There is a way to "lock" an electric motor.
.
In normal operation the brushes magnetize a set of windings a few degress ahead of the stationary magnet and the magnetic force pulls the rotor towards the magnet. As it moves, the brushes move to the next set of windings and the process continues and the motor shaft turns.
A locking motor has a second set of brushes that energize the winding right next to the magnet and hold it in place. This second set of brushes will usually have a circuit to reduce the amperage through the windings in this position.
I have only seen these types of motors on old (very old) manufacturing and machining equipment. I doubt they would be effective in holding a train.
-Kevin
Living the dream.
Overmod xdford With AC motors, it is possible to effectively short circuit the terminals which means an infinite load on the generating ability so the motor is stalled in its tracks. This makes utterly no sense as written. A typical locomotive AC traction motor has neither permanent magnets (as in some modern HSR motors) or excited field or armature interaction, so "shorting the terminals" would produce nothing electrical for counter-EMF under any circumstances. To get the effect of dynamic at low speed the inverter synthesizes phase that essentially 'motors backward' until the locomotive comes to a standstill. Not really different from how the inverter produces the effect of 'locked-rotor torque' at very low road speed without significant motor overheating. I encourage other readers to provide more specific electrical information.
xdford
With AC motors, it is possible to effectively short circuit the terminals which means an infinite load on the generating ability so the motor is stalled in its tracks.
This makes utterly no sense as written.
A typical locomotive AC traction motor has neither permanent magnets (as in some modern HSR motors) or excited field or armature interaction, so "shorting the terminals" would produce nothing electrical for counter-EMF under any circumstances. To get the effect of dynamic at low speed the inverter synthesizes phase that essentially 'motors backward' until the locomotive comes to a standstill. Not really different from how the inverter produces the effect of 'locked-rotor torque' at very low road speed without significant motor overheating. I encourage other readers to provide more specific electrical information.
As I understand it, having trained as an Electrical Fitter in South Australia, one aspect that was used to help describe was the nature of a dynamic brake was that any electrical motor can be either a motor or generator depending on whether it was being powered by a generator or turned by another motor.
I believe that an AC motor can be stopped ala dynamic braking if the terminals are shorted, partly because there would be a small amount of residual magnetism which would cause a motor to work in generator mode ... weakly admittedly while the rotor is moving but none the less there would be a small voltage output which I was trying to figure out.
At the main workshops, there were two overhead cranes for lifting of locomotives both of which had a dynamic brake arrangement which dated from the mid 1920's or so but still worked well into the early 70's when I was there.
I have no direct experience with AC motors as applied to Railway Traction let alone dynamic braking in a Railway application as I put myself through College after time as an apprentice and tradesman.
However I was trying to make sense of how a motor could bring to, and I presume hold a train at a stand still without air and effectively loading the motor by effectively shorting it or at least very heavily loading it to me anyway seemed a logical way of stopping it using what I had learned.
Your explanation is very plausible and unless I am mistaken (nothing unusual for me) there would be no dissipation of heat through resistors etc and as you note where you say " To get the effect of dynamic...) rather than term it directly dynamic braking.
In effect you seem to be saying it is akin to backing a manual car, then placing it in first and easing out the clutch to bring it to a stop and (in the case of the car) starting to move forward... not a good practice but people have been known to do it!
As you also point out, the heavier AC motors should be able to take this without significant overheating but as you describe it, it seems to be a form of electrical brake rather than a dynamic braking in the conventional sense... but maybe I have misinterpreted you as well!
Anyway I too am looking forward to hearing more specific information...
Regards from Australia
Trevor
Dynamic brakes on trains perform the same basic function as compression brakes (Jake Brakes) on class 8 trucks.
Both of these devices use the forward motion of the vehicle to create drag on the drive axles, thus braking the vehicle.
How they do this is 100% different. A compression brake releases the potential energy created by the compression stroke of an internal combustion engine. The dynamic brake turns the drive motor into a generator and then creates a high amperage load for that circuit.
There are other devices that also do the same thing, hydraulic driveline retarders and output shaft loading devices on automatic transmissions are two.
The regenerative effect of hybrid vehicles also works as braking.
Since these types of brakes use the forward motion of the vehicle as the energy source for braking, the effectiveness of these devices is increased with:
1) Heavier loads
2) Downhill grades
3) Higher speeds
Since the relative weight of a passenger train is much lower, and they lose speed quickly, the effectiveness of a dynamic brake is reduced. The only place it would be of any usefulness would be if it operated on a continuous downgrade.
I believe that is why they were used less frequently on passenger locomotives.
xdfordWith AC motors, it is possible to effectively short circuit the terminals which means an infinite load on the generating ability so the motor is stalled in its tracks.
