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ECE 8830 Electric Drives

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Q1-On; Q2 - Chopping; D3,Q1 freewheeling. Single-Phase Inverters (cont'd) ... Q4 - Chopping; D2,D1 freewheeling. Single-Phase Inverters (cont'd) ... – PowerPoint PPT presentation

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Title: ECE 8830 Electric Drives


1
ECE 8830 - Electric Drives
Topic 6 Voltage-Fed Converters
Spring 2004
2
Introduction
  • Voltage-fed converters convert dc input to ac
    output. These converters can operate
    bidirectionally as either an inverter or as a
    rectifier.
  • The input voltage should be stiff -
    indicating that the Thevenin impedance should be
    close to zero. Often the term Voltage Stiff
    Inverters (VSI) is used to describe these types
    of converters.

3
Introduction (contd)
  • Input Sources
  • Utility line or ac generator through
  • rectifier/filter.
  • Battery, fuel cell, PV array
  • Output Format
  • Single phase/Polyphase
  • Square wave, sine wave, PWM wave,
  • stepped wave, or quasi-square wave.

4
Single-Phase Inverters
  • Half-Bridge Inverter
  • One of the simplest types of inverter.
    Produces a square wave output.

5
Single-Phase Inverters (contd)
  • Full Bridge (H-bridge) Inverter
  • Two half-bridge inverters combined.
  • Allows for four quadrant operation.

6
Single-Phase Inverters (contd)
  • Quadrant 1 Positive step-down converter
  • (forward motoring)
  • Q1-On Q2 - Chopping D3,Q1 freewheeling

7
Single-Phase Inverters (contd)
  • Quadrant 2 Positive step-up converter
  • (forward regeneration)
  • Q4 - Chopping D2,D1 freewheeling

8
Single-Phase Inverters (contd)
  • Quadrant 3 Negative step-down converter
  • (reverse motoring)
  • Q3-On Q4 - Chopping D1,Q3 freewheeling

9
Single-Phase Inverters (contd)
  • Quadrant 4 Negative step-up converter
  • (reverse regeneration)
  • Q2 - Chopping D3,D4 freewheeling

10
Single-Phase Inverters (contd)
  • Phase-Shift Voltage Control - the output of
    the H-bridge inverter can be controlled by phase
    shifting the control of the component
    half-bridges. See waveforms on next slide.

11
Single-Phase Inverters (contd)
12
Single-Phase Inverters (contd)
  • The waveform of the output voltage vab is a
    quasi-square wave of pulse width ?. The Fourier
    series of vab is given by
  • The value of the fundamental, a1
  • The harmonic components as a function of phase
    angle are shown in the next slide.

13
Single-Phase Inverters (contd)

14
Three-Phase Bridge Inverters
  • Three-phase bridge inverters are widely used
    for ac motor drives. Two modes of operation -
    square wave and six-step. The topology is
    basically three half-bridge inverters, each
    phase-shifted by 2?/3, driving each of the phase
    windings.

15
Three-Phase Bridge Inverters (contd)

16
Three-Phase Bridge Inverters (contd)

17
Three-Phase Bridge Inverters (contd)
  • The three square-wave phase voltages can be
    expressed in terms of the dc supply voltage, Vd,
    by Fourier series as

18
Three-Phase Bridge Inverters (contd)
  • The line voltages can then be expressed as

19
Three-Phase Bridge Inverters (contd)
  • The line voltages are six-step waveforms and
    have characteristic harmonics of 6n?1, where n is
    an integer. This type of inverter is referred to
    as a six-step inverter.
  • The three-phase fundamental and harmonics are
    balanced with a mutual phase shift of 2?/3.

20
Three-Phase Bridge Inverters (contd)
  • If the three-phase load neutral n is isolated
    from the the center tap of the dc voltage supply
    (as is normally the case in an ac machine) the
    equivalent circuit is shown below.

21
Three-Phase Bridge Inverters (contd)
  • In this case the isolated neutral-phase
    voltages are also six-step waveforms with the
    fundamental component phase-shifted by ?/6 from
    that of the respective line voltage. Also, in
    this case, the triplen harmonics are suppressed.

