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MCQs on Power Electronics

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01․ The typical value of SCR for modern alternator is
1.5.
0.5.
1.0.
1.2.

Always the typical value of SCR for modern alternator is 0.5.
02․ An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be
1500 A.
6000 A.
2121.32 A.
4242.64 A.

03․ An SCR is considered to be a semi controlled device because
it can be turned OFF but not ON with a gate pulse.
it conducts only during one half cycle of an alternating current wave.
it can be turned ON but not OFF with a gate pulse.
it can be turned ON only during one half cycle of an AC.

During positive half cycle SCR is in forward blocking mode. By applying gate pulse the SCR can be turned ON during forward blocking mode. But SCR can not be turned OFF by applying gate pulse. It automatically turns off when anode current is below the Holding current. That is why it is called semi controlled rectifier.
04․ 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°.
30° to 180°.
60° to 120°.
60° to 180°.

So, control range is θ to ( π- θ ) i.e. 30° to 150°.
05․ A single phase full bridge inverter can operated in load commutation mode in case load consist of
RL.
RLC underdamped.
RLC overdamped.
RLC critically damped.

In this case reading current will flow in circuit and it will became zero so thyristor will be load commutated .
06․ A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is
0.32.
0.67.
0.45.
0.27.

For step-up chopper, E0=Edc/(1-α), E0 = output voltage and Edc = output voltage,α = 1-(110/150) ⇒ 0.26 where α = duty cycle. recheck
07․ Which statement is true for latching current ?
It is related to turn off process of the device.
It is related to conduction process of device.
It is related to turn on process of the device.
Both C and D.

Latching current is the minimum required anode current to turn on SCR.
08․ If holding current of a thyristor is 2 mA then latching current should be
0.01 A.
0.002 A.
0.009 A.
0.004 A.

Generally latching current is 2 or 3 times of holding current.
09․ 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, J3 is in reverse bias and J2 is in forward bias.
junction J1 and J2 is in forward bias and J3 is in reverse bias.

In reverse blocking mode J1 and J2 is in reverse bias and J2 is in forward bias.
10․ In forward 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 is in reverse bias.
Junction J1, J3 is in reverse bias and J2 is in forward bias.
Junction J1 and J2 is in forward bias and J3 is in reverse bias.

In forward blocking mode junction J2 acts in reverse bias whereas J1 and J3 act in forward bias.
11․ Leakage current flows through the thyristor in
forward blocking mode.
reverse blocking mode.
both forward and reverse blocking mode.
forward conduction mode.

In forward blocking mode junction J2 and in reverse blocking mode junction J1 and J3 act as a capacitor hence leakage current present in both mode.
12․ Which triggering is the most reliable?
Forward voltage triggering.
Gate triggering.
dV / dt triggering.
Thermal triggering.

Gate triggering process is the most reliable as junction temperature does not rise to high value in this process.
13․ Light triggering mainly used in
low - voltage direct current transmission.
medium voltage direct current transmission.
high voltage direct current transmission.
all of these.

Light triggering combined with gate triggering mainly used in HVDC as this combination is very powerful triggering process.
14․ In a P1N1P2N2 thyristor which layer is less doped ? width=176
P1.
N1.
P2.
N2.

H2 ⇒ Highly doped. P2 ⇒ Less highly doped. N1 ⇒ Less doped. P1 ⇒ Same as P2.
15․ If a latching current for the circuit shown in figure is 2 mA. Obtain the value of minimum width of the property turn ON the SCR? mcq-power-electronics-8
3 µs.
3.1 µs
3.2 µs
3.3 µs

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16․ 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 >
Vc1 > Vc2 > Vc3 when Ig1 < Ig2 < Ig3.
Vc1 = Vc2 = Vc3 any value of Ig.
Vc1 > Vc2 > Vc3 when Ig1= Ig2

If gate current is increased then thyristor will shift from forward blocking mode to forward conducting mode at a lower voltage.
17․ SCR will be turned off when anode current is
< 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.
both (A) and (B).

When anode current is in between holding current and latching current then SCR is off only if gate signal is absent. When anode current is below holding current then it is properly turned off.
18․ After proper turn on of thyristor
gate signal is always present.
gate signal must be removed.
gate signal should present but can be removed.
none of the above.

After proper turn on of thyristor gate signal must be removed to reduce the loss in gate circuit.
19․ Gate circuit or triggering circuit of a thyristor is
lower power circuit.
high power circuit.
magnetic circuit.
may be low power or high power circuit.

Thyristor circuit is high power circuit. But its triggering circuit is low power circuit.
20․ A thyristor will be triggered when Vg = 1.5 volt and Ig = 100 mA in the given figure. Calculate the value of R in ohm is mcq-power-electronics-9
65.
3.714.
37.14.
60.

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