Inverters (DC-AC)

Power Electronics
EE 368 Lecture-6
DC to AC Power
processing DC-AC Invertors
These slides are compiled from the material collected from the text book and web resources:

An inverter refers to a power electronic device that converts DC input
voltages to a symmetrical AC output voltages at the required
magnitude and frequency.
DC- AC power Processing
Inverter
Single Phase AC supply from a DC source
or
Three phase AC supply from a DC source
Inverter circuits can produce:

DC to AC Power
processing
DC motor coupled with 3 phase
AC Alternator is used to
produce AC voltages from DC
input
Rotary Converter

Three phase AC
alternator driven by DC
Motor
Efficiency ????

Losses, Losses, Losses
Mechanical DC-AC
conversion using rotary-
motor-alternator set face
heavy energy losses.
Better Idea!!!!!!
Solid-state switches using PWM
DC
AC

Solid state inverters are more efficient with minimal losses.

What is an Inverter?
• A static device that converts DC power into AC power at desired
output voltage and frequency is called an Inverter.
• Applications
• Adjustable – speed AC drives
• Automobiles (EV)
• Induction Heating,
• Aircraft power supplies,
• PV (Photovoltaic Systems)
• UPS, Air-conditioning units

Inverter Application in
modern days Air-
condition unit

Classification of Inverters
• According to the method of Commutation
• Line Commutated Inverter
• Force Commutated Inverter
• According to the method of Connections
• Series Inverter
• Parallel Inverter
• Bridge Type Inverter
• According to the nature of DC source feeding the Inverter
• Voltage source Inverter
• Current Source Inverter

Voltage Source Inverters
• VSI has a constant voltage at its input terminals.
• Its output voltage does not depend on load.
• Its output current depends on the type of load.
• CSI has a constant current at its input terminals.
• Its output current does not depend on load.
• Its output voltage depends on the type of load.
Current Source Inverters

Voltage Source Inverters
Single phase Inverter
Half Bridge Inverter
Full Bridge Inverter
Three phase voltage source inverter
180 degree mode
120 degree mode

There are three basic types of dc-ac converters
depending on their AC output waveform:
1. Square wave
2. Modified sine wave
3. Pure sine wave
THE TYPES.
Total harmonic distortion (THD)
Square wave = THD≈45%
Modified sine wave = THD is 24%
Pure sine wave:
By adding two more voltage levels, a designer
can reduce THD from 24% to typically 6.5%

Basic DC-AC Converter Connections (Square-Wave Operation)

Low-power-inverter
working
Low-power inverter uses only 9 parts and turns
10 to 16 Vdc into 60-Hz, 115-V square-wave
power to operate ac equipment up to 25 W
The two transistors drive the transformer in push-pull fashion. When one transistor is biased-on, the
other is cut-off. The transformer is a 120 V/18 VCT unit that is connected backwards, so that it steps the
voltage up rather than down. Oscillator circuit U1, R1, R2, and C1 operates from about 4 to 16 V with a
very ~stable output.
Easy to Understand

Basics
Inverters are DC to AC converters
We can use inverters to generate
• A dc supply
• Single-phase AC supply
• Three-phase AC supply
from a single dc source.
The basic building block is the inverter ‘leg’.
An inverter leg is shown. Vdc is the input, Vout the AC
output.

Inverter switching
s
on
dcout
T
t
VV ,1

• T1 and T2 are NEVER turned on together. Why?
• T1 and T2 are switched using PWM in a complementary
manner (T2 ON, T1 OFF)
• Vout is then a switched waveform, just like the basic step-
down converter earlier.

