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SHEAR CUTTING MACHINE

A bench shear, also known as a lever shear , is a bench mounted shear with a comound

mechanism to increase the mechanical ad!anta"e# It is usuall$ used %or cuttin" rou"h shaes

out o% medium si&ed ieces o% sheet metal, but cannot do delicate work# 'or the small shear, it

mostl$ desi"ned %or a wide %ield o% alications# (i"ht wei"ht and eas$ e%%icient oeration,

$et !er$ sturd$ in construction# The cuttin" blades %itted are care%ull$ and accuratel$ "round

to "i!e eas$, clean )uick cuts, and %ree o% burrs# These secial %eatures hel the oerators sa!e

a "reat deal o% their ener"$# *ut some shearin" machines can cut sheet bar and %lat bar u to

+mm# It is electricall$ welded to"ether to make it a sturd$ stable unit caable to withstand

hi"hest stresses due to hea!$ dut$ usa"e# The %ootlates are rein%orced with bracin" an"les so

that the$ "i!e %irm stabilit$ to the shear# The machine is ro!ided with section kni!es with

slidin" blades which can be ad-usted b$ hand to make . cuts on an"les and T/sections o%

di%%erent si&es as well as with oenin"s %or cuttin" round and s)uare bars#  

https://en.wikipedia.org/wiki/Workbenchhttps://en.wikipedia.org/wiki/Workbench

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The machine used is called a s)uarin" shear, ower shear, or "uillotine# The machine ma$ be

%oot owered 0or less commonl$ hand owered1, or mechanicall$ or h$draulicall$ owered# It

works b$ %irst clamin" the material with a ram# A mo!in" blade then comes down across a

%i2ed blade to shear the material# 'or lar"er shears the mo!in" blade ma$ be set on an an"le

or 3rocked3 in order to shear the material ro"ressi!el$ %rom one side to the other4 this an"le

is re%erred to as the shear an"le# This decreases the amount o% %orce re)uired, but increases

the stroke# A 5 de"ree shear an"le decreases the %orce b$ about 67# The amount o% ener"$used is still the same# The mo!in" blade ma$ also be inclined #5 to 6#58, this an"le is called

the rake an"le, to kee the material %rom becomin" wed"ed between the blades, howe!er it

comromises the s)uareness o% the ed"e#9:; As %ar as e)uiment is concerned, the machine

consists o% a shear table, work/holdin" de!ice, uer and lower blades, and a "au"in" de!ice#

The shear table is the art o% the machiner$ that the workiece rests on while bein" sheared#

The work/holdin" de!ice is used to hold the workiece in lace and kee it %rom mo!in" or

 bucklin" while under stress# The uer and lower blades are the iece o% machiner$ that

actuall$ do the cuttin", while the "au"in" de!ice is used to ensure that the workiece is bein"

cut where it is suosed to be#

https://en.wikipedia.org/wiki/Rake_anglehttps://en.wikipedia.org/wiki/Rake_anglehttps://en.wikipedia.org/wiki/Shear_(sheet_metal)#cite_note-degarmo-4https://en.wikipedia.org/wiki/Rake_anglehttps://en.wikipedia.org/wiki/Shear_(sheet_metal)#cite_note-degarmo-4

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The desi"n o% ress tools is an en"ineerin" comromise# A shar ed"e, stren"th and durabilit$

are ideal, howe!er a shar ed"e is not !er$ stron" or durable so blades %or metal work  tend to

 be s)uare/ed"ed rather than kni%e/ed"ed# T$ical workiece materials include aluminum,

 brass, bron&e, and mild steel because o% their outstandin" shearabilit$ ratin"s, howe!er,

stainless steel is not used as much due to its tendencies to work/harden#

Power shears

A ower shear is electricall$ or   neumaticall$ owered hand tool desi"ned to blank  lar"e

 ieces o% sheet metal# The$ are desi"ned to cut strai"ht lines and relati!el$ lar"e radius

cur!es# The$ are ad!anta"eous o!er a bandsaw because there is not a si&e limit# (ar"e

!ersions can cut sheet metal u to +6 "au"es#

An alternati!e to the hand tools are h$draulicall$ owered tools attached to hea!$ machiner$#

The$ are usuall$ used to cut materials that are too bulk$ to be transorted to a cuttin" %acilit$,

too bi" or dan"erous %or the hand tools and are stored at remote locations 0e#"# mines,

