# Power Electronics Questions and Answers – Single Phase Semi-Converters-3

This set of Power Electronics Problems focuses on “Single-Phase Semi-Converters-3”.

1. A single-phase semi-converter is having continuous conduction, as such each thyristor will conduct for an angle of
a) α
b) π
c) α+π
d) π-α

Explanation: From 0 to α there is no conduction. SCRs are gated at α and than naturally commutated at π. Hence, conduction angle = π-α.

2. Consider the following statements regarding phase controlled converters:
i) They do not provide smooth variation of output voltage.
ii) They inject harmonics into the power system.
iii) They draw non-unity power factor current for finite firing angles.
Which of the above statements are correct?
a) All the three
b) i and ii only
c) i and iii only
d) ii and iii only

Explanation: All the above given statements are correct. They all are the major drawbacks of using phase controlled converters.

3. Consider the following statement:
The overlap angle of a phase controlled converter will increase ___________
i) if the firing angle is increased
ii) if the supply frequency is increased
ii) if the supply voltage magnitude is lowered
Of these statements
a) all are correct
b) only the first and the last one are correct
c) only the first one is incorrect
d) all are incorrect

Explanation: The period during which both the incoming and outgoing SCRs are conducting is called as an overlap angle. This happens when inductance comes into picture either on the load or the source side.

4. All the modern AC-DC converters are using GTOs instead of SCRs because
a) Temperature is an issue in SCRs
b) Switching losses are high in SCRs
c) GTOs have reliable commutation
d) GTOs are cheaply available today

Explanation: GTOs unlike conventional SCRs can be turned off by providing a negative gate pulse. Hence, using GTOs the turn off as well as the turn-on time can be controlled.

5. Find the average output dc voltage of a single-phase semi-converter with Vs=230 V and firing angle of 30°. The converter is operating under continuous conduction.
a) 193 V
b) 256 V
c) 0 V
d) 230 V

Explanation: Vo = √2Vs (1+cosα)/π.
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6. In the below given circuit, the average value of output voltage is 191 V. Find the value of the average current through the given load if E = 100 V and R = 5 Ω. Assume that the load current is continuous. a) 0 A
b) 10.2 A
c) 18.2 A
d) 29.2 A

Explanation: Vo = 191 V
Io = Vo-E/R = 191-100/5 = 18.2 A.

7. In the below given circuit, the average value of output voltage is 191 V. Find the value of the power delivered to the voltage source E. Take E = 100 V and R = 5 Ω. Assume that the load current is continuous. a) 1.82 kW
b) 18.2 kW
c) 0.182 kW
d) 18.2 W

Explanation: Note that as the load current is continuous, L can be neglected.
Vo = 191V.
Io = Vo-E/R = 191-100/5 = 18.2A
P = E x Io = 100 x 18.2 = 1.82 kW.

8. A single-phase semi-converter is operated from a 240 V, 60 Hz, AC source. It is fired at an angle of 45°. Find the value of average output voltage.
a) 176 V
b) 184 V
c) 167 V
d) 148 V

Explanation: For a semi-converter, the output voltage Vo = √2Vs (1+cosα)/π.

9. A single-phase semi-converter is operated from a 230 V, 60 Hz, AC source. A DMM connected at its output terminals read 219.3 V. Find the value of FF (form factor).
a) 1.24
b) 0.735
c) 1.11
d) 1

Explanation: A DMM always reads the RMS value. Therefore, Vrms = 219.3 V
Vo = √2Vs (1+cosα)/π = 176.72 V
FF = Vrms/Vo = 1.241.

10. Name the circuit shown below. a) Half controlled semi-converter symmetrical configuration
b) Half controlled semi-converter asymmetrical configuration
c) Half controlled full-converter asymmetrical configuration
d) Half controlled full-converter symmetrical configuration

Explanation: It is using only two SCRs, hence it is a half controlled semi-converter. It is an asymmetrical configuration as each leg has same devices.

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