizing circuit.
- Circuit breaker.
Answera
- What happen due to high di / dt?
- None of these.
- Insulation failure.
- Local hot spot.
- Breakdown of junction.
Answerc
- What may happen high dV / dt?
- Breakdown of J2 junction.
- Anyone of these.
- Both
- Unwanted turn ON.
Answerc
- Why resistor is used in Snubber circuit ?
- all of these
- To minimize the discharging current.
- To minimize the loss
- To minimize the charging current.
Answerb
- Snubber circuit is used with SCR
- in parallel.
- in series.
- either series or parallel.
- anti parallel.
Answera
- Delay time is defined by the interval when
- all of these.
- anode current reaches 10 % from forward leakage current.
- anode voltage drops from 100 % to 90 % of its actual value.
- gate current increases from 90 % to 100 % of its final value.
Answera
- Rise time is defined by the interval when
- both
- anode voltage drops from 90 % to 10 % of its initial value.
- none of these
- anode current rises 10 % to 90 % of its final value.
Answera
- Spread time is defined as the interval during which
- anode current rises from 90 % to its final value.
- none of these
- anode current rises from 10 % to 90 % of its final value.
- anode voltage drops from 10 % of its initial value to zero.
Answerd
- Maximum power loss occurs during
- rise time.
- spread time.
- all.
- delay time.
Answera
- The typical time of rising time lies between
- 40 – 60 µs.
- 1 – 4 µs.
- 10 – 20 µs.
- 90 – 100 µs.
Answerb
- During which time maximum conduction spreading take place in the thyristor during turn ON?
- Same for every case.
- Spread time.
- Delay time.
- Rise time.
Answerb
- During reverse recovery time
- charge carrier of junction J3 is swept out.
- charge carrier of junction J1&J3 is swept out.
- charge carrier of junction J1 is swept out.
- charge carrier of junction J2 recombined.
Answerb
- During gate recovery time
- charge carriers of J3 junction is removed.
- charge carrier of J1 junction removed.
- charge carriers of J2 junction is swept out.
- charge carriers of J2 junction recombined.
Answerd
- Typical range of thyristor turn OFF time is
- 3 – 10 µs.
- 3 – 50 µs.
- 3 – 100 µs.
- 3 – 500 µs.
Answerc
- Which statement is true ?
- All of these.
- Reverse recovery time ( trr ) > gate recovery time (tgr).
- Device turn OFF time ( tq ) > reverse recover time (trr).
- Circuit turn OFF time > device turn OFF time ( tq ).
Answera
- ON state voltage drop across SCR lie between the range
- 0 – 0.5 V.
- 1.5 – 2 V.
- 1 – 1.5 V.
- 0.5 – 1 V.
Answerc
- SCRs are used in series to meet
- high voltage demand
- low current demand.
- high current demand.
- low voltage demand.
Answera
- By which one of the following we can measure the reliability of a string?
- Factor of safety.
- String efficient.
- Derating factor.
- 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- 6, 4.
- 4, 6.
- 5, 7.
- 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- 83.3 %.
- 91.6 %.
- 76.3 %.
- 90.9 %
Answerc