inverter lecture 4

8/17/2019 Inverter Lecture 4

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( DC to AC Converters)

PWM Inverters

Inverters


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Voltage Source Inverters : Pulse

Width Modulated Inverters

EEE/INSRT F342 - Power ElectronicsR. Femi, Lecturer, EEE Department

2


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Single-phase half-bridge inverter

nics R. Femi, Lecturer, EEE Department

3


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rms value of the fundamental component

1,3,5,..

2sin so

n

V v n t

n

4

nics R. Femi, Lecturer, EEE Department

= 1.273

Vs=Vd


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Square-Wave Mode of Operation

EEE/INSRT F342 - Power ElectronicsR. Femi, Lecturer, EEE Department5


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6

Single Phase Inverter Square-wave

V dc

Q1

Q2

Q’ 1

Q’ 2

vout

+

+

-

-

q1(t)

q2

(t)1-q1

(t)

1-q2(t)

vout(t)

-Vdc

Vdc

dcdcout V V V 27.14

1

t

1,3,5,..

4sindco

n

V v n t

n

EEE/INSRT F342 - Power Electronics R. Femi,

Lecturer, EEE Department


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7

THD = 0.48

Characteristics: - High harmonic content.

- Low switching frequency.

- Difficult filtering.

- Little control flexibility.

Single Phase Inverter Square-wave




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Pulse-Width Modulated VSI

Objective of PWM

Disadvantages of PWM

Increase of switching losses due to high PWM frequency

Reduction of available voltage

EMI problems due to high-order harmonics

Control of inverter output voltage

Reduction of harmonics

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Voltage Control of Single-Phase Inverters

Commonly-used Techniques Single-Pulse-Width-Modulation

Multiple-Pulse-Width-Modulation

Sinusoidal-Pulse-Width-Modulation

Modified-Sinusoidal-Pulse-Width-Modulation

9 EEE/INSRT F342 - Power ElectronicsR. Femi, Lecturer, EEE Department


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Single-Pulse-Width-Modulation



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One Pulse per Half-Cycle

Pulse Width Controls the Output Voltage

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Carrier and Reference Signals

Compare the Reference Signal with the Carrier

Frequency of the Reference Signal determines the frequencyof the Output Voltage

Modulation Index (amplitude modulation ratio) = M = Ar/Ac

Gate PulseGate Pulse

EEE/INSRT F342 - Power ElectronicsR. Femi, Lecturer, EEE Department 12


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rms value of the Output Voltage

1

22

2

2

2( )

2

0 180

0

o s

o s

o s

V V d t

V V

V V


Quasi Square wave Inverter


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Fourier Series for the Output Voltage

1,3,5,...

4( ) sin sin2

so

n

V nv t n t n



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Multiple-Pulse-Width-Modulation



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Multiple Pulses per Half-Cycle of Output Voltage

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Gate Signal Generation

Compare the Reference Signal with the Carrier

Frequency of the Reference Signal determines the Output Voltage

Frequency Frequency of the Carrier determines the number of pulses per half-

cycle

Modulation Index controls the Output Voltage

EEE/INSRT F342 - Power ElectronicsR. Femi, Lecturer, EEE Department 17


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Gate Signals and Output Voltage

Number of pulses per half cycle = p = f c/2f o = mf /2

where mf = frequency modulation ratio



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rms Value of the Output Voltage1

2( ) / 2

2

( ) / 2

2( )

2

0 1

0

2

0

0

p

o s

p

o s

o s

pV V d t

pV V

M

T

p

p

V V



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Fourier Series of the Output Voltage

1,3,5,...

2

1

( ) sin

4 3sin sin ( ) sin (

4 4 4

o n

n

p

sn m m

m

v t B n t

V n B n n

n



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Sinusoidal Pulse-Width Modulation

In multiple-pulse modulation, all pulses are the same width

Vary the pulse width according to the amplitude of a sine

wave evaluated at the center of the same pulse



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Generate the gating signal

2 Reference Signals, vr , -vr 22




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Comparing the carrier and reference signals

Generate g1 signal by comparison with vr Generate g4 signal by comparison with -vr



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Comparing the carrier and reference signals

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Potential problem if Q1 and Q4 try to turn ON

at the same time!



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If we prevent the problem

Output voltage is low when g1 and g4 are

both high

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This composite signal is difficult to generate

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Generate the same gate pulses with one

sine wave

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Alternate scheme

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rms output voltage

Depends on the modulation index, M

2

1

p

m

o S S m

pV V V

Where δm is the width of the mth pulse

30EEE/INSRT F342 - Power Electronics

R. Femi, Lecturer, EEE Department


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Fourier coefficients of the output voltage

2

1

4 3

sin sin sin4 4 4

1, 3, 5,..

pS m m m

n m m m

V n

B n n n

n

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Pulse-Width Modulation (PWM)

Fig. 5 Pulse-width modulation.10

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I. Voltage Source Inverter (VSI)B. Pulse-Width Modulated VSI (3)

Inverter output voltage

When vcontrol > vtri, VA0 = Vdc /2

When vcontrol < vtri, VA0 = -Vdc /2

A01A0

10

Vof componentfrequecnylfundamenta:)(Vwhere,

,2/

)(

dc

A

tri

control

V

V of peak

v

vm

Modulation Index (m)

Control of inverter output voltage

Amplitude is controlled by the peak value of vcontrol

Fundamental frequency is controlled by the frequency of vcontrol

PWM frequency is the same as the frequency of vtri

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Characteristics: - High harmonic content.

- Low switching frequency.- Difficult filtering.

- More control flexibility.

THD = 0.3

Example with Vout-1=1.21Vdc

Single Phase Inverter Single Phase Inverter

SquareSquare--wave “Modulation” (4)wave “Modulation” (4)



O L f S i h M d DC AC


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One Leg of a Switch-Mode DC-AC

Inverter

EEE/INSRT F342 - Power Electronics

R. Femi, Lecturer, EEE Department35

S th i f Si id l O t t b


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Synthesis of a Sinusoidal Output by

PWM




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Details of a Switching Time Period


R F i L EEE D38

inverter lecture 4

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