Power Electronics Unit 10: Converters
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Questions and Answers

What type of output does an inverter typically produce?

  • Chopped AC voltage with zero DC component (correct)
  • Pure AC voltage with no harmonics
  • Constant DC voltage without distortion
  • Sine wave output with high frequency signals

Which of the following harmonics are considered particularly difficult to filter?

  • 2nd and 4th
  • 3rd and 5th (correct)
  • 7th and 9th
  • 1st and 6th

What characteristic is true about harmonics in the spectra of a square wave?

  • Harmonics increase linearly with frequency
  • Even harmonics are absent (correct)
  • Harmonics only consist of even frequencies
  • All harmonics are present in equal measure

If the fundamental frequency is 50 Hz, what is the nearest harmonic?

<p>150 Hz (C)</p> Signup and view all the answers

Why is good filtering required in some applications of inverters, such as UPS?

<p>To achieve high purity sine wave output (B)</p> Signup and view all the answers

What is the primary function of a rectifier in power electronic converters?

<p>Convert constant AC voltage to variable DC output voltage (A)</p> Signup and view all the answers

Which type of converter would you use to control the speed of a DC motor?

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

Which device is specifically designed for converting DC voltage to AC voltage?

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

What is an application of inverters?

<p>AC power supply from photovoltaic array (B)</p> Signup and view all the answers

Which of the following describes a characteristic of power transistors?

<p>They operate in the saturation region (A)</p> Signup and view all the answers

What type of converter can output power at a different frequency without converting to DC first?

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

Which of the following power semiconductor devices can handle a voltage of up to 6000 V?

<p>Thyristor (A), Power Diode (D)</p> Signup and view all the answers

What is a key advantage of using power diodes?

<p>High mechanical and thermal reliability (D)</p> Signup and view all the answers

What is one advantage of BJTs over other types of transistors?

<p>High switching frequencies (C)</p> Signup and view all the answers

Which of the following statements about IGBTs is true?

<p>IGBTs combine the advantages of BJTs and MOSFETs (D)</p> Signup and view all the answers

Why are BJTs not suitable for parallel operation?

<p>They induce negative temperature coefficient issues (A)</p> Signup and view all the answers

What is a common application for silicon controlled rectifiers (SCRs)?

<p>High voltage and power control (C)</p> Signup and view all the answers

What state is the IGBT in when the gate-emitter voltage (VGE) is less than the gate-emitter threshold voltage (VGET)?

<p>Off state (C)</p> Signup and view all the answers

Which component creates a magnetic field in the transformer when the DC voltage is applied to the inverter?

<p>T1 in saturation (A)</p> Signup and view all the answers

What is a key drawback of BJTs mentioned?

<p>Complex drive circuits (D)</p> Signup and view all the answers

What is the purpose of the harmonic filter L1 and C2 in an inverter?

<p>To smooth the output AC voltage (C)</p> Signup and view all the answers

What characteristic of IGBTs contributes to their efficiency?

<p>Low on-state voltage (B)</p> Signup and view all the answers

Which of the following describes a characteristic of total harmonic distortion (THD)?

<p>It is the total contribution of harmonic frequencies (B)</p> Signup and view all the answers

What limitation is associated with the triggering angle for SCRs?

<p>It is limited to 90 degrees (D)</p> Signup and view all the answers

In what applications can an inverter be used?

<p>For industrial induction heating and UPS systems (B)</p> Signup and view all the answers

Which of the following is a disadvantage of using a BJT in applications?

<p>Problems with charge storage limiting switching frequencies (D)</p> Signup and view all the answers

When T1 is OFF in the inverter circuit, what happens next?

<p>T2 will switch to the ON state (D)</p> Signup and view all the answers

What does the term 'cut-off' refer to in the context of IGBT operation?

<p>The device is completely non-conducting (A)</p> Signup and view all the answers

Which parameter is the controlling factor for the IGBT?

<p>Gate-emitter voltage (VGE) (A)</p> Signup and view all the answers

Flashcards

What are converters?

Converters are electronic circuits that transform AC power to DC power, or vice versa, or modify AC voltage and frequency.

What are rectifiers?

Rectifiers convert AC to DC voltage. They are used in power supplies for electronic devices, like those on a factory floor.

Choppers

Choppers are DC to DC converters that adjust the output voltage, often used to control the speed of motors.

Inverters

Inverters convert DC to AC voltage and are used in applications like variable frequency drives, UPS systems, and power generation.

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AC Voltage Controllers

These converters control the voltage of an AC signal, but keep the frequency the same.

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Cycloconverters

These converters directly change the frequency of AC power without converting to DC first.

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IGBT's

Insulated Gate Bipolar Transistors (IGBT's) are power semiconductor devices that combine the features of MOSFET's and BJT's for high-power switching.

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Diodes

Diodes are semiconductor devices that allow current to flow in one direction only, they are often used in rectifiers.

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Transistor Switching Speed

Modern transistors switch much faster than thyristors, making them ideal for high-speed applications like DC-DC and DC-AC converters.

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Transistor Power Ratings

Transistors handle lower voltage and current than thyristors, making them suitable for low to medium power applications.

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Bipolar Junction Transistor (BJT)

A type of transistor that uses a base current to control a larger collector current. They're known for their high switching frequencies and controlled turn-on/off.

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What are the advantages of BJTs?

BJTs offer high switching frequencies, low turn-on losses, controlled turn-on/off, and don't require complex commutation circuits.

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What are the disadvantages of BJTs?

