EE 318 Chapter 4: Three-Phase Controlled Rectifiers
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Questions and Answers

What is the primary power source for three-phase controlled rectifiers?

  • Three-phase AC supply (correct)
  • High voltage DC transmission
  • DC voltage source
  • Single-phase AC supply

    • What is an advantage of three-phase controlled rectifiers compared to single-phase rectifiers?

  • Lower DC output voltage
  • Higher ripples in the output voltage and current
  • Lower DC output power
  • Higher DC output voltage and higher DC output power (correct)

    • What is a common application of three-phase controlled rectifiers?

  • Residential power supply
  • Radio frequency amplifiers
  • Electrochemical processes (correct)
  • Low-power communication systems

    • What is a characteristic of the voltage sources in a three-phase AC supply?

    <p>They are 120° displaced from each other in time</p> Signup and view all the answers

    What is a benefit of using three-phase controlled rectifiers in high-power applications?

    <p>They provide lower ripples in the output voltage and current</p> Signup and view all the answers

    What type of equipment uses three-phase controlled rectifiers?

    <p>Traction equipment and motor drives</p> Signup and view all the answers

    What is the condition for the load current to flow through the supply phase winding 'a - n' and through thyristor T1?

    <p>When thyristor T1 is triggered at 𝑤𝑡 = (30 + 𝛼)</p> Signup and view all the answers

    What happens to thyristor T1 when thyristor T2 is triggered?

    <p>It becomes reverse biased</p> Signup and view all the answers

    What is the condition for the phase voltage 𝑉𝑐𝑛 to appear across the load?

    <p>When thyristor T3 is triggered at 𝑤𝑡 = 270 + 𝛼</p> Signup and view all the answers

    Why is it necessary to know that L>>R in a three-phase controlled rectifier?

    <p>To derive an expression for the average output voltage</p> Signup and view all the answers

    What is a characteristic of a three-phase controlled rectifier with RL load when L>>R?

    <p>It never has discontinuous conduction mode</p> Signup and view all the answers

    What is the purpose of triggering thyristor T1 at 𝑤𝑡 = (30 + 𝛼)?

    <p>To allow the phase voltage 𝑉an to appear across the load</p> Signup and view all the answers

    What is the term often used to describe the voltage between two lines?

    <p>Line voltage</p> Signup and view all the answers

    What is the phase difference between voltage A and voltage B in a three-phase supply?

    <p>120 degrees</p> Signup and view all the answers

    What is the purpose of the firing current pulse in a three-phase half wave converter?

    <p>To turn on the thyristor</p> Signup and view all the answers

    What is the common point that connects the supply and the load in a three-phase half wave converter?

    <p>Common neutral point</p> Signup and view all the answers

    What is the term used to describe the three-phase half wave converter circuit?

    <p>3-pulse converter</p> Signup and view all the answers

    What is the effect of delaying the firing pulse by an angle α in a three-phase half wave converter?

    <p>Controls the load voltage</p> Signup and view all the answers

    What is the configuration of the supply transformer in a three-phase half wave converter?

    <p>Star connected</p> Signup and view all the answers

    What is the condition for a thyristor to conduct in a three-phase half wave converter?

    <p>Anode-to-cathode voltage is positive and a firing current pulse is applied</p> Signup and view all the answers

    What is the formula to calculate the average output voltage in circuits with RL load?

    <p>Vdc = 3Vm / 2p * (-cos(wt) + sin(wt))</p> Signup and view all the answers

    What is the purpose of the formula for average output voltage in circuits with RL load?

    <p>To calculate the average output voltage</p> Signup and view all the answers

    How is the average output voltage and current calculated in circuits with RL load?

    <p>Using the trigonometric relationship</p> Signup and view all the answers

    What is the symbol 𝜔 in the formula for average output voltage?

    <p>Angular frequency</p> Signup and view all the answers

    What is the relationship between the average output voltage and current in circuits with RL load?

    <p>They are proportional to the cosine of the angle 𝛼</p> Signup and view all the answers

    What is the purpose of simplifying the formula for average output voltage in circuits with RL load?

    <p>To make the calculation more simple</p> Signup and view all the answers

    What is the significance of the angle 𝛼 in the formula for average output voltage in circuits with RL load?

    <p>It is the phase angle between the output voltage and current</p> Signup and view all the answers

    What is the RMS component of the output voltage and current waveforms determined from?

    <p>The average output voltage and current</p> Signup and view all the answers

    Study Notes

    Three-Phase Controlled Rectifiers

    • Three-phase controlled rectifiers have a wide range of applications, including electrochemical processes, motor drives, traction equipment, and controlled power supplies.
    • Features of three-phase controlled rectifiers:
      • Operate from 3-phase AC supply voltage
      • Provide higher DC output voltage and power
      • Provide lower ripples in the output voltage and current
      • Require lower values of capacitance for filtering
      • Are extensively used in high-power variable-speed industrial DC drives

    Review of 3-Phase Supplies

    • Three-phase supplies usually represent the voltages generated by three coils (physically displaced by 120° from each other) in an AC rotating machine (alternator).
    • The three-phase supply waveforms are 120° displaced from each other in time.
    • Assuming a maximum phase voltage of Vm, the phase sequence is A-B-C, and B lags A by 120°, C lags B by 120°, and A lags C by 120°.

    Controlled Three-Phase Half-Wave Rectifiers

    • A 3-phase half-wave converter combines three single-phase half-wave controlled rectifiers in one single circuit feeding a common load.
    • The 3-phase input supply is applied through a star-connected supply transformer.
    • The thyristor will conduct (ON state) when the anode-to-cathode voltage is positive, and a firing current pulse is applied to the gate terminal.
    • Delaying the firing pulse by an angle α controls the load voltage.

    Operation of Controlled Three-Phase Half-Wave Rectifiers

    • When thyristor T1 is triggered, the phase voltage Van appears across the load when T1 conducts.
    • When thyristor T2 is triggered, T1 becomes reverse-biased and turns off, and the load current flows through thyristor T2 and the supply phase winding 'b − n'.
    • When thyristor T3 is triggered, T2 becomes reverse-biased and turns off, and the load current flows through thyristor T3 and the supply phase winding 'c − n'.

    Circuits with RL Load

    • There is no discontinuous conduction mode for three-phase controlled rectifiers if L >> R.

    • The average output voltage can be calculated using the following equation:

      Vdc = (3Vm / 2p) * [sin(wt + α) + sin(wt - α)]

    • The average output voltage and current can be calculated as follows:

      Vdc = (3Vm / 2) * cos α Idc = (3Vm / 2) * cos α

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    Quiz on three-phase controlled rectifiers, their applications, and uses in electrochemical processes and motor drives.

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