%orests1#

Nibbler

A nibbler, or nibblers,9+; is a tool %or cuttin" sheet metal with minimal distortion# ?= mm aart,

rollin" u the waste in a ti"ht siral as it cuts# Nibblers ma$ be manual 0hand oerated1 or

 owered#

@ower nibblers are o%ten owered b$ comressed air , thou"h electrical t$es also e2ist# A

common IB nibbler tool is an electric drill attachment, which con!erts the rotar$ motion o%

the drill into a recirocatin" motion o% the -aw#

https://en.wikipedia.org/wiki/Machine_presshttps://en.wikipedia.org/wiki/Metalworkinghttps://en.wikipedia.org/wiki/Mild_steelhttps://en.wikipedia.org/wiki/Stainless_steelhttps://en.wikipedia.org/wiki/Pneumatichttps://en.wikipedia.org/wiki/Pneumatichttps://en.wikipedia.org/wiki/Hand_toolhttps://en.wikipedia.org/wiki/Blanking_(metalworking)https://en.wikipedia.org/wiki/Bandsawhttps://en.wikipedia.org/wiki/Hydraulicshttps://en.wikipedia.org/wiki/Hydraulicshttps://en.wikipedia.org/wiki/Heavy_machineryhttps://en.wikipedia.org/wiki/Heavy_machineryhttps://en.wikipedia.org/wiki/Nibbler#cite_note-1https://en.wikipedia.org/wiki/Saw#Terminologyhttps://en.wikipedia.org/wiki/Compressed_airhttps://en.wikipedia.org/wiki/Compressed_airhttps://en.wikipedia.org/wiki/Do_it_yourselfhttps://en.wikipedia.org/wiki/Electric_drillhttps://en.wikipedia.org/wiki/Machine_presshttps://en.wikipedia.org/wiki/Metalworkinghttps://en.wikipedia.org/wiki/Mild_steelhttps://en.wikipedia.org/wiki/Stainless_steelhttps://en.wikipedia.org/wiki/Pneumatichttps://en.wikipedia.org/wiki/Hand_toolhttps://en.wikipedia.org/wiki/Blanking_(metalworking)https://en.wikipedia.org/wiki/Bandsawhttps://en.wikipedia.org/wiki/Hydraulicshttps://en.wikipedia.org/wiki/Heavy_machineryhttps://en.wikipedia.org/wiki/Nibbler#cite_note-1https://en.wikipedia.org/wiki/Saw#Terminologyhttps://en.wikipedia.org/wiki/Compressed_airhttps://en.wikipedia.org/wiki/Do_it_yourselfhttps://en.wikipedia.org/wiki/Electric_drill

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CHAPTER-2

APPARATUS

2.1 Dc motors

At the most basic le!el, electric motors e2ist to con!ert electrical ener"$ into

mechanical ener"$# This is done b$ wa$ o% two interactin" ma"netic %ields / one stationar$,

and another attached to a art that can mo!e# A number o% t$es o% electric motors e2ist, but

most *EAM bots use C motors+ in some %orm or another# C motors ha!e the otential %or 

!er$ hi"h tor)ue caabilities 0althou"h this is "enerall$ a %unction o% the h$sical si&e o% the

motor1, are eas$ to miniaturi&e, and can be 3throttled3 !ia ad-ustin" their sul$ !olta"e# C

motors are also not onl$ the simlest, but the oldest electric motors#

The basic rinciles o% electroma"netic induction were disco!ered in the earl$ +Ds

 b$

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%rom rechar"eable batteries, ro!idin" the moti!e ower %or the %irst electric !ehicles# Toda$

C motors are still %ound in alications as small as to$s and disk dri!es, or in lar"e si&es to

oerate steel rollin" mills and aer machines#

2.2 Principles of peration of DC !otor

In an$ electric motor, oeration is based on simle electroma"netism# A current/

carr$in" conductor "enerates a ma"netic %ield4 when this is then laced in an e2ternal

ma"netic %ield, it will e2erience a %orce roortional to the current in the conductor, and to

the stren"th o% the e2ternal ma"netic %ield# As $ou are well aware o% %rom la$in" with

ma"nets as a kid, oosite 0North and South1 olarities attract, while like olarities 0North

and North, South and South1 reel# The internal con%i"uration o% a C motor is desi"ned to

harness the ma"netic interaction between a current/carr$in" conductor and an e2ternal

ma"netic %ield to "enerate rotational motion#

(etDs start b$ lookin" at a simle 6/ole C electric motor 0here red reresents a

ma"net or windin" with a 3North3 olari&ation, while "reen reresents a ma"net or windin"

with a 3South3 olari&ation1#

"i#$re 2.1 Sectional %iew of DC !otor

E!er$ C motor has si2 basic artsF a2le, rotor 0a#k#a#, armature1, stator, commutator,

