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

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

• ECE 452
• Power Electronics

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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.

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• 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.

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• 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.

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180 degree Conduction

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

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120 degree Conduction

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

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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.

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• 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

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Single Pulse-Width Modulation

• There is one pulse per half-cycle, and its width
  is varied.

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

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

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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).

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

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

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

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

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Modified Sinusoidal Pulse-Width Modulation

• This utilizes a different method of modulation.

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• The harmonic profile is shown below.

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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.

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