Power Electronics: Three-Phase Inverters
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Questions and Answers

Which thyristor conducts first in the 180° conduction mode when T1 is fired at 0°?

  • T4 (correct)
  • T3
  • T2
  • T5

How long does each thyristor conduct in the 120° conduction mode?

  • 60°
  • 180°
  • 120° (correct)
  • 240°

What is the main function of a three-phase inverter?

  • To synchronize three phases
  • To convert AC to DC
  • To increase voltage levels
  • To convert DC to three-phase AC (correct)

During the 180° conduction mode, what is the overlapping period of the three thyristors?

<p>60° (A)</p> Signup and view all the answers

In the voltage equations given for the loads, what is the value of VCN in the 180° conduction mode?

<p>0 (B)</p> Signup and view all the answers

What is the purpose of forced commutation in power electronic applications?

<p>To control the flow of current through the semiconductor device (B)</p> Signup and view all the answers

Which circuit uses a combination of inductance and capacitance for forced commutation?

<p>LC Resonant Circuit (B)</p> Signup and view all the answers

In a voltage commutation circuit, how is the turn-off of the semiconductor device achieved?

<p>By applying a reverse voltage across the device (A)</p> Signup and view all the answers

How does the Parallel Capacitor Commutation method function?

<p>It provides a low-impedance path for current during commutation. (D)</p> Signup and view all the answers

What role do auxiliary circuits play in the forced commutation process?

<p>They assist in triggering the main device to turn off. (B)</p> Signup and view all the answers

Which method includes applying a gate pulse to induce a non-conductive state in the semiconductor?

<p>Gate Pulse Commutation (D)</p> Signup and view all the answers

Which of the following circuits employs a resistor and capacitor to achieve forced commutation?

<p>RC Commutation Circuit (A)</p> Signup and view all the answers

What are the main components involved in a series inverter?

<p>Thyristors and capacitors (C)</p> Signup and view all the answers

In what condition does a series inverter operate under the commutation method?

<p>Natural commutation only (B)</p> Signup and view all the answers

What is the effect of exceeding the maximum operating frequency in a series inverter?

<p>Short-circuits the DC input source (A)</p> Signup and view all the answers

What is the requirement for an RLC circuit to be classified as underdamped in a series inverter?

<p>R &lt; 4L/C (C)</p> Signup and view all the answers

What type of waveform does a series inverter ideally produce?

<p>Distorted waveform (B)</p> Signup and view all the answers

During which mode does the capacitor voltage remain constant in a series inverter?

<p>Mode II (B)</p> Signup and view all the answers

Which factor affects the load current flow in a series inverter?

<p>Components L and C (D)</p> Signup and view all the answers

Why is the load regulation poor in a series inverter?

<p>Because load current is discontinuous (A)</p> Signup and view all the answers

What type of commutation is used in a series inverter?

<p>Class-A commutation (B)</p> Signup and view all the answers

What happens when thyristor T2 is triggered while T1 is still ON in a modified series inverter?

<p>T1 gets reverse biased and turns OFF. (D)</p> Signup and view all the answers

Which advantage does the modified series inverter have over the basic series inverter?

<p>It can increase output frequency beyond the resonant frequency. (B)</p> Signup and view all the answers

In a parallel inverter, what role does the commutating capacitor serve?

<p>It prevents instantaneous discharging of the inductor. (D)</p> Signup and view all the answers

What type of output waveform does a parallel inverter produce from a DC input?

<p>Square wave (C)</p> Signup and view all the answers

What dictates the operation of the output voltage waveform in a series inverter?

<p>The characteristics of the load (A)</p> Signup and view all the answers

Which of the following statements correctly describes the basic function of the thyristors in a parallel inverter?

<p>They alternate to allow current flow through the load. (A)</p> Signup and view all the answers

Why is the operation of improved series inverters advantageous over simple series inverters?

<p>They provide continuous power supply from dc across both half cycles. (C)</p> Signup and view all the answers

In the context of a modified series inverter, what must happen when the load current becomes zero?

<p>The voltage across C1 and C2 reverses. (C)</p> Signup and view all the answers

What frequency range are parallel inverters best suited for?

<p>Up to 100 kHz (D)</p> Signup and view all the answers

Which application is NOT typically associated with a series inverter?

<p>Wind power generation (D)</p> Signup and view all the answers

What occurs across thyristor T2 when thyristor T1 is triggered in Mode-III?

<p>A voltage equal to -2Vdc appears across T2 (B)</p> Signup and view all the answers

Which characteristic describes the output voltage of a parallel inverter?

<p>It is approximately rectangular (C)</p> Signup and view all the answers

What is the primary benefit of using a parallel inverter compared to a series inverter?

<p>It employs complementary voltage commutation (B)</p> Signup and view all the answers

What is a disadvantage of parallel inverters when operated at low frequencies?

<p>The transformer core gets saturated (A)</p> Signup and view all the answers

In a single-phase PWM inverter, which of the following is a key component used for modulation?

<p>Carrier frequency (B)</p> Signup and view all the answers

How does the harmonic content compare between the half-bridge and full-bridge inverters?

<p>Full-bridge inverters have lower harmonics (A)</p> Signup and view all the answers

What is the firing interval of thyristors in a basic three-phase bridge inverter?

<p>60° interval (A)</p> Signup and view all the answers

What type of load current behavior is observed when a parallel inverter supplies power to an inductive load?

