Three Phase Rectifiers Questions and Answers

Q: In a 3-φ controller bridge rectifier, the maximum conduction of each thyristor is :

Each thyristor conducts for at most 120°.In a three-phase fully controlled bridge rectifier, two thyristors conduct at any time, one from the upper group and one from the lower group. A new thyristor is fired every 60°, and each one stays in conduction for two of these intervals, 120° of each cycle, when the load current is continuous.60° is the interval between successive firings, not the conduction period. 180° is the conduction angle of switches in a 180°-mode inverter or of a diode in a single-phase circuit, and 90° does not apply to the three-phase bridge.

Q: A six pulse thyristor rectifier bridge is connected to a balanced 50 Hz three-phase ac source. Assuming that the dc output current of the rectifier is constant, the lowest harmonic component in the ac source line current is :

The lowest harmonic in the line current is 250 Hz.With constant DC current, each line of a six-pulse bridge carries a quasi-square wave of 120° positive and 120° negative blocks. Its Fourier series has no even harmonics (half-wave symmetry) and no triplen harmonics (the three phases are balanced and 120° apart), so only harmonics of order\[ h = 6k \pm 1 = 5, 7, 11, 13, \ldots \]remain. The lowest is the 5th:\[ 5 \times 50 = 250\ \text{Hz} \]100 Hz would be the 2nd harmonic and 150 Hz the 3rd, both absent. 300 Hz is the 6th harmonic, the lowest ripple frequency on the DC side of a six-pulse bridge, not in the AC line current.

Q: It is required to drive a d.c. shunt motor at different speeds in both the directions (forward and reverse) and also to break it in both the directions which one of the following would you use ?

The right choice is a dual converter.Driving a dc motor at variable speed in both directions and braking it regeneratively in both directions needs four-quadrant operation: both polarities of armature voltage and both directions of armature current. A dual converter, two fully controlled bridges connected in anti-parallel, provides this; one bridge carries forward current and the other reverse current, and each can rectify or invert.A single full-controlled bridge can reverse its voltage (inverting) but cannot reverse the current, so it gives only two quadrants. A half-controlled bridge gives one quadrant, and an uncontrolled diode bridge gives no control at all.

For a 3-phase semi-converter, if the firing angle is \(80^\circ\), then the free-wheeling diode conducts for ______.

A.\(90^\circ\)
B.\(60^\circ\)
C.\(40^\circ\)
D.\(20^\circ\)

Exam Year(s)

DSSSB AE - 2023,

Answer: D

In a three phase semi-converter, each SCR carries current for 120 degrees at most, and the free wheeling diode starts to conduct when the output voltage tries to go negative, which happens only for firing angles above \(60^\circ\). For \(\alpha > 60^\circ\) the diode conducts for \(\alpha - 60^\circ\) in each of the three intervals of the supply cycle.

\[ \alpha - 60^\circ = 80^\circ - 60^\circ = 20^\circ \]

The conduction of the diode is zero for \(\alpha \le 60^\circ\), so 60 degrees and 90 degrees cannot be the answers, and 40 degrees would correspond to \(\alpha = 100^\circ\). The answer is \(20^\circ\), option D.

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The 3-phase, half-wave converter will require ______ SCRs whereas the 3-phase, full-wave converter will require ______ SCRs.

A.three; three
B.six; six
C.three; six
D.six; three

Exam Year(s)

DSSSB AE - 2023,

Answer: C

A three phase half wave converter has one SCR in each of the three phases, so it needs three SCRs. A three phase full wave (bridge) converter has two SCRs in each of the three legs, one in the upper group and one in the lower group, so it needs six SCRs. The correct pair is three and six.

The other options give six for the half wave converter or three for the bridge, which reverses the numbers, or the same number for both, which is wrong since the bridge has twice as many. The answer is option C.

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A three-phase full wave controlled rectifier is operated from a three-phase star connected 208 V, 50 Hz supply and the load resistance is \(100\ \Omega\). What is the value of delay angle if it is required to obtain an average output voltage of 50% of the maximum possible output?

