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Inverter Circuits. Provide a variable voltage, variable frequency AC output from a DC input Very important class of circuits. Extensively used in variable speed AC motor drives for example (see H5CEDR)

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inverter circuits
Inverter Circuits
  • Provide a variable voltage, variable frequency AC output from a DC input
  • Very important class of circuits. Extensively used in variable speed AC motor drives for example (see H5CEDR)
  • We have already seen how the fully controlled thyristor converter can operate in the inverting mode ( > 90O) - however that is limited:
    • Can only invert into an existing AC supply
    • Voltages must already be present to provide natural commutation of thyristors
  • The circuits we will look at here are much more versatile and can provide an AC output into just about any kind of load
  • Three phase and single phase versions are possible - principles are the same
basic inverter leg 1

Q1

D1

E/2

IAC

DC Supply (E)

O

X

Don’t worry about where

current goes yet

E/2

D2

Q2

Basic Inverter Leg (1)
  • Basic building block is the “2-level inverter leg”
  • Capacitor does not have to be split - O provides a convenient place to reference voltages to for understanding
  • Obviously never gate Q1 and Q2 at the same time! - “shoot through” causes destruction
  • Normal mode is to use complementary gating for Q1 and Q2
    • In practice a small delay must be introduced between turning Q1 off and Q2 on (and vice versa) to avoid “shoot through” due to finite switching times
    • We will ignore the effect of this and assume perfect switching
basic inverter leg 2
Basic Inverter Leg (2)
  • Output voltage depends on gated device only and not on current direction
  • Circuit produces 2 voltage levels
  • Equivalent circuit:
  • Not often used on its own - but provides basic building block for other circuits
single phase inverter h bridge 1

IDC

DC LINK

D1

D3

E/2

Q1

Q3

IAC

DC Supply (E)