%ield ma"net0s1, and brushes# In most common C motors 0and all that *eamers will see1, the

e2ternal ma"netic %ield is roduced b$ hi"h/stren"th ermanent ma"nets# The stator is the

stationar$ art o% the motor, this includes the motor casin", as well as two or more ermanent

ma"net ole ieces# The rotors 0to"ether with the a2le and attached commutator1 rotate with

resect to the stator# The rotor consists o% windin"s 0"enerall$ on a core1, the windin"s bein"

electricall$ connected to the commutator# The abo!e dia"ram shows a common motor la$out

with the rotor inside the stator 0%ield1 ma"nets#

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The "eometr$ o% the brushes, commutator contacts, and rotor windin"s are such that

when ower is alied, the olarities o% the ener"i&ed windin" and the stator ma"net0s1 are

misali"ned, and the rotor will rotate until it is almost ali"ned with the statorDs %ield ma"nets#

As the rotor reaches ali"nment, the brushes mo!e to the ne2t commutator contacts, and

ener"i&e the ne2t windin"# Gi!en our e2amle two/ole motor, the rotation re!erses the

direction o% current throu"h the rotor windin", leadin" to a 3%li3 o% the rotorDs ma"netic %ield,

dri!in" it to continue rotatin"#

The C Motor or irect Current Motor to "i!e it its %ull title, is the most commonl$

used actuator %or roducin" continuous mo!ement and whose seed o% rotation can easil$ be

controlled, makin" them ideal %or use in alications were seed control, ser!o t$e control,

andor ositionin" is re)uired# A C motor consists o% two arts, a 3Stator3 which is the

stationar$ art and a 3Rotor3 which is the rotatin" art# The result is that there are basicall$

three t$es o% C Motor a!ailable#

i. &r$she' !otor / This t$e o% motor roduces a ma"netic %ield in a wound rotor 0the

 art that rotates1 b$ assin" an electrical current throu"h a commutator and carbon

 brush assembl$, hence the term 3*rushed3# The stators 0the stationar$ art1 ma"netic

%ield is roduced b$ usin" either a wound stator %ield windin" or b$ ermanent

ma"nets# Generall$ brushed C motors are chea, small and easil$ controlled

#

ii. &r$shless !otor / This t$e o% motor roduce a ma"netic %ield in the rotor b$ usin"

 ermanent ma"nets attached to it and commutation is achie!ed electronicall$# The$

are "enerall$ smaller but more e2ensi!e than con!entional brushed t$e C motors

 because the$ use 3Hall e%%ect3 switches in the stator to roduce the re)uired stator 

%ield rotational se)uence but the$ ha!e better tor)ueseed characteristics, are more

e%%icient and ha!e a lon"er oeratin" li%e than e)ui!alent brushed t$es#

iii. Ser%o !otor / This t$e o% motor is basicall$ a brushed C motor with some %orm o% 

 ositional %eedback control connected to the rotor sha%t# The$ are connected to and

controlled b$ a @M t$e controller and are mainl$ used in ositional control

s$stems and radio controlled models#

 Normal C motors ha!e almost linear characteristics with their seed o% rotation bein"

determined b$ the alied C !olta"e and their outut tor)ue bein" determined b$ the

current %lowin" throu"h the motor windin"s# The seed o% rotation o% an$ C motor can be

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!aried %rom a %ew re!olutions er minute 0rm1 to man$ thousands o% re!olutions er minute

makin" them suitable %or electronic, automoti!e or robotic alications# *$ connectin" them

to "earbo2es or "ear/trains their outut seed can be decreased while at the same time

increasin" the tor)ue outut o% the motor at a hi"h seed#

2.( &r$she' DC !otor

A con!entional brushed C Motor consist basicall$ o% two arts, the stationar$ bod$

o% the motor called the Stator and the inner art which rotates roducin" the mo!ement called

the Rotor or )Armature) %or C machines# The motors wound stator is an electroma"net

circuit which consists o% electrical coils connected to"ether in a circular con%i"uration to

 roduce the re)uired North/ole then a South/ole then a North/ole etc, t$e stationar$

ma"netic %ield s$stem %or rotation, unlike AC machines whose stator %ield continuall$ rotates

with the alied %re)uenc$#

The current which %lows within these %ield coils is known as the motor %ield current#

