 Open Access
 Total Downloads : 556
 Authors : C. S. Aswini, Mrs. L. Jayasree, Dr. M. Sasikumar
 Paper ID : IJERTV2IS4133
 Volume & Issue : Volume 02, Issue 04 (April 2013)
 Published (First Online): 17042013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Modified Single Stage ThreeLevel ACDC Converter for DC Machine Drives
1C. S. Aswini, 2Mrs. L. Jayasree, 3Dr. M. Sasikumar
1P.G Scholar, 2Assistant Professor, 3Professor, Jeppiaar Engineering College, Chennai 600 119, Tamil Nadu, India
Abstract
A performance of threelevel singlestage AC/DC converter based DC machine drive system for industrial applications has been discussed. The difficulty of high power component stress caused due to high rising intermediate bus voltages at light load conditions is not found which is found in other singlestage converters because of its threelevel structure. Though little harmonic distortion is found in input current, the operation is over a wider load range with significantly less output inductor current ripple. Here, the operation of the AC/DC converter is explained and analyzed, its results are discussed.
Index terms: ACDC power conversion, LC Filter.

INTRODUCTION
The concept of multilevel converters has been introduced since 1975. Subsequently, many other multilevel converter topologies are developed. Though, the basic concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Multiple dc voltage sources includes capacitors, batteries, and renewable energy voltage sources. The power switches combine the multiple dc sources in order to achieve high voltage at the output. But the rated voltage of the power semiconductor switches depends only upon the rating of the dc voltage sources to which they are connected. By adding passive filter elements to the traditional passive diode rectifier/LC filter input combination these standards are satisfied. The resulting converter would be very bulky and heavy due to the size of the lowfrequency inductors and capacitors. Currentfed converters are connected with a boost inductor which is connected to the input of the fullbridge circuit, this can achieve a nearunity input power factor, but they lack an energystorage capacitor across the primaryside dc bus, which can result in the appearance of high voltage overshoots and ringing across the dc bus. Multilevel converters can reduce the peak voltage stresses of the converters. They do not have the problem of discontinuous output current, distorted input currents and variable switching frequency.

CIRCUIT DESCRIPTION
Resonant converters, it can be controlled by varying switchingfrequency control, which makes it difficult to optimize their design as they must be able to operate over a wide range of switching frequency. These converters have two conversion stages that allow a portion of the power that is transferred from the input to the output to be processed only once. It is for this reason that they are considered by some to be singlestage converter, but they have two converter stages and so have the cost and complexity associated with twostage converter. Voltagefed, single stage, pulsewidth modulation converters with a large energystorage capacitor connected across their primaryside dc bus. The drawbacks of resonant and currentfed converters are not present here. They operate with fixed switching frequency, voltage overshoots and ringing from appearing across the dc bus is prevented by the bus capacitor. Voltage fed converters have the following drawbacks such as: The primaryside dc bus voltage of the converter may become excessive under highinputline and low outputload conditions. The high dc bus voltage results in the need for higher voltage rated devices and very large bulk capacitors for the dc bus. The new voltage fed converter has more advantages than the conventional converters. The peak voltage stresses of the converter devices is reduced, as the switch voltage is limited to half the dcbus voltage. It is free from distorted input currents, discontinuous output current, and the use of variable switching frequency. The proposed converter has wide output load variation (from 10% of full load to a full load that is greater than
500 W), PWM control, excellent pf, a continuous
output inductor current, without its components being exposed to excessive peak voltage stresses. The multilevel converter has dc bus voltage which can be split equally among the capacitors so that the capacitors and the converter switches are not exposed to the full dc bus voltage, but are exposed to half of it. This allows greater flexibility in the design of the converter as there is less need to constrain the dc bus voltage. It has greater flexibility in the converter design that allows for improvements in the performance of a singlestage fullbridge converter. The proposed converter is shown in Fig. 2.1. It consists of an AC input section, a threelevel dcdc converter, and dc link circuitry that is based on auxiliary windings taken from the main power transformer and that contains an inductor Lin and two diodes.
Fig 2.1 Proposed single stage AC/DC Converter
Diode D3 only conducts current to charge the dc bus capacitor when the converter starts up. This does not operates when the converter is in steadystate. The dc link circuit acts like the boost switch in an ac dc PFC boost converter. When the two converter switches are ON, a voltage is impressed across each auxiliary winding so that the voltage across one of the windings cancels out the voltage across the dc link capacitors which is the sum of the voltage across C1 and C2. This is opposite to the boost switch being ON and current in Lin rises.

THEORY OF OPERATION
During this mode, switches S1 and S2 are ON and energy from the dclink capacitor C1 flows to the output load. Since the auxiliary winding generates a voltage that is equal to the total dclink capacitor
voltage (sum of C1and C2), the voltage across the auxiliary inductor is the rectified supply voltage. The characteristics of single stage AC/DC converter is shown in fig. 3.1. This allows energy to flow from the ac mains into the auxiliary inductor during this mode, and the auxiliary inductor current increases, according to
where vs,k  is the rectified ac supply voltage during switching cycle interval k. The supply voltage can be considered to be constant within a switching cycle as the switching frequency is much higher than the line frequency. The current in the auxiliary inductor Lin at the end of Mode 1 is
Duty cycle, D, is defined as the time when S1 and S2 are both ON during the first half cycle or when S3 and S4 are both ON during the second half cycle. These two cases correspond to energy transfer modes of operation. Since D is defined with respect to a half switching cycle Tsw /2 or 1/2fsw , (where fsw is the switching frequency) the duration that is used in (2) is D/(2fsw ).
Similarly, the output inductor current can be expressed as
where Vbus is the average dclink voltage, VL is the load voltage and N is the transformer ratio between input and output (N =Npri /Nsec ). If the output inductor current is continuous then peak ripple current can be expressed as
Fig 3.1 Waveforms of Three stage AC/DC Converter

SIMULATION RESULTS
The simulation model of proposed circuit is given in fig 4.1.
Fig 4.1.Proposed circuit
The input voltage, current of proposed circuit is shown in fig 4.2 & 4.3.
Fig 4.2 Input voltage
Fig 4.3 Input current
Fig 4.4 Applied voltage and current
The primary and secondary voltage of proposed circuit is shown in fig 4.5 & 4.6.
Fig 4.5.Primary voltage of proposed circuit
Fig 4.6. Secondary voltage of proposed circuit
The output current, voltage and power of proposed circuit is shown in fig 4.7 4.9.
Fig 4.7 Output current of proposed circuit
Fig 4.8 Output voltage of proposed circuit
Fig 4.9. Output power of proposed circuit
Closed loop with motor load
Fig 4.10 Closed loop with motor load
Input Voltage
Fig 4.11 Input Voltage of Closed loop with motor load
Fig 4.12. Speed of Closed loop with motor load
Fig 4.13. Torque of Closed loop with motor load
Fig.4.14 Comparative analysis of Proposed system with Conventional System

CONCLUSION
Thus a singlephase, singlestage voltagefed, three level, ac/dc converter which operates with a single controller was shown. This converter can operates with a better efficiency and with less output inductor current ripple and universal input voltage and wide load operating range. The results of simulation for conventional and proposed method with R load and closed loop with motor is presented and the results are obtained.
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