EEB 451 & EEB 516
Prof. E. Matlotse
Inverters
Inverter
• DC to AC Converters
- Converters Modulating Power From A DC
Source to Provide AC Power Are Called
Inverters.
- Two Parameters of Importance in Inverters
Are Output Voltage and Output frequency.
- Switch-Mode Inverter Is The Most Common
Inverter Found In Service.
Inverter
• Power Semi-Conductor Switches, In Different
Configurations, Chop the DC Supply Waveform So
That The Load Experiences Rectangular Waves
That Periodically Change Polarity To Give AC
Voltage.
• Concern Is Given To The Quality Of Power
Absorbed By The Load.
• Mostly, A Sinewave of A Single Frequency Is
Desired To Minimize Losses Produced By Other
Harmonics.
Inverter
• In Switch-Mode Inverters, There Are Always
Some Harmonics Becos Output Waveform Is
Synthesized From Rectangular Waveforms.
• Techniques To Produce A Near-SineWave Are
Called Pulse-Amplitude Modulation (PAM) Or
Pulse-Width Modulation (PWM) Methods.
Inverter
• Centre-Tapped Source Inverter
Fig.1(a): Centre-Tapped Source Inverter With Resistive Load
Fig.1(b): Centre-Tapped Source Inverter Waveforms
Inverter
• Fig. 1 Depicts A Single-Phase Inverter With A
Centre-Tapped Supply And a Resistive Load.
• This Inverter Is Also Known As A Single-Phase,
Half-Bridge Inverter.
• Overall Cct Can Be Viewed As Two Choppers
With A Common Load R.
• One Chopper Cct Is Comprised of the
Elements Vs1, TH1 and R.
Inverter
• This Chopper Provides An Adjustable +Ve Voltage
VL at the Load.
• Second Chopper Is Comprised of the Elements
Vs2, TH2 and R.
• This Chopper Provides An Adjustable -Ve VL at
the Load.
• If Each Chopper Has Its Switch Turned On for An
Interval ton over a period T, But Out of Phase
180⁰ (Equivalent to T/2 Seconds) From Each
Other, Waveforms Are Like Those Shown In Fig.
1(b).
Inverter
• Both Thyristors Are Off Initially.
• Thyristor TH1 Is Turned On At Time t = 0, It’s
Allowed To Conduct The Load Current For An
Interval ton and It’s Then Turned Off.
• At Time t = T/2, While Thyristor TH1 Remains
Off, Thyristor TH2 Is Turned On to Conduct the
Reverse Load Current.
• TH2 Remains On For An Interval ton and is
Turned Off.
Inverter
• TH1 and TH2 Remain Off For the Rest Of the
Period T.
• At Time t = T, The Switching Cycle is Repeated.
• Instantaneous Output Voltage Can be
Expressed In Fourier Series As
2VS
VL = sin nt
n =1,3,5
n
=0 for n = 2,4,..........
Inverter
• where: = 2f is the frequency load voltage in
rad/sec.
• In Other Applications, It’s Important That The
Load Voltage Waveform Is Sinusoidal.
• Load Voltage Waveform In Fig. 1 Is Far From
Sinusoidal.
• There Is A Fundamental Sinewave Component At
Frequency f But There Are Also Substantial
Harmonic Components At Frequency 3f, 5f, 7f,
e.t.c.
Inverter
• Some of the Harmonics Can Be Filtered Out
Using LC Components.
• Output Of Practical Inverters Contain
Harmonics and the Quality of an Inverter Is
Normally Evaluated In Terms of the Following
Performance Parameters.
• Harmonic Factor of nth Harmonic (HFn): It’s a
Measure of Individual Harmonic Contribution
and It’s Defined As
Inverter
Vn
HFn =
V1
• where: V1 is the rms value of the fundamental
component
Vn is the rms value of the nth
harmonic component
• Total Harmonic Distortion (THD): It’s A Measure
of Closeness In Shape Between A Waveform and
It’s Fundamental Component and It’s Defined As
Inverter
1/ 2
1 2
THD =
Vn
V1 n=2,3,.....