Pulse-width Modulation
τsw
ton
Carrier
waveform
Modulating
waveform
sw
swf

1

sw
ont

 
T1,on
T2,on

Current Paths
Current path if T2 ON, or T1 and T2 ‘OFF’
Current path if T1 ‘ON’
• Two switches with freewheel diodes
provides uni-directional voltage and
bi-directional current control.
• Only when T1 is ON is energy
supplied from the source.
• When T2 ‘ON’, the energy stored in
inductor dissipated through T2 in
parallel with D2 current is start
reducing but still in +
• With positive current flow →
T2 ‘ON’

Current Paths
Current path if T2 ON
Current path if T1 ON, or T1 and T2 OFF
• When T1 is ON (or T1 and T2 OFF)
energy has to be absorbed by the
source.
• When T2 ON, a zero voltage loop is
applied.
• With negative current flow →

Source: https://www.youtube.com/watch?v=5YiCHLbNSos
SINGLE PHASE HALF BRIDGE INVERTER (with resistive load)

Source: https://www.youtube.com/watch?v=5YiCHLbNSos
SINGLE PHASE HALF BRIDGE INVERTER (with RL load)
T1 ON
T1 ON
Inductor dissipating through D2
T2 ON

SINGLE PHASE HALF BRIDGE INVERTER (with RL load)

Switching signal and the output voltage
Power Circuit of a half wave bridge inverter
Pulse Width Modulated Inverters
Two Level Inverters (One Leg or half Bridge)
1. Single Phase Half Bridge Inverters
2. Single Phase Full Bridge Inverters
3. Three-phase PWM voltage source inverter
V
I
The basic bridge leg
can operate in two
quadrants of the VI
graph.

Average Output Voltage
A single inverter leg produces an average output voltage:
Define a duty cycle or modulation index
Hence
m must be between 0 and 1.
We can make m vary in time therefore we can produce any
voltage and any frequency we desire (within the bounds
fixed by the switching frequency and Vdc).
s
on
dcout
T
t
VV ,1

s
on
T
t
m ,1

dcout mVV 

‘LEG’ is a basic building
block of an inverter
‘LEG’

Current and Voltage in an Inductor
How much induced voltage will be produced by
the inductor depends upon the rate of current
change. In our tutorial about Electromagnetic
Induction, Lenz’s Law stated that: “the direction
of an induced emf is such that it will always
opposes the change that is causing it”. In other
words, an induced emf will always OPPOSE the
motion or change which started the induced
emf in the first place.
So with a decreasing current the voltage polarity will be acting as a source and with an increasing current the
voltage polarity will be acting as a load. So for the same rate of current change through the coil, either
increasing or decreasing the magnitude of the induced emf will be the same.
Inductor Example
A steady state direct current of 4 ampere passes through a solenoid coil of 0.5H. What would be the back emf
voltage induced in the coil if the switch in the above circuit was opened for 10mS and the current flowing
through the coil dropped to zero ampere.

Single Phase Full Bridge Inverter
With Restive Load

With Inductive and Resistive Load
T1, T2 ON
Stored Energy in L
dissipated through
D3, D4
‘L’ source D3, D4 ON

DC- AC power Processing T3, T4 Conducting
With Inductive and Resistive Load

3-Phase Bridge Inverter
The 180 degree mode and the 120
mode of operation are two different
strategies for switching the power
electronic switches to get the
desired AC output waveform.
180 degree mode of operation:
if you divide a 360 degree cycle into
6 intervals of 60 degree each you
will have 3 devices conducting in
each interval
120 degree mode of operation:
Each switch is on for an interval of
120 degrees in each cycle and two
switches are on simultaneously.

120 degree Operation
Only two switches are used at
any given time

3-Phase Bridge Inverter
180 degree Operation
Only three switches are
used at any given time

https://www.homemade-circuits.com/arduino-spwm-generator-circuit/
Arduino Based Sine Wave
Inverter Circuit

Pulse Generator using
555 Timer
100K POT is used to control
the PULSE Width

Hall Effect Sensor
3-Phase DC Motor
https://www.digikey.com/en/articles/techzone/2013/mar/an-introduction-to-brushless-dc-motor-control

3-Phase DC motor
Stater Winding

https://www.digikey.com/en/articles/techzone/2013/mar/an-introduction-to-brushless-dc-motor-control

Dual Full Bridge PWM Motor Driver IC DRV 8402
By Texas Instrument

Inverters (DC-AC)

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