brakes and jacks

8/9/2019 Brakes and Jacks

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Brakes and JacksBrakes and Jacks


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Importance ofbrakeImportance ofbrake

A large generator running at rated speed and thenA large generator running at rated speed and thentripped off the busbar either prior to routine shuttripped off the busbar either prior to routine shut

down or due to an emergency may take half an hourdown or due to an emergency may take half an houror more to come to rest unless a braking force,or more to come to rest unless a braking force,additional to friction and windage is applied.additional to friction and windage is applied.

During the last minute or so before the rotor finallyDuring the last minute or so before the rotor finally

comes to rest the rotor and thus the bearing thrustcomes to rest the rotor and thus the bearing thrustface is crawling round, resulting in possible wiping offace is crawling round, resulting in possible wiping ofthe white metal of the bearing pads.the white metal of the bearing pads.


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If, due to debris, the turbine gates do not closeIf, due to debris, the turbine gates do not closecompletely but allow a leakage of water correspondingcompletely but allow a leakage of water correspondingto, say, 1 per cent of full load torque, then the rotorto, say, 1 per cent of full load torque, then the rotormay continue to crawl for an indefinite period.may continue to crawl for an indefinite period.

For these reasons hydroelectric generators are alwaysFor these reasons hydroelectric generators are alwaysprovided with friction brakes usually of the typeprovided with friction brakes usually of the typeshown in Fig.shown in Fig.


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Brake PadsBrake Pads

HH -- Circular Brake TrackCircular Brake TrackTT -- Square ShoeSquare Shoe

RR -- Asbestos Base Friction MaterialAsbestos Base Friction Material


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Each unit consists of a pistonEach unit consists of a piston

and cylinder which is suppliedand cylinder which is supplied

with compressed air at awith compressed air at a

pressure of 6 to 7 kg/cm2. Thepressure of 6 to 7 kg/cm2. The

top of the piston carries a squaretop of the piston carries a square

shoe T faced with a disc ofshoe T faced with a disc ofasbestosasbestos--base friction material R.base friction material R.

Admission of air to the cylinder forces the friction discAdmission of air to the cylinder forces the friction discagainst the circular brake trackH, which is made inagainst the circular brake trackH, which is made insegments both for ease of manufacture and also to allowsegments both for ease of manufacture and also to allowcircumferential expansion, since with prolonged brakingcircumferential expansion, since with prolonged brakingthe temperature of the track may rise to 300the temperature of the track may rise to 300C or more.C or more.


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Brake Jacks & Piping


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Brake Dust Exhaust


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With high speed generators having relatively smallWith high speed generators having relatively smalldiameter rotors it is difficult to dissipate the energydiameter rotors it is difficult to dissipate the energy

involved in the restricted area of the brake trackinvolved in the restricted area of the brake trackwithout the danger of brake track distortion.without the danger of brake track distortion.

In these cases the brake track consists of a steelIn these cases the brake track consists of a steelcylinder attached to the underside of the rotor. Acylinder attached to the underside of the rotor. Anumber of caliper brakes are fitted in which eachnumber of caliper brakes are fitted in which eachbrake applies the friction torque to both sides of thebrake applies the friction torque to both sides of thecylinder thus producing heat on both sides of thecylinder thus producing heat on both sides of thecylinder and avoiding distortion.cylinder and avoiding distortion.

The axial length of the cylinder can obviously beThe axial length of the cylinder can obviously bemade to suit the amount of energy to be dissipated.made to suit the amount of energy to be dissipated.


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An important point in brake design is the speed atAn important point in brake design is the speed atwhich the brake should be applied during the runwhich the brake should be applied during the run--down.down.

To check this, calculations were made on a 100 MVA,To check this, calculations were made on a 100 MVA,428 rpm generator and the results are shown in Fig.428 rpm generator and the results are shown in Fig.

The energy to be dissipated from any given speed toThe energy to be dissipated from any given speed tostandstill was calculated and this was taken as a directstandstill was calculated and this was taken as a directmeasure of the relative brake wear since actual wear ismeasure of the relative brake wear since actual wear is

difficult to predict.difficult to predict. In determining the braking effect, in all cases theIn determining the braking effect, in all cases the

retarding torque of the friction and windage losses ofretarding torque of the friction and windage losses ofthe rotor and bearings losses were included.the rotor and bearings losses were included.

Braking SpeedBraking Speed


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It can be seen from Fig.It can be seen from Fig. curve 1that the brake wear with thebrakes applied at 100 per centspeed is reduced by more, than70 per cent with the brakesapplied at 50 per cent speed andby more than 90 per cent with

application at 25 per cent speed. This shows that with the brakes

applied at 100 per cent speedthe stopping time was 7.5

minutes compared to about 15and 21 minutes, with brakeapplication at 50 per cent and 25per cent speed respectively.

1. Friction Braking only2. Friction Braking and Dynamic Braking(75% rated armature current)

3. Friction Braking and Dynamic Braking

(100% rated armature current)


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Dynamic BrakingDynamic Braking

In this, after the generator has been tripped from theIn this, after the generator has been tripped from thebars, the three line terminals are shortbars, the three line terminals are short--circuited and thecircuited and the

rotor winding is excited from a separate DC supply.rotor winding is excited from a separate DC supply. The generator excitation system cannot be used for thisThe generator excitation system cannot be used for this

purpose since the excitation voltage decreases rapidlypurpose since the excitation voltage decreases rapidlywith falling speed.with falling speed.