A.\(30^\circ\)
B.\(60^\circ\)
C.\(45^\circ\)
D.\(75^\circ\)

Exam Year(s)

DSSSB AE - 2023,

Answer: B

For a three phase full controlled (six pulse) bridge converter the average output voltage is \(V_{dc} = V_{dc,max}\cos\alpha\), where \(V_{dc,max}\) is the value at \(\alpha = 0\).

For an average output of 50 per cent of the maximum

\[ \cos\alpha = 0.5 \Rightarrow \alpha = 60^\circ \]

The supply voltage and the load resistance do not matter for the delay angle, since only the ratio matters. The angle \(30^\circ\) would give 86.6 per cent, \(45^\circ\) about 70.7 per cent and \(75^\circ\) about 25.9 per cent. The answer is \(60^\circ\), option B.

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A 3-phase half-wave rectifier is supplied with a 440 V, 60 Hz supply. Find the output ripple frequency.

A.30 Hz
B.60 Hz
C.120 Hz
D.180 Hz

Exam Year(s)

DSSSB - 2023,

Answer: D

The output ripple frequency is 180 Hz. In a 3-phase half wave rectifier there are three pulses in each cycle of the supply, so the ripple frequency is three times the supply frequency.

\[ f_{ripple} = 3 \times 60 = 180\ \text{Hz} \]

The value 60 Hz would be the ripple of a single phase half wave circuit, 120 Hz would be that of a single phase full wave circuit, and 30 Hz is not a ripple frequency of any such circuit at 60 Hz. The voltage of 440 V does not change the ripple frequency. So the fourth option is the answer.

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In a 3-φ controller bridge rectifier, the maximum conduction of each thyristor is :

A.90°
B.120°
C.180°
D.60°

Exam Year(s)

RSMSSB JE - 2025, RSMSSB - 2025,

Answer: B

Each thyristor conducts for at most 120°.

In a three-phase fully controlled bridge rectifier, two thyristors conduct at any time, one from the upper group and one from the lower group. A new thyristor is fired every 60°, and each one stays in conduction for two of these intervals, 120° of each cycle, when the load current is continuous.

60° is the interval between successive firings, not the conduction period. 180° is the conduction angle of switches in a 180°-mode inverter or of a diode in a single-phase circuit, and 90° does not apply to the three-phase bridge.

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A six pulse thyristor rectifier bridge is connected to a balanced 50 Hz three-phase ac source. Assuming that the dc output current of the rectifier is constant, the lowest harmonic component in the ac source line current is :

A.100 Hz
B.150 Hz
C.250 Hz
D.300 Hz

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

The lowest harmonic in the line current is 250 Hz.

With constant DC current, each line of a six-pulse bridge carries a quasi-square wave of 120° positive and 120° negative blocks. Its Fourier series has no even harmonics (half-wave symmetry) and no triplen harmonics (the three phases are balanced and 120° apart), so only harmonics of order

\[ h = 6k \pm 1 = 5, 7, 11, 13, \ldots \]

remain. The lowest is the 5th:

\[ 5 \times 50 = 250\ \text{Hz} \]

100 Hz would be the 2nd harmonic and 150 Hz the 3rd, both absent. 300 Hz is the 6th harmonic, the lowest ripple frequency on the DC side of a six-pulse bridge, not in the AC line current.

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It is required to drive a d.c. shunt motor at different speeds in both the directions (forward and reverse) and also to break it in both the directions which one of the following would you use ?

A.a half-controlled thyristor-bridge
B.a full-controlled thyristor-bridge
C.a dual converter
D.a diode bridge

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

The right choice is a dual converter.

Driving a dc motor at variable speed in both directions and braking it regeneratively in both directions needs four-quadrant operation: both polarities of armature voltage and both directions of armature current. A dual converter, two fully controlled bridges connected in anti-parallel, provides this; one bridge carries forward current and the other reverse current, and each can rectify or invert.

A single full-controlled bridge can reverse its voltage (inverting) but cannot reverse the current, so it gives only two quadrants. A half-controlled bridge gives one quadrant, and an uncontrolled diode bridge gives no control at all.

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