BJTs have complex drive circuits, charge storage limitations, and negative temperature coefficient issues that prevent parallel operation.

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Silicon Controlled Rectifier (SCR)

A four-layer solid-state device used for controlling high voltage and current in AC power applications like motor control.

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Insulated Gate Bipolar Transistor (IGBT)

A transistor combining the best of both BJT and MOSFET worlds, offering high input impedance, low conduction losses, and fast switching speeds.

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IGBT Applications

IGBTs are used in high-power applications like motors, inverters, and welding systems due to their high voltage and current handling capabilities.

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Inverter Output

The output of an inverter is a 'chopped' AC voltage, meaning it's a square wave with rapid transitions between positive and negative values. Notably, it has no DC component (average value is zero) and contains harmonics.

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Harmonics in Inverter Output

Harmonics are unwanted frequencies present in the inverter output due to the square wave nature of the signal. They occur at multiples of the fundamental frequency and contribute to distortion in the output voltage.

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LC Filter for Inverter

An LC filter, consisting of inductors and capacitors, is used to reduce high-frequency harmonics in the inverter output. This filter acts as a low-pass filter, allowing the desired fundamental frequency to pass through while attenuating the higher frequencies (harmonics).

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Lower Order Harmonics

Lower order harmonics (like the 3rd and 5th harmonics) are difficult to filter due to the large size and complexity of the required filter. These harmonics can significantly distort the output voltage, leading to performance issues.

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Fourier Series

A mathematical tool used to analyze complex waveforms (like the square wave output of an inverter) by breaking them down into a sum of sine waves with varying amplitudes and frequencies. This allows for the identification and understanding of the individual harmonics present in the signal.

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Gate-Emitter Threshold Voltage (VGET)

The minimum gate-emitter voltage required to turn an IGBT on. Below this voltage, the IGBT remains in the off state.

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IGBT Transfer Characteristics

The relationship between the gate-emitter voltage (VGE) and the collector current (IC) of an IGBT. It shows how the current changes as the gate voltage increases.

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DC to AC Converter (Inverter)

A circuit that transforms direct current (DC) into alternating current (AC) by switching the DC input to generate a varying AC output.

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Inverter Applications

Used in various applications like UPS systems, industrial AC motor drives, HVDC power transmission, induction heating, and AC supply from solar arrays.

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How does an Inverter work?

An inverter employs transistors to switch DC input on and off in a sequence, causing current to flow through a transformer's primary winding in alternating directions, inducing an AC voltage in the secondary winding.

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Harmonic Filtering

A process used to remove unwanted harmonic frequencies from the output of an inverter, ensuring clean and stable AC output.

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Total Harmonic Distortion (THD)

A measure of the overall distortion caused by harmonic frequencies in a waveform. A lower THD indicates cleaner AC output.

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Study Notes

Unit 10 Converters

  • Converters are circuits used to change AC voltage to DC voltage, or vice versa.
  • Types of converters include rectifiers (AC to DC), choppers (DC to DC), inverters (DC to AC), AC voltage controllers, and cycloconverters.
  • Rectifiers convert constant AC voltage to variable DC voltage.
  • Choppers convert fixed DC voltages to controllable (variable) DC voltage and are used to control DC motor speed.
  • Inverters convert fixed DC voltage to AC voltage and are used in applications like variable frequency AC motor drives, power supplies, induction heating, and welding.
  • AC voltage controllers convert fixed AC voltage to variable AC voltage at the same frequency.
  • Cycloconverters convert input power at one frequency to output power at a different frequency without first converting to DC.

Power Semiconductor Devices

  • Power devices include diodes, transistors (BJTs, MOSFETs, Thyristors, IGBTs).
  • Diodes (6000V, 4500A) are beneficial due to high mechanical/thermal reliability, high peak inverse voltage, low reverse current, low forward voltage drop, high efficiency, and compactness.
  • Transistors (BJTs, MOSFETs, Thyristors, IGBTs) have controlled turn-on and turn-off characteristics and are used as switching devices. -BJTs excel in high switching frequencies, low turn-on losses, controlled turn-on/off characteristics, and lack of commutation circuits.
  • Thyristors (Silicon Controlled Rectifiers), are four-layer solid-state current control devices used for high-voltage and high-power applications (e.g., motor control).

IGBTs (Insulated Gate Bipolar Transistors)

  • IGBTs combine the advantages of BJTs and MOSFETs, including high input impedance, low switching energy, and small/low on-state voltage.
  • IGBTs are voltage-controlled devices, switching on when gate-emitter voltage (VGE) exceeds the threshold voltage (VGET).
  • IGBT characteristics show how voltage controls the device.

DC to AC Converter (Inverter)

  • An inverter converts DC to AC power by switching the DC input at a predefined sequence to produce AC voltage or current output.
  • Typical applications for inverters include UPSs, industrial induction motor drives, HVDC power plants and stations, induction heating, and AC power supplies from photovoltaic arrays.

Harmonics Filtering

  • Harmonics in a circuit are periodic waveforms with frequencies that are multiples of the fundamental frequency (e.g., 60Hz).
  • Total Harmonic Distortion (THD) represents the overall contribution of all harmonic currents to the total current frequency.
  • Filtering is often required for inverter applications to reduce harmonic distortion and ensure a pure sine wave output. LC filter circuits are often used to prevent high-frequency harmonics from being output.

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Unit 10 Converters PDF

Description

Explore the various types of converters used in power electronics including rectifiers, choppers, inverters, and more. This quiz will test your understanding of how these devices function and their applications in converting voltage types. Perfect for students of electrical engineering.

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