RSS: thanx for tutorial on AC and DC differences on the additional circuit components needed to effectively have these big beast locos work the dynamic brake systems.
I ran into a retired Westinghouse EE @ B&O museum in Baltimore recently. He explained how and why it too so long for AC systems to be applied for big diesel locomotives. DC series winding motors had the guts out of the box to get serious tonnage moving..and as the speed built up to convert the ole amp/volt ratios smoothly to keep the knuckles tight.. the AC phase motors needed a lot of years to line up components to give it too the guts for loco use. He did say AC motrs had better adhesion and lasted longer tween shopping..and too dynamic braking was actually easier to convert.Both neede the venilation and or fan powered toater grids to get rid of the heat end byproduct/
So depending on a whole bunch of factors/cost, profile.tonnage, traffic,etc......both systems still have certain advantages when ordereing new locs.
If I'm not mistaken PCCs were also field tapped making it possible to use increasing or decreasing parts of the fields to regulate the amount of motor flux. On a freight locomotive there are no field taps. The quick and easy way to tell if a motor has field taps is to count the number of leads coming out of the motor, anything more than 4 leads (2 armature-2 field) the motor likely has multiple field taps. Some freight locomotives did have field shunting but that was done externally from the motor.
RSS
Randy Stahl There is no special generator for field excitation, the AR-10 is sufficiently controllable to use for that application.
There is no special generator for field excitation, the AR-10 is sufficiently controllable to use for that application.
Randy,
I had a vague recollection that some early diesels used a separate generator, but I do appreciate your comment about the AR-10 being sufficiently controllable. The important point is that the traction motors are separately excited in dynamic braking. Interestingly enough, the dynamic braking on PCC's did not make use of separate excitation, control of braking was maintained by the starting/accelerating resistors, which could be rapidly switched in order to maintain desired braking rates. This was based on work Westinghouse had done in the late 1920's for an improved streetcar motor control.
The original Milwaukee electrics (pelicans) used a separate generator for blended excitation of the traction motors, with the generator driven by the 3,000V motor used for the blowers. The Westinghouse passenger locomotives for the Milwaukee were the ones using an axle driven generator for field excitation. The Bipolars used one of the traction motors for excitation and I'm not sure what the Little Joe's used.
Dynamic braking control on modern AC locomotives is relatively simple - adjust the inverters so that the synchronous speed of the motors is less than the rotational speed and you now have an induction generator. The tricky part, IIRC, is that the DC bus on the inverters is typically run at a constant voltage, so the effective resistance would need to be varied - which could be implemented with a fixed resistor and chopper.
- Erik
There is no special generator for field excitation, the AR-10 is sufficiently controllable to use for that application. The DC traction motor will in fact self excite and the results can be very destructive if not controlled. In fact a DC traction motor will begin to regenerate as soon as it starts to spin, this is called counter electro-motive force.
I have personally seen examples of traction motors connected wrong , by wrong I mean that that leads for the fields were connected to the outputs for the armatures, in effect one of the motor armatures was connected to the reversing contactor and one field was connected to the main gen. In normal power operation you wouldn't know because the relationship between the fields and armatures allowed the motor to operate correctly, but as soon as you placed the selector handle in dynamic brake set up things went crazy. In dynamic brakes the fields are connected in one big series loop with the AR-10 as the field exciter, in my scenario now you also have a spinning armature acting as an additional exciter (one that cannot be controlled of regulated). The switch gear on all locomotive sets up all of the high power circuits for the operation, all of the contactors close ! The only contact that remains open is the generator field. The motor armatures are all connected to the grids and the fields are connected in series with the AR-10 as soon as the handle is placed in DB setup.
Thomas Edison knew early on that the properties of his arc lighting dynamos made it possible to run them as motors as well as generators thereby discovering regenerative braking.
On the Milw electric locomotives there was a small generator on an axle to supply excitation power in many cases.
