Inverters (With Pics)

7/28/2019 Inverters (With Pics)

1/19

Inverter Circuits

Provide a variable voltage, variablefrequency 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 thatis 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


2/19

Basic Inverter Leg (1)

Basic building block is the 2-level inverter leg

Dont worry about wherecurrent goes yet

ODC Supply (E) X

E/2

E/2

IAC

D2

D1Q1

Q2

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 introducedbetween 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


3/19

Basic Inverter Leg (2)

Output voltage depends on gated device only and

not on current direction

Circuit produces 2 voltage levels

Equivalent circuit:

-E/2Q2-Q2

-E/2D2+Q2

+E/2D1-Q1

+E/2Q1+Q1

VXOConducting

Device

Direction of

IAC

Gated Device

Not often used on its own - but provides basic

building block for other circuits


4/19

Single Phase Inverter

H-bridge (1) Uses 2 inverter legs

-Q2 Q4

+Q2 Q4

-Q2 Q3

+Q2 Q3

-Q1 Q3

+Q1 Q3

-Q1 Q4

+Q1 Q4

Energy

flow

Polarity

of IDC

VACConducting

Devices

Polarity

of IAC

Gated

Devices

DC Supply (E)E/2

E/2

IAC

D2

D1

Q1

Q2

DC LINK

loadOX

Y

Q3

VAC

Q4

D3

D4

IDC

Energy flow in both directions possible - circuit

can be used as a rectifier - see later


5/19


H-bridge (2)

VXO, VYO are 2-level waveforms (E), VXY can be a3-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


6/19


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 frequencyhas 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


7/19


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:


8/19

Inverter Application

Examples Single Phase

Three Phase


9/19


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 atthe 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


10/19

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 usednow)

Most applications now use a microprocessor,

microcontroller or DSP to generate the PWM

pattern using a digital modulation technique.


11/19

Natural Sampling 1

See handout for detail of comparison process

Definitions:

Comparator

+

-

PWM

Q1,Q4

Q2,Q3

Carrier Wave c(t)

Modulating Wave m(t)

Natural Sampling Generation of 2-level PWM

)(MIndexModulationwavePWMofAmplitude

componentmodulatingofPeak

:

)(MDepthModulationc(t)ofPeak

m(t)ofPeak

)(FratioFrequencyFrequencyModulating

frequencyCarrier

I

D

R

=

=

=

M waveFor any PW


12/19

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 inverteroutput 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


13/19

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 VXOand VYO and therefore cancel in VXY

Other components are in anti-phase in VXO and VYOand therefore add in VXY

3-level produces less distortion for given carrier

(switching) frequency see Handout

Comparator

+

-

PWM

Q1

Q2

Carrier Wave c(t)

Modulating Wave m(t)

Natural Sampling Generation of 3-level PWM

+

-

PWM

Q3

Q4Comparator

-1


14/19

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 ofwidth 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).


15/19

Regular Sampling

Symmetric 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.

TC TC TC

TC/2

K-1K-1

K K+1K K+1

( )

( )

( )

depthModulationtheis

etc14

14

14

111

111

D

KDC

kK

KDC

kK

KD

C

kK

M

SMT

SMT

SMT

+++

+==

+==

+==

Simple equations define the pulsewidths

OK for real time digital implementation.

MD MI for regular sampling


16/19

Regular Sampling

asymmetric 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.

TC TC TC

TC/2

K-1K-1

K K+1K K+1

( ) ( )

( ) ( )

( ) ( )

depthModulationtheis

etc14

,14

14

,14

14,14

1111

1111

D

BKDC

kAKDC

K

BKDC

kAKDC

K

BKDC

kAKDC

K

M

SMT

SMT

SMT

SMT

SM

T

SM

T

++++

+=+=

+=+=

+=+=

Asymmetric PWM produces less distortion than

symmetric PWM for a given carrier (switching

frequency)

MD MI as for symmetric sampling


17/19

PWM Miscellaneous Choice of carrier frequency

Compromise depending on switching losses in theinverter 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 controlledinduction 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


18/19

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 VAOetc 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.

DC LINK

3-phase load

ODC Supply

(E)A B C


19/19

3-phase AC to AC

(rectifier - inverter)

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.

3-PHASE

SUPPLY

3-PhaseAC Load

RECTIFIER DC LINK INVERTER

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