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Practical Marine Electrical Knowledge_4
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Chapter four: Motors andstartersEhctrlcMotorsConstnrctlonMolorEnclosuresThtEffectof Shefl LoedMotorRetlngslnductlon Motor - Besic 0peretlonControl EquipmentIXrcct-on-Llne SterterStrr-Delte StsrterAutotrsnsformer St mil ngMoiorProlectionMofor Speed ControlMotor md Strrter Maintenrncc
4fr1lr4124lt4144ls41.4174174lr.4l124l ts4lr.
Ehctric motor's
The drive lxrwer for compressorst pumps and fans aboard ship comes from electric motors.By far lhc most c()mmon type of motor is the 3-phase. 'cage-rotor' induction motor. It ispopular because it is simple. tough and requires very little attention. Another advantage irthat starting and stopping these motors can be done with simple. reliable direct-on-linecontactor starters.f nduction motors are usually supplied at 440V. fflHz, but 3.3kV,60H2 is sometimes used forlarge drives such as bow thrusters and calgo pumpi.Special types of molor can also be found on board ships. DC commutator motors are oftenused for driving deck machinery where speed control is important and single-phase ac motorsare used in low [nwer drives such as galley equipment.This section willdeal principa{ly with the 3-phasc, cage rotor induction motor, together withits control and protection. Additionally, the more common types of motor speed control areoutlined. followed by maintenance procedures for motors and starters.
Construclion
The induction m()tor has two main comprrnents, the STATOR and the ROTOR. The statorcarries the 3-phase winding in slots cut into a laminaled steel magnetic core. The ends of thestator windings are terminated in the stator terminal box where they are connected to theincoming calle from the 3-phase supply.
4 l l
The rotor has an uninsulated 'cage' winding which consists of copper or aluminium conductorbars which are connected ttlgethcr.at the cnds bv'c'nd rings'.'I 'he conductor bars are set in alaminated stecl magnetic core. Thc csse ntiul reliability tlf thc induclirln nrotor comes fromhaving this type of simple. tough rotor which has no insulation and does not have anytroublesome current collection gear like brushes. c()mmulalor or sliprings.The exploded diagram below shows all the components of a tvpical totally encloscd. fanventilated. induction motor.
Componcnrt of ,ypical cata-roror ndccrion motot@
I EndthEA, dilrint.nd2 Crarc nipplct Grc.tc tclicf tcrcv
end nu,t5 Atrti-bvmp weth.?t' 6 E,,ll bcoiaj, drivinl end7 Fcbe bceilni thonldtrt Flumc9 Roror on thcl,
l0 D''�rn plutI! VolG w�ath or wrlhout lc.l' ' 12 Eycbonti In dc cep, non.drinnr.ndl1 D.ll bc.r,nt, ^on-driviat crd
Molor enchcurcs
l5 Ctrchp
l6 Endthtcld, noa'drtvnt cnd
I 7 lnttdc cap rr.||t
19 Fca cotcr20 Lubnatot ertcsturr 9tPe2l Tataiul bot corcr22 Tcrmlml box cotcr gtlct
2l Tctmrnel boctd
21 Tcrmtool bot
25 ' lcrmrncl bor gttlct
20 Rec.*sy pbt.
2, Rac.||at pblc tctrctlE D lbntc29 C locc lbntc
1 End tccur'^t boh, or lhrouth boll lt Ftn vith pc1 or lcy
Enclosure protection for electrical equipmcnt is dcfined in tcrms of its opptrsili()n lo lh€ingress of solid particles and liquids and against mcchirnical impact.
'l'he cnclosure proteclion
is defined by the lngress Protection (lP) Codc wherc a 2 or 3 figurc numbe r is uscd to indicatethe degree of proteclion as shown below. (A 2 figurc lP crxle numhcr indicates eltclosureprotection againsl the ingress of solids antl l iquids only.)
td tlgur.:prot.clion ag.an.l aolkl bodilr
IP
2nd figurc:prol.clion .grinrl llquadr
3.d llgura:mcch.nicrl prolcclaon
r;-rJ\
""'',:in\o r ( t pu l ' t ' r r '9 r ,
IP
o
I
2
I r i , ' i l ? r t r ! ! t t j ' , r l
{. 1
,:l\ i - . / .-;: l:f,"@
o
IP
o
I
4 1 2
Drip prtxrf oF en veniilated motors lre used where the risk of dripping liquids from overheadpipes and valves nrav hc n prohlcm. Air is drawn into the machine by an internal fan toprovide crxrling. The ventilation ducts are fitted with mesh screens to prevent any objectsfrom entering the motor and causing damage. These screens must always be kep clean andfree from dust otherwise the molor will overheat due to inadequate v€ntilation.
When a greater degree of protection is re
Motor rrtings
The motor converts clectrical cnergy takcn from thc supply int() r()lational mcchanical energyat the motor shaft. Losses txrur during the encrgy conversion wltich rcsults in thc productionof heat in the motor. These losses increasc when thc load on thc m(tl(rr increase s because themotor takes more current from the supply. The life of the insulating ntaterials used on moiorwindings depends on the temperature at which it is operated. Insulating mate rials are selectcdto have an adequate life-span on lhe assumption that the temperature limit associated with aparticular insulation class is not exceeded.
lnsulationclass
LimitingtemP.nseK
Insulationmaterial
A 60 Cotton, natural silk, synthetic silk,presspan
E N Wire enamels with a base ot polyvlnyl
acetal, epoxy oy-polyamide resins
B m Mica productq, wtre enamels wllh abase of polyterephthalate, laminatedslass-fibre materials
F lm Mica products, glass fibre, a$bestos,wire enamels with a base of imide-oolvester and esterimide
H 125 Mita products, glass fibre, asbestos.wire enamels with a base of purepolyimide
MAX AMBIENT TEMPERATURE 4O'C
Ratcd Currcnt -
Retcd Voltage -
Rated Frequency -
Powcr Rating -
Ratcd Spced -
IP Number -
This is the maximum value of currenl that the motor can con-tinuously take from the supply without exceeding lhc tcmPcnlulc
limit for the insulating materials used.The motor has been designed to oPerate successfully when con-
nected to rhis value of supply voltage. If the rated voltage is nol
applied, overheating, stalling and burn-out can result.The motor speed is directly affected by the supply frequencyi so are
the molor losses. lf thc motor is operated at other than ratedfrequency overhealing can occur.Thii ls rhe ihati powcr ourput of the motor when it is connecled to
raled voltage and frcguency and drawing rated current from the
fi$|'l'the fult loarl speerl of the molor whcn connecred to ratcd
voltage and frequency.Indicites the degree of protecrion given by the motor cnclosure.The rating details arc shown on the molor nameplale'
414
lnductlon molor - hslc operrtlon
When the -l-phase supply is connected to the stator winding the currents which flow in thewindings produce a multi-yxrle magnetic field. This field rorares at a speed catled SYNCHRO-NOUS SPEED. The value of synchronous speed depends on how many magnetic poles thcstalor winding produces and on the freguency of the supply connected to the stator winding.
fn =-j-where n is the synchronous speed (rev./sec.)
f is thc supply frequency (Hz)p is the number of pole-pain
QUESTION:
What is the synchronous speed of a 6pole motor supplied at 60H2.