On old vehicles with DC generators they would sometimes need their fields flashed to get things started, not unlike the aux gens in locomotives!
tdmidget "DC traction motors do not automatically become generators when no power is being supplied and the shaft is being turned by an outside force. Part of the arrangement of regenerative or dynamic braking is the resetting of the control circuits to allow this to occur." Yes it does. Maybe ,most here aren't old enough to remember when motor vehicles had generators instead of alternators. The field has enough residual magnetism to enable the motor (or generator, there's no difference in DC machines) to produce current. When you installed a new generator you would have to "polarize" it with just short flow of current to produce this residual magnetism.
"DC traction motors do not automatically become generators when no power is being supplied and the shaft is being turned by an outside force. Part of the arrangement of regenerative or dynamic braking is the resetting of the control circuits to allow this to occur."
Yes it does. Maybe ,most here aren't old enough to remember when motor vehicles had generators instead of alternators. The field has enough residual magnetism to enable the motor (or generator, there's no difference in DC machines) to produce current. When you installed a new generator you would have to "polarize" it with just short flow of current to produce this residual magnetism.
While you're correct in saying that there is some residual magnetism in the motor frame (and field), there is a problem in that most DC traction motors are series wound. Series motors do not lend themselves to stable operation as generators without some form of assistance. Note that car generators are shunt wound. One of the major advancements with the Milwaukee electrification was a means for stabilizing regeneration with series motors.
If I'm not mistaken, dynamic braking in diesel locomotives was accomplished by using a special generator to excite the field windings on the traction motors.
So a locomotive rating to start a train is the force it can overcome at rest thru the conversion of fire box thermal release into the boiler to create the steam that rods the drivers to move the tonnage=pounds that now go distance at speed and this moving energy can be rated in horsepower(work that the locomotive can and is accomplishing)!
Ok/ so that's why in the diesel throttle loco control panel you have 8 notches for pulling thru the traction motors//and 8 notches to change the polarity to create the dynamic braking.
Ok/ so that's why different wheel size and alignment freight steam locomotives had a certain speed when they could develop their maximum horsepower effeciency.
solidsnowshed Terminology in footpounds means force to overcome a standing mass and put into motion=railroad. Pound feet indeed a differenf form of measurement in terms of levering an object into motion=capacity of an electrical winch and cable===torque
Terminology in footpounds means force to overcome a standing mass and put into motion=railroad.
Pound feet indeed a differenf form of measurement in terms of levering an object into motion=capacity of an electrical winch and cable===torque
A foot-pound is a unit of energy, not a measure of force. Tractive effort is a measure of force and the usual measure is in pounds (force).
If you apply some tractive effort over a distance, it will consume some energy (force x distance). How fast you can cover that distance depends on how much power you can apply (force x distance/time).
550 foot-lb/sec = 1 HP
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
Origin of Dynamic braking was letting horse go slower in front of cart, the regular braking was what is still employed today a block on a wheel.
basicly no improvement since before Roman empire ;-)
Regarding the origin of dynamic braking:
I would think that dynamic braking would have been understood and self-evident from the moment the first generator was invented. If you turn a generator and make electricity, use that electricity to do work, it takes physical force to turn the generator.
So a working generator resists rotation. If you connect it to your coasting wheels, it will slow them down. This was known from the start, but there was no way to apply it until the advent of the diesel-electric locomotive.
In the rating system days of steam locomotives...and diesels for that matter...tractive effort=the rating a locomotive would have to start standing tonnage=was in footpounds. the maximum horsepower the locomotive could generate was at a certain speed in mph.Same thing for aerial trams(ski lifts)(rope tows)..footpounds the system could genrate(load) at various speeds.