22
Three-Phase Bridge Inverters (contd)
  • For a linear and balanced 3? load, the line
    currents are also balanced. The individual line
    current components can be obtained from the
    Fourier series of the line voltage. The total
    current can be obtained by addition of the
    individual currents. A typical line current wave
    with inductive load is shown below.

23
Three-Phase Bridge Inverters (contd)
  • The inverter can operate in the usual
    inverting or motoring mode. If the phase current
    wave, ia, is assumed to be perfectly filtered and
    lags the phase voltage by ?/3 the voltage and
    current waveforms are as shown below

24
Three-Phase Bridge Inverters (contd)
  • The inverter can also operate in rectification
    or regeneration mode in which power is pushed
    back to the dc side from the ac side. The
    waveforms corresponding to this mode of operation
    with phase angle 2?/3 are shown below

25
Three-Phase Bridge Inverters (contd)
  • See Bose text for Input Ripple and Device
    Ratings.

26
Three-Phase Bridge Inverters (contd)
  • The phase-shift voltage control principle
    described earlier for the single-phase inverter
    can be extended to control the output voltage of
    a three-phase inverter.

27
Three-Phase Bridge Inverters (contd)
  • The circuit shows three H-bridge inverters
    (one for each phase winding) where each H-bridge
    is connected to the primary winding of a
    transformer. The output voltages are derived from
    the transformers secondary windings connected in
    a wye configuration.

28
Three-Phase Bridge Inverters (contd)

29
Three-Phase Bridge Inverters (contd)
  • The three waveforms va0,vb0, and vc0 are of
    amplitude 0.5Vd and are mutually phase-shifted by
    2?/3.
  • The three waveforms ve0,vf0, and vg0 are of
    similar but phase shifted by ?.

30
Three-Phase Bridge Inverters (contd)
  • The transformers secondary phase voltages,
    vA0, vB0, and vc0 may be expressed as follows
  • where m is the transformer turns ratio
  • ( Ns/Np). Note that each of these waves is a
    function of ? angle.

31
Three-Phase Bridge Inverters (contd)
  • The output line voltages are given by
  • While the component voltage waves va0, vd0, vA0
    etc. all contain triplen harmonics, they are
    eliminated from the line voltages because they
    are co-phasal. Thus the line voltages are
    six-step waveforms with order of harmonics 6n?1
    at a phase angle ?.

32
Three-Phase Bridge Inverters (contd)
  • The Fourier series for vA0 and vB0 are given
    by

33
Three-Phase Bridge Inverters (contd)
  • The Fourier series for vAB is given by
  • Note that the triplen harmonics are removed in
    vAB although they are present in vA0 and vB0.

34
Three-Phase Bridge Inverters (contd)
  • See text for a description of Voltage and
    Frequency Control for the three-phase H-bridge
    inverter.

35
Three-Phase Bridge Inverters (contd)
  • A twelve-step inverter can be created by
    combining two six-step inverters.

36
Three-Phase Bridge Inverters (contd)
  • Features of the 12-step inverter
  • The lower bridge is phase shifted by ?/6 with
    respect to the upper bridge.
  • Each inverter is connected to the primary delta
    winding of each transformer.
  • Upper bridge transformer only has one secondary
    winding per phase whereas lower bridge
    transformer has two secondary windings per phase.
  • Note the difference in turns ratio for the two
    transformers.

37
Three-Phase Bridge Inverters (contd)
  • Phasor diagram for voltage synthesis and
    output voltage waveform are shown below

38
Three-Phase Bridge Inverters (contd)
  • The output phase voltages are obtained by the
    interconnection of three secondary windings to
    satisfy the phasor diagram on the previous slide,
    e.g. vNA vabvde-vef

39
Three-Phase Bridge Inverters (contd)
  • Since the lower bridge lags by ?/6,
    considering vab as the reference, the Fourier
    series of the component voltages are given by
  • where n is the turns ratio of the upper
    transformer.

40
Three-Phase Bridge Inverters (contd)
  • The fundamental component of vNA is given by
  • The output phase voltage Fourier series can be
    expressed as
  • Note the lower harmonic content compared to
    the six-step inverter.

41
Three-Phase Bridge Inverters (contd)
  • See Bose text for 18-step inverter.

42
Simulating Three-Phase Bridge Inverters
  • The below figure shows a SIMULINK block diagram
    for a 3-phase voltage-fed inverter.
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