dynamic braking

8/10/2019 dynamic braking

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UNIT 6 DYNAMIC BRAKINGOBJECTIVES

After completion of this unit, you should be able to: Understand the principle of operation of Resistive Brake Learn a motor can run as a generator Appreciate the braking control Understand the effectiveness of Dynamic Brake Appreciate the characteristic of Dynamic Brake Appreciate the requirement of au iliary systems

STRUCTURE

!" #ntroduction: $rinciple of operation%" &otors as generators'" Braking control(" Re)calibration of system*" Dynamic braking effectiveness+" $rotection against heel sliding-" Braking effort and its characteristics." Au iliary systems: Resistor cooling and motor cooling/" 0rain brake application ith dynamic brake


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1. INTRODUCTION PRINCIPLE OF OPERATION-RESISTIVE BRAKING

0he installation of dynamic braking on diesel electric locomotives has becomequite common" By taking advantage of the traction motors ability to act as a generator,

the diesel electric locomotive offers a form of braking po er hich, ithout the use of air, can be used as a speed controlling brake on grades or a slo ing brake on level track"Use of dynamic brake lessens brake shoe and heel ear on both locomotive and train"1n long do n grades dynamic brake operation enables a train to be handled ith fe er air applications" 0his results in safer train operation, due to the locomotive and car heelsrunning cooler" #n 2ig"! the action of heels ith dynamic braking is sho n" 0hemomentum of the train turns the heels" 0his drives traction motor as generator andforces current through the braking resistors as sho n by the arro s and the resistors heatup" 0he traction motor, orking as a generator, resists the turning of the heels and tendsto stop it, so the motor is used to do the same thing as the brake shoes" #n this case the

braking resistor and motor instead of the brake shoes and heel get hot" 3ence, the

blo ers must cool them" 0he heels and brake shoes do not ear because there is norubbing" 0o have dynamic braking the heels must be turning" 0his is because generator generates only hen it is turning" 4o the dynamic braking cannot be used to hold or stoptrain" 2or this purpose air brakes are to be used"

#n 2ig"%, ho dynamic braking orks on a four motor locomotive is sho n" 0hemomentum of the train pushes the locomotive and turns the heels, hich drives themotors" 0he output of the motors is fed into the braking resistors" 0he driver controls the

braking by moving the selector handle" A load)meter sho s him ho much brakingcurrent he is getting"

2. MOTORS AS GENERATORS5e kno that D6 machine can be used as either a motor or a generator" 2ig"' 7a8

sho s the motoring connections" 6urrent is being pumped through the motor armatureand field by the generator" 0his causes the motors to turn and move the locomotive" #n2ig"' 7b8 s itches have been sho n to change the motor connections" 9o the generator

pumps current through the motor fields only" 0 o things have happened to the armature)

!" #t as cut off from the generator,%" #t as connected across the braking resistor"

9o e have separated the motor field from its armature and are pumping currentthrough the field only" #f the locomotive is moving, the heels are turning and driving thearmature" #t is connected across a resistor so that it has load" #n electrical language ehave a separately e cited generator ith a load resistance"

#n 2ig"' 7b8 e see that the field current is flo ing in the same direction as in'7a8" But the armature current is reversed in 7b8" #f ee reverse the field current '/7c8, thearmature current ill flo in the same direction as in motoring 7a8 i"e" if e change from


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motoring to braking if the field current stays the same, the braking current ill reverse" #f the field current is reversed, the braking current ill stay the same" 0here are some

points about the 2ig"' circuit to be kno n for understanding dynamic braking: )

0he more current e put through the motor field, 7 ithin certain limits8 the more

braking current e ill get"

0he lo er the braking resistance, the higher the braking current 7this resistance isfi ed hen the locomotive is built8"

'" 0he faster the armature turns, the higher is the braking current"(" 0he higher the braking current, the more braking e get"

3. BRAKING CONTROL

0he key to controlling the output of almost any generator is its field" #n this case,

it is the traction motor field" Ref" 2ig"', the main generator supplies the current to thetraction motor field" #n e citation system chapter e have seen ho the main generator output is controlled in motoring" 0he same control is used for braking, but the generator is connected to the motor field only" By controlling this e control the dynamic brakingcurrent" 0he driver does this by moving the selector handle"

2or e ample at high train speed e need a eak motor field to hold the brakingcurrent to a reasonable value" 0his calls for a small main generator output" 0he driver achieves this by moving the selector handle" At lo speed e need a strong motor field toget full braking current" 3e gets this by moving the selector handle further into the

braking sector" 0he driver can get the amount of braking he ants for any speed bymoving the selector handle"