Recalling a anecdotal case/ the C&O Alleghenys (2-6-6-6) locomotive..near the peak of steam design evolution..once got mistakenly red boarded with a max tonnage no helper coal drag on a 2% WVA grade...and restarted and completed the run...forcing the head of engineering dept. to calculate that that design had exceeded its outshopped maximum footpound rating by 11%..delivering 120,000 ft/lbs for the @ grade start.
solidsnowshed every once and awhile..this ole thread gets a new practical app. posting...a good thing that takes me back to the fundamentals of thermodynamlic Law. Energy transforming itself=gravity,footpounds,motion,heat,friction,magnetic fields,combustion. Heat starts the traction process/and becomes the product to get rid of in braking. HAH!
every once and awhile..this ole thread gets a new practical app. posting...a good thing that takes me back to the fundamentals of thermodynamlic Law. Energy transforming itself=gravity,footpounds,motion,heat,friction,magnetic fields,combustion. Heat starts the traction process/and becomes the product to get rid of in braking. HAH!
Johnny
oltmannd softail86mark: Anybody notice that the new cars, the hybrids, are just now advertising regenerative/dynamic braking on their cars??? Only a hundred years or so behind the rail industry.... Regenerative braking is the whole reason for hybrid cars. The only two locomotives with on board regenerative braking energy storage are the NS 999 battery powered locomotive and the GE road locomotive prototype.
softail86mark: Anybody notice that the new cars, the hybrids, are just now advertising regenerative/dynamic braking on their cars??? Only a hundred years or so behind the rail industry....
Anybody notice that the new cars, the hybrids, are just now advertising regenerative/dynamic braking on their cars??? Only a hundred years or so behind the rail industry....
And (as previously mentioned )many electric locomotives.. Also the unsuccessful (at least so far) Railpower Green Goats...Regenerative braking is also used,on a smaller scale, to provide extra power to the auxiliary systems, on Brookvilles "Cogen" series locomotive and the "Powerhaul" units GE is building for export to the UK..
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
KERS is returning to F1 for the 2011 season. With the rules that surround its use, as a racer, I view it more as a gimmick than as a effective racing tool. At some point in time it may become developed into a drivers benefit, but at present it is just a gimmick.
GP40 Slightly Off topic but related. In 2009 the boys in Formula One Grand Pix racing are mandated to have some form of dynamic braking in their open wheel race cars. Kind of the sort that is present in today's hybrid car. They call it KERS, which stands for Kinetic Energy Recovery Systems. It is a monumental PITA for the engineers, techs and pitcrews. Besides wearing full Nomex suits for the high octane racing gas fires now have wear heavy rubber gloves because of the high voltage these systems use.
Slightly Off topic but related.
In 2009 the boys in Formula One Grand Pix racing are mandated to have some form of dynamic braking in their open wheel race cars. Kind of the sort that is present in today's hybrid car.
They call it KERS, which stands for Kinetic Energy Recovery Systems. It is a monumental PITA for the engineers, techs and pitcrews. Besides wearing full Nomex suits for the high octane racing gas fires now have wear heavy rubber gloves because of the high voltage these systems use.
Never too old to have a happy childhood!
softail86mark Anybody notice that the new cars, the hybrids, are just now advertising regenerative/dynamic braking on their cars??? Only a hundred years or so behind the rail industry....
I believe the Milwaukee road electric boxcabs had dynamic braking.They were first built in 1915 and were used all the way into the 1970's.I know when coasting downhill the electric motors would turn into generators and pump voltage back into the grid that was feeding them.Milwaukee used to say the train coasting downhill is powering the one coming up the hill.
WP Lives
Not to split hairs but from a mechanical engineering view point dynamic braking strictly refers to the system used on most diesel electric locomotives (i.e the current from the traction motors is dissipated as waste heat into the atmosphere). Any system that reuses the energy from the braking system is termed "regenerative braking".
So the "Jake Brake"(using engine compression) on a semi truck is a form of dynamic braking while a Prius employs regenerative braking....
spokyone Hobby shops many years ago (40?) had slot-car tracks. The cars had 12 vdc motors, some mounted sideways & some longitudely. The controller was a pistol grip type that had 3 wires. At full speed down the straight, when the throttle was released quickly, the car would act like it had brakes. If the third wire was not connected, the car would coast for many feet. Was this a form of dynamic braking?
