Low Cost Converter To Improve Power Factor Of Ac Motor

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A NEW SINGLE PHASE TO THREE-PHASE CONVERTER WITH ACTIVE INPUT CURRENT SHAPING FOR AC MOTOR DRIVES

By V. MAHALAKSHMI SUNDARI Electrical & Electronics Engineering Roll No. 05H71A0221 Contact No. : 9490768425 Email ID: [email protected]

&

S. SOWJANYA Electrical & Electronics Engineering Roll No. 05H71A0242 Email ID: [email protected] DEVINENI VENKATA RAMANA & Dr.HIMA SEKHAR

MIC COLLEGE OF TECHNOLOGY

DocumentToPDF trial version, to remove this mark, please register this software. CONTENTS

1. INTRODUCTION

2. PROPOSED SINGLE PHASE TO THREE PHASE CONVERTER AND ITS OPERATION

2.1 FRONT END RECTIFIER

2.2 FOUR SWITCH THREE PHASE INVERTER TOPOLOGY

2.3 SELECTION OF DC LINK FILTER CAPACITORS

3. COMPARISION CHART

4. CONCLUSION

5. BIBLIOGRAPHY

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ABSTRACT

In this paper a new single phase to three-phase converter for ac motor is proposed. The proposed converter employs only six switches and incorporates active input current shaping feature, which results in sinusoidal input current at close to unity power factor. The front-end rectifier in the proposed converter permits bidirectional power flow and therefore provides for excellent regulation against fluctuations in source voltage. This feature also facilitates regenerative braking of ac motor drive. An easy to implement control strategy to maintain near unity power factor over the full operating range is detailed. Suitable design guide for the selection of filter component is presented.

DocumentToPDF trial version, to remove this mark, please register this software. 1. INTRODUCTION:

unavailable. Previous work on single phase to three phase conversion suggest

With the recent progress in power semiconductor

device the

technology

followed

by

advancements

electronic

control methods,

in

variable

speed inverter-fed ac motor drives are finding wide variety of applications. Typical applications include the variable speed operation of fans, air-compressors, pumps etc with excellent energy saving benefits. Also recent research reports published by electric power research institute (EPRI) cites energy potential saving

figures

for

operating

the

residential heating ventilation and air conditioning

(HVAC)

systems

at

variable speed. The report also suggest that for wide spread application of this technology for HVDC systems, the power converter for the ac motor must satisfy low cost and low harmonic pollution requirements.

and

energy

capacitor

balance

elements.

inductor The

and

principle

disadvantage of this scheme is its fixed frequency

operation.

The

present

topology for adjustable speed HVDC systems employs a diode bridge rectifier and a regular six-switch PWM inverter to power a three-phase induction motor from

single-phase

ac

mains.

This

approach suffers from poor input current quality and lack of bidirectional power flow capability. In response to future regulations and strict harmonic current standards on single-phase ac mains, the additional of a boost switch and a blocking diode to shape the input current has

been

suggested

in

several

investigations. The need for additional power devices to maintain input power quality adds to the cost and power

Despite the enormous application potential of variable speed ac motor

losses.

Several

resonant

circuit

topologies are suggested. The increase in component count to reap the additional

drives in residential HVDC systems a simplified,

low

converter

topology

cost-efficient which

power draws

sinusoidal input current from single phase ac mains and powers a variable speed

the employment of four thyristor devices

three

phase

ac

motor

is

benefit of low switching losses is apparent from these topologies. A direct conversion scheme to convert 1-phase ac to 3-phase output by using a forced commutated

cycloconverter

(FCC)

DocumentToPDF trial version, to remove this mark, please register this software. structure is suggested. The primary

switches is proposed. The proposed

disadvantage of the FCC structure is low

configuration incorporates a front-end

voltage gain of the power conversion

half bridge active rectifier structure,

stage

of

which provides the dc link with active

bidirectional switches at the present

input current shaping feature. Further,

time. Several other low cost converters

the

for ac motor drives do not provide for

directional power flow between the dc

active input current shaping and require

link and ac mains. A four-switch inverter

additional stage to achieve this objective

configuration with split capacitors in the

and

the

unavailability

front-end

rectifier

allows

bi-

dc-link provides a balanced three-phase output to the ac motor load at adjustable voltage and frequency. Since transistors and IGBT switches can operate at high frequency, advanced PWM techniques known for inverter control can be used. Therefore the proposed converter can be controlled to draw sinusoidal input current at close to unity power factor and to simultaneously deliver high quality high quality three phase output voltages to the load. The

advantages

of

the

proposed

converters are: 1. Employs only six transistor or IGBT switches for single phase to three phase variable and

variable

voltage frequency

conversion, hence low cost. In this paper a new single phase

2. Bi-directional

power

flow

is

to three-phase converter that employs

possible between the ac mains

only six transistors or IGBT type

and the dc link. This

feature

DocumentToPDF trial version, to remove this mark, please register this software. provides for excellent voltage

The above figure illustrates the proposed

regulation

voltage

1-phase to 3-phase converter employing

mains

six transistor or IGBT type devices. A

and also facilitates regenerative

split capacitor bank forms the dc-link.

braking of the ac motor drive.

