# Electric Drives Questions and Answers – Solid-State Devices – di/dt and dv/dt Protection

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This set of Electric Drives Questions and Answers for Campus interviews focuses on “Solid-State Devices – di/dt and dv/dt Protection”.

1. Calculate the value of output r.m.s voltage for Single phase Half-bridge inverter if the DC supply is 80 V.
a) 40 V
b) 20 V
c) 10 V
d) 60 V

Explanation: The output voltage of a single phase half bridge inverter is a square wave with amplitude is equal to vD.C÷2. The r.m.s value of output voltage is vD.C÷2=80÷2=40 V.

2. High d(i)÷d(t) causes _______________
a) Local hotspots
b) Breakdown
c) High cost
d) Low cost

Explanation: High d(i)÷d(t) causes local hotspots. Local hotspots are the small areas with high temperature due to overcrowdedness of charge generation. This cause high heat generation.

3. Calculate the value of output r.m.s voltage for Single phase Full-bridge inverter if the DC supply is 25 V.
a) 26 V
b) 25 V
c) 27 V
d) 28 V

Explanation: The output voltage of a single phase full bridge inverter is a square wave with amplitude is equal to vD.C. The r.m.s value of output voltage is vD.C=25 V.

4. High d(v)÷d(t) causes _______________
a) Local hotspots
b) False triggering
c) High cost
d) High-speed operation

Explanation: High d(v)÷d(t) causes false triggering. The high value of d(v)÷d(t) causes the breakdown of the junction J2. The thyristor will trigger automatically.

5. A step-down chopper has input voltage 12 V and output voltage 2 V. Calculate the value of the duty cycle.
a) 0.26
b) 0.16
c) 0.33
d) 0.41

Explanation: The output voltage of the step-down chopper is Vo = Vin×(D). The value of the duty cycle is less than one which makes the Vo < Vin. The step-down chopper is used to step down the voltage. The value of the duty cycle is 2÷12=.16.
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6. Calculate the De-rating factor if the string efficiency is 90 %.
a) .4
b) .1
c) .7
d) .8

Explanation: De-rating factor is used to measure the reliability of a string. The value of the De-rating factor is 1-(string efficiency)=1-.9=.1.

7. Calculate the string efficiency if the de-rating factor is .22.
a) 78 %
b) 42 %
c) 20 %
d) 90 %

Explanation: The string efficiency is calculated for series and parallel connection of SCRs. The value of string efficiency is 1-(De-rating factor)=1-.22=78 %.

8. 25 V rated n SCRs are connected in series. The operation voltage of the string is 300 V and the string efficiency is 40 %. Calculate the value of n.
a) 27
b) 30
c) 34
d) 36

Explanation: The string efficiency can be calculated using the formula operation voltage÷(Number of SCRs×Rated voltage). The value of n is 300÷(25×.4)=30.

9. The most reliable method for thyristor triggering is thermal triggering.
a) True
b) False

Explanation: Gate triggering is more reliable than any other triggering method. The risk of false triggering of SCR reduces in the case of gate triggering.

10. SCR is unipolar.
a) True
b) False

Explanation: The SCR is uni-directional, bipolar power electronics device. It is bipolar in nature because of its reverse and forward mode blocking capability.

11. What is the formula for thermal voltage?
a) K×e÷T
b) K×T÷e
c) K2×e÷T
d) K×e2÷T

Explanation: The formula for thermal voltage is K×e÷T where K is the Boltzmann constant, e is the charge on the electron, T is the absolute temperature.

12. Device with highest d(i)÷d(t) and d(v)÷d(t) capability is _____________
a) GTO
b) BJT
c) SCR
d) SITH

Explanation: SITH stands for static induction thyristor. It has a buried gate structure in which the gate electrode is placed in the n-base region. They have high d(i)÷d(t) and d(v)÷d(t) capability.

13. ____________ is the switching frequency of SITH.
a) 55 KHz
b) 100 KHz
c) 91 KHz
d) 105 KHz

Explanation: 100 KHz is the switching frequency of static induction thyristor. Static Induction Thyristor has a low frequency because of lower turn-off current gain.

14. Calculate the value of Crest factor if Vpeak=1.5 V and Vr.m.s=2.5 V.
a) .4
b) .6
c) .5
d) .7

Explanation: The value of the crest factor is Vpeak÷Vr.m.s=1.5÷2.5=.6 V. It signifies the peak value is .6 times than the r.m.s value.

15. Calculate the output voltage of the Boost converter if the supply voltage is 1 V and duty cycle value is .9.
a) 10 V
b) 20 V
c) 40 V
d) 60 V

Explanation: The output voltage of the buck converter is Vo = Vin×(D). The value of the duty cycle is less than one which makes the Vo < Vin . Vo = Vin×(1-D)=10 V.

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