ANSWER:
20 rev./sec. or 12il) rev./min.'[he
rotating magnetic field cuts rhrofoh the rotor conductors and induces emfs in them.Since the rolor conduclors are connecteb together dt the ends, the induced emfs set up rotolc'trrrents. The rotor currenls produce a magnetic field whigh interscts with the rotatingmagnelic field and torgue is exerted on the rotor conductors. the direction of the totquc onrolor conduclors causes the rolor to rotate in the same direction es the rotating magneticfield.
QUESTTON:
How is the rotor direction reversed?
ANSWER:
Simply try swopping over any two suppty line connections at the stetor terminel bor. Thisreverses the direction of the rotating magnetic fietd.
An induction molor cannol run normally at synchronous speed. This is because the rotorconductors would then be stationary with respect to the rotating magnetic field. No emfwould be indueed in the rotor and lhere would be no rotor currentind rio torque developed.-Even when the motor is on no-load the rotor s;rced has to be slightly less than synchronous sotha tcur ren tcant rc induced in thero torandtorqueproduceJ . � �SLIP is the difference hetween the rotor speed and iynchronous speed.SLIP is usually expressed as a percentage of synchronous speed.
SLIP---.9IJIICHRONOUS SpEED - ROTOR SpEEO x r(nZoSYNCHRONOUS SPEED
QUF,sTTON
If a 6-pole motor issupplied at 60Hz and runs with aslipof 57o, what isthe actual rotorspeed?
ANSIVF]R . :
Thc synchronttus speed is l2(X) rpm. and rhe rotor slips by 5./c of 1200. i.e. by 60 rpm so therolor runs at I140 rpm.
At no-load ihe rotor speed is only slightly less thansyfthronous speed since the only torque
4ls
required is that net'ded to overcome lhe r()tuti()nll losscs of friction and windugc. Whcn lttudis applied l() lhe m()tor shaft thc rolor tends to sklw down. l 'his t lkrws ihc constant spcedrotating field to cut the rolor conduct()rsltf a fastcr ratc which irtduccs larger rolor currcnts.The result is an incrcascd tt lrque ()utput at u slightly rctluccd spccd.
The above curve shows the variation of torque with slip for a standard cage-typc inductionmotor. Also shown is a typical load characteristic which indicates the torque necessary todrive the load at different speeds.At start the motor develops more tor<iue than is trcqssary to turn the load so the motor andload accelerate. The speed increases until, at the intirsection of the two characteristics, thetorque developed by the motor is the same as ihe torque required by the load at that speed.The motor and load will run at this constanl speed.
Ccttol equlpmcnt
When the stator windings of an intluction motor are conncctcd directly to its 3-phasc supply. avery large currenl (6-8 x full load current) flows initially.
'fhis surgc currcnt reduccs as the
motor accelerales up to its running speed.Induction molors can be Direct-on Line (DOl-) sturtcd in this way. Thc starting currcnt willnot cause damage to the motor unless lhc ntotor is rcpeatedly sturtcd and stopJred in a shorlspacc of time. This is calhd 'fast cycling'.When very large molors are started direct-on-line they causc a disturbance of vrlltage ( voltagedip) on the supply lines due to thc large sturting currcnt surgc.This voltage disturbance may resuh in the malfunction of othcr clcctrical cquaprlcnt con-nected to the supply.To limit thp starling currenl some largc inductron m()t()rs arc slarlcd at rcduccd voltage andthen have the full supply voltage reconncclcrl whcn thcy havc run up t() neirr rutcd sp,ced.Most induction motors on ships arc startcd DIRE(-|'-ON-LINE (DOL).Reduced voltage starting is used tbr largc nlotors driving krads likc carg,() punrps and btlwlhrusters. Two methods of reduccd voltagc starling arc S'l 'AR-t)ELl'A S'l-ART'lN(i andAUTOTRANSFORMER S'I 'ARl ' ING.Contactors perform the switching aclion in startcrs t() conncct und disconncct thc lx)wcrsupply to the motor. The contaclor is an clcclrornaglrcticallv rlpcrltr:tl 3-;rolc su'ilch irtitiulcdfrom local and/or remole stop/start push buttons. lf thc currcnt gocs lhovc thc ratcd currcnlfor the motor the contactot *il l h" rripped ()ul lutonlaticully to disc.rnncct the ruttttlr lrom thesupply (SEE MOTOR PRO'I'E(*flON ).
' l 'hc. cssr.,ntiul conrlxrrre nts rlf i i c()ntitcl()] arc shown
below:
1,,6
Large m<ltors use contaclors of this typc which are mounted on insulated boards insidcswitchgear comparlments called starlers.A compact. sclf-containcd conlaclor frrr use with small and mcdium sized motors isshownbelow in expkxled view.
IXrccl-on-Unc Slrrter
t ,lr.r.rrtlt
L t t t
Ga. lac la r aaa l
The contactor coil is connected in series with a start button. stop hutton and overload tripconlacls. I 'his is callcd the control ciJcuit and is energised from two lines of the 3-phase Supply- usuallv via a step-d<)wn transformer. Whcn the start bulton is pressed the control Supply isconnectcd lo lhc contactor coil. The contactor closes flnd starls the motor. When the startbutton is releasqd its c()ntacts spring open. An auxil iarv contact on the contactor keep thecontact()r coil encrsiscd after thc st:rrt button is released.Pressing lhe stop hullon hreaks (he control circuit to the contactor coil: the contactor tripsand the m()t()r st()ps.lf the motor takes tcxr nruch current because it is mechanically overloaded or stalled. theoverload c<li ls wil l eithcr magneticallv or thcrmalty open the oveiload trip contacts which wil lstop lhe m(tl()r and prevenl overheating. N()te. the correct term is 'overcurrent' rather thanlhe commonly used 'overload'.
Star-delta starter
l f a nt r l tor is d i rect - t ln- l inc s tar tcd wrth thc stator winding star connccted. i t wi t l on ly takertnc-third of the starting currcnt lhat it would takc if the. windings were delta connected.Thc sti lrt inI currqnl ()f a motor \r 'hich is dcsigned to run deha crlotrcclcd can be reduced in thisway.Star-dclta sli lrtcrs rnilv he opcrated hv a rnanual changeover switch or thcy can be aut()mata-cally switclred using corrtact()rs controllctl by a timing relay.
