An inverter converts DC power to AC power by switching the DC input voltage in a predetermined sequence to generate an AC output voltage. It has applications in providing backup power to AC equipment during failures and for controlling the speed of induction motors. A simple square wave inverter uses transistors as switches to alternately pass the DC voltage to generate an AC waveform. Harmonics filtering is needed as the inverter output contains harmonics. An LC filter is used to reduce high frequency harmonics. The inverter can vary both the frequency and voltage of the AC output by adjusting the input DC voltage and switching period.

1. BACK UP SYSTEMS
CHAPTER 16.5
INVERTERS
By
Dinesh Kumar Sarda

2. WHAT IS AN INVERTER ??
Also known as DC to AC Converter.
DEFINITION: Converts DC to AC power by
switching the DC input voltage (or current) in a
pre-determined sequence so as to generate
AC voltage (or current) output.

3. GENERAL BLOCK DIAGRAM
IDC
Iac
+

VDC Vac
+

DC INPUT
AC OUTPUTINVERTER

4. APPLICATIONS OF INVERTER
• Used in industries for providing back up supply to ac
equipments in case of ac failure
How does an inverter work-.mp4
• Used in Air conditioners for providing supply to compressor
motor to regulate temperature
Principles & Benefits & DC Inverter Technology - Powrmatic-Midea.mp4
• Used to control speed of induction motor in industries
• Used for harmonics filtering….Etc.

5. Simple square-wave inverter to illustrate the concept
of AC waveform generation
VDC
T1
T4
T3
T2
+ VO -
D1
D2
D3
D4
SQUARE-WAVE
INVERTER
IO
S1 S3
S2S4
EQUIVALENT
CIRCUIT

6. VDC
S1
S4
S3
+ vO

VDC
S1
S4
S3
S2
+ vO 
VDC
vO
t1 t2
t
S1,S2 ON; S3,S4 OFF for t1 < t < t2
t2 t3
vO
-VDC
t
S3,S4 ON ; S1,S2 OFF for t2 < t < t3
S2

7. What are harmonics ????
A harmonic of a wave is a component
frequency of the signal that is an integer
multiple of the fundamental frequency, i.e. if
the fundamental frequency is f, the harmonics
have frequencies 2f, 3f, 4f, . . . etc.
Harmonics Filtering

9. Harmonic voltages and currents in an electric
power system are a result of non-linear
electric loads. Harmonic frequencies in the
power grid are a frequent cause of power
quality problems. Harmonics in power systems
result in increased heating in the equipment
and conductors, misfiring in variable speed
drives, and torque pulsations in motors.
Reduction of harmonics is considered
desirable.

10. • Output of the inverter is “chopped AC voltage
with zero DC component”. It contain
harmonics.
• An LC section low-pass filter is normally fitted
at the inverter output to reduce the high
frequency harmonics.
vO 1
+

L
C
vO 2
(LOW PASS) FILTER
+

vO 1
vO 2
BEFORE FILTERING AFTER FILTERING
INVERTER LOADDC SUPPLY

11. Variable Voltage Variable Frequency Capability
T1 T2 t
Vdc1
Vdc2 Higher input voltage
Higher frequency
Lower input voltage
Lower frequency

12. • Output voltage frequency can be varied by
“period” of the square-wave pulse.
• Output voltage amplitude can be varied by
varying the “magnitude” of the DC input
voltage.
• Very useful: e.g. variable speed induction
motor drive

13. Rating of an Inverter