Great video on airbrakes and dynamic brakes used going down Southern Railway's Saluda grade!
https://www.youtube.com/watch?v=9IABM8UPplY
jeffhergert With the modern AC locomotives, it is possible to stop some trains with dynamics alone. It will depend on how heavy the train is and/or where it is. Jeff
With the modern AC locomotives, it is possible to stop some trains with dynamics alone. It will depend on how heavy the train is and/or where it is.
Jeff
Hello Jeff,
Dynamic Braking has been used in industrial applications for perhaps even longer than they have been used on trains and used to stall the motors
I am not familiar with the modern version of dynamics as such but wonder if the circuit is loaded up at the very low end to the point that the motor is stalled. Given the capacity of AC motors to withstand very high current outputs and no need to maintain a minimum continuous speed as do their DC counterparts, I can see where that would work.
Can anyone add to this to clarify the facts of the matter please?
Cheers and Thanks in Anticipation from Australia
Hi Dean,
No need to apologise and actually I can understand the confusion.
I can remember seeing the same Alco units (DL500's - a 6 axle version of an FPA-4 but 3 years earlier) come into stations with the dynamics still on but the cooling fan going slower because they were driven by the power from the motors. The drivers (engineers to you) were possibly trying to extract maximum braking effort and conserving air as the reservoirs were not that big. It would be easy to think the dynamics brought the train to a stop from casual observation.
The whole control system dropped out if the air pressure dropped below 42psi. A colleague of mine was sent "up the hill" to clean up a generator that had flashed over and it was heavily raining. The driver stopped it at Mt Lofty as he did not think he could get the load "home" with 2 units. My mate cleaned up the generator problem, then rode the train back down and noted the train pipe air pressure along with the dynamics did not get ABOVE about 50 psi after starting the down grade and hovered very close to 44/45psi for the most part of the trip... it was one of his hairier experiences!
Sorry, Trevor, please chalk it up to Old Age (I haven't seen the Trains magazine article in more than 30 years), typing things in the middle of the night, when my internal "Logic-checker" isn't working (probably disengaged entirely!), and rampant nostalgia (Geeps and Esdees were my favorite locos in the '50s, with EMD and Alco F-units coming in third).
Naturally, dynamic braking by turning traction motors into generators only works when the wheels are turning--and at a pretty good speed. I think probably the M&K Branch Esdees used dynamic braking to retard their trains, reserving the air to stop, when necessary.
I think that article was a 2-part job and the writer managed to paint a word picture of 3-unit F's helping 4-unit road engines (or was it the other way round?), having to stop on the grade going up, to blow the traction motors cool, and leaving traction sand all over the place going up and down, still appeals to me. I could practically smell the smoking brake shoes and Diesel smoke, with snowflakes dancing around the windshields and caboose windows!
Deano
OT Dean When I saw the headline, I immediately thought of my favorite of old, the B&O's helper operation out of M&K junction. In one of the articles, as I recall, it was said that the actual M&K Branch was usually serviced by SD7s (or Nines), equipped with dynamic braking that could bring a 60-car freight train to a halt on the M&K's grades (I don't remember if this was the same 2% as the main line), without air braking ... Deano
When I saw the headline, I immediately thought of my favorite of old, the B&O's helper operation out of M&K junction. In one of the articles, as I recall, it was said that the actual M&K Branch was usually serviced by SD7s (or Nines), equipped with dynamic braking that could bring a 60-car freight train to a halt on the M&K's grades (I don't remember if this was the same 2% as the main line), without air braking ...
Hello Dean,
While not being able to put a time line on it, Extended Range Dynamic Brakes did not come in till around the late sixties as I recall.
I doubt it would have been possible to bring a train back then to a halt on a grade with earlier diesels (such as an SD7 or 9) using Dynamic Brakes alone as it cannot be physically done now. Dynamics require a voltage to be generated by the motors being pushed by the train and once the effective generating capacity of those motors goes because of too low a speed, so does the braking effort.
My experience with Alcos in Australia was that DB was OK down to around 12 mph before it became ineffective descending 2.5% grades day in day out for about 17 miles. Later units with Extended range DB bought that down to about 6-7mph but air was/is still needed to bring a train to a complete halt. Seeing we use a lot of the same technology and have done for many years, I see no reason that the US would be any different. There is a basics tutorial shown on http://trn.trains.com/railroads/abcs-of-railroading/2006/05/dynamic-braking
Going back to the original question, there were a couple of advantages to dynamic brakes. On long downgrades they would provide some or most of the braking effort and overheated wheels and shoes were not a big problem. They are also a carman's friend, since brake shoes last much longer. Changing 24 brake shoes on a 6-axle heavyweight costs in both labour and materials. CPR retrofitted dynamic brakes to the F-units that haul the Royal Canadian Pacific fleet of business cars, and they must have calculated that costly investment was easily justified by the maintenance savings. I have heard after a return trip between Calgary and Vancouver it was often questionable whether the shoes had enough life for a repeat.