L r
417
. '-... 'aai rr- o.i
When thc opcrating handle is phed in the 'start' position the motor stator windings areoonnectcd in star acroclr the supply. As the motor approaches norma! running speed thcopcrator must quickly change the handle to the run position which changes the rnotorconnection from star to dclta. If the operator does not move the handle quickly from start lorun then the motor may be dicconnected from the supply long enough for the motor speed tofell conridcrably. When the handle is eventually put into the run position the motor will take alarge current and acceleratc up to speed again. This surge current may be large enough tocau$e apprcciablc voltage dip. To prevent this, a mechanical interlock is fitted to theopcrating handle. The handle must be moved quickly from start to run othcrvisc thcintcrlock jams the handlc in the start position.An lutomatic changeover ir preferable and this is achieved by using contactors.
!
Star-Delta $tarter sequence:Opcrator closes motor isolator lS then presses start button.Start button connects the supply io contaclor coil S.Contactor contacts S close and auxiliary contacts Sl close.Supply is connected through Sl to contactor coil L. Contactor contacts L close. motor
windings are star connegted to 3-phasc supply. molor slarls.Auxiliary contacts Ll close at lhe same tirne as contactor c()nlacts L. The opcralor may nowrelearc the start button since supply to l- is nraihtained through Ll.After a time interval which allows lhe mottlr to run up to speed. auxiliary contscts L2 openand.L3 close.Contacior coil S is de-energised and its contacts S trpen; so do lhe auxiliary contacts Sl.
Contactor coil D is energised and thc m()tor is now dclta connectctl to thc 3-phasc supply.ln some cases a mec'hanical intqrlock is ftttcd bctwccn thc contact()r c()ntact$ S and D srl lhatboth cannot be closed at the samc tine.
QUESTION
Why is i t essent ial thal c()nt i tctr)rs S irrrd i ) i r rc not hot l r ckrscd at lhc samc t intc ' . )
418
ANSWER
lf you check the stitrtcr diagranr vou will see that a full short-circuit is applied across thesupply in this condition.
At srart. when the supply has just been switched on and the motor has not yet started torotate. there is no mechanicll outJntt from the motor. The only factors which determine thecurrent tnken by the motor nre the supply voliage (V) and the impedance of the motorwindings (Z).Cohpare the starting current when star connected to the starting currenl when delta cpn-nected:STARTI NC CUR RET.{T WH ENDELTA CONNECTED
STARTING CURRENTWHEN
Phase current at start $ =X
Line starting current lir = V3lJ = $E
So the ratio Line Startiqt Current in Delta
Pftase current et start Ii=Jv3Z
linc starting current Ij - l.t
Line srarringcurrenr = {#
. v 3 y # = +Line Sterting Current in Slar
This shoss thrt the strrtint currenl of r dclta connected motor cen be rcduced to o'rc third lftftc rnotor is s|lr connected for starting. (Unfortunately, thc torguc ir ebo reduced toone-thlrd resulting in rn increrrcd runtp timc for the motor end load.)Whcn an induction motor is running on losd it is converting electrical encrgy inprt torncchanical energy output.The input current is now determined by the load on the motor shaft.An induction motor will run at the same speed when it is star coinected as it will when it isdelta connected.This means that th€ tnwer output from the motor is the same when the motor is starconnecled as when the motor is delta connected. so the power inputs and line curr€nts mustbe the same when running in either connection.
I t . l O
But. the phase current which flows in each stator winding. | nts
v 3
and ls. which equals ls1. t irr the star connection.
lf thc motor is designed to run in dclta but is star connected. and on full load. then each statorwindins wilt hc carrving un ovcrkrrd of 1.73 x Ratcd ci.rrrent.' l 'his
wil l causc ovcrhcilt ing lnd cvcntual hurnout unlcss tripped by the ()vercurrent relay.Rcrncrnhcr that thc m()rr)r 'c()pf^-r iosscs irrc'prrxtuced hy thc l:Rcffclct so the motor wil l run Jtimcs hottcr if lctt trr run in thc star c()nneclion whe.n dcsigned for delta rutrning. Thisntalfunctiorr nlit\ ' (rccut if thc crxrtrol t inring sequencg"is n()l c(mpleled ttr thc' slar conlactolrctnairrs stuck in whilc u nrcchunical intcrlrrck prevcnls the delta c()ntactor from ckrsing.
STARCONNECTED
4le
For correct overcurrenl proiection. thc ovcrcrrrcnt rclavs must be fittcd in the phaseconnections and not in thc line conncctitrns. Chcck thc ptlsilion of thc ovcrcurrcnt dcviccs rnthe previous schematic diagrtrn for an itut()matic stur-dclta rtilrtL.r.
Autotrensformcr Sterting
Starting large motors with long run-up pcriods demands a vcry high currcnt surgc from thesupply Senerator. This causes A :icverc voltuge dip which affects cvery load on thc system.Reduced voltage starting will l imit the starting surgc current. One way to reduce the initialvoltage supplied to the motor is to step it down using a transformer. Then. when rhe motor hasaccelerated up to almost fullspeed, the reduced voltage is replaced by the full mains volrage.Thc transformer used in this starter is not the usual type with separate primary and secondarywindings. It is an autotransformer which uses only one winding frrr both input and output.This arrangement is cheaper. smaller and lighter than an equivalent double-wound transfor-mer. For induction molor starting, the autotransformer is a 3-phase unit. and, bccause of€xpenie, this method is only used with large motor drives. e.g. electric cargo pumps.
8a,$.thra c.istte||
fk|..tarla c.|o.trrct
The supply voltage is connected across thr' complcte winding and the motor is connected tqthe reduced voltage iapping. A numtrcr of tappings arc usually availablc on the transformerwinding, giving voltage outpuls ranging from about 5{)% to tt0rzn of the mains supply voltage.e.g. adJa/o tap on an autotfansltrrmer supplied ill .l4OV would providc an outpul of .
.f,f{) = 26*ly
The aulolransformer with irs rangc o[ rupping Jxrints givcs a sct rilngc of stlrting voltages lolimit the motor starling surgc currcnt t() it reitsoniihlc vuluc..
QUESTTON
Estimate and comparc thc likclv st.lrting currcnt surgcs lirr l mrltor thll trrkes ltXlA on fullload when started (a) D()L. (hl Strrr- l )c l t : r . (c l / \utolr iurst i l rnrcr with r 5t l ' i t lpping.
ANSWER
(a) When star t ing Di)L thc in i t iu l surgc currcnt is ahtu l 5 x F l - . i 'e ' l (x l ( lA '(b) A star-dc l ta s lar ter reduccs thc tn i t i l l s tar t ing surgc t ( ) ( )nc- th i rd of thc cquiv l lcnt DOI-
value. i .e . to atx lut lS l lA in th is casc.(c) . T 'hc aulot ran ' r i for rncr mcthod rct luccs thc in i t r l l s l i l r l ing surgc l ( ) (x) j . l txr t . whcrc x *
tapping point . In th is cx lnrp le r = | f .5 . ;o thc surgr l currcnt lcvc l is t l . . i r r l l l l l | lA = l .5 l lA.