These electroma"netic coils which %orm the stator %ield can be electricall$ connected in

series, arallel or both to"ether 0comound1 with the motors armature# A series wound C

motor has its stator %ield windin"s connected in series with the armature# (ikewise, a shunt

wound C motor has its stator %ield windin"s connected in arallel with the armature as

shown#

2.(.1 Series an' Sh$nt Connecte' DC !otor

"i#$re 2.2 Series an' Sh$nt DC !otor

The rotor or armature o% a C machine consists o% current carr$in" conductors

connected to"ether at one end to electricall$ isolated coer se"ments called the commutator#

The commutator allows an electrical connection to be made !ia carbon brushes 0hence the

name 3*rushed3 motor1 to an e2ternal ower sul$ as the armature rotates#

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The ma"netic %ield setu b$ the rotor tries to ali"n itsel% with the stationar$ stator %ield

causin" the rotor to rotate on its a2is, but cannot ali"n itsel% due to commutation dela$s# The

rotational seed o% the motor is deendent on the stren"th o% the rotors ma"netic %ield and the

more !olta"e that is alied to the motor the %aster the rotor will rotate# *$ !ar$in" this

alied C !olta"e the rotational seed o% the motor can also be !aried#

2.(.2 Con%entional *&r$she'+ DC !otor

"i#$re 2.( Con%entional *&r$she'+ DC !otor

@ermanent ma"net 0@MC1 brushed motors are "enerall$ much smaller and cheaer 

than their e)ui!alent wound stator t$e C motor cousins as the$ ha!e no %ield windin"# In

 ermanent ma"net C 0@MC1 motors these %ield coils are relaced with stron" rare earth

0i#e# Samarium Cobalt, or Neod$mium Iron *oron1 t$e ma"nets which ha!e !er$ hi"h

ma"netic ener"$ %ields# This "i!es them a much better linear seedtor)ue characteristic than

the e)ui!alent wound motors because o% the ermanent and sometimes !er$ stron" ma"netic

%ield, makin" them more suitable %or use in models, robotics and ser!os#

Althou"h C brushed motors are !er$ e%%icient and chea, roblems associated with

the brushed C motor is that sarkin" occurs under hea!$ load conditions between the two

sur%aces o% the commutator and carbon brushes resultin" in sel% "eneratin" heat, short li%e

san and electrical noise due to sarkin", which can dama"e an$ semiconductor switchin"

de!ice such as a M

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2., &r$shless DC !otor

The brushless C motor 0*CM1 is !er$ similar to a ermanent ma"net C motor,

 but does not ha!e an$ brushes to relace or wear out due to commutator sarkin"# There%ore,

little heat is "enerated in the rotor increasin" the motors li%e# The desi"n o% the brushless

motor eliminates the need %or brushes b$ usin" more comle2 dri!e circuits were the rotor 

ma"netic %ield is a ermanent ma"net which is alwa$s in s$nchroni&ation with the stator %ield

allows %or a more recise seed and tor)ue control# Then the construction o% a brushless C

motor is !er$ similar to the AC motor makin" it a true s$nchronous motor but one

disad!anta"e is that it is more e2ensi!e than an e)ui!alent 3brushed3 motor desi"n#

The control o% the brushless C motors is !er$ di%%erent %rom the normal brushed C

motor, in that it this t$e o% motor incororates some means to detect the rotors an"ular 

 osition 0or ma"netic oles1 re)uired to roduce the %eedback si"nals re)uired to control the

semiconductor switchin" de!ices# The most common ositionole sensor is the 3Hall E%%ect