When I saw the headline, I immediately thought of my favorite of old, the B&O's helper operation out of M&K junction. In one of the articles, as I recall, it was said that the actual M&K Branch was usually serviced by SD7s (or Nines), equipped with dynamic braking that could bring a 60-car freight train to a halt on the M&K's grades (I don't remember if this was the same 2% as the main line), without air braking. However, since you're interested in passenger service, I'll just throw this information out there for anyone who's interested.
Trevor, EMD did build some E-8s and E-9s with dynamic brakes. I offer the following taken from "Extra 2200 South--The Locomotive Newsmagazine":
EMD 952 E-8A, demonstrator, sold to Rock Island 643
MILW 18 E-9A, 6 E-9B
SP 1 E-8A
SP 9 E-9A
SOU 10 E-8A
UP 18 E-8A, 28 E-8B
UP 35 E-9A, 34 E-9B
FRRYKidWrong part of Montana.
My mistake
Ed
NHTX An early streamliner as you describe it would probably run about six cars long, per unit. ... Torpedo tube GP's were not a rarity so go for one!
An early streamliner as you describe it would probably run about six cars long, per unit. ... Torpedo tube GP's were not a rarity so go for one!
The train that I have in mind would be five cars. (That just happens to be what fits the track space that I have.) I also have two GP20s (with steam generator cars) and an F7A in the passenger fleet.
gmpullman If I were modeling the Montana area
If I were modeling the Montana area
Wrong part of Montana. I protolance in the southern part of Montana. (The idea is that the new BN decided after a few years to lease off the old NP lines which would form a new railroad. Something similar to the modern MRL. One still would see THe BN rainbow the new railroad also is seen as the primary.)
Dynamic braking on earlier generation diesels regardless of whether they were in North America or Australia or anywhere else in the world was a revolution on long grades but for the most part difficult to initiate above 40mph.
The traction motors could act like a large DC arc welder by flashing between the brush holders and set off ground relays but that depends on what the gearing of the loco would have been.
Locos here in Australia on Standard and Broad Gauges were usually geared for 71 mph or alternatively a max of 89 mph for a couple of loco classes (trains usually stuck to 60 mph for a variety of reasons here but that is another issue) so an early generation loco geared for 100 mph would have a higher threshold that the Dynamic could be applied but I would think not that much higher.
I cannot answer this as I have no knowledge of it but were any/many/all E units fitted with Dynamic Brakes? I am of the impression that there may have been a few from E8's and E9's from the EMD option... that may well dictate how you operate your braking.
Have fun in any case,
Cheers from Australia,
FRRYKidSo I don't know if that would fall under your newer technology definition.
Not in the geeps, more like the F40-PH.
If I were modeling the Montana area I'd go for one of these!
https://www.pwrs.ca/announcements/view.php?ID=7925
(The nice Big Sky Blue one in the middle photo)
Cheers, Ed
An early streamliner as you describe it would probably run about six cars long, per unit. The speed required to meet the schedule, the need for steam to heat or cool the cars,the ruling grade, the frequency of starts and stops, are some of the factors the dictate motive power requirements. Most passenger road switchers of the era you envision were geared for 80 mph, and since horsepower translates into speed, a rule of thumb of one unit per six cars, especially on a model railroad, is reasonable. Also, if you do find you need more HP for a train heavier than six cars, you could couple up a freight unit with dynamic brakes, and no steam generator. The restriction is it must not be between the steam generator and cars unless it is equipped with steam-pass through lines. I rode a railfan special on the Central Vermont with this configuration back in the early 1960s. Torpedo tube GP's were not a rarity so go for one!
The technology I am looking at would be early streamliners (represented by Athearn Streamliners and a Con-cor dome-obs) and the engine would be a GP7/9 with roof air tanks. So I don't know if that would fall under your newer technology definition.
Newer technology (1970 +) in passenger braking uses blended braking where both automatic air and dynamic braking are each used to their best advantage for smoother ride qualities, higher speeds (friction shoes get hot fast at speeds over 100) and more efficient stopping time with reduced wheel and brake shoe/pad wear.
This page has a good explanation of it:
http://www.railroad.net/articles/columns/shopnotes/blendedbrake/index.php
Good Luck, Ed
Passenger trains never were equal in weight to freight trains. Although the speeds of passenger trains were higher than freight trains, the lighter weight could be safely controlled by ordinary air brakes, thus no need for the extra expense of DB.
OK Forum friends, this may have been covered many times before but here goes. On a passenger train, as an example, what would the operational differences be between a unit with dynamic brakes and one without? I am looking to add an additional hood unit to my passenger engine fleet and it wouldn't have dynamic brakes based on the prototype I am looking to replicate.
As usual, thank you in advance for any assistance that can be provided.