17-04-2012, 03:58 PM
IMPLEMENTATION OF PWM BASED FIRING SCHEME FOR MULTILEVEL
INVERTER USING MICROCONTROLLER
project_Report.pdf (Size: 1.49 MB / Downloads: 153)
INTRODUCTION
Ac loads require constant or adjustable voltages at their input terminals. When such loads are fed
by inverters, it’s essential that output voltage of the inverters is so controlled as to fulfill the
requirements of AC loads. This involves coping with the variation of DC input voltage, for
voltage regulation of inverters and for the constant volts/frequency control requirement. There
are various techniques to vary the inverter gain. The most efficient method of controlling the
gain (and output voltage) is to incorporate pulse-width modulation (PWM) control within the
inverters. The carrier based PWM schemes used for multilevel inverters is one of the most
straight forward methods of describing voltage source modulation realized by the intersection of
a modulating signal (Duty Cycle) with triangular carrier wavefroms.The Alternative PWM
strategies with differing phase relationships are:
PROJECT OUTLINE
This project aims at generation of carrier based PWM scheme using POD strategy through the
means of an AT89C51 microcontroller. The salient features are:
· Firstly, both the high-frequency triangular carrier wave & the sinusoidal reference signal
are being generated in the microcontroller.
· A digital to analog converter (DAC 0808) is then employed for converting them into their
analog signal forms.
INVERTERS
A device that converts DC power into AC power at desired output voltage and frequency is
called an Inverter. Phase controlled converters when operated in the inverter mode are called line
commutated inverters. But line commutated inverters require at the output terminals an existing
AC supply which is used for their commutation. This means that line commutated inverters can’t
function as isolated AC voltage sources or as variable frequency generators with DC power at
the input. Therefore, voltage level, frequency and waveform on the AC side of the line
commutated inverters can’t be changed. On the other hand, force commutated inverters provide
an independent AC output voltage of adjustable voltage and adjustable frequency and have
therefore much wider application.
APPLICATIONS
· DC POWER SOURCE UTILIZATION
Inverter designed to provide 115 VAC from the 12 VDC source provided in an automobile. The
unit provides up to 1.2 Amps of alternating current, or just enough to power two sixty watt light
bulbs.
An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells
to AC electricity. The electricity can be at any required voltage; in particular it can operate AC
equipment designed for mains operation, or rectified to produce DC at any desired voltage.
UNINTERRUPTIBLE POWER SUPPLIES
An uninterruptible power supply is a device which supplies the stored electrical power to the
load in case of raw power cut-off or blackout. One type of UPS uses batteries to store power and
an inverter to supply AC power from the batteries when main power is not available. When main
power is restored, a rectifier is used to supply DC power to recharge the batteries.
It is widely used at domestic and commercial level in countries facing Power outages.
· INDUCTION HEATING
Inverters convert low frequency main AC power to a higher frequency for use in induction
heating. To do this, AC power is first rectified to provide DC power. The inverter then changes
the DC power to high frequency AC power.
PULSE-WIDTH MODULATION
In PWM the pulses representing successive sample values of s(t) have constant amplitudes but
vary in time duration in direct proportion to the sample value. The pulse duration can be changed
relative to fixed leading or trailing time edges or a fixed pulse center. To allow for time-division
multiplexing, the maximum pulse duration may be limited to a fraction of the time between
samples (Fig. 1c).
PULSE-POSITION MODULATION
PPM encodes the sample values of s(t) by varying the position of a pulse of constant duration
relative to its nominal time of occurrence. As in PAM and PWM, the duration of the pulses is
typically a fraction of the sampling interval. In addition, the maximum time excursion of the
pulses may be limited
ADVANTAGES OF PWM
Using pulse width modulation has several advantages over analog control.
I. The entire control circuit can be digital, eliminating the need for digital-to-analog
converters.
II. Using digital control lines will reduce the susceptibility of your circuit to interference.
III. Finally, motors may be able to operate at lower speeds if you control them with PWM.
When you use an analog current to control a motor, it will not produce significant torque
at low speeds.
IV. The output voltage control can be obtained without any additional components.
V. With this method, lower order harmonics can be eliminated or minimized Along with its
output voltage control.
CONCLUSION
The background study regarding the various aspects of the PWM firing scheme was studied. The
carrier based PWM scheme using the POD strategy based on AT89C51 was simulated with the
help of “TOP VIEW SIMULATOR (1.2H)”. Hardware involving the power circuit of AT89C51
was fabricated. Power circuit of AT89C51 was interfaced with DAC0808 for the generation of
analog signal. The analog signals (sinusoidal and triangular) so generated were compared using a
comparator (KF351) thereby generating PWM waves which is fed as triggering pulses to the
inverters.