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Test: Thyristor Commutation Techniques - Electrical Engineering (EE) MCQ


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10 Questions MCQ Test GATE Electrical Engineering (EE) 2024 Mock Test Series - Test: Thyristor Commutation Techniques

Test: Thyristor Commutation Techniques for Electrical Engineering (EE) 2024 is part of GATE Electrical Engineering (EE) 2024 Mock Test Series preparation. The Test: Thyristor Commutation Techniques questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Thyristor Commutation Techniques MCQs are made for Electrical Engineering (EE) 2024 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Thyristor Commutation Techniques below.
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Test: Thyristor Commutation Techniques - Question 1

To turn-off or commutate a thyristor

Detailed Solution for Test: Thyristor Commutation Techniques - Question 1

To turn-off a thyristor, anode current must be reduced below holding current and a reverse bias must be applied across thyristor for a finite period of time. If all these two conditions are not met simultaneously then forced commutation can be used to turn-off the thyristor.

Test: Thyristor Commutation Techniques - Question 2

Most SCRs can be turned-off by voltage reversal during negative half cycle of the ac supply for

Test: Thyristor Commutation Techniques - Question 3

Turn-off time of a thyristor

Test: Thyristor Commutation Techniques - Question 4

Turn-on time (ton) of an SCR is related to its turn-off time (toff) in which of the following way?

Detailed Solution for Test: Thyristor Commutation Techniques - Question 4

The turn-on time of an SCR depends on time constant (L/R) of the load.

Test: Thyristor Commutation Techniques - Question 5

Assertion (A) : It is necessary to keep the thyristor reverse biased for a finite period before a forward anode voltage can be reapplied.
Reason (R) : Once the SCR starts conducting an appreciable forward current, the gate has no control on it and the device can be brought back to the blocking state only by reducing the forward current level below that of the holding current.

Detailed Solution for Test: Thyristor Commutation Techniques - Question 5

It is necessary to keep the thyristor reverse biased for a finite period before a forward anode voltage can be reapplied because if a forward voltage is applied immediately after reducing the anode current to zero, it will not block the forward voltage and will start conducting again, although it is not triggered by a gate pulse. Reason is also a true statement but not the correct explanation of assertion.

Test: Thyristor Commutation Techniques - Question 6

Match List-I (Commutation techniques) with List-ll (Alternative name) and select the correct answer using the codes given below the lists:
List-I
A. Class-E
B. Class-F
C. Class-B, C and D
D. Class-A

List-ll
1. Forced commutation
2. Load commutation
3. External-pulse commutation
4. Line commutation

Test: Thyristor Commutation Techniques - Question 7

Consider the following statements associated with various commutation techniques of a thyristor:
1. In forced commutation, the commutating components L and C carry load current continuously.
2. Forced commutation is usually employed in dc choppers.
3. In load commutation, L and Care connected in parallel with the load or C in series with the load.
4. Load commutation is employed in series inverters.
5. In class-C commutation, the main thyristor that is to be commutated is connected in parallel with the load.

Which of the statements given above are correct?

Detailed Solution for Test: Thyristor Commutation Techniques - Question 7
  • In forced commutation, the commutating components L and C do not carry load current continuously.
  • Forced commutation is usually employed in dc choppers because natural current zero is not available in the circuits containing dc choppers.
  • In load commutation. L and Care connected in series with the load or C in parallel with the load such that overall load circuit is underdamped.
  • Load commutation is employed in series inverters.
  •  In class-C commutation, the main thyristor that is to be commutated is connected in series with the load while the complementary thyristor is connected in parallel with the main thyristor as shown below.

Thus, statements 2 and 4 are only correct.

Test: Thyristor Commutation Techniques - Question 8

For the class-B commutation circuit shown below, the initial voltage across capacitor is 230 volt. For a constant load current of 300 A, the conduction time for the auxiliary thyristor is approximately equal to

Detailed Solution for Test: Thyristor Commutation Techniques - Question 8

Given, L = 5μH, C = 20μF
Resonant frequency,

∴ Conduction time for auxiliary thyristor

Test: Thyristor Commutation Techniques - Question 9

For the circuit shown below, the peak value of the current through thyristors T1 and T2 are

Detailed Solution for Test: Thyristor Commutation Techniques - Question 9

Given, Vs = 200 V, R= 10 Ω, and R2 = 100 Ω
Given circuit represents a class-C commutation circuit.
The peak value of current through thyristor T1 is


The peak value of current through thyristor T2 is

Test: Thyristor Commutation Techniques - Question 10

The SCR in the circuit shown below is forced commutated by a circuitry not shown in the figure. The SCR has minimum charging current of 5 mA to turn it on and its junction capacitance is 25 pF.

Detailed Solution for Test: Thyristor Commutation Techniques - Question 10

Under steady-state condition, voltage across capacitor is Vs = 200 V and SCR conducts having a current through it equal to

When the SCR is forced commutated, the capacitor voltage at time t is


 ...(i)
Let Cj be the junction capacitance of SCR. Now, when SCR get turned-on, the

or, 
= 0.02 μF
Thus, the minimum value of C so that SCR does not get turn-on is 0.02 μF.
If C > 0.02 μF, SCR will be turn-on.

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