colchai starting methods.docx

8/16/2019 ColchaI Starting methods.docx

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Synchronous motors and Synchronous condenser

Abstract — Many methods can be used to start large

AC induction motors. Choices such as full voltage,reduced voltage either by autotransformer or wye-

delta, a soft starter, or usage of an adjustable speed

drive can all have potential advantages and trade

offs. Reduced voltage starting can lower the starting

torue and help prevent damage to the load.

Additionally, power factor correction capacitors can

be used to reduce the current, but care must be ta!en

to si"e them property. #sage of the wrong capacitors

can lead to significant damage. Choosing the proper

starting method for a motor includes an analysis of

the power system as well as the starting load to

ensure that the motor is designed to deliver theneeded performance while minimi"ing its cost. $his

paper e%amines the most common starting methods

and their recommended applications.

Index Terms —

I. INTRODUCTION

A 3-phase induction motor is theoretically sel startin!. The

stator o an induction motor consists o 3-phase "indin!s#

"hich "hen connected to a 3-phase supply creates a rotatin!ma!netic ield. This "ill lin$ and cut the rotor conductors

"hich in turn "ill induce a current in the rotor conductors and

create a rotor ma!netic ield. The ma!netic ield created %y the

rotor "ill interact "ith the rotatin! ma!netic ield in the stator

and produce rotation. Thereore# 3-phase induction motors

employ a startin! method not to pro&ide a startin! tor'ue at

the rotor# %ut %ecause o the ollo"in! reasons(

)* Reduce hea&y startin! currents and pre&ent motor rom

o&erheatin!.

+* ,ro&ide o&erload and no-&olta!e protection. There are

many methods in use to start 3-phase induction motors. Some

o the common methods are(

• Direct On-ine Starter DO*

•Star-Delta Starter

• Auto Transormer Starter

• Rotor Impedance Starter

• ,o"er /lectronics Starter

&irect 'n-(ine )tarter *&'(+

The Direct On-ine DO* starter is the simplest and the most

ine0pensi&e o all startin! methods and is usually used or

s'uirrel ca!e induction motors. It directly connects the

contacts o the motor to the ull supply &olta!e. The startin!

current is &ery lar!e# normally 1 to 2 times the rated current.

The startin! tor'ue is li$ely to %e .45 to + times the ull load

tor'ue. In order to a&oid e0cessi&e &olta!e drops in the supply

line due to hi!h startin! currents# the DO starter is used only

or motors "ith a ratin! o less than 567 There are saety

mechanisms inside the DO starter "hich pro&ides protection

to the motor as "ell as the operator o the motor.The po"er

and control circuits o induction motor "ith DO starter are

sho"n in i!ure )*.

6)8 8ain contactor

)tarting methods of induction motors Decem%er

+)5*Colcha I&9n , IEEE # National ,olytechnic School

)


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The DO starter consists o a coil operated contactor 6)8

controlled %y start and stop push %uttons. On pressin! the start

push %utton S)# the contactor coil 6)8 is ener!i:ed rom line

). The three mains contacts )-+*# 3-;*# and 5-1* in i!. )*

are closed. The motor is thus connected to the supply. 7hen

the stop push %utton S+ is pressed# the supply throu!h the

contactor 6)8 is disconnected. Since the 6)8 is de-

ener!i:ed# the main contacts )- +*# 3-;*# and 5-1* are

opened. The supply to motor is disconnected and the motor

stops.

)tar-&elta )tarter

The star delta startin! is a &ery common type o starter and

e0tensi&ely used# compared to the other types o the starters.

This method used reduced supply &olta!e in startin!.


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2. Method of consequent poles

In this method a sin!le stator "indin! is di&ided into e"

coil !roups. The terminals o all these !roups are %rou!ht out.

The num%er o poles can %e chan!ed "ith only simple chan!es

in coil connections. In practice# the stator "indin! is di&ided

only in t"o coil !roups. The num%er o poles can %e chan!ed

in the ratio o +).


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Single Phase Motors

As the name su!!ests# these motors are used on sin!le =

phase supply. Sin!le phase motors are the most common type

o electric motors# "hich inds "ide domestic# commercial

and industrial applications. Sin!le phase motors are small si:e

motors o raction = $ilo"att ratin!s. Domestic applications

li$e ans# hair driers# "ashin! machines# mi0ers# reri!erators#

ood processors and other $itchen e'uipment employ thesemotors. These motors also ind applications in air =

conditionin! ans# %lo"ers oice machinery etc. Sin!le phase

motors may %e classiied into the ollo"in! %asic types

). Sin!le phase induction motors

+. AC. Series motor uni&ersal motor*

3. Repulsion motors

;. Synchronous motor

Single Phase Induction Motor

A sin!le phase induction motor is &ery similar to 3 = phase

s'uirrel ca!e induction motor. It has a s'uirrel = ca!e rotor

identical to a 3 - phase s'uirrel ca!e motor and a sin!le =

phase "indin! on the stator. Unli$e 3 = phase induction motor#

a sin!le phase induction motor is not sel startin! %ut re'uires

some startin! means.


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slips o less than unity are !reater than those at slips o more

than unity# the resultant tor'ue "ill %e in the direction o the

rotation o the or"ard lu0. Thus i the motor is once started#

it "ill de&elop net tor'ue in the direction in "hich it has %een

started and "ill unction as a motor.

Starting of Single Phase Induction Motors

The sin!le phases induction motors are classiied %ased on

the method o startin! method and in act are $no"n %y thesame name descripti&e o the method.

). Split = phase Induction 8otor

The stator o a split = phase induction motor has t"o

"indin!s# the main "indin! and the au0iliary "indin!. These

"indin!s are displaced in space %y F electric de!rees as

sho"n in i!ure ;-a*.

The au0iliary "indin! is made o thin "ire so that it has a

hi!h RL ratio as compared to the main "indin! "hich has

thic$ super enamel copper "ire. 7hen the t"o stator "indin!s

are ener!i:ed rom a sin!le = phase supply# the current Im and

Ia in the main "indin! and au0iliary "indin! la! %ehind the

supply &olta!e H# and Ia leadin! the current Im as sho"n in

i!ure ;-%*.

This means the current throu!h au0iliary "indin! reaches

ma0imum &alue irst and the mm or lu0 due to Ia lies alon!

the a0is o the au0iliary "indin! and ater some time the

current Im reaches ma0imum &alue and the mm due to Im lies

alon! the main "indin! a0is. Thus the motor %ecomes a + =

phase un%alanced motor. >ecause o these t"o ields a startin!

tor'ue is de&eloped and the motor %ecomes a sel startin!

motor. Ater the motor starts# the au0iliary "indin! is

disconnected usually %y means o centriu!al s"itch that

operates at a%out 45E o synchronous speed.


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a!acitor – Start Motor

Capacitors are used to impro&e the startin! and runnin!

perormance o the sin!le phase inductions motors

The capacitor = start motor is identical to a split = phase

motor e0cept that the startin! "indin! has as many turns as the

main "indin!. 8oreo&er# a capacitor C is connected in series

"ith the startin! "indin! as sho"n in i!ure 5-a*.

The &alue o capacitor is so chosen that Ia leads Im %y

a%out Fo


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Theoretically# optimum startin! and runnin! perormance can

%e achie&ed %y ha&in! t"o capacitors. The startin! capacitor is

lar!er in &alue and is o the ac electrolytic type. The runnin!

capacitor permanently connected in series "ith the startin!

"indin!# is o smaller &alue and is o the paper oil type.

Typical &alues o these capacitors or a .5 hp are Cs B3

colchai starting methods.docx

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