0he capacity of braking resistors is limited by the amount of heat it can ithstand"0o prevent them from getting damaged blo er fans are used" 0he po er for driving thesefans is used from the po er generated during the application of dynamic brake" 5henthere is no braking current, the fans do not run, but as the braking increases, ith that the

braking current and heat increases and also the fan speed increases hence giving morecooling" Braking resistors usually require little attention" Dirt and ater are the biggestsource of trouble"

4. RECALIBRATION OF SYSTEMS

Assume that a train is drifting do ngrade, and that the engine)man is preparing toapply the dynamic brake moves the throttle to #DL;< position" 5ith the throttle in =idle>all po er contactors drop out and the motor)generator circuits become completely de)energi?ed"

9o the engine)man moves the selector handle from hatever motoring positionit as in, to the =off> and then to the =Big D> position" &oving the 43 to =Big D>accomplishes the follo ing:


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i8 6auses braking s itch 7B@08 to thro reconnecting the po er circuits"a8 6onnects all motor fields in series,

b8 6onnects series motor fields across main generator,c8 6onnects motor armatures across braking grids"

ii8 ;nergi?es braking relay 7B@R8 hich:a8 Raises engine speed to ( th notch,

b8 Replaces engine speed signal ith braking control signal at reference)mi er circuit"

c8 Re)calibrates control circuits as required,

iii8 4ets up braking potentiometer 7B@6$8:

2igure"* sho s a typical motor)generator circuit for a ( motor locomotive" All po er contactors are in their braking positions and the directions of current flo are

sho n" 9ote that the starting field is in the circuit ith the generator armature and motor

fields during braking" 0his is done to make the generator more stable at the very lorange of voltages used in braking" 6urrent in the starting field creates an opposite effectto that in the shunt field consequently much more e citing terminal voltages than ouldother ise be needed" 0his makes it possible to operate the e citer at output levels morenearly comparable to those in motoring"

5. DYNAMIC BRAKING EFFECTIVENES

&a imum braking e citation 7selector handle all the ay for ard8 ill givema imum braking effort for any locomotive speed" #f, for e ample, the engine)manimposes ma imum e citation at a speed of say a fe miles per hours the braking effort

ill be relatively lo because the armatures are turning slo ly"

#f, on the contrary, the train is moving faster, the higher rotational speed of thearmatures ill tend to cause a higher current to flo through them, therefore, a greater

braking effort ill follo s" #n cases in hich the tonnage is such that the train speed on agrade cannot be controlled fully ith dynamic braking, and ould tend to acceleratedespite the use of dynamic braking, the engine)man can use the air brakes on the train tocomplement the action of dynamic braking"

6. PROTECTION AGAINST !EEL SLIDING

#t is desirable to provide protection against the possibility of heel sliding" 5hent o separately e cited generators are connected in series ith each other across a loadand the speed of one is permitted to slo belo the speed of the other, the faster generator feeds current through the armature of the slo er generator" 0he latter generator thus has current flo ing through its armature from a separate source 7the faster generator8and e citation current in its field" 0his causes it to act as a motor ith a torque applied to


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it and it slo s further" ;ventually, it might possibly stop and skid the heels or evenreverse them" ;ven if heel skidding does not occur, the fact that one motor and itsconnected heels are rotating slo er than the other heels of the locomotive, ill causethe slo er heels to be dragged along, thus causing hat is best described a rollingslides air brake system" #ts function is to

prevent the locomotive>s air brakes from system" #ts function is to prevent thelocomotive>s air brakes from being automatically applied hen dynamic braking is beingused and a train line reduction is made"


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1&.SUMMARY

A diesel locomotive equipped ith dynamic brake is very useful in controllingrate of acceleration in grades and speed in level track ithout using conventional air

brake, hich leads to e cessive un anted ear of brake blocks and heels" Resistive brake ith automatic limit is very common for diesel locomotives, hich provides afacility of braking effort control through driversC selector handle" 0he braking effortachieved ith resistive brake has a typical characteristic of depending on train speed" 0he

braking grids 7resistors8 have self)cooling arrangement, hich does not involve separateenergy source" 0o cool the traction motors during their operation as generators, the enginedriven traction motor blo ers are run as higher speed by increasing the engine R$&"

11.SELF-ASSESMENT E$ERCISES

!" ; plain the principle of resistive braking"%" 5hat do you mean by braking control and ho it is achieved in diesel electric

locomotives'" ; plain the effectiveness of resistive braking"(" ; plain the characteristic of resistive braking ith necessary diagram"*" 5hat are the au iliary systems involved in dynamic brake system

dynamic braking

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