The D() l - s tar tcr is s implc i rnd chcap hut causcs, i r l t rgc st l r t ing surgc. Star- t lc l t l s tar t ingretJuce s thc surge hul is sorncvvlri l t lnorr. conrplcx. re tluiring lhrcc contaclors lrttd it t intcr.
-l 'hc
autr)lr i lnsf()rmcr mr"'th(xl ciln he irrrirngc't l t(t nlit lch lhe. ntolrlr trrrgc e rrricrrl uil( l rurl-ttP pcrirtd
l ( ) mcct the rupplv l in t i tu t ions hv u sui tuble choicc of vo l l lqc ' t lpp ing. lh is s tar tcr isct tns idernbly mrt rc cxpe| ls i |c th i l r r lhc othcr t$ ' ( ) s l i t r ter tvpc\ .
,s"
4 / 1 0
As with the star-tJcltil starter. the autotrdnsformer mily use what is called ei open transilionswitching sequence or a chxed transition switching sequence between thc start and runconditions. ln the fonner. the reduced voltage is supplied to the motor iit start then discon-nected and the full supplv voltage rapidly reconnected to lhe motor.The circuit diagram helow shows a manually operated open transition. autotransftrrmerstsrler.
The problem with open transition is that a yery large surge current can flow after thetremition fiom reduced to full voltage.
QUESTTONWhat causcs the large currenl surge in open transition startcrs when going from the start tothe run condition?
ANSWER
All motors generile a hack emf against the supply voltage when they are running. When thesupply is removed from a running induction motor the magnetic field does not immediatelycollapse. The molor begins to slow down but still generates an emf. When the supply is
'reconnecled. lhe supply voltage and motor emf are not necessarily in phase (the conditiort issimilar to synchronising a generator onto the bus-bars). So each time the starter is operated adifferent surge current will be produced - sometimes very large. sometimes quite small,Large surges could cilus€ appreciable voltage dip on the supply and so affect other equip"ment. Closed transition slarlers overcome this because the motor is never disconnected fromthe supply during the srarting cycle. '
An arrangement which overc()mes the transition switching problem is the closed transition'Korndorfer' starting mcthod. A typical circuit is shown below:
4 / l I
When the start button is pressed the first stage c(;ntactor coil is energised which closcs themain lsr conlacts and also lst/l and lst/2.
'fhe timer relay coil and thc secontl stagc cr)ntactor
coil are alsrr energised. The main 2st contacts ckrse . which applies a reduced vrtltlgc from theautotransformer to the motor windings.
"I'he mtltor starts. After a preset tintc intcrval lhe
timer rclay opens tr/2 which dnlps out the second staBe contactor. The star Jxtint of thetransformcr is opened by 2st so that transtirrmer action rto longcr takes place. 'I'hc transfor-mer winding now just acts as an inductive .roltagc dropping impedance in the supply lincs toihe motor. The voltagc applied lo thc motor is now higher than be fore but is stil l less than thefull supply volrage. After a further time interval the timer relay closes trl3. which energisesthe changeover relay. The changeover relay closes the run contactor which puts full voltageon to the motor. Auxiliary contacts rnl? and rn/l on this contactor drop oul the first stagecontactor and maintain the supply to the run contachx coil. respectively. The stop button orovercurrent relay trips out the run contactor to stop the motor.
Moltr proS.ctloo
frotecting an electric molor basically involves preventing the motor from getting too hot.Remember. every lffC above the maximum recommended tempcralure of the insulation canreduce its working life by half . Obviously. the trcst way to protect a motor against overheatingis to directly monitor the tempe rature of the motor windings. lf the temperature exceeds themaximum set value for the motor its contactor is tripped to allow the motor to stop and crxrldown.There are a number of temperature depe ndent protection devices available. but THERMIS-TOFPROTECTION is probably the most common. Thermistors are small beads of semi-conductor material which are embedded into all three of the molor stator windings duringmanufactur.e. When thermistors get hot their resislance changes dramatically. They areconnected so lhat if the motor temperature gets too high the startcr conlaclor will be trippedby a special thermistor protection relay to stop the motor.Direct thermistor prolectk)n is usually only fitted to large motors. e.g. bow thrusters. FD fansetc.Most motors ar€ prolected by monitoring the tcmpcraturc indirectly by mcusuring thecurrent flowing in the rupply lincs. This mclhod uses thcrmal or clccromagnetic time delayovercurrent relays (OCRs) in the nK)tor startcr. The system is dcsigncd $o that if the motortakes too mirch current because it is mechanically overloaded, the OCR will trip the contactorcoil, after a certain time delay, before scvere overheating can occur. The largest OVER-LOAD posrible is the current taken when the molor hasstalled. This. of course . is the startingcurrent of the motor which will bc' about six times thc full load current. Thc contactor iscapable of tripping this stalled crurrcnl quickly and sat'ely. lf a short-circuit rrccurs in thcmolor, the starter. or the supply cablc. thcn u hugc FAUt-1'currenl wi l l f low.lf the contactor tries to ()pcn unde r shori-circuit condilions mas-sivc arcing will trccur at itsconlacts such that it will fail t() intcrrupt thc fault currcnl.
'fhc prolongt'd shorl-circuit currenl' wil l cause serious damagc to thc mot()r. startcr and cablc with thc altendanl risk of an
.1 electrical f ire. To prevent this. fuses are fittcd which wil l hlow bcforc lhc conlactor ()Pcns
1 {uring a short-circuit fault.I l t isimportant that the tripping characteristics of the'()CR and fuscs are matchcd so that theI contaclor trips on overload whilc the fuscs intcrupt shrlrt-circuit fault currcnt\. [ 'his'ctlntac-
ltor-fuse' arrang,ement is sometinres callcd 'fuse back-up protccti()n
4l r2
QULSTTON
At whut value' of current shrlukl the O('R be ser'.,
ANSWER"I'o proteci u modc'rn MCR (CONTINI-JOUS MAXTMUM RATING) moror the thermalOCR should he set rl the full-load current of lhe nrotor. This will ensure that tripping will notoccur within 2 hours al l0-57o FLC. At l2lY/o FLC tripping willoccur within'i hJurs.
It must be emphasized that the fuses are not chosen for their rated current but for their inversccurrent/lime characterislic. T'his means that lhe current ratingof fuses used lo protecl a motordoes not appear to have any direct relationship to the full-load curqent raring of the motor.The table helow shows some recommended fuse ralings for protecting mot;rs and startersagainst short-circuits.
Direct on-tine start
Maxitnum motor full-loadcurrent (Amp) 2.5 t0 20 60 135 2m 330 570
Recommended standardFuse l ink rat ing (Amp) r0 2s 40 t00 200 3t5 450 700
QUESTTON
A ntolor is prolected bv a thermalOCR and hack-up fuses. Can the motor exceed irs ratedlemperature wirhout being'tripped by the prorection?
