Starting Methods of a DC Motor

Starting Methods Of A DC Motor By Kiran Daware DC Machines

Basic operational voltage equation of a DC motor is is given as E = Eb + Iaa an! hence

Ia = "E # Eb$ % a

&ow' when the motor is at rest' obviously' there is no bac( emf Eb' hence armature current will be high at starting) *his ecessive current will# ,) blow out the fuses an! may !amage the armature win!ing an!%or win!ing an!%or commutator brush arrangement) -) pro!uce very high starting torque t orque "as torque is !irectly proportional to armature current$' an! this high starting toque will pro!uce huge centrifugal force which may throw off the armature win!ings) *hus to avoi! above two !rawbac(s' starters are use! for starting of DC machine.

Starting Methods Of A DC Motor *hus' to avoi! the above !angers while starting a DC motor ' it is necessary to limit the starting current) .or that purpose' starters are use! to start a DC motor) *here are various starters li(e' / point starter' 0 point starter' &o loa! release coil starter' thyristor starter etc) *he main concept behin! every DC motor starter is' is' a!!ing eternal resistance to the armature win!ing at starting) 1. 3 Point Starter

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*he internal wiring of a 3 point starter is is as shown in the figure)

1hen motor is to be starte!' the lever is turne! gra!ually to the right) 1hen lever touches point ,' the fiel! win!ing gets !irectly connecte! across the supply' an! the armature win!ing gets win!ing gets connecte! with resistances , to 2 in series) 3ence at starting full resistance is a!!e! in series with armature) *hen as the lever is move! further' the resistance is gra!ually is cut out from the armature circuit) &ow' as the lever reaches to position 4' all the resistance is cut out from the armature circuit an! armature gets !irectly connecte! across the supply) *he electromagnet E "no voltage coil$ hol!s the lever at this position) *his electromagnet electromagnet releases the lever when there is no "or low$ supply voltage) 1hen the motor is overloa!e! beyon! a pre!efine! value' overcurrent release electromagnet D gets activate!' which short circuits electromagnet E ' an! hence releases the lever an! motor is turne! off)

2. 4 Point Starter:





*he main difference between a 3 point starter and a 4 point starter is that the no voltage coil is not connecte! connecte! in series with fiel! coil) coil) *he fiel! gets !irectly !irectly connecte! to the supply' as the lever moves touching the brass arc) *he no voltage coil "or 3ol! on coil$ is connecte! with a current limiting resistance h) *his arrangement ensures that any change of current in the shunt fiel! !oes not affect the current through hol! on coil at all) *his means that electromagnet pull of the hol!#on coil will always be sufficient so that the spring !oes not unnecessarily restore the lever to the off position) *his starter is use! where fiel! current is to be a!5uste! by means of a fiel! rheostat)

3. DC series motor starter 6 Constrction of DC series motor starters is very basic as shown in the figure) 7 start

arm is simply move! towar!s right to t o start the motor) *hus *hus at first maimum resistance is connecte! in series with the armature an! then gra!ually !ecrease! !ecrease! as the start arm moves towar!s right) *he no loa! release coil hol!s the start arm to the run position an! leaves it at no loa!)





8pee! Control Metho!s 9f DC Motor By Kiran Daware DC Machines

8pee! 9f 7 DC Motor 1e (now' bac( emf of a DC motor E motor Eb is the in!uce! emf !ue to rotation of the armature in magnetic fiel!) *hus value of the Eb can be given by the EM. equation of a DC generator )

Eb =

:;&<

%47

"where' := no) of poles' ;=flu%pole' &=spee! in rpm' <=no) of armature con!uctors' con!uctors' 7=parallel paths$ paths$ Eb can also be given as' Eb = ># Iaa thus from above equations

&=

%:;<

E 47 b

but' for a DC motor 7' : an! < are constant & ? K

%;

E b

"where' K=constant$

thus' it shows spee! is !irectly proportional proportional to bac( emf an! inversely proportional to the flu per pole)





Speed Contro! Methods Of DC Motor 8pee! Control 9f 8hunt Motor ,) .lu Control Metho!

It is seen that speed of the motor is is inversely proportional to flu) *hus by !ecreasing flu spee! can be increase! an! vice versa)

*o control the flu' a rheostat is a!!e! in series with the fiel! win!ing' as shown in the circuit !iagram) 7!!ing more resistance in series with fiel! win!ing will increase the spee!' as it will !ecrease the flu) .iel! current is relatively small an! hence I - loss is small' hence this metho! is quiet efficient) *hough spee! can be increase! by re!ucing flu with this metho!' it puts a limit to maimum maimum spee! as wea(ening of of flu beyon! the limit will a!versely affect the commutation)

-) 7rmature Control Metho!





Speed of the motor is is !irectly proportional to the bac( emf Eb an! Eb = ># Iaa) *hat is

when supply voltage > an! armature resistance a are (ept constant' spee! is !irectly proportional to armature current Ia) *hus if we a!! resistance in series with armature' Ia !ecreases an! hence spee! !ecreases) @reater the resistance in series with armature' greater the !ecrease in spee!)

/) >oltage Control Metho! 7$ Multiple voltage voltage control6 In this metho! the' the' shunt file! is connecte! connecte! to a fie! fie! eciting voltage' an! armature is supplie! with !ifferent voltages) >oltage across armature is change! with the help of a suitable switchgear) *he spee! is approimately proportional to the voltage across the armature)

B$ "ard#$eonard S%stem6

*his system is use! where very sensitive speed contro! of motor is is require! "e)g electric ecavators' elevators elevators etc)$ *he arrangement of this system is as require! in the figure besi!e)

M- is the motor whose spee! control is require!) M, may be any 7C any 7C motor or or DC motor with with constant spee!) @ is the generator !irectly !irectly couple! to M,) In this metho! the output from the generator @ is fe! to the armature of the motor







this metho!)

8pee! Control 9f 8eries Motor ,) .lu Control Metho! 7$ .iel! !ivertor !ivertor 6

7 veritable veritable resistance resistance is connecte! connecte! parallel to the series fiel! as shown in fig "a$) *his variable resistor is calle! as !ivertor' as !esire! amount of current can be !iverte! through this resistor an! hence current through fiel! coil can be !ecrease!) 3ence flu can be !ecrease! to !esire! amount an! spee! can be increase!)

B 7rmature !ivertor6





7s shown in fig "c$ "c$ fiel! coil is tappe! tappe! !ivi!ing number number of turns) *hus *hus we can select select !ifferent value of ; by selecting !ifferent number of turns)

D$ :aralleling fiel! coils6 In this metho!' several spee!s can be obtaine! by regrouping coils as shown in fig "!$)

-) >ariable esistance In 8eries 1ith 7rmature By intro!ucing resistance in series with armature' voltage across the armature can be re!uce!) 7n! hence' spee! re!uces in proportion with it)

/) 8eries#:arallel Control *his system is wi!ely use! in electric traction' where two or more mechanically couple! series motors are employe!) .or low spee!s' motors are 5oine! in series' an! for higher spee!s motors are 5oine! in parallel) 1hen in series' the motors have the same current passing through them' although voltage across each motor is !ivi!e!) 1hen in parallel' voltage across each motor is same although current gets !ivi!e!)

Starting Methods of a DC Motor

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