Sensor3, but some motors also use otical sensors#

Usin" Hall E%%ect sensors, the olarit$ o% the electroma"nets is switched b$ the motor 

control dri!e circuitr$# Then the motor can be easil$ s$nchroni&ed to a di"ital clock si"nal,

 ro!idin" recise seed control# *rushless C motors can be constructed to ha!e, an e2ternal

 ermanent ma"net rotor and an internal electroma"net stator or an internal ermanent ma"net

rotor and an e2ternal electroma"net stator# Ad!anta"es o% the *rushless C Motor comared

to its brushed cousin are hi"her e%%iciencies, hi"h reliabilit$, low electrical noise, "ood

seed control and more imortantl$, no brushes or commutator to wear out roducin" a much

hi"her seed# Howe!er their disad!anta"e is that the$ are more e2ensi!e and more

comlicated to control#

2. DC Ser%o !otor

C Ser!o motors are used in closed loo t$e alications were the osition o% the

outut motor sha%t is %ed back to the motor control circuit# T$ical ositional 3'eedback3

de!ices include Resol!ers, Encoders and @otentiometers as used in radio control models such

as airlanes and boats etc# A ser!o motor "enerall$ includes a built/in "earbo2 %or seed

reduction and is caable o% deli!erin" hi"h tor)ues directl$# The outut sha%t o% a ser!o motor 

does not rotate %reel$ as do the sha%ts o% C motors because o% the "earbo2 and %eedback 

de!ices attached#

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2. DC !otor &eha%ior

At a simlistic le!el, usin" C motors is rett$ strai"ht %orward / $ou ut ower in,

and "et rotar$ motion out# (i%e, o% course, is ne!er this simle / there are a number o% 

subtleties o% C motor beha!ior that should be accounted %or in *EAMbot desi"n#

2..1 Hi#h-spee' o$tp$t

This is the simlest trait to understand and treat most C motors run at !er$ hi"h

outut seeds 0"enerall$ thousands or tens o% thousands o% R@M1# hile this is %ine %or some

*EAM bots 0sa$, hoto oers or solar rollers1, man$ *EAM bots 0walkers, heads1 re)uire

lower seeds#

2..2 &ac/ E!"

Just as uttin" !olta"e across a wire in a ma"netic %ield can "enerate motion, mo!in"

a wire throu"h a ma"netic %ield can "enerate !olta"e# This means that as a C motorDs rotor 

sins, it "enerates !olta"e the outut !olta"e is known as back EM'# *ecause o% back EM', a

sark is created at the commutator as a motorDs brushes switch %rom contact to contact#

Meanwhile, back EM' can dama"e sensiti!e circuits when a motor is stoed suddenl$#

2..( 0oise *ripple+ on power lines

A number o% thin"s will cause a C motor to ut noise on its ower linesF

commutation noise 0a %unction o% brush commutator desi"n K construction1, rou"hness in

 bearin"s 0!ia back EM'1, and "earin" rou"hness 0!ia back EM', i% the motor is art o% a "ear 

motor1 are three bi" contributors#

E!en without these a!oidable %actors, an$ electric motor will ut noise on its ower 

lines b$ !irtue o% the %act that its current draw is not constant throu"hout its motion# Goin"

 back to our e2amle two/ole motor, its current draw will be a %unction o% the an"le between

its rotor coil and %ield ma"netsF

Since most small C motors ha!e > coils, the coilsD current cur!es will o!erla$ each other,

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Added to"ether, this ideal motorDs current will then look somethin" like this,

Realit$ is a bit more comle2 than this, as e!en a hi"h/)ualit$ motor will disla$ a

current transient at each commutation transition# Since each coil has inductance 0b$

de%inition1 and some caacitance, there will be a sur"e o% current as the commutatorDs brushes

%irst touch a coilDs contact, and another as the brushes lea!e the contact 0here, thereDs a sli"ht

sark as the coilDs ma"netic %ield collases1#

As a "ood e2amle, consider an oscilloscoe trace o% the current throu"h a Mabuchi

''/>@N motor sulied with 6 L 0+ms er hori&ontal di!ision, #5 mA er !ertical

di!ision1

"i#$re 2., C$rrent Ripple

In this case, the eak/to/eak current rile is aro2imatel$ #6. mA, while the

a!era"e motor current is -ust under >+ mA# So under these conditions, the motor uts about

less than +7 o% current rile onto its ower lines 0and as $ou can see %rom the 3clean3 traces,

it oututs essentiall$ no hi"h/%re)uenc$ current noise1# Note that since this is a >/ole motor,

and each coil is ener"i&ed in both directions o!er the course o% a rotor rotation, one re!olution

o% the rotor will corresond to si2 o% the abo!e cur!es 0here, = 2 6#: ms #+:: sec,

corresondin" to a motor rotation rate o% -ust %ewer than :6 R@M1#

http://www.solarbotics.net/starting/200111_dcmotor/pix/Mabuchi_2.007V.jpghttp://www.solarbotics.net/starting/200111_dcmotor/pix/Mabuchi_2.007V.jpg

