LECTURE 16 (Ch. 6) PULSE-WIDTH-MODULATED INVERTERS - PowerPoint PPT Presentation

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LECTURE 16 (Ch. 6) PULSE-WIDTH-MODULATED INVERTERS

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LECTURE 16 (Ch. 6) PULSE-WIDTH-MODULATED INVERTERS ECE 452 Power Electronics * Three-Phase Inverters Three-phase inverters are used for high power applications. – PowerPoint PPT presentation

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Title: LECTURE 16 (Ch. 6) PULSE-WIDTH-MODULATED INVERTERS


1
LECTURE 16 (Ch. 6) PULSE-WIDTH-MODULATED INVERTERS
  • ECE 452
  • Power Electronics

2
Three-Phase Inverters
  • Three-phase inverters are used for high power
    applications.
  • Three-single phase inverters can be connected in
    parallel to form a three-phase inverter.
  • This arrangement will require 12 transistors, 12
    diodes, and three single-phase transformers.

3
  • The gating signals of the single phase inverters
    should be 120 degrees with respect to each other.
  • The transformer primaries are isolated from each
    other, while the secondaries may be connected in
    wye or delta.

4
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5
  • A three-phase output can be obtained from a
    configuration of six transistors and six diodes.
  • Two types of control signals can be applied to
    the transistors 180o conduction or 120o
    conduction.

6
180 degree Conduction
  • For this mode of operation, each device conducts
    180 degrees.
  •  
  • The sequence of firing is 123, 234, 345, 456,
    561, 612.
  • The gating signals are shifted from each other by
    60 degrees.

7
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8
120 degree Conduction
  • In this mode, each transistor conducts for 120
    degrees.
  • The sequence of firing is 61, 12, 23, 34, 45,
    56, 61.

9
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10
Voltage Control of Single-Phase Inverters
  • In industrial applications, it is required to
    control the output voltage of inverters.
  • There are various techniques to vary the inverter
    gain.
  • The most efficient method of controlling the gain
    is to incorporate PWM control within the
    inverters.

11
  • The commonly used techniques are
  • Single-pulse-width modulation
  • Multi-pulse-width modulation
  • Sinusoidal pulse-width modulation
  • Modified sinusoidal pulse-width modulation
  • Phase-displacement control

12
Single Pulse-Width Modulation
  • There is one pulse per half-cycle, and its width
    is varied.

13
  • The modulation index is
  • The rms output voltage is
  • The following figure shows the harmonic profile
    with variation of the modulation index M.

14
  • The dominant harmonic is the third.
  • DF increases significantly at a low output
    voltage.

15
Multiple-Pulse-Width Modulation
  • The harmonic content can be reduced by using
    several pulses in each half-cycle of output
    voltage.
  • This type of modulation is also known as
    uniform-pulse-width modulation (UPWM).

16
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17
  • The number of pulses per half cycle is found
    from
  • where mf is defined as frequency modulation
    ratio.
  • The rms output voltage is

18
  • The following figure shows the harmonic profile
    against the variation of modulation index, and
    p5.
  •  

19
Sinusoidal Pulse-Width Modulation
  • Instead of maintaining the width of all pulses
    the same, the width of each pulse is varied in
    proportion to amplitude of a sine wave.
  • This kind of modulation is known as SPWM.

20
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21
  • The rms output voltage is
  • The DF and LOH are reduced significantly, as
    shown below.

22
  • This type of modulation eliminates all harmonics
    less than 2p-1.

23
Modified Sinusoidal Pulse-Width Modulation
  • This utilizes a different method of modulation.

24
  • The harmonic profile is shown below.

25
Phase-Displacement Control
  • Voltage control can be obtained by using multiple
    inverters and summing the output voltages of
    individual inverters.
  • A single-phase full-bridge inverter can be
    perceived as the sum of two-bridge inverters.

26
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28
  • A 180 degrees phase displacement produces c.
  • A delay angle of a produces e.
  • Then the rms output voltage is
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