MCQs on Power Electronics
02․ An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be
03․ An SCR is considered to be a semi controlled device because
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
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
In this case reading current will flow in circuit and it will became zero so thyristor will be load commutated .
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 ?
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
Generally latching current is 2 or 3 times of holding current.
09․ In reverse blocking mode of a thyristor
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
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
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?
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
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 ?
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?
16․ Let of a thyristor Vc1, Vc2, Vc3 are forward break over voltage for gate current Ig1, Ig2, Ig3 respectively. Then
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
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
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
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