<p>Current lags behind the voltage (B)</p> Signup and view all the answers

What is a common application of parallel inverters?

<p>High current, low voltage applications (C)</p> Signup and view all the answers

What is a significant drawback of using extra feedback diodes in a parallel inverter circuit?

<p>They trap energy in the circuit (B)</p> Signup and view all the answers

Flashcards

Forced Commutation

Turning off a semiconductor device using an external circuit

Resonant Commutation

Uses L and C to turn off a device at the resonant frequency.

LC Resonant Circuit

Circuit using inductance and capacitance for resonant commutation.

Voltage Commutation

Using an external voltage source to turn off a semiconductor device.

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Voltage Commutation with Transformer

Utilizes a transformer to increase voltage for faster commutation.

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Current Commutation

Diverts current away from a device to turn it off.

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Gate Pulse Commutation

Using a gate signal to force a device into a non-conductive state.

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Series Inverter

An inverter where commutation components are connected in series with the load, using class-A or resonant commutation for natural current decay.

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Class-A Commutation

A commutation method used in series inverters to turn off thyristors using the natural decay of load current, only works in dc-supplied circuits

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Commutation Components (Series Inverter)

Inductors (L) and capacitors (C) in a series inverter crucial for managing current flow and voltage transitions during switching.

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Underdamped RLC Circuit

A series RLC circuit where oscillations occur due to its design, which has the condition R^2 < (4L/C).

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Series Inverter Modes

The three stages of operation of a series inverter: T1 triggering, commutation and T2 triggering.

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Series Inverter Output Waveform

An output form with distortions and harmonics rather than a perfect sine wave.

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Series Inverter Limitations

Distorted output voltages, frequency limitations, discontinuous current flow from the supply, and high component ratings result from the resonant nature of the circuit

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Resonant Frequency

The frequency at which maximum oscillation is observed from an underdamped circuit.

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Thyristor T1 and T2

Two thyristors in a series inverter responsible for carrying the load current in alternate directions

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Modified Series Inverter

An inverter circuit where two inductors are closely coupled, allowing thyristors to conduct in an overlapping period, increasing output frequency range beyond resonant frequency.

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Improved Series Inverter

A modified series inverter circuit that draws power from the DC supply in both output waveform half-cycles, eliminating intermittent current flow, reducing harmonic content and peak current needs, leading to more efficient use of DC input.

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Parallel Inverter

An inverter where commutating components (inductor and capacitor) are connected in parallel, suitable for low-frequency applications up to 100kHz, producing square wave output voltage from DC input.

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Parallel Inverter Mode I

Mode in a parallel inverter where a thyristor (T1) is triggered, load current flows through inductor (L) and T1, voltage across the primary winding of the transformer becomes 2Vdc, capacitor charges to 2Vdc.

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Parallel Inverter Mode II

Mode where T2 is triggered, T1 is reversed biased (-2Vdc), T1 is commutated, load current only flows through T2 and inductor.

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Commutating Capacitor

A capacitor used in an inverter circuit to provide voltage for triggering and commutation of thyristors during switching.

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Load Commutation

A self-commutation process in parallel inverter; a capacitor connected across the load causes an underdamped load circuit enabling inverter to commutate itself.

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Thyristor Commutation

The process of switching a thyristor off by applying a reverse voltage across it, usually aided by elements like inductor and capacitor.

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Series Inverter Application

Series inverters are used in induction heating, ultrasonic equipment, and cycloconverters.

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Output Frequency Range

The range of frequencies an inverter can produce (or operate at).

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180° Conduction Mode

A three-phase inverter mode where each thyristor conducts for 180° of the output voltage cycle.

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Three-Phase Inverter

An electronic circuit that converts DC input to three-phase AC output.

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Thyristor Firing Sequence

The specific order in which thyristors are turned on/off to create the desired output waveform.

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120° Conduction Mode

A three-phase inverter mode where thyristors conduct for 120° of the output voltage cycle.

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Output Voltage Waveform

The alternating waveform created at the output terminals of the three-phase inverter. Each phase has its voltage.

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Parallel Inverter

A type of inverter where the switching devices are connected in parallel to the load, using complementary voltage commutation.

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Parallel Inverter Advantages

Simple design, small size, low cost, and better output voltage compared to series inverters.

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Parallel Inverter Disadvantages

Requires a large transformer for high current application, needs feedback diodes, and not suitable for fluctuating loads.

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Single-Phase PWM Inverter

An inverter that generates a variable AC voltage using pulse width modulation (PWM) technique, with a DC power source.

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Half-Bridge Inverter

A type of single-phase PWM inverter with bipolar output voltage, featuring a simplified circuit using an IGBT/diode pair.

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Full-Bridge Inverter

A single-phase PWM inverter with better harmonic performance than half-bridge, producing a monopolar waveform in a similar topology.

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Three-Phase Bridge Inverter

A device that converts DC power into three-phase AC output, typically using six thyristors (six-step bridge) for conversion.

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PWM (Pulse Width Modulation)

A technique to control the output voltage waveform by varying the width of pulses.

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Harmonics

Extra frequencies present in the output waveform, usually multiples of the fundamental frequency, of a converter circuit.

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Inductive Load

A load that has inductance, causing the load current to lag behind the load voltage in a parallel inverter.

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