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Test: Phase Controlled Rectifiers - Electrical Engineering (EE) MCQ


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10 Questions MCQ Test Power Electronics - Test: Phase Controlled Rectifiers

Test: Phase Controlled Rectifiers for Electrical Engineering (EE) 2024 is part of Power Electronics preparation. The Test: Phase Controlled Rectifiers questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Phase Controlled Rectifiers MCQs are made for Electrical Engineering (EE) 2024 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Phase Controlled Rectifiers below.
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Test: Phase Controlled Rectifiers - Question 1

How many thyristors are there in a three-phase full wave controller?

Detailed Solution for Test: Phase Controlled Rectifiers - Question 1

Three-Phase Full Converters:

If all diode is replaced by thyristor, a three-phase full converter bridge formed as shown below,

  • A three-phase input supply is connected to the terminal A, B, C, and RLE load is connected across the output terminal.
  • It worked as three-phase AC to DC converter for firing angle delay 0 <α ≤ 90.
  • The positive group of SCRs is fired at an interval of 120° and a similarly negative group of SCRs fired at an interval of 120°, but SCR of both groups are fired at an internal of 60° or commutation occurs at every 60°.
  • At any time tow SCRs, one from the positive group and one from the negative group must be conducted together for the source to energize the load.

Waveform:

Voltage and Current Equation:

The average output voltage (V0) is given by,

Where Vm is the maximum value of line voltage.

Voltage and current waveform for α = 30° and for constant current can be drawn as,

From waveform RMS value of Thyristor current (IT = iT1) will be,


Where I0 is constant DC load current.

Test: Phase Controlled Rectifiers - Question 2

In a single Phase full converter with resistive load and firing angle α. The load current is non-zero and zero, respectively for-

Detailed Solution for Test: Phase Controlled Rectifiers - Question 2

Concept:

Phase-controlled converters:

  • It is a device which is used to convert A.C sinusoidal into D.C.
  • In this the A.C input is fed from transformer stepdown side and then input is converted into D.C output.
  • Here we are talking about single phase fully controlled converter.

Single phase full converter with resistive load:

  • An A.C voltage source of Vs = Vmsin(wt) is connected.
  • Four controlled thyristors and a resistive load is connected.
  • Let Vo be the output voltage.

Working:

  • Let thyristor T1 and T4 are triggered at a firing angle of 'α'.
  • During the first cycle from '0 to π' the current starts flowing at firing 'αand flows through Vs→ T1​→ RL​→ T4​→ Vs.
  • During the second cycle from 'π→2π' the thyristor T2 and T3 are triggered at (π + α)
  • The current flows through Vs→ T​2→ RL​→ T3​→ Vs.

Current flowing through a resistive load will be 

I = Vo/R

We can see from the above graph that the current will be zero from '0 to α' and the current will be non-zero from α to π.

So, the current will be non-zero and zero from 'π - α' and 'α' respectively.

*Answer can only contain numeric values
Test: Phase Controlled Rectifiers - Question 3

3 phase, 6 pulse converter shown in the figure, the load is taking ripple free constant current of 10 A. The average output voltage of the converter is 150 V at a firing angle α = 30º. Find the value of line resistance R in ohms. (Up to two decimal places)


Detailed Solution for Test: Phase Controlled Rectifiers - Question 3

Concept:

The average output voltage of 3 phase 6 pulse converter is given by 

here Vml is the maximum value of line-to-line voltage 

The voltage drop in the resistance is given as

2RI0, here 2 resistance will come in to picture in every 60º conduction of two phases

so this voltage drop is subtracted from the final output of this converter.

Solution:

The average output voltage is equal to

R = 4.19 Ω 

Test: Phase Controlled Rectifiers - Question 4

In a dual converter, if the firing angle of one bridge is 30°, then the firing angle of second bridge is ______.

Detailed Solution for Test: Phase Controlled Rectifiers - Question 4

Concept:

  • A dual converter is an electronic converter or circuit which comprises of two converters.
  • One performs as a rectifier and the other performs as the inverter.
  • Two full converters are arranged in an anti-parallel pattern and linked to the same DC load.
  • These converters can provide four-quadrant operations.

