Test: Thyristors Ratings & Thyristors Protection

# Test: Thyristors Ratings & Thyristors Protection

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## 20 Questions MCQ Test Power Electronics | Test: Thyristors Ratings & Thyristors Protection

Test: Thyristors Ratings & Thyristors Protection for Electrical Engineering (EE) 2023 is part of Power Electronics preparation. The Test: Thyristors Ratings & Thyristors Protection questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Thyristors Ratings & Thyristors Protection MCQs are made for Electrical Engineering (EE) 2023 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Thyristors Ratings & Thyristors Protection below.
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Test: Thyristors Ratings & Thyristors Protection - Question 1

### The voltage safety factor (VSF) for an SCR is the ratio of

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 1

It is peak repetitive reverse voltage/ the maximum value of input voltage(Vm).

Test: Thyristors Ratings & Thyristors Protection - Question 2

### The forward dv/dt rating of an SCR

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 2

If the temperature is high, lesser dv/dt is required to turn on the device as the higher temperature has already excited few of the holes & electrons.

Test: Thyristors Ratings & Thyristors Protection - Question 3

### The finger voltage of an SCR is

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 3

Finger voltage is the minimum value of Vak (anode to cathode voltage) to turn on the device with gate triggering, it is to be avoided for accidental turn-on of the device.

Test: Thyristors Ratings & Thyristors Protection - Question 4

Which among the following anode current waveforms will have the minimum junction temperature?

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 4

N % DC is nothing but a wave with n % duty cycle. Lower the Duty cycle lesser is the current flowing & lesser is the temperature dissipation.

Test: Thyristors Ratings & Thyristors Protection - Question 5

An SCR has half cycle surge current rating of 3000 A for 50 Hz. Calculate its one-cycle surge current rating

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 5

By equating the energies involved in one cycle & subcycle
I2.T = Isb2.t
Isb = 3000 A
T = 1/50
t = T/2.

Test: Thyristors Ratings & Thyristors Protection - Question 6

For a SCR the maximum rms on-state current is 35 A. If the SCR is used in a resistive circuit for a rectangular wave with conduction angle of 90°. Calculate the average & rms currents respectively.

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 6

First convert the conduction angle into N for rectangular wave.
N = 360°/Conduction angle = 4
Therefore, Iavg = I/4
Irms = I/√4 = I/2.

Test: Thyristors Ratings & Thyristors Protection - Question 7

For an SCR the average & rms values of current are I/4 & I/2 respectively. Calculate the average on-state current rating (ITAV). Take maximum RMS on-state current = 35 A.

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 7

Form factor = RMS/Average vaules of current
Therefore, FF = 2
ITAV = 35/2 = 17.5.

Test: Thyristors Ratings & Thyristors Protection - Question 8

The amp2-sec rating of the SCR specifies

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 8

The amp2-sec is the energy that the device can handle before the fault is cleared. Its value decides how fast the fault has to be cleared to avoid damage to the device.

Test: Thyristors Ratings & Thyristors Protection - Question 9

The maximum rms current of an SCR is 50 A. For a 120° sine wave conduction the form factor (FF) = 1.878

Find the average on-state current rating (ITAV).

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 9

ITAV = 50/FF.

Test: Thyristors Ratings & Thyristors Protection - Question 10

The thermal resistance between junction & the SCR (θjc) has the unit

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 10

Thermal resistance always has the unit degree temperature per watt.

Test: Thyristors Ratings & Thyristors Protection - Question 11

di/dt protection is provided to the thryistor by

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 11

By placing the di/dt inductor (L) in series with the load, the change in the anode current can be limited to a small value.

Test: Thyristors Ratings & Thyristors Protection - Question 12

The local hot spot formation in the cross-section of the SCR is avoided by

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 12

Applying the higher gate current spreads the ions quickly and avoids hotspot formation.

Test: Thyristors Ratings & Thyristors Protection - Question 13

The dv/dt protection is provided in order to

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 13

Accidentally some voltage spike or noise may occur in the vicinity of the device, if the magnitude is large enough it may turn on the SCR.

Test: Thyristors Ratings & Thyristors Protection - Question 14

dv/dt protection is provided to the SCR by

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 14

Snubber circuit R-C in parallel with SCR is connected for dv/dt protection.

Test: Thyristors Ratings & Thyristors Protection - Question 15

Figure below shows SCR having dv/dt and di/dt protection, when the switch(shown in green) is closed the current through Rl =

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 15

As soon as the switch is closed, C acts like a S.C & the voltage equation gives
Vs = (Rs + Rl) I + L di/dt
Solve the above D.E.

Test: Thyristors Ratings & Thyristors Protection - Question 16

The effect of over-voltages on SCR are minimized by using

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 16

Varistors are non-linear voltage clamping devices, RC circuits across the loads can also be used.

Test: Thyristors Ratings & Thyristors Protection - Question 17

Over-current protection in SCRs is achieved through the use of

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 17

FACLF stands for Fast Acting Current Limiting Fuse.

Test: Thyristors Ratings & Thyristors Protection - Question 18

False triggering of the SCRs by varying flux & noise is avoided by using

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 18

Shielded cables provide isolation from outside noise.

Test: Thyristors Ratings & Thyristors Protection - Question 19

The thyristor has the following specifications
Vs = 400 V
(di/dt)max = 25 A/μsec
Find the value of L

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 19

As soon as the switch is closed, C acts like a S.C & the voltage equation gives,
Vs = (Rs + Rl) I + L di/dt
Solve the above D.E.
di/dt = Vs/L e-t/τ
di/dt is maximum at t = 0, substitute the above given values & find L

Test: Thyristors Ratings & Thyristors Protection - Question 20

Thyristors are used in electronic crowbar protection circuits because it possesses

Detailed Solution for Test: Thyristors Ratings & Thyristors Protection - Question 20

Crowbar protection circuits have high surge current capabilities.

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