# Power Electronics Question and Answers – 3P-Diode Rectifiers-1

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “3-Phase Diode Rectifiers-1”.

1. In a three-phase half wave rectifier usually, the primary side of the transformer is delta connected because
a) it has no neutral connection
b) we can get greater output voltage
c) it provides a path for the triplen harmonics
d) it provides better temperature stability

Explanation: The delta connected winding help circulating and eliminating the triplen (3rd order) harmonics.

2. The diode rectifier circuit given below is that of a a) three-phase half wave common cathode arrangement
b) three-phase half wave common anode arrangement
c) three-phase full wave common cathode arrangement
d) three-phase full wave common anode arrangement

Explanation: Common anode arrangement because all the anodes are connected to the load side.

3. In a three-phase half wave diode rectifier using 3 diodes, each diode conducts for
a) 90 degrees
b) 120 degrees
c) 180 degrees
d) 360 degrees

Explanation: Each diode conducts for 120 degrees, starting from ωt = 30 degrees.

4. In the below shown diode rectifier circuit, The diodes D1, D2 & D3 are connected to phases R,Y and B respectively as shown
The phase sequence is R-Y-B.
The diode D1 would conduct from
a) 0 to 90 degrees
b) 30 to 150 degrees
c) 0 to 180 degrees
d) 30 to 180 degrees

Explanation: It conducts from 30 to 150, for 90 degrees. D1 starts conducting first as it will be the most positive as it is connected to the R phase.

5. In the below shown diode rectifier circuit, The diodes D1, D2 & D3 are connected to phases R,Y and B receptively as shown
The phase sequence is R-Y-B.
The diode D3 conducts from
a) 0 to 270 degrees
b) 270 to 390 degrees
c) 270 to 450 degrees
d) 270 to 360 degrees

Explanation: It conducts from 270 to 450, for 120 degrees. D1 starts conducting first (from 30 degrees) as it will be the most positive as it is connected to the R phase & likewise.

6. In a three-phase half wave diode rectifier using 3 diodes,
a) All diodes conduct together
b) Only two diodes conduct at a time
c) Only one diode conducts at a time
d) None of the above mentioned

Explanation: 3 diodes, each conduct for 120 degree at a time.

7. In a three-phase half wave diode rectifier, if Vmp is the maximum phase voltage, then the output voltage on a R load varies from
a) 0 to Vmp
b) 0.5 Vmp to Vmp
c) Vmp to 3Vmp
d) –Vmp to Vmp

Explanation: The voltage value is positive and varies from (1/2)Vmp to Vmp.

8. The average value of the output voltage, in a 3-phase half wave diode rectifier with Vml as the maximum line voltage value, is given by the expression
a) Vml/3π
b) 2Vml/3π
c) 3Vml/2π
d) 3Vml

Expression: The average value can be obtained by
3 x [ 1/2π x Vml sin ωt d(ωt) ] The integration runs from π/6 to 5π/6 as the diode is conducting for 120 degrees each.

9. In a three-phase half wave 6-pulse mid-point type diode rectifier, each diode conducts for
a) 120°
b) 60°
c) 90°
d) 180°

Explanation: In a six-pulse rectifier, each diode conducts once every one cycle, 60° x
6 diodes = 360°.

10. A step-down delta-star transformer, with per-phase turns ration of 5, is fed from a 3-phase, 1100 V, 50 Hz source. The secondary of this transformer through a 3-pulse type rectifier feeds a R load of 10 Ω. Find the maximum value of the load current (phase).
a) √2 x 22 A
b) 1 x 11 A
c) √2 x 11 A
d) 1 x 22 A

Explanation:
Vph = 1100/5 = 220 V (Transformer ratio = 5)
Vmp = √2 x 220 V
Imp = Vmp/R.

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