ANSWER
YES! Although overheating is usually indicated by the current drawn by the motor risinggbove its rated value. a number of other situations can contributc to motoi overheating. Foiexample - very high ambient lemperaiure; inadequate ventilation; a stardeltastarter rtirainsin the star connection: stopping and starting too often; worn or dry shaft bearings.
The molor windings can only be protected against these conditions by using direct thermalproteclion ( thermist<trs).There are two:types of overcurrent retay (OCR) used for motor protection: THERMAL andELECTROMAGNETIC.Although elcctromagnetic devices with rime delays can give adequate protection tgainstlarge. sustained overloads to motors which are operated well below theif maximum o-utputand temperature. they have been found to be inadequate for modern continuous maximlmrated (MCR) motors. These motors are protecred by thermal OCRs. Delayed thermal OCRsusually work wilh bi-mctal strips. The strips are heatcd by the motor current ly hnd benddepending on the temperature. If the motor takes an overload current the stripa operate apair of normally closed trip contacts which open-circuits the contactor coil circuit to stop thcmot()r. The minirnum tripping current of such a device can be adjusted over a small range.1:his ad;ustment alte rs the distance the strips have to bend before operating the rip conracts.
I l i a a l a l a t r r r at i a a l a tt r . ca l i i o l a l i o r t i l i lI a l r , c r r c g r l a t r r a i lla ml t , G9r r ta l
l t t irrra
4lr3
F'or larger motors. thc heaters do nol carrv the full mtrtor currcnl. 'I 'hcv arc supplicd fromcurrenl lransformers (CTs) which stcp down the m()tor current so that smallcr hcatcrs may heused.To operate correctly, induction motors must be connected to a 3-phasc supply. C)ncc startetlthey will continue to run even if one of the three supply leads bccomes disconnectcd. 'I 'his
iscalled 'single-phasing' and can result in motor burn-out. Single-phasing is usually causedwhen one of the three back-up fuses blows or if onc of the contactor conlacts is ope n-circuil.The effect of single-phasing is to increase the current in the two remaining lines and cause themotor to become very noisy due to the uneven torque produced.An increase in line current due to single-phasing will be detected by the protective OCR. Forstar connected motor windings the phase and line currents are equal so the line connectedOCR is correctly sensing the winding current. If the overcurrent setting is exceeded during asingle-phase fault the motor will be tripped off.Thc situation is not so simple with a delta connected motor. Normally the line currenl dividesvectorially between two phases of the motor windings. The phase current is just over half theline current,
Iph
When one of the lines becomes open-circuited a balanced 3-phase condition no longer exists.Now the currents in the phases are no longer equal to one another.
1 9 r ' n 9 0 . . a r e r a
: I ' = n . 5 7 i r ,'fr
L I
The table belowshows typical values of line and phase currents at various le vels of load duringa single-phase fault:
HEALTHY SINGLE PHASED CONDITION
7o bf motor rated fullload current on 3 phasc
supply
/r OF RATED FULL [-OAD CURRENT
l r : & l r . 1 l A & 1,, l(
(,{)70
l{x)
t (12l-1( l2.r3
6l79
t29
r 3 ll 6 tIn5
Particularly not'e that the current in winding C is considcrably highcr than that in thc othertwo windings.Look at the condition where the motor is at 6(l(Z of full loatJ whcn single-phlsing occurs. Thei ine currents are l l l2 lo of the fu l l - loat l va luc but the currcnt in winding ( ' is lS lc? of i tsfull- load value. The l(127r" l ine currcnt wil l probably not aclivutc t l inc c()nncctc(l OCR andthe motor remains connected. However. thc local overhcrting in winding C rrf the mrltor wil lquickly result in damagc.This is why star connected motors havc. irr the pirst. trcen prclerrcd to dclta conncctetl m()lorsfor use on ships. However. delta conncctcd motors can he. pr()tectcd against this condition byusing a'differential 'typc relay. In fact. nrost motlern thcrmal ()CRs tirr nrolors havc thisprotection incrlrptxated as a normal feature .The thrce the rma! q lements of thc O( 'R arc arr r rn{cd in such a wlv thal u l rc tgual hct t ing o[the b i -mcta l s t r ip i causcs a d i f tcrcnt iu l rnovcnrcnt whieh t r ips thc corr t le lor .
o t 4 a r a t o a r r a c l r o .
4 l t 4
_l --{- *:,"^-,'ffi
Llndcr normal hllancetJ conditions lhc currcnts in lhe three overcurrent healers are equal andthc bi-mttalcnds movc togcther to lhe n()rmll load position. The complete assembll movesbut drrcs not rcxch thc fixcd stop. lf l nlot()r ovcrkrad occurs. all three currenls increaseequallv. Thc hi-rrrctals move lhr'conrplcte rrssembly until the lower trip bar reaches the fixedstop. The upper trip har continue's t() move causing the bellcrank to pivot about 'B'. Theroller lifis l<r ()pr'n th(' r('lily contacts to trip the molor contaclor. lf single-phasing occurs. onerlf thc bi-mctirls will crrol down.
'l 'his moves the lower trip bar to the left. The bellcrank pivots
ahrut 'A'. Again. the rollt 'r l ifts and trips the contactor.lf single-phasing txcurs when thc motor is running the motor keepa on running unless theprotectien trips the contaclor. lf the motor is stopped. it will ndt restarl. When the contactoris closed. the motor will teke a large starting current but develop no torque. Thc OCR is sct toellow the starting current to flow long enough lor the moior. under normal conditions, to runup to qreed. With no ventilation on the stationary molor. this tirne delay will result in rapidrnd severe overheating. Worse still. if the ()perator makes several attempts to slart the motorit will hurn out. lf i l motor fails to stan after two atlempts. you must investigate the cause.Undervoltlge pr()tccti()n is nccessary in a distrihution systcm that supplies motois. lf thcre isa 'black-out' all the nrotors must hc disconnected from the supply. This is to prevcnt all themotors restarring togcther whcn thc supply voltage is reslored. which would result in a hugecurrent surgc. tripping lhc gencralor lgnin.Motors musl be restarted in n controlled sequence after u supply failure.Undervoltage protection is provided by the spring-loaded motor contsctor bccausc it wllldrop out-when the supply voltage is lost. Whcn the supply relurns lhe motor will not restartuntil its contactor coil is re-energised. This may require lhe operator to press thc '3terl' buttonor. if it is an esscntial ltnd. it may be done automatically b) a SEQUENCE RESTARTSYSTEM. This system ensurcs lhat cssential services are restarted automatictlly on rcstora-tion of supply ftrllowing a blackout.