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Motor ower rile can wreak ha!oc in N! nets b$ destabili&in" them inad!ertentl$#

'ortunatel$, this can be miti"ated b$ uttin" a small caacitor across the motorDs ower lines

0$ouDll onl$ be able to %ilter out 3sike$3 transients this wa$, thou"h / $ouDll alwa$s see cur!es

like the ones abo!e bein" imosed on $our ower1#

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2.. Sh$nt wo$n' motor

A shunt wound motor has a hi"h/resistance %ield windin" connected in arallel with

the armature# It resonds to increased load b$ tr$in" to maintain its seed and this leads to an

increase in armature current# This makes it unsuitable %or widel$/!ar$in" loads, which ma$

lead to o!erheatin"#

2.. Series wo$n' motor

A series wound motor has a low/resistance %ield windin" connected in series with the

armature# It resonds to increased load b$ slowin" down4 the current increases and the tor)ue

rises in roortional to the s)uare o% the current since the same current %lows in both thearmature and the %ield windin"s# I% the motor is stalled, the current is limited onl$ b$ the total

resistance o% the windin"s and the tor)ue can be !er$ hi"h, but there is a dan"er o% the

windin"s becomin" o!erheated# Series wound motors were widel$ used as traction

motors in rail transort o% e!er$ kind, but are bein" hased out in %a!or o% AC induction

motors sulied throu"h solid state in!erters#

The counter/EM' aids the armature resistance to limit the current throu"h the

armature# hen ower is %irst alied to a motor, the armature does not rotate# At that instant,

the counter/EM' is &ero and the onl$ %actor limitin" the armature current is the armature

resistance# Usuall$ the armature resistance o% a motor is less than + O there%ore the current

throu"h the armature would be !er$ lar"e when the ower is alied# There%ore the need

arises %or an additional resistance in series with the armature to limit the current until the

motor rotation can build u the counter/EM'# As the motor rotation builds u, the resistance

is "raduall$ cut out#

The outut seed tor)ue characteristic is the most notable characteristic o% series

wound C motors# The seed bein" almost entirel$ deendent on the tor)ue re)uired to dri!e

the load# This suits lar"e inertial loads as the seed will dro until the motor slowl$ starts to

rotate K these motors ha!e a !er$ hi"h stallin" tor)ue#

2..5 Permanent ma#net motor

@ermanent/ma"net t$es ha!e some er%ormance ad!anta"es o!er direct/current,

e2cited, s$nchronous t$es, and ha!e become redominant in %ractional horseower 

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alications# The$ are smaller, li"hter, more e%%icient and reliable than other sin"l$ %ed

electric machines#

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outut %ilter is sometimes installed to smooth the a!era"e !olta"e alied to the motor and

reduce motor noise#

Since the series/wound C motor de!elos its hi"hest tor)ue at low seed, it is o%ten

used in traction alications such as electric locomoti!es, and trams# Another alication is

starter motors %or etrol and small diesel en"ines# Series motors must ne!er be used in

alications where the dri!e can %ail 0such as belt dri!es1# As the motor accelerates, the

armature 0and hence %ield1 current reduces# The reduction in %ield causes the motor to seed

u until it destro$s itsel%# This can also be a roblem with railwa$ motors in the e!ent o% a

loss o% adhesion since, unless )uickl$ brou"ht under control, the motors can reach seeds %ar 

hi"her than the$ would do under normal circumstances# This can not onl$ cause roblems %or 

the motors themsel!es and the "ears, but due to the di%%erential seed between the rails and

the wheels it can also cause serious dama"e to the rails and wheel treads as the$ heat and cool

raidl$# 'ield weakenin" is used in some electronic controls to increase the to seed o% an

electric !ehicle#

The simlest %orm uses a contactor and %ield/weakenin" resistor4 the electronic

control monitors the motor current and switches the %ield weakenin" resistor into circuit when

the motor current reduces below a reset !alue 0this will be when the motor is at its %ull