The switches

against

fluctuations in the ac-

and

form the front

3. Compact size, since the six

end rectifier. The inverter to convert the

switches converter structure is

dc-link voltage to a balanced three phase

available in a modular package

output with adjustable voltage and

from

frequency features is configured with

several

device

manufacturers.

four switches

4. Provides high quality balanced

and

respectively.

2.1 FRONT END RECTIFIER:

three phase output voltages. 5. Draws near sinusoidal current for the ac mains at close to unity power

factor

and

therefore satisfies strict harmonic current standards. The single phase ac input which is of

2. PROPOSED SINGLE PHASE

fixed frequency is rectified by the front

TO

end rectifier switches

THREE

CONVERTER OPERATION:

PHASE AND

ITS

and

.The

split capacitor bank in the dc-link is charged through the diodes present in and . The switches operated

on

a

and PWM

are pattern

synchronized to the ac mains to shape the input current to be sinusoidal. The filter inductor order

current

aids in filtering higher harmonics.

The

fundamental component of the voltage at points 'o' and 'c' is

which is

DocumentToPDF trial version, to remove this mark, please register this software. essentially the reflected voltage due to

pattern SLN1.The chosen programmed

the PWM operation of

and

. An

pattern selectively eliminates up to 59th

technique

which

harmonic.3rd graph shows the sinusoidal

lower

input current as a result of PWM

advanced

PWM

selectively

eliminates

several

operation of the front end rectifier.

order harmonics is used to control and

.From the the input current

is

given

by

The real power

From Eqn (1) the power factor angle is given

by,

and the input power factor pf

is,

Now

as,

flowing from the ac

mains into the dc-link can be expressed as,

à

(2)

defining

a

constant

k

A constant value of’k’implies that the

This equation indicates that the input

dc-link voltage is regulated to maintain a

power

constant value

is bilateral between the input ac

as given by Eqn

mains and dc-link for positive and

(5).Now from Eqn (2) to Eqn (5) we get

negative

these

values

Further, for

of

respectively.

expressions

=0 the real power input is

zero. From Eqn (2) to Eqn (5) the value of ** to maintain unity input power factor is,

From Eqn (6) to Eqn (8) it is clear that for Above simulated results illustrates the simulation results of the front end rectifier operating on an advanced PWM

,

unity input

pf

can be

maintained only for a particular value of the input power

.The input voltage

can be assumed to be one per-unit to

DocumentToPDF trial version, to remove this mark, please register this software. infer this result. Further, for Zero input

options for the four switch inverter in

power i.e.,

conjunction with the front end rectifier

and

, Eqn (6)

and Eqn (7) suggest that

and

are dealt in the paper.

.Hence from the point

By controlling the

is not particularly

switches T3&T4 in PWM fashion the

a good choice . Therefore in order to

output voltage Vac can be defined.

maintain close to unity pf and for

Further switches T3 &T6 determine the

the input of view of input

changes in input power

from 0 to 1

per-unit,

strategy

the

following

is

Vbc voltage .In order to generate balanced three phase output voltages, the output is phase shifted by –60 degrees

suggested.

from Vac. Thus control of switches 2.2FOUR SWITCH THREE PHASE INVERTER TOPOLOGY: The output side of the proposed single to three-phase converter

consists

of

four

switch

(T3&T6) inverter. The center point of the capacitor forms the third phase c. A detailed comparison of the four-switch inverter with the conventional six switch configurations is given in following graphs.

T3&T6 to have phase shift of 60degress phase shift between Vac & Vbc voltages ensure the voltage to have the same magnitude &power phase in accordance with the three phase laws .Fig 10 illustrates the inverter output voltages Vac, Vbc & Vab and the line currents from R-L load. It is noted that voltage Vab is three level swinging between Vo, 0, -Vo.On the other hand the voltages Vab&Vbc are two level type swinging between

Vo/2&-Vo/2.

Further,

the

fundamental content is the same in the three-phase output voltage. 2.3 SELECTION OF DC LINK FILTER CAPACITORS: The split capacitor bank in the DC link is essential to minimize the required power semiconductor switches in the front-end However, the operation & control

rectifier as well as in the output inverter

DocumentToPDF trial version, to remove this mark, please register this software. stage. Further the capacitors must be

near UPF. The proposed topology thus

able to accept an effective alternating

realizes the following features

current of 0.707Ic.If Ic, ripple is the

•

Employs only six switches.

front end per unit ripple current in the

•

Draws near sinusoidal current

capacitor ,then the voltage drop in the capacitor

can

be

expressed

as

from the ac mains at close to UPF. •

Permits

bi-directional

power

flow, and hence facilitates regenerative braking. .

•

3. COMPARISION CHART

Common

neutral

connection

between input and output. Suitable design equations are for the proposed topology. Finally, selected results have been verified experimentally.

5. BIBLIOGRAPHY: 1. B.K.Bose."Adjustable speed ac drive systems." 2. S.B.Dewan and M.Showleh,"A Novel static Single phase to Three phase converter". 3. Jin He. and N.Mohan”Input current Shaping

4. CONCLUSION: A

new

single

phase to three-phase converter topology has

been

proposed

.The

proposed

converter is capable of powering a threephase adjustable speed ac motor drives from a single-phase ac mains while maintaining sinusoidal input current at

Resonant

in

Line

Rectification

Converter".

by