• Distortion Factor (DF): This Indicates the
Amount of Harmonic Distortion That Remains
In A Particular Waveform After the Harmonics
Of That Waveform Have Been Subjected To A
2nd Order Attenuation (i.e. divided by n2)
Inverter
• Thus DF Is A Measure of Effectiveness In
Reducing Unwanted Harmonics Without
Having to Specify the Values of a 2nd Order
Load Filter and It’s Defined As
1/ 2
1 2
Vn
DF = 2
V1 n=2,3,..... n
• DF of An Individual (or nth) Harmonic
Component Is Defined as
Inverter
Vn
DFn =
V1n 2
• Lowest-Order Harmonic (LOH): This is That
Harmonic Component Whose Frequency Is
Closest To The Fundamental One and Its
Amplitude Is Greater Than or Equal to 3% of
the Fundamental Component.
Inverter
• Single-Phase Bridge Inverter
Fig.2(a): Basic Single-Phase Bridge Inverter Cct
Fig.2(b): Basic Single-Phase Bridge Inverter Square-Wave Output
Inverter
• Generation of a Square Wave Load Voltage With An
Inductive Load Is Shown In the Waveforms of Fig.2(b).
• The Thyristors Are Fired By A Continuous Train of Gate
Pulses for 180⁰ of the Inverter Output Voltage.
• Looking At The Latter Part of the +Ve Half Cycle, The Load
Current Is +Ve and Growing Exponentially, However,
When T3 and T4 Are Gated To Turn Off Thyristor T1 and T2,
The Load Voltage Reverses But Not The Load Current.
Inverter
• The Only Path For The Load Current Is Via Diodes
D3 and D4, Which Connects The DC Source To The
Load, Giving A Reverse Voltage With The Stored
Inductive Energy Of The Load Being Returned To
The DC Source Until The Load Current Falls To
Zero.
• Once Load Current Ceases, Thyristors T3 and T4
Can Conduct So As To Feed Power Into The Load,
The Load Current Now Growing Exponentially.
Inverter
• Control Of The Voltage Can Be
Obtained From A Fixed-Level DC
Source By Introducing Zero Periods
Into The Square-Wave Giving A
Shape Known As A Quasi-Square
Wave (Refer To The Recommended
Text Book).
Inverter
• Three-Phase Inverters
Fig.1(a): Three-Phase Inverter Cct Diagram
Fig.1(b): Three-Phase Inverter Waveforms & Conduction Intervals
Fig.2: Equivalent Ccts (a) TH5, TH6 & TH1 ON (b) TH6, TH1 & TH2 ON
Inverter
• Fig.1 Depicts An Ideal Cct Diagram of A Three-
Phase Bridge Inverter With A Resistive Wye-
Connected Load.
• Phase Voltage Van Is +Ve During The Interval 0 to
π Radians.
• To Implement This Requirement In Practice, TH1
Must Be ON and TH4 Must Be OFF.
• At the Same Time, Either TH6 or TH2 Must Be ON
To Complete The Path To The Supply and Allow
Conduction In Phase a.
Inverter
• On Fig.1, TH1 Is Shown To Have A Conduction
Interval From 0 to π Radians.
• For Cyclic Repetition, TH1 Must Be Conducting
Again For The Interval 2π to 3π Radians.
• In the Same Figure, Voltage Van Is –Ve Over
The Interval π to 2π Radians.
• This Can Be Accomplished By Having TH4 ON
and TH1 OFF.
Inverter
• Consequently, TH4 Is Shown To Have A
Conduction Interval From π to 2π Radians.
• If We Repeat This Argument For All +Ve and –
Ve Voltages of Phases b and c, We Can Arrive
At A Conduction Pattern For All The Thyristors
Covering All Intervals of Time.
• In Terms of Each Individual Thyristor, It’s ON
for π Radians Each Period.
• This Is Called 180⁰ Switching.