With dynamic braking, the field being set to give ratedWith dynamic braking, the field being set to give ratedcurrent, and friction brakes applied at 25 per cent speed.current, and friction brakes applied at 25 per cent speed.


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Fig. shows that theFig. shows that thetemperature rise of thetemperature rise of the

winding over and abovewinding over and abovethat existing at the momentthat existing at the momentof shut down, is 48of shut down, is 48C.C.

Even with a generatorEven with a generator

having a 60having a 60C rise thisC rise thisincrease would beincrease would beundesirable. With 75 perundesirable. With 75 percent rated current in thecent rated current in thearmature winding andarmature winding andfriction brakes applied atfriction brakes applied at25 per cent speed the25 per cent speed thetemperature rise is reducedtemperature rise is reducedto 36to 36C.C.

1. Friction Braking only

2. Friction Braking and Dynamic Braking(75% rated armature current)

3. Friction Braking and Dynamic Braking(100% rated armature current)


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Brake TransformerBrake Transformer


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The braking time is about 11The braking time is about 11minutes, so that allowing forminutes, so that allowing forthe assumption in thethe assumption in the

calculations that there is nocalculations that there is nocooling by the ventilating aircooling by the ventilating airduring runduring run--down, the use ofdown, the use ofdynamic braking with 75 perdynamic braking with 75 percent armature current pluscent armature current plus

friction braking from 25 perfriction braking from 25 percent speed would becent speed would besatisfactory.satisfactory.

Stopping times with Braking.


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Brake PlatesBrake Plates

The brake plates, on the underside of the rotor, areThe brake plates, on the underside of the rotor, arepreferably made of a steel with about 1 per centpreferably made of a steel with about 1 per centmanganese addition.manganese addition.

Experience shows that this material produces lessExperience shows that this material produces lessdust and is less liable to heat distortion than milddust and is less liable to heat distortion than mildsteel.steel.

The plates should be bolted to the rotor bodyThe plates should be bolted to the rotor bodythrough elongated slots to allow peripheral expansionthrough elongated slots to allow peripheral expansiondue to the heat produced by braking.due to the heat produced by braking.

It is also important that each plate should span anIt is also important that each plate should span anintegral number of spider arms so that each end of theintegral number of spider arms so that each end of theplate is firmly supported. Failure to do this may leadplate is firmly supported. Failure to do this may leadto an undesirable clatter when the brakes are applied.to an undesirable clatter when the brakes are applied.


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JacksJacks During erection and maintenance it is necessary to liftDuring erection and maintenance it is necessary to lift

the rotor, so that the thrust face is clear of the bearingthe rotor, so that the thrust face is clear of the bearingpads to avoid any damage to latter.pads to avoid any damage to latter.

In some cases it may be necessary to lift the rotorIn some cases it may be necessary to lift the rotorbefore starting in order to establish an oil film betweenbefore starting in order to establish an oil film between

the thrust bearing surfaces.the thrust bearing surfaces. This lifting has in many cases in the past beenThis lifting has in many cases in the past been

accomplished by providing a number of hydraulic jacksaccomplished by providing a number of hydraulic jacksarranged in between the brake assemblies.arranged in between the brake assemblies.

Depending on the weight of the rotor, size and numberDepending on the weight of the rotor, size and number

of jacks, the oil supply may be at pressures up to 100of jacks, the oil supply may be at pressures up to 100kg/cm2.kg/cm2.

After jacking up, the oil supply to the jacks is cutAfter jacking up, the oil supply to the jacks is cut--offoffand the rotor allowed to rest on suitable packingand the rotor allowed to rest on suitable packing

pieces.pieces.


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It may be noted that theIt may be noted that thestrippers in Fig. preventstrippers in Fig. prevent

the bearing pads beingthe bearing pads beinglifted due to suction andlifted due to suction andthus displaced during thethus displaced during theraising of the rotor.raising of the rotor.


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A better and widely used formA better and widely used formof jack is shown in Fig. In thisof jack is shown in Fig. In this

the brake cylinder for thethe brake cylinder for thecompressed air is also madecompressed air is also madesuitable for the high pressuresuitable for the high pressureoil for jacking.oil for jacking.

In addition an outer cylinderIn addition an outer cylinderK is split on a helix T so that after the rotor is raisedK is split on a helix T so that after the rotor is raisedthe cylinder is rotated on the helix until it is again inthe cylinder is rotated on the helix until it is again incontact with the undercontact with the under--surface of the flange J.surface of the flange J.

A radial dowel is then fitted in a hole in the inner andA radial dowel is then fitted in a hole in the inner andouter cylinders to lock the outer cylinder in positionouter cylinders to lock the outer cylinder in positionon the helix. The oil pressure is then cuton the helix. The oil pressure is then cut--off. In alloff. In allrespects this arrangement is superior to the use ofrespects this arrangement is superior to the use ofseparate jacks and packers.separate jacks and packers.


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brakes and jacks

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