desi"n seed1#

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almost an$ en!ironment where "ood seed control was re)uired, %rom assen"er li%ts throu"h

to lar"e mine it head windin" "ear and e!en industrial rocess machiner$ and electric

cranes# Its rincial disad!anta"e was that three machines were re)uired to imlement a

scheme 0%i!e in !er$ lar"e installations, as the C machines were o%ten dulicated and

controlled b$ a tandem !ariable resistor1# In man$ alications, the motor/"enerator set was

o%ten le%t ermanentl$ runnin", to a!oid the dela$s that would otherwise be caused b$ startin"

it u as re)uired#

Althou"h electronic 0th$ristor1 controllers ha!e relaced most small to medium ard/

(eonard s$stems, some !er$ lar"e ones 0thousands o% horseower1 remain in ser!ice# The

%ield currents are much lower than the armature currents, allowin" a moderate si&ed th$ristor 

unit to control a much lar"er motor than it could control directl$# 'or e2amle, in one

installation, a > am th$ristor unit controls the %ield o% the "enerator# The "enerator outut

current is in e2cess o% +5, ameres, which would be rohibiti!el$ e2ensi!e 0and

ine%%icient1 to control directl$ with th$ristors#

2..7 Protection

To e2tend a #C# motorPs ser!ice li%e, rotecti!e de!ices and motor controllers are

used to rotect it %rom mechanical dama"e, e2cessi!e moisture, hi"h dielectric stress and hi"h

temerature or thermal o!erloadin"# These rotecti!e de!ices sense motor %ault

conditions and either annunciate an alarm to noti%$ the oerator or automaticall$ de/ener"i&e

the motor when a %ault$ condition occurs# 'or o!erloaded conditions, motors are rotected

with thermal o!erload rela$s# *i/metal thermal o!erload rotectors are embedded in the

motorDs windin"s and made %rom two dissimilar metals# The$ are desi"ned such that

the bimetallic stris will bend in oosite directions when a temerature set oint is reached

to oen the control circuit and de/ener"i&e the motor#

Heaters are e2ternal thermal o!erload rotectors connected in series with the motorPs

windin"s and mounted in the motor contactor# Solder ot heaters melt in an o!erload

condition, which cause the motor control circuit to de/ener"i&e the motor# *imetallic heaters

%unction the same wa$ as embedded bimetallic rotectors# 'uses and circuit breakers are o!er 

current or short circuit rotectors# Ground %ault rela$s also ro!ide o!er current rotection#

The$ monitor the electrical current between the motorPs windin"s and earth s$stem "round#

In motor/"enerators, re!erse current rela$s re!ent the batter$ %rom dischar"in" and

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motori&in" the "enerator# Since #C# motor %ield loss can cause a ha&ardous runawa$ or o!er 

seed condition, loss o% %ield rela$s are connected in arallel with the motorPs %ield to sense

%ield current# hen the %ield current decreases below a set oint, the rela$ will deener"i&e the

motorPs armature# A locked rotor condition re!ents a motor %rom acceleratin" a%ter its

startin" se)uence has been initiated# istance rela$s rotect motors %rom locked/rotor %aults#

Under !olta"e motor rotection is t$icall$ incororated into motor controllers or starters# In

addition, motors can be rotected %rom o!er!olta"es or sur"es with solation

trans%ormers, ower conditionin" e)uiment, M

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2.5.1 !an$al-startin# rheostat

"i#$re 2. !an$al Startin# Rheostats

hen electrical and C motor technolo"$ was %irst de!eloed, much o% the

e)uiment was constantl$ tended b$ an oerator trained in the mana"ement o% motor s$stems#

The !er$ %irst motor mana"ement s$stems were almost comletel$ manual, with an attendant

startin" and stoin" the motors, cleanin" the e)uiment, reairin" an$ mechanical %ailures,

and so %orth#

The %irst C motor/starters were also comletel$ manual, as shown in this ima"e#

 Normall$ it took the oerator about ten seconds to slowl$ ad!ance the rheostat across the

contacts to "raduall$ increase inut ower u to oeratin" seed# There were two di%%erent

classes o% these rheostats, one used %or startin" onl$, and one %or startin" and seed

re"ulation# The startin" rheostat was less e2ensi!e, but had smaller resistance elements that

would burn out i% re)uired to run a motor at a constant reduced seed#

This starter includes a no/!olta"e ma"netic holdin" %eature, which causes the rheostat

to srin" to the o%% osition i% ower is lost, so that the motor does not later attemt to restart

in the %ull/!olta"e osition# It also has o!er current rotection that tris the le!er to the o%% 

 osition i% e2cessi!e current o!er a set amount is detected#

2.5.2 Three-Point starter

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"i#$re 2.5 Three Point Starters