X

O

VAC

load

Y

Q2

Q4

E/2

D2

D4

Single Phase Inverter H-bridge (1)
  • Uses 2 inverter legs
  • Energy flow in both directions possible - circuit can be used as a rectifier - see later
single phase inverter h bridge 2
Single Phase Inverter H-bridge (2)
  • VXO, VYO are 2-level waveforms (E), VXY can be a 3-level waveform
    • Note: this is called a “2-level” circuit since each leg is a 2-level leg
    • Circuit can produce +E, 0 and -E in response to gating commands, regardless of current direction
  • We can synthesize (on average) any waveform we like by switching for varying amounts of time between +E, 0, -E
  • For example, for variable DC we could use:
    • Q1, Q4 gated 0 < t < dT, Q2, Q3 gated dT < t < T
  • Average (DC) output = Ed - E(1-d) = E(2d-1)
  • Used like this (or similarly) circuit is called a “Chopper” - see H5CEDR for application to DC motor drives
single phase inverter h bridge 3
Single Phase Inverter H-bridge (3)
  • To get AC output, we could operate like described previously, but dynamically vary the duty cycle (d) to follow an AC demand
  • This is called Pulse-Width Modulation (PWM) - see lhandout for what the waveform looks like
  • For this to be effective, the switching frequency has to be an order of magnitude greater than the demand frequency
  • PWM produces an output waveform with a spectrum consisting of the wanted component + distortion components clustered (sidebands) around the switching frequency and its multiples
single phase inverter h bridge 4
Single Phase Inverter H-bridge (4)
  • Some sort of filtering action is required to extract the desired component and eliminate the distortion
  • To produce an AC voltage we could use:
  • For an inductive load that requires a smooth current (eg an electrical machine), the machine inductance provides the filtering:
inverter application examples
Inverter Application Examples
  • Single Phase
  • Three Phase
single phase inverter square wave operation
Single Phase Inverter Square wave operation
  • Return to PWM later - simplest method of voltage/frequency control is “quasi-squarewave”
  • Used to be very popular when power devices were slow and high switching frequencies were not possible
  • Gate each side of the bridge with a squarewave at the desired output frequency
  • Adjust phase shift between the two sides to get voltage control
  • See handout for waveforms
  • See handout on relationship between AC side and DC side harmonics
pwm techniques
PWM Techniques
  • 2 Basic forms for single phase (H-bridge) inverter
  • 2-level PWM.
    • Each diagonal pair of switches is operated together.
    • Output is either +E or –E (hence name 2-level).
    • Gating pattern is Q1Q4  Q2Q3  Q1Q4.
  • 3-level PWM
    • All possible (allowable) gating patterns are used.
    • Output can be +E, 0 or –E.
  • Generation of PWM gating pattern.
    • Easiest method to understand is Natural Sampling (analogue method not often used now)
    • Most applications now use a microprocessor, microcontroller or DSP to generate the PWM pattern using a digital modulation technique.
natural sampling 1
Natural Sampling 1
  • See handout for detail of comparison process
  • Definitions:
natural sampling 2
Natural Sampling 2
  • Frequency ratio (FR) can be integer (synchronous PWM) or non-integer (asynchronous PWM).
    • It is normal now to keep the carrier frequency fixed as the modulating frequency is varied – hence most PWM today is asynchronous.
  • Modulation Index (MI) tells us how large the modulating frequency component at the inverter output will be for a given DC link voltage.
  • Modulation Depth (MD) tells us how much we have modulated the pulses by (compared to an unmodulated 50% duty cycle carrier frequency squarewave).
  • For Natural Sampling MI = MD (provided MD < 1)
    • Hence control of amplitude and frequency of the modulating wave, provides direct frequency and voltage control at the inverter output.
  • Spectrum of 2-level PWM: Modulating component + sidebands around carrier frequency + sidebands around 2 times carrier frequency etc – see Handout
natural sampling 3
Natural Sampling 3
  • 3-level  use the same carrier for both sides of the H-bridge, but invert the modulating wave (180O shift).
  • VXO and VYO are 2-level, VXY is 3-level.
  • Components clustered as sidebands around odd multiples of the carrier frequency are in-phase in VXO and VYO and therefore cancel in VXY
  • Other components are in anti-phase in VXO and VYO and therefore add in VXY
  • 3-level produces less distortion for given carrier (switching) frequency – see Handout
digital pwm
Digital PWM
  • Natural sampling is not suitable for a microprocessor implementation.
    • Switching instants occur at the natural intersection between a triangle wave and a sinewave.
    • Equation determining the switching instants has no analytical solution (transcendental equation) and can only be solved by iteration – no good for real time calculation.
  • Microprocessor implementation uses the Regular Sampling method (or something similar).
    • There are no continuous modulating or carrier waves.
    • Time is divided into a sequence of carrier periods of width TC.
    • The modulating wave exists as a series of samples, sampled either every TC (symmetric PWM) or every TC/2 (asymmetric PWM).
    • One pulse is produced within each carrier period.
    • Pulsewidth depends on either one sample of the modulating wave (symmetric PWM) or two samples of the modulating wave (asymmetric PWM).
regular sampling symmetric pwm
Regular SamplingSymmetric PWM
  • Let SK-1, SK, SK+1 etc be the samples of the modulating wave sampled at rate (1/TC).
  • Assume the modulating wave is scaled so that its peak amplitude is unity.
  • Simple equations define the pulsewidths – OK for real time digital implementation.
  • MD  MI for regular sampling
regular sampling asymmetric pwm
Regular Samplingasymmetric PWM
  • Let SAK-1, SBK-1, SAK, SBK, SAK+1, SBK+1 etc. be the samples of the modulating wave sampled at rate (2/TC).
  • Assume the modulating wave is scaled so that its peak amplitude is unity.
  • Asymmetric PWM produces less distortion than symmetric PWM for a given carrier (switching frequency)
  • MD  MI as for symmetric sampling
pwm miscellaneous
PWM Miscellaneous
  • Choice of carrier frequency
    • Compromise depending on switching losses in the inverter and output waveform distortion.
    • Also depends on the switching device technology used.
    • Typical values: 16kHz (1kW), 5kHz (100kW), 1kHz (1MW) – assuming IGBT devices.
  • Other types of PWM (not a complete list)
    • Space Vector PWM
      • Similar to regular sampling, but derived from the “space-phasor” representation of 3-phase quantities. Popular in “Vector controlled” induction motor drives (see H54IMD)
    • “Optimised PWM”
      • Spectrum of PWM is defined mathematically in terms of the pulsewidths. Numerical techniques are then used to calculate the pulsewidths to meet a particular performance target.
      • For example: eliminate certain harmonics, minimise weighted sum of harmonics etc.
      • Not popular except in some special applications
3 phase inverter

DC LINK

DC Supply (E)

O

A

B

C

3-phase load

3-phase Inverter
  • VAO etc are 2-level (±E/2), VAB etc are 3-level (±E and 0).
  • Each leg is modulated using the same carrier, but with modulating waves 120o apart (3-phase).
  • The large carrier frequency component in VAO etc cancels in VAB etc.
  • PWM control of inverter gives variable voltage and variable frequency output.
  • Average power flow can be bidirectional if the DC source can accept power input.
3 phase ac to ac rectifier inverter
3-phase AC to AC(rectifier - inverter)

RECTIFIER

DC LINK

INVERTER

3-PHASE

SUPPLY

3-Phase

AC Load

  • Industry “workhorse” - made from a few kW to MW - particularly for Induction Motor drives.
  • Unidirectional power flow since diode rectifier can't accept power reversal.
  • Energy can only be extracted from motor (braking) if some form of resistor is connected across the DC link during this mode. Common practice in industrial drives - known as dynamic braking.
  • AC supply current waveforms are poor because of diode rectifier.