Hobby shops many years ago (40?) had slot-car tracks. The cars had 12 vdc motors, some mounted sideways & some longitudely. The controller was a pistol grip type that had 3 wires. At full speed down the straight, when the throttle was released quickly, the car would act like it had brakes. If the third wire was not connected, the car would coast for many feet.
Was this a form of dynamic braking?
Yes it was. There are many uses of this effect all around us. This is what stoppes the CD in your drive when you push eject. It slows some brands of power saws when you release the trigger. It control the back tape tension on many tape recorders and VCR's. It's everywhere.
I think it is safe to assume that AC has better and more effective dynamic braking than DC. Main reason is that braking can theoretically be maintained to zero speed with AC, but below a certain speed the braking energy will be dissipated in the rotor of the induction motor - but the dissipation would be the same as what would be dissipated at the same braking effort (force) at higher speeds.
Is the dynamic braking power "better" or "more effective" on AC vs DC?
Dan
In 1853 Elisha Otis invented a "Safety Device" that prevents a "Lift" or "Hoist" from falling if the Hoist Ropes (it was rope in the 1850s) parted. He called his invention a "Safety Elevator" making it possible to move Passengers safely and the Skyscraper possible. Like any machine, properly maintained, an Elevator can not fall. Today, all brands of Elevators are safe. With 11 Billion in Sales (2007), Otis is still the world leader.
Don U. TCA 73-5735
DMUinCT erikem DMUinCT Elevators have used "Dynamic Breaking" from World War I on. I remember a comment in an electrical machinery course about the terms "dynamic braking" and "regenerative braking" predating their use in locomotives. The PCC car control systems incorporated dynamic braking - while not a diesel locomotive, it is an example of dynamic braking use prior to the FT. P.S. Don, your comments on elevator motor control technology are always welcome by me. Oh yes, the good old days. I retired 6 years ago after 47 years with the Otis Elevator Division of United Technologies. I started on elevators with walls of relays and ended up correcting problems on computer controlled elevators. Our largest DC Traction Motors (Hancock Tower Boston, World Trade Center NY, CN Tower, etc.) stand 14 feet high to the top of the Brake, and the Motor Field Ring is 6 feet in diameter, take out the Armature and you can walk through it. A little bigger than on a locomotive.
erikem DMUinCT Elevators have used "Dynamic Breaking" from World War I on. I remember a comment in an electrical machinery course about the terms "dynamic braking" and "regenerative braking" predating their use in locomotives. The PCC car control systems incorporated dynamic braking - while not a diesel locomotive, it is an example of dynamic braking use prior to the FT. P.S. Don, your comments on elevator motor control technology are always welcome by me.
DMUinCT Elevators have used "Dynamic Breaking" from World War I on.
Elevators have used "Dynamic Breaking" from World War I on.
I remember a comment in an electrical machinery course about the terms "dynamic braking" and "regenerative braking" predating their use in locomotives. The PCC car control systems incorporated dynamic braking - while not a diesel locomotive, it is an example of dynamic braking use prior to the FT.
P.S. Don, your comments on elevator motor control technology are always welcome by me.
Oh yes, the good old days. I retired 6 years ago after 47 years with the Otis Elevator Division of United Technologies. I started on elevators with walls of relays and ended up correcting problems on computer controlled elevators. Our largest DC Traction Motors (Hancock Tower Boston, World Trade Center NY, CN Tower, etc.) stand 14 feet high to the top of the Brake, and the Motor Field Ring is 6 feet in diameter, take out the Armature and you can walk through it. A little bigger than on a locomotive.
I'm OT here but I wonder just how many times over those 47 years you had to say to non-technical folks: "No, the Elevator will not plummet down if the cables snap"? LOL....
Since I started this thread, my extensive education so far in this engineering application field, needs one area of clarification: namely...... what different settings does a diesel locomotive engineer make on his control panel when cresting a grade with a heavy freight---from generating notch 8 juice into the traction motors....to ??? to engage the traction motors into dynamic braking on the downgrade. ??/////////// solidsnowshed
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