The incomin" ower is indicated as (+ and (6# The comonents within the broken lines

%orm the three/oint starter# As the name imlies there are onl$ three connections to the

starter# The connections to the armature are indicated as A+ and A6# The ends o% the %ield

0e2citement1 coil are indicated as '+ and '6# In order to control the seed, a %ield rheostat is

connected in series with the shunt %ield#

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The ossibilit$ o% accidentall$ oenin" the %ield circuit is )uite remote# The %our/oint

starter ro!ides the no/!olta"e rotection to the motor# I% the ower %ails, the motor is

disconnected %rom the line#

2.5., Ener# 8osses in DC !otors

(osses occur when electrical ener"$ is con!erted to mechanical ener"$ 0in the motor1,

or mechanical ener"$ is con!erted to electrical ener"$ 0in the "enerator1# 'or the machine to

 be e%%icient, these losses must be ket to a minimum# Some losses are electrical, others are

mechanical# Electrical losses are classi%ied as coer losses and iron losses4 mechanical

losses occur in o!ercomin" the %riction o% !arious arts o% the machine#

Coer losses occur when electrons are %orced throu"h the coer windin"s o% the

armature and the %ield# These losses are roortional to the s)uare o% the current# The$ are

sometimes called I6R losses, since the$ are due to the ower dissiated in the %orm o% heat in

the resistance o% the %ield and armature windin"s#

Iron losses are subdi!ided in h$steresis and edd$ current losses# H$steresis losses are

caused b$ the armature re!ol!in" in an alternatin" ma"netic %ield# It, there%ore, becomes

ma"neti&ed %irst in one direction and then in the other# The residual ma"netism o% the iron or 

steel o% which the armature is made causes these losses# Since the %ield ma"nets are alwa$s

ma"neti&ed in one direction 0dc %ield1, the$ ha!e no h$steresis losses#

Edd$ current losses occur because the iron core o% the armature is a conductor 

re!ol!in" in a ma"netic %ield# This sets u an EM'# across ortions o% the core, causin"

currents to %low within the core# These currents heat the core and, i% the$ become e2cessi!e,

ma$ dama"e the windin"s# As %ar as the outut is concerned, the ower consumed b$ edd$

currents is a loss# To reduce edd$ currents to a minimum, a laminated core usuall$ is used# A

laminated core is made o% thin sheets o% iron electricall$ insulated %rom each other# The

insulation between laminations reduces edd$ currents, because it is 3trans!erse3 to the

direction in which these currents tend to %low# Howe!er, it has no e%%ect on the ma"netic

circuit# The thinner the laminations, the more e%%ecti!el$ this method reduces edd$ current

losses#

2.5. A'%anta#es of DC !otor

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i. Spee' control o%er a wi'e ran#e both abo%e an' below the rate' spee'3 The

attracti!e %eature o% the dc motor is that it o%%ers the wide ran"e o% seed control both

abo!e and below the rated seeds# This can be achie!ed in dc shunt motors b$

methods such as armature control method and %ield control method# This is one o% the

main alications in which dc motors are widel$ used in %ine seed alications such

as in rollin" mills and in aer mills#

ii. Hi#h startin# tor9$e3 dc series motors are termed as best suited dri!es %or traction

alications used %or dri!in" hea!$ loads in startin" conditions# C series motors will

ha!e a starin" tor)ue as hi"h as 57 comared to normal oeratin" tor)ue#

There%ore dc series motors are used in the alications such as in electric trains and

cranes#

iii. Acc$rate steep less spee' with constant tor9$e3 Constant tor)ue dri!es is one suchthe dri!es will ha!e motor sha%t tor)ue constant o!er a "i!en seed ran"e# In such

dri!es sha%t ower !aries with seed#

i%. Quick startin", stoin", re!ersin" and acceleration

%. 'ree %rom harmonics, reacti!e ower consumtion and man$ %actors which make dc

motors more ad!anta"eous comared to an AC induction motors#