-l'imer rclays in the starters of esscntial motor circtits will
initiate starl-up in a conlrolled sequencc. For a vessel with sieam lurbine propulsion a typicalSequential Start List might includc:
I) SE('ONDS B0I I -F ,R WATER ( ' IRC. PUMPA U X . F E E D P U M PF O. SUPPI -Y PUMPMAIN CENERATOR LUB. OIL PUMPSl I - .F .RIN( ; ( ;HARIVIATN ] 'RN NS}'ORMER ' I
A[J 'T0MA'I ' ION AND ALARMINTERIOR ( 'OMMUN ICATIONRADIO EOUIPMENTNAVIGA'T ION EQUIPMENTMAIN LUB. OIL PUMPAUX.COOLIN(; FRESI{WATER PUMPJACKET ( 'OOLINC WATER PUMPTVIAIN SEAWATER COOLING PUMPAUX. ( 'ONDENSER CIRCULATINC PUMPCONTROL AIR COMPRESSORMAIN AIR COMPRESSORENGINE ROOM VENT FANS
lllolor speed control
Thc stlndard cagc-rotor induction motor ()perates as an almost constant qreed drive over itsload rangc. T'his leature is satisfnctorv f<rr most of the ship's auxil iary services supplyingp()wcr to vcnti lation firns and circulating. pumps. Variuble speed control is necessary forcrancs. winchcs. windlass. capslitns. frrrccd-draught fons etc.J'rr 'o main forms of spcd contrrtl are availahle :(a) l 'olc-chlrnging to si1.' i$() or mrlrc fixed specds. c.g. 3-spccd forccd-draught fans and
.l-rpcctl u' irrchcs.(b) ( 'ontinuouslv variahlc spced control. c.g. smorrihcontrol of deck cranes and winches.
5 SECONDSI I I SE( 'ONDSI3 SE( -ONDSr5 st_('oNl)slr| st_('()NDS]5 SE('ONDS'] I t SE('ONDS-10 sF_c()NDS
4 l t s
Fixed set s1rceds can bc obtained from a cage-rot()r'induction m()lor by using a dual-woundstator winding: each winding heing designed to crcate a different numher of magnctic prrles.
QUESTION
A dual-wound induction molor is arrangcd lo create either 6 Sxrles or l{) glles. Estimate thcrated speeds assuming thal the rotor slips by 5/c and the F)wer supply is at a frcqucncy of60H2.
ANSWERFrom n- = f/p (n. = speed of rotating flux pattern)High speed (6-pole): tr, = 6O = 20 rev/sec or 12fi) rpm
3but rotor runs at 95 o/o x 12fi)= !!LO tpt
lmlow speed (10 pole): o, = 6O = 12 revlsec or 720 rpm
sbut motor runs at 95 o/o x 72ll-- 6ti4 rpm
l(x)
A 3-speed prle-change winch motor can be arranged by having lwo cage rolors mounted rrn
the same drive shaft. One stator winding (usually 24-ple) gives a low speed while the othr.:r is
dual wound to give medium speed (tl-pole) and high speed (a-pole) outputs. Speed coalrol
and drive direction are achieved by a set of switching and reversing contactors operaterj from
thewinchcontro l grdesta l . Remembcrthal tor€versetherotat ionof aninduct ionmotor i t is
necessary to switch over lwo of thc supply lines to lhe stator winding.
An alternative method giving two fixed spccds in a 2 : I ratio from a cage'r(ttor induction
motor is to use a single stator winding which has centre-tap connections availablc on each
phase. This mc.thod uscs a startcr with a sct ()f c()nlact('rs to switch the phase windings into
either single-star (low spced) or doublc-slar (high spccd). Thc supply l incs to the stalor
windings arc shown bclow. Notc that two of thc supply l incs arc interchanged in the
double-star connecti()n - this is tt l maintuin thc sanrc dircction rrf rotation as in the low speed
connection.
r l a . l a t o r a r a a d l a o . l l a r l a f l f r r l l r t t a . a l
A continuousty variable specd range of rnotor c()ntr()l involvcs ttt{)rc c(lt l lpltcatirln and
expense than that reqUired to ()trl i l in a cuuplc ()f sct spccds. Vitrious nrr'thods itrc avli lahle
which inclurle:(a) Electro-hydraul icdr ive.(b) Wound-rotor resistancc conlrol. ttf induction In()lers.(c) Ward-l-eonard dc nrotor drivc.(d) Variable-frcqucncy induction nl()t()r c()rt lr()1 .
(a) The electrt;-hydraulic t lr ivc. oltcn uscd for dcck cnrnc c()nlrol . l t irs it rclt l ivclv sirnplc
e lectrical sccti{)n. ' t 'his
is a c()nslallt sirrglc-spcctl inductittn tt l()l()r su;rplicd lrortl i t I)()l-
or s tar-dc l t i t s t i l r ter . ' lhc ntotor ru l ts conl i r ruouslv l t t n ta inta i r t o i l p lcsst t tc t ( ) lhc
var iablc-spccd hvdr i ru l ic n l ( ) l ( ) rs .
(b) A crr rdc forru o l 'specd c()nt r ( r l is prov i t lc t l l r r thc n 'ot t r td to lor indt tc t io t t t t l r r lor . l 'hc
rotor has a 3-p l r : rse r r in t l ing (s int i l i r r to i ts r t : r lor u i r rd i r rg, ) r rh ic l | is cr l t t t tec led l t r 1
s l rpr i r lgs l l l ( )u , l lc ( l o l t l l tc rh l l l " \ r r r ' r ie t i ia l -1-Phi tse rcs is l t r r l r i t t tL t r c( t l l l lcc lc( l l ( l
brushcs on thc rotor s l ipr ings. r \ rc t ( r l c( rnt i tc t ( ) rs or- l s l idc r r - i ; rcr ( l i r r \ l l l i l l l l l l ( ) t ( l rs)
. var ics lhc antot tnt o l ' r t ' r is l i t t tcc ; t t l t l . ' r l l t r lhc ro lor t ' i rcr r i t .
1 n 6
. lncreasing the value of external resistance decreases the rotbr speed. Cenerally. the
stsrters nf wound-rotor motors are interlocked to allow start-up only when maximum
rotor resistance is in circuit. This has the benefits of reducing the starting current surge
while providing a high starting torque'The *ound-rotor airangement is more expensive than an equivalent cage-rotor
machinc. lt requires moie mainrenalrce on accounl of the sliprings and the eltcJnel
resiiror trank which may require special cooling facilities.
(c) W6ere conrinuously ""ii"bt. rp".d ttut to be combined with high torque' smooth
rcteleration. irrcluiing inching mntrol and regenerative braking. it is necessary to
consider the mcrirs oie dc notot drive. Speed and lorque control of I dc motol is
basically simple requiring the varialion of armature voltage and field current. The
problem is: where does the necessary dc prrwer supply come fnrm on a ship with an ac
electrtcal sYstem?A poprlar nrcthorl for lifts. cranes and winches is found in the Ward-Leonard drive'
Here a constant speed induction motor drives a dc generator which in turn supplies cme
or mofie dc morors. The generaror output voltage is controlled by adiusting its small
excitation current via the lpeed regulator. The dc motol speed is directly controllcd by
thc aenerator voltage'
obviously itre .triotor-ceneralor .(M-c) set requires space and maintenance' '{n
alfernative is to replace the rotary M-G sef with a static electlonic thyristor controllef
which is supplie{ with constant ac voltage and deliveJs a variable dc output voltage to
the drive molor.
