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Post by Christopher J on Apr 24, 2005 19:09:33 GMT
Hey everybody, Can anybody provide me with any information on how the Rheostatic Brake works? All I really know about the Braking system is that the motors are used to stop the Train, how it happens, I don't have a clue, and that on the C/D Stocks it's used down to a certain speed (5-10MPH IIRC) before an EP/Friction brake takes over by air from the Mainline Pipe being pushed into Brake Cylinders along the Train, which then pushes a Brake Pad/Disk against the wheels of the Train. Could anyone in the know of how the Rheo Brake works please help me understand what happens when the brake is activated? Answers much appreciated, Christopher.
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Deleted
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Post by Deleted on Apr 24, 2005 19:15:07 GMT
A rheostatic brake is essentially a great big aircooled resistor matrix connected to the motors via the control circuitry used to increase speed. When the driver makes a call for braking, the control circuitry is reversed and the motors are connected backwards through the resistor matrix; thus, they become generators, and the electricity they generate is converted to heat in the matrix and dissipated into the atmosphere.
As the motor continues to run backwards and burn off its momentum in the resistor matrices, it eventually begins to slow down due to the combination of resistors connected across its circuit, as well as friction on the rail; eventually, the motor is spinning too slowly to pump its energy through the resistor matrices anymore, and the rheo 'fades'.
<fx: stampede of T/Ops coming to poke holes>
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Post by piccadillypilot on Apr 24, 2005 19:26:56 GMT
<fx: stampede of Motormen coming to poke holes> As well as passing through the resistors the back EMF* is used to drive the opposite motor on that bogie in reverse thereby increasing the braking effect. The sequence is that the driver selects a braking position. The air brake applies on all cars (hopefully ;D ) when it has proved to be working the Rheo brake cuts in and the air brake drops out on the motor cars. When speed has reduced to a level where the rheo is no longer effective the air brake reapplies and the rheo drops out. Back EMF = Electro Motive Force, the current generated by a motor when being turned mechanically.
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solidbond
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'Give me 118 reasons for an Audible Warning on a C Stock'
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Post by solidbond on Apr 24, 2005 20:45:00 GMT
In addition to this, the air brake will continue to apply on the trailer cars as needed to provide the rate of braking demanded by the position of the TBC, taking into account various factors including speed and weight of the whole train as well as weight of individual cars
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Post by Deleted on Apr 24, 2005 23:03:06 GMT
I don't have a clue, and that on the C/D Stocks it's used down to a certain speed (5-10MPH IIRC) before an EP/Friction brake takes over by air from the Mainline Pipe being pushed into Brake Cylinders along the Train, which then pushes a Brake Pad/Disk against the wheels of the Train. Rheo braking uses the fat that a motor when turned becomes a generator or dynamo. A load is placed across the output of the traction motor, which has the effect of slowing down the motor, and therefore the wheels of all motor cars. Rheo braking is provided on all motor cars. When the TBC is placed in a service brake position (1 - 4), the westcode brake will apply on all car, in addition the rheo brake will apply on the motor cars. When the rheo brake has proved electrically to the train that it is applying the westcode on the motor cars is gradually released as the rheo brake takes over the braking on that car. When the speed of the train reaches approx 10 mph, the rheo barke braking effort fades and the westcode brake applies to bring the train to a stand. The rheo brake will not operate if braking is iniated when the speed of the train is less than 21mph nor will it operate at any speed above 50 mph. If the rheo brake were to fail, the westcode would still operate fully. The advantages of the rheo barke are that it reduces wear and tear on the brake blocks and the wheels, it also reduces brake dust emission and therefore is environmentally friendly. While we are here, here is a bit about westcode brakes: The westcode braking system is an electrically controlled air brake system and completely fail safe. The train op controls the westcode braking system by placing the TBC to one of the braking positions (service 1 - 4). Contacts now make beneath the TBC which sends 50 v dc along wires running the length of the train and on every car a feed is taken to a local brake feed to apply or release the brake on that car. On every car a tapping is taken from the main line air pipe and air passes through to the following pieces of equipment: The variable load valve which will increase the air supplied to the 7 step relay to vary the brake pressure depending on the loading. The Non return valve prevents loss of braking due to loss of main line air. The brake isolating cock is operated to isolate the brakes on that car (a bleed hole allows air to drain from the brakes slowly). The brake supply resevoir stores sufficient air supply for 10 emergeny applications of the brake. The seven Step relay controls the amount of main line air to the brake cylinders. The brake release valve if operated allows air to drain from the brakes ralidly. Dump valves, which are operated rom the CDU in the cab, dump air from the brakes on that particular car. The brake cylinders actually applies the brake shoe against the wheels. Finally the brake cylinder gauge enables a reading of the brake cylinder pressure. All of the above is related to a 73ts, however it is much the same as a D stock, which a few excpetions.... If you want to know about Spring Applied Parking Brakes, let me know!
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Post by Deleted on Apr 25, 2005 1:40:21 GMT
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Post by Deleted on Apr 25, 2005 22:08:39 GMT
Bit of both! I know about it, but took a lot of it from the stock book, so i got the wording right!