Modes of operation of Dual converter:

Non-circulating current mode:

  • There is no circulating current between the converters in this mode as only one converter will perform at a time
  • During converter 1 operation, firing angle (α1) will be between 0 to 90°
  • During converter 2 operation, firing angle (α2) will be between 0 and 90°

Circulating current Mode:

  • In this mode, there is a circulating current as the two converters will be in the ON condition at the same time.
  • The firing angles are adjusted such that
  • Firing angle of converter 1 (α1) + firing angle of converter 2 (α2) = 180°
  • Converter 1 as rectifier when 0° < α1 < 90° and converter 2 as inverter when 90° < α2 < 180°
  • Converter 1 as inverter when 90° < α1 < 180° and converter 2 as rectifier when 0° < α2 < 90°
  • Four quadrant operation of dual converter is shown below
     

Application of dual converter:

  • Direction and speed control of DC motors
  • Applicable wherever the reversible DC required
  • Industrial variable speed DC drives.

Calculation:

Given that, firing angle of one bridge is 30° (i.e. α1 = 30°)

We know that firing angles can never be greater than 180°.

i.e. α1 + α2 = 180°

∴ 30° + α2 = 180°

α2 = 180° - 30°

α2 = 150°

  

Test: Phase Controlled Rectifiers - Question 5

A delayed full-wave rectified sinusoidal current has an average value equal to half its maximum value. Find the delay angle θ.

Detailed Solution for Test: Phase Controlled Rectifiers - Question 5

Concept:

The average output voltage of a full-wave controlled rectifier with R load is given by:

Where Vm is the maximum value of supply voltage

α is the firing angle or delay angle

Average load current:

Calculation:

Given that, the average value of the average load current is equal to half of its maximum value.

Test: Phase Controlled Rectifiers - Question 6

A delayed full-wave rectified sinusoidal current has an average value equal to one – third its maximum value. Find the delay angle.

Detailed Solution for Test: Phase Controlled Rectifiers - Question 6

Concept:

Considered Vm is the maximum value of AC input voltage of converter and V0 is the average output voltage converter and α is delay angle.

For single-phase semi converter or delayed full-wave rectifier,


Calculation:

Given that,

V0 = Vm/3

Hence, the equation becomes,

or,
π/3 = (1 + cosα)
cos α = 0.047
α = cos-1(0.047)

Test: Phase Controlled Rectifiers - Question 7

Distortion factor (DF) and total harmonic distortion (THD) are related by.

Detailed Solution for Test: Phase Controlled Rectifiers - Question 7

Total harmonic distortion:

  • It is a common measurement of the level of harmonic distortion present in power systems.
  • THD can be related to either current harmonics or voltage harmonics.
  • It is defined as the ratio of r.m.s value of all the harmonic components to the r.m.s value of the fundamental component.
  • Mathematically, it can be represented as


Where,
Vor = Total rms value

01 = Fundamental rms value

Distortion factor (DF): It is the ratio of the fundamental component to r.m.s value of the waveform.

Distortion factor (DF) and total harmonic distortion (THD) are related as

Test: Phase Controlled Rectifiers - Question 8

A freewheeling diode in a phase-controlled rectifier:

Detailed Solution for Test: Phase Controlled Rectifiers - Question 8

Freewheeling diode:

  • A freewheeling diode placed across the inductive load will provide a path for the release of energy stored in the inductor while the load voltage drops to zero.
  • The freewheeling diode prevents the load voltage from becoming negative. Whenever load voltage tends to go negative, FD comes into play.
  • As a result, the load current is transferred from the main thyristor to FD, allowing the thyristor to regain its forward blocking capability.
  • The advantages are the input power factor is improved, load current waveform is improved and better load performance.
Test: Phase Controlled Rectifiers - Question 9

A single-phase, full-bridge diode rectifier fed from a 230 V, 50 Hz sinusoidal source supplies a series combination of finite resistance, R, and a very large inductance, L. The two most dominant frequency components in the source current are:

Detailed Solution for Test: Phase Controlled Rectifiers - Question 9

Concept:

Fourier series of source current in a full bridge rectifier is given by


For n = 1, 3, 5, ......

The most dominant frequency components in the above expression are f, 3f

Application:

A single-phase, full-bridge diode rectifier fed from a 230 V, 50 Hz sinusoidal source

Dominant frequencies = f, 3f

= 50 and 150 Hz

Test: Phase Controlled Rectifiers - Question 10

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

Detailed Solution for Test: Phase Controlled Rectifiers - Question 10

In a six-pulse thyristor bridge rectifier, the harmonics present are = 6 k ± 1

So, the harmonics are = 5, 7, 11, 13, ...

lowest harmonic component = 5th harmonic supply frequency = 50 Hz

5th harmonic frequency = 5f = 250 Hz

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