4lr7
(d) Althouglt thc Ward-Lconard dc schcmc providcs un exccllent p()wcr drivc, thc mainte-nance problcms are high duc to lhc comrnulator and brushgcar itspccls of the 4cmachines. To eliminatc thcse prohlems melns returning to the simplicity of the cage-rotor induction motor. I lowever. the only way to achicve I continuously variable sJxc6output by electrical control is to varl, the input supply frequency. A sratic elcctroniclhvristor controller may be used to pcneratc such a variable frequency output to dircctlycontrol the speed of the nlotor.
A € - A CI lrt. i atCt
It a r .a
I ra ecaac t
r l a r a b l I
f r a q r a i c tJ.
t t a o d G o n l r o l
! ttraaal .c . r r9 t t t
4;r Os'
The result is a relatively maintenance-frec system with a lot of electronic circuitry.
Motor and sterter mainlenance
The mainienance rcquirements for cage-r(rtor induction motors are very simple:
Keep insulation resistances high and conlact resistanccslow. Lubricate correctly and maintain an even air gap.Ensure both the interior and exterior are alwavs cleanand dry.
Provided these requirements are met. an induction motor should give trcuble-frec serviceduring its long life.
QUESTTON
What is the mosl common cause 0f induction m(x()r failure'l
At\SWER
Fai lure of s ta l ( ) r insulat ion duc t ( ) dampncss is i r nra jor prohlcnr wi th mar inc nro l r r rs . ( )pcn
vent i la ted m() t ( ) rs are mosl at r isk. par t icu lar lv u 'hcn thcv l re n() t uscd l i r r k lng per iods.Ant i -condcnsal ion heaters shr lu ld bc rcgular ly chcckcd t ( ) scc th: t t thcv nrc actu i t l lv wt l rk ingand keeping the motor dry.
For all nl()tors. cleanlincss is ncxt to godlincss. A rcgular clcanilrg routi lrc is rcquired toreniove harmful de;xrsits ttf dust. dirt. grcasc atrd oil from both insidc and oulsit lc lhc nr()t()r.' fhe
c leaning of the e x lqrnal sur lacc is cspcci l l lv i rnpt l r tanl l i r r t t l l l l lv cnch)sc( l nrotors whichrun continuttusly. l-he heiit gcncratcd in thcsc nl()t()rs is removcd through thc cxtcrnalsurface. A thick layer of dust wil l rctlucc thc heiit dissipation and rcsulr in vcrv hightemperatures. Internal dust and dirt in open l 'cnti latctJ motors must tx. ncgularly rcmoved bytrlowing or cxtraclion and ventilation scrcclrs und ducts clcrrcd rtul,lf motrlrs are l() br: blt lwn oul. the i i ir uscd nrust be al'rsolutclv drv irntl thc prcssurc shrluld nolt re morc than 1.75 t rar . l f thc pressurc is h ighcr than th is i t forccs thc dust in t ( ) lhc windinginsulat i r ln rathcr th tn removing i t .Whcn b lowint t ( )ut a m() lor r r 'menrbcr l ( ) e() \ 'c r up othcr nr lch incs in lhc arct l ( ) pr( ) tcct thcmfrom f ly ing dust . Suct ion c lcaning is bct te r than hkrwing our .
QUESTION
l low r l l tclr shoukl i t nt() l()r bc clcanctl . '
4 l l t l
ANSWER
Basically this will he tletcrmined by the locril conditions and the tvpe rrf ventilation. Only theexternal surfaces rrf totallv cncltxed nrotors will reqrtire regulnr cleaninq. But both theoutside and insitle rtf open ventilated motors will require routine attention. The inside of at<ltally enclosed molor can be cleancd if the motor has been dismantled for bearing replace-ment. Molors in areas where considerable amounts of air-txrrne dust are exSrected, hatchcover motors are an example, will obviouslv require more frequent cleaning than othermolors.
Contamination by oil and grease from moror bearings is often a iause of insulation failure.The insulation should be cleaned by brushing or spraying with one of the many proprietarybrands of cleaning fluid which are available. Badly contaminated motors may require totelimmersion of the stator windings in cleaning fluid.Broken oi missing bearing covers must be repaired or replaced to pievenr grease escaping.When a motor has been dismantled for cleaning and overhaul it should be thoroughlyinspected. ln this way faults can be detected before they lead to breakdowns.STATOR - on the stator windings look for damaged insulation caused by careless replace-ment of lhe rotor into the stator. Discoloured insulation is an indication that the winding hasbeen overheating. The cause of overheating must be found and corrected before putting themolor back into service. Carefully examine the stator core for signs of rubbing with the rotorcaused by a worn bearing. Even slight rubbing of the rotor against iire stator wilt generateenough heat to deslroy the stator insulation. Renew bearings hefore putting back intoservice. Core plates which have been badly scored may cause a local hot spot to be generatedwhen the molor is running. This is because the iron losses will increase in the damaged aree.After the motor has treen put back into service with new bearings check the motor runningtemperature. After a short period of service dismantle the motor and check for discolourationat the core damage which will indicate local heating. lf you suspect core hot spots then themotor core will need to be dismantled for the laminations to bi: cleaned and re-insulated - ashore job.The insulation resistance reading is the best indication as to the presence of moisture in themotor windings. Breakdowns due lo insulation failure usually result in an eailh fautt,short-circuited turns in a phase or phase-to-phase faults.
QUESTTON
How do you check the insulation resistance between phases on an induction motor?
ANSWER
Larger motors are usually 'six terminal', which means that all six ends of the stator windingsare brought out to the terminal block. Links between the terminals are used to star ordelta-connect the motor. Disconnect the supply leads and remove the links. Test betweenphases with an insulation resistance tester.
: | :
A I _ B IR I - C IA I _ C I
@o
Tcst Betwscn
4 l t e
A problcnr cun ar isc on snta l l . ' thr t 'c ' tcr r r t inu l n lo lors uhcre lh . ' \ l i t r ( ) r r lc l l i t cot tnt :c l ion is
madc ins idc thc motor . ( )n l r ,onc cnd t l f t ' ; rch u i r td ing is l r i t i lah lc i t t lhc lernt t t tu l hkrc l .
Phase-to-phase insu.lation rcsistancc clnn(rt ltc clrcckcd. l l l thrcc tcrnrinrl t l l() l()r is lr l bc
rewound ask thc repaircr to c()nvcrt it trt i t sir tcrntinal arri l l lEclrtcnl.
BEARINGS - lnduction rn()t()rs arc fitt,:d $ith ball alrd/or rtt l lcr bearings.' lhcsc bcrrings
are tough and reliablc and should givc vcry l itt lc trouhlc prrrvidcd thcy lrc propcrly f ittcd,
kept absolutc lv c lcan and lubr icr tcd crr r . rcct lv . Many cngincJrs i l rguc that i f a hc i t r i t tg secnts
to be operating corrcctly it should not bc t:rnrpcred with. It is not possihlc to prcdict with any
degree of certainty the unexpired l itt 'ot bcarings that have already run for some time. Also.
inspection may not show damage to raccways and roll ing elements in arcas hidden frrtnt view.