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Post by Deleted on Apr 26, 2005 0:20:46 GMT
Bit of both! I know about it, but took a lot of it from the stock book, so i got the wording right! Taking it out of the stock book is CHEATING Chris!! ;D A very accurate description, but the westcode/rheo system as fitted to '73' and 'D' stock has one major design fault. If emergency braking is suddenly needed, like when running into a station, the rheo instantly drops-out... leaving you with NO brake at all on the motor cars for half-a-second or so. OK it doesn't sound that bad I know, but when somebody falls onto the track in front of you and you drop the deadman, you want the train to stop quickly without leaping forward first. This happened to me at Picc Circus EB, and the woman lived because 2 blokes were quick and grabbed her out of the way just in time. Not an experience I'd like to repeat. One reason I'm happy to drive 'A' stock now, but does this still happen on more modern stocks?? Pete
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Post by piccadillypilot on Apr 26, 2005 8:24:03 GMT
Taking it out of the stock book is CHEATING Chris!! ;D Yeah, especially when some of us can do it from memory (despite not being anywhere near them for mumble mumble years). ;D ;D
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Post by Deleted on Apr 26, 2005 9:40:59 GMT
A very accurate description, but the westcode/rheo system as fitted to '73' and 'D' stock has one major design fault. If emergency braking is suddenly needed, like when running into a station, the rheo instantly drops-out... leaving you with NO brake at all on the motor cars for half-a-second or so. Yes that is annoying with the 73ts or the D stock. The train kangeroos forward aas ell, which doesnt exactly help if you want to stop rather on the quickly side! When i was getting some experience on the picc, the I/O reccomended that it was best to keep the train in a service 4 application for as long as possible, as you then get the rheo to help slow you down. IIRC the 95ts jumps forward as well, as when you use EMCY on them, you only get a straight air brake, as you do with a 73ts. On a C stock, you should always go to EMCY as opposed to dropping it, as if you use the position on the TBC, you will get a full application of westinghoue along with the discharge of train line air. Yeah, especially when some of us can do it from memory (despite not being anywhere near them for mumble mumble years). ;D ;D Thats probably because i have only started to learn about stock since the begining of the year.
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Post by piccadillypilot on Apr 26, 2005 10:40:47 GMT
Thats probably because i have only started to learn about stock since the begining of the year. 'S OK, we all started from zero. IIRC when I joined my knowledge was limited to the fact that trains ran on rails, and that was about it. ;D
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Post by Christopher J on Apr 26, 2005 13:10:09 GMT
IIRC the 95ts jumps forward as well, as when you use EMCY on them I can't talk for the 95's, but I can for the 96's. (Technically the same really) When you drop the handle on a 96 the Train jumps forward and the brake progresses on as more air is let into the Brake Cylinders. When I had my Cabride on the Jubilee, the I/O showed me what it feels like to have the handle dropped, he then showed me what it feels like to bring the handle right back to full Rheostatic braking, and believe it or not a full Rheo application seemed to be more effective and quicker to apply than dropping the handle and letting the air brake do all the work!
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Post by Deleted on Apr 26, 2005 23:42:28 GMT
When I had my Cabride on the Jubilee, the I/O showed me what it feels like to have the handle dropped, he then showed me what it feels like to bring the handle right back to full Rheostatic braking, and believe it or not a full Rheo application seemed to be more effective and quicker to apply than dropping the handle and letting the air brake do all the work! In most cases full brake is more effective than EMCY, especially on the newer stock.
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Post by Dmitri on Apr 27, 2005 11:03:08 GMT
When I had my Cabride on the Jubilee, the I/O showed me what it feels like to have the handle dropped, he then showed me what it feels like to bring the handle right back to full Rheostatic braking, and believe it or not a full Rheo application seemed to be more effective and quicker to apply than dropping the handle and letting the air brake do all the work! A bit of international insight . On xUSSR metros, primary method of braking is Rheostatic, as it is really quicker to apply; if Rheo is ineffective on a car for whatever reason, Substitution Valves apply air brakes instead.
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Post by igelkotten on May 4, 2005 22:43:31 GMT
And I can only concur with Dmitri: On all Stockholm rolling stock, rheostatic braking ("elbroms", electrical brake) is the primary brake used, with the mechanical friction brake (air) only coming into action as the rheo fades out at low speed, and as a low-friction tread brake to keep the wheel surfaces clean.
On all Stockholm tube stock, a full service application is more efficient than a emergency application, since the full service also appplies full rheostatic braking. The emergency brake is there to provide failsafe functionality, and a way of braking no matter what happens to the control circuitry, electricity supply or whatever.
The speed at which the rheostatic braking fades into uselessness and friction braking takes over is very much a function of the design and construction of the equipment used to control the traction motors, but as you said, around 7-15 km/h is a commonly used threshold.
There is also regenerative braking -an old concept which has recently grown more and more popular. Basically, the electricity generated by the traction motors during electrical braking is not sent to resistor grids and burned off, but rather fed back to the traction current supply, feeeding other trains. This can give energy savings of 25% and more in ideal cases.
There are several problems with this kind of operation, though: First of all, the power supply and substations have to be able to recieve and redistribute the regenerated traction current, something that can necessitate extensive and expensive rebuilding of the power supply system. A common compromise solution is to allow regenerative braking feed only when there is another user -i e another train- on the same power feed subsection, and otherwise lettign the electricity be burnt off in the rheostatic grids.
Another problem is that the regen brake is dependant on a continous electric circuit with the third rail, with loss of contact leadign to a loss of braking. This is normally circumvented by letting the trains swirch over to rheostatic braking automatically if they lose contact with the third rail, but it can lead to momentary jerking and loss of brake power.
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Post by trainopd78 on May 5, 2005 10:11:57 GMT
if Rheo is ineffective on a car for whatever reason, Substitution Valves apply air brakes instead. On the District, we always have the "District Flick" of course.
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