1. 1
Arif Hussain COMSATS Atd. Pakistan.
Inverter
A power inverter, or inverter, is an electronic device/circuitry that changes direct
current (DC) to alternating current (AC). The input voltage, output voltage and
frequency, and overall power handling capacity depends on the design of the circuitry.
The inverter does not produce any power. Instead, the power is provided by the DC
source.
A power inverter changes DC power from a battery into conventional AC power.
it converts DC to AC through a switching process. Purely sine wave is the ideal/desired
waveform for the output AC power.
VSI and CSI
CSI gets input from a DC source with current at constant level, regardless of voltage
variations. For such purpose we insert a large inductor in series with the input dc
source. While VSI gets input from a DC source with Voltage at constant level,
regardless of current variations. We insert a large capacitor in parallel to input dc
source.
General types of inverters
There are three major types of inverters - sine wave (sometimes referred to as a "true"
or "pure" sine wave), modified sine wave (actually a modified square wave), and
square wave. A sine wave is what you get from your local utility company and (usually)
from a generator.
three phase inverter Principle
It changes DC input to three phase variable frequency variable voltage AC output. It has
six switches and freewheeling diodes.
Types are 120’ and 180’ conduction types, depending on switching periods of the
switches.
Topology
A three-phase inverter consists of 3 legs; each leg includes switches in either the up or
down position. The resulting eight possible switching configurations give rise to 6 active
voltage space vectors and two zero vectors. The animation shows a specific
sequencing of the 8 states where the active vectors rotate in discrete 60 o
steps. Each
leg is delayed by 120’.A three phase inverter with star connected load is shown below.
2. 2
Arif Hussain COMSATS Atd. Pakistan.
Output waveforms of three phase inverter
Pure Sine Wave inverters (ideal)
Output voltage wave form is pure sine wave with very low harmonic distortion and clean
power, like utility-supplied electricity. It has many advantages so we need to have a sine
wave output waveform of voltage. It has many advantages as it enhances the
performance of loads and increases their lifetime as well. Further it Reduces audible
and electrical noise in fans, fluorescent lights, audio amplifiers, TV, Game consoles,
Fax, and answering machines. Pure output waveform can be achieved by two ways,
i. by adding a filter circuit at the output side of the inverter, that filter must be
large enough to be able to handle large output power of the inverter, this will
then add extra-large circuitry to the inverter thus causing power losses and
lower efficiency of inverter.
ii. Another method to get a sinusoidal output waveform is the PWM technique.
This technique uses a specific switching scheme within the inverter to obtain
a desired output waveform that is a sine wave In this case.
iii.
Uses
The inverter is used in UPS.
It changes DC power from a battery into conventional AC power, AC power which then
derives all kinds of AC loads.