The best policy is to renew the bearings as part of a planned maintenance programmc. lf this rs
not possible because of cost or a shortage of replacements, then bearings should be removed.
cleaned and inspected for signs of clamage before a decision to refit or renew is taken. Before
opening up a bearing make sure that the complete area around tlre housing i i,clcan and dry.
Bearing manufacturers recommend that trearings should be remtlved frttn thc shaft us
seldom as possible. But cleaning and inspcction is best d<tne with thc bcariltq off thc shaft. lf
the correct size of wedges or pulle rs is used. thcn rcm()val should n()t cause any <lamage.
Bearings should be cleaned try immersirln in a solvcnt such as clcan whitc spirit or clcan
paraffin. then thoroughly dried in a jet of clean. dry comprcssed air. Bearings should not be
spun by the air jet because skidding can damage the roll ing c'le me nts and raccways. ()nce dry,
the bearing must be l ightly oiled. Any traces of metal particles. such as brass. ittdicate cage
wear and the bearing must be discardcd. lf therc is no evidence of mctal particlcs. carcfully
examine the raceways and roll ing elements for signs of wear or damage. Hold the inner race in
one hand and slowly turn the outer race. lf there is any sticking, ()r unevenness in the rtttalion
re-wash the bearing and rotate it in the clcuning fluid. lf the sticking pe rsists thc hcaring rnust
be rejected. Simitarly, hearings with visihlc signs of ctlrrosion. ovc'rhcating or darnagc. and
those with a noticeable degree of r<lughncss in rotatirtn should also hc discardcd.
When f i t t ing a bc 'ar ing to a shaf t . f i rs t c lcan thc shaf t and applv a th in f i lnr o l l icht o i l . St ' l the
bearing square on the shaft and. with a tuhular drift. force the hcaring against thc shati
sh6ulder. ' l 'he
drift should hcar on thc i lrncr race ls closc to thc shaft its possihlc. l-argc
bearings can be heatcd tbr l( l-t5 minutt 's in clc.ln mincrrl oil up to tl()"C to flci l i tatc fitt ing.
Lubricate the bearings with the correct typc nnd quantity of greasc as rcc()mmcndcd by the
manufacturer. Fil l the bearing about one third to one half full with g,rease. ()vergreasing
caus€s churning and friction which results in hcating. oxidation of the grcase and possible
leakage (hrough the seats. Because of the high anrhient tcmpcrature and exccssive vibration
which marine motors endure. greasc l ifc can bc shrlrt and fresh grcase shttuld be applied at
regular intervals. Unlcss the bearing housing has a vcnt httle io ulkrw c'xccss grcilsc to escilpc.
it wil l be necessary to clcan out thc bcaring housing bcfttrc charging with frcsh grease.
Because of the vibration on ships. bearings can bc danragcd whcn lhc ln()t()r is t lol runntng.
The shafts tlf stationafy motors should be pcriodicit l lv rotatcd l qtlartcr lurl l t() minimise
vibration damage to the bcarings.ROTOR - As you wil l have gathered. mitintcnilncc ()f cagc-rotor intluction l l l() l()rs tcnds t0
mainly involve the stator windings and bcarings. Cage-rolrlrs requirc l i tt lc t lr n() spccial care
in noimal service. Inspcct for signs of damugc and overhcaring in thc cagc winding lnd
laminated core. Make sur ! . that a l l core vent i la t ing ducf s urc c lcan and e lcar . l f ln i r l tcrn i t l f i ln
is f ittcd it must bc in good conditit ln if i t is to providc i ldcqualc cooling.
QUESTION i
A cagc-rol<l/ inducfion m()lor has tlecn fltxxJcd with sca wulcr. lttsulatiolt rcsislitt lcc is dou'tr
lo zero MQ. What is the prtrcedurc tirr putting the rtrolor back ittto scrvicc.'
ANSWER
T'he main pr t l l r l ! .m is tg rcstgrc thc insulat ion v1luc.( ) f lhc \ t l t ( ) r u i r t r l i r rg to l r l t iu l t v i r l r rc . I h is
is achievcd in three stagcs.( a ) C l can ing
( i i l D rY rng( i i i ) Rc-varn ishinS
' l
4120
Sall contirminllion cun be rcmovetl bv wrtshing with clearr. lrcsh wlt.'r. Any grease or oil onthe windings has to hc removed using a dcgrensant liquirl such ir Armaclean. Dry the siatorwindings with low fxrwer electric heaters or lamps with plcnty rlf ventilation to alklw thedanrpncss t() cscape. Altcrnatively. the windings can be henterl by current injection from awelding set or from a special injection transfrrrmer. Be sure to keep the injccted current levelwell helow lhe mot()r's full load rating.With the windings clean and tlry. tnd if the lR lest remains high over a few hours. apply acouple of c()als of grxxl quality air-drying insulating varnish.
Starter and other motor control gear should be regularly inspcted to check and maintain thefollowing items:(a) Enclosure - Check for accumulations of dirt and rust. Any corroded parts must b€
cleaned and repainted. Examine the starter fixing tnlts and its earth bonding connec-tion - pilrticularly where high vibration is present. e.g. in the steering flat and thefrx'sle.
(b) Contactors nnd relays - Check for any signs of overheating and loose connections.Remove any tlust and grease from insulating components to prevent voltage breakdownby surface tracking. Ensure that the magnet armature of contectors movcr frccly. Removcrny dirt ()1 rust from magnet frccs which mry prevent correct closing.
(c) Contacts - Examine for excessive pitting and roughness due to burning. Copper, contacts may be sm<\thed using a fine file and copper oxide. which acts as a high
resislance. can be removed using glass-paper. DO NOT file silver alloy contacts orremove silver oxide as it acts a$ a gxrd conductor. A thin smear of electrical contactlutrrication helps to prolong the life of all contacts. When contects have to be replaced,always replace txlh fixed and rtoving conlacts in pairs.Check contact spring pressure and compare adjacent coniact sets for equal pressure.Check fnwer and control fuse contacls for signs of overheating - lutrr icate movingconlacts on fux'-hokler.
(dl Connections - Examine all pttwer and control connedions for tightness and signs ofoverheating. Check flexible leads for fraying and brittleness.
(c) Overcurrent relays - Check for proper size (relate to motor full-load current). Inspectfor dirt. greas€ and corrosion and for freedom of movement. A thorough OCR per-formance test can only be carried out by dalibrated current injection.
(f) Control op€ration - Watch the sequence of operation during a normal start-up, control- and shut-down of lhe motor. Particularly look for excessive contact sparking. Remem-her to chc'ck operation of emergency stop and auto restart functions.
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