15-05-2013, 12:17 PM
DESIGN AND SIMULATION OF THREE PHASE FIVE LEVEL AND SEVEN LEVEL INVERTER FED INDUCTION MOTOR DRIVE WITH TWO CASCADED H-BRIDGE CONFIGURATION
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Abstract
This paper deals with study of Three phase Five Level and Seven Level inverter fed induction motor drive . Both
five level and seven level are realized by cascading two H- bridges. The poor quality of voltage and current of a conventional
inverter fed induction machine is due to the presence of harmonics and hence there is significant level of energy losses. The
Multilevel inverter is used to reduce the harmonics. The inverters with a large number of steps can generate high quality
voltage waveforms. The higher levels can follow a voltage reference with accuracy and with the advantage that the generated
voltage can be modulated in amplitude instead of pulse-width modulation. An active harmonic elimination method is applied
to eliminate any number of specific higher order harmonics of multilevel converters with unequal dc voltages. The
simulation of three phase five and seven level inverter fed induction motor model is done using Matlab/Simulink. The FFT
spectrums for the outputs are analyzed to study the reduction in the harmonics.
INTRODUCTION
Adjustable Speed Drives (ASDs) are the
essential and endless demand of the industries and
researchers. They are widely used in the industries to
control the speed of conveyor systems, blower
speeds, machine tool speeds and other applications
that require adjustable speeds. In many industrial
applications, traditionally, DC motors were the work
horses for the Adjustable Speed Drives (ASDs) due to
their excellent speed and torque response. But, they
have the inherent disadvantage of commutator and
mechanical brushes, which undergo wear and tear
with the passage of time. In most cases, AC motors
are preferred to DC motors, in particular, an induction
motor due to its low cost, low maintenance, lower
weight, higher efficiency, improved ruggedness and
reliability. All these features make the use of
induction motors a mandatory in many areas of
industrial applications. The advancement in Power
Electronics and semiconductor technology has
triggered the development of high power and high
speed semiconductor devices in order to achieve a
smooth, continuous and step less variation in motor
speed. Applications of solid state converters/inverters
for adjustable speed induction motor drive are wide
spread in electromechanical systems for a large
spectrum of industrial systems.[3],[6],[10].
MULTILEVEL INVERTER
Multilevel inverters have drawn tremendous
interest in the power industry. They present a new set
of feature that are well suited for use in reactive
power compensation. Multilevel inverters will
significantly reduce the magnitude of harmonics and
increases the output voltage and power without the
use of step-up transformer. A multilevel inverter
consists of a series of H-bridge inverter units
connected to three phase induction motor. The
general function of this multilevel inverter is to
synthesize a desired voltage from several DC sources.
The AC terminal voltages of each bridge are
connected in series. Unlike the diode clamp or flyingcapacitors
inverter, the cascaded inverter does not
require any voltage clamping diodes or voltage
balancing capacitors.[1-4],[10],[13]. This
configuration is useful for constant frequency
applications such as active front-end rectifiers, active
power filters, and reactive power compensation.
Choosing appropriate conducting angles for the H
bridges can eliminate a specific harmonic in the
output waveform.
INDUCTION MOTOR DRIVE
Synchronous speed of Induction Motor is
directly proportional to the supply frequency. Hence,
by changing the frequency, the synchronous speed
and the motor speed can be controlled below and
above the normal full load speed. The voltage
induced in the stator, E is proportional to the product
of slip frequency and air gap flux. The motor terminal
voltage can be considered proportional to the product
of the frequency and flux, if the stator voltage is
neglected. Any reduction in the supply frequency
without a change in the terminal voltage causes an
increase in the air gap flux. Induction motors are
designed to operate at the knee point of the
magnetization characteristic to make full use of the
magnetic material. Therefore the increase in flux will
saturate the motor. This will increase the magnetizing
current, distort the line current and voltage, increase
the core loss and the stator copper loss, and produce a
high pitch acoustic noise. While any increase in flux
beyond rated value is undesirable from the
consideration of saturation effects, a decrease in flux
is also avoided to retain the torque capability of the
motor. Therefore, the variable frequency control
below the rated frequency is generally carried out by
reducing the machine phase voltage, V, along with
the frequency in such a manner that the flux is
maintained constant.
CONCLUSION
Five level and Seven level inverter fed induction
motor drive are simulated using the blocks of
simulink. The results of five level and seven level
systems are compared. It is observed that the total
harmonic distortion produced by the seven level
inverter system is less than that of a five level inverter
fed drive system. Therefore the heating due to seven
level inverter system is less than that of a five level
inverter fed drive system. The simulation results of
voltage, current, speed and spectrum are presented.
This drive system can be used in industries where
adjustable speed drives are required to produce output
with reduced harmonic content. The scope of this
work is the modeling and simulation of five level and
seven level inverter fed induction motor drive
systems. Experimental investigations will be done in
future. Seven level inverter system is a viable
alternative since it has better.