When a locomotive is going up a hill, the diesel engine is supplying power to the traction motors to propel the train. Going down the hill, the traction motors are used to slow the train. In turn, they produce excess current, that is disipated by the grids and cooling fans. Am I right so far? What is the diesel engine doing, when the dynamic brakes are being applied? Is it kicked into neutral(?), and just idling?
When going downhill in dynamic the traction motors become generators. The current they produce is routed to the dynamic brake grids and disipated as heat. Think of the grids as a large toaster. The diesel engine depending on the model locomotive is anywhere from idle to full rpm. For example:
Most SD40-2s are in idle up to aprox 5-6th position on the controller then the engine goes to 4 throttle rpm (I forget what the exact rpm number is). The engine speed increase is to furnish more air to cool traction motors the DB grids have their own cooling fans. If you reduce the DB controller say to number 3 position the diesel eng will stay reved up for about a minute before going back to idle speed.
The AC4400s will rev up to the same rpm in DB as they would for the corresponing throttle position under power, positions 1-8.
Some older EMDs will run in 4 throttle all the time you are in DB.
I’ve always been under the impression that you can vary the amount of dynamic braking - the simulators run that way, and this article by Al Krug bears that out.
Increasing the throttle increases the current in field windings of the traction motors, making them more powerful generators, thus increasing their braking power.
How that’s applied on specific locomotives I don’t have the knowledge to discuss. Only one of the locomotives we are using right now has dynamics, and it’s at the other end of the operation so I never see it.
That is what it sounded like on the Modern Marvels segment last night where they rode along in the BNSF train going over Cajon Pass.
When motoring, the traction motor field (stator) and armature (rotor) are electrically connected in series with the main generator.
When in DB, the TM field is connected directly to the main gen and the armature is connected directly to the DB grids. You control the DB by varying the TM field. On the older locomotives, the control system would just rev the engine up to the max speed it could ever need and then just regulate by varying the main generator excitation. The old GE Uboats and early Dash 7s used the engine cooling fan to cool the DB grids, too, so they revved up to notch 8 to provide enough cooling air.
In order to save some fuel, both EMD and GE went to a more complicated control scheme where they would vary the engine speed depending on how much main gen excitation was needed. GE also went to a separate DB blower ala EMD on the later Dash 7s.
If you look under a locomotive, you will see 4 cables connected to each traction motor. Two for the field and two for the armature. They are connected to contactors (GE) or rotary braking switch (EMD) so that the motors can be configured for either braking or motoring. If you didn’t need to do dynamic braking, you’d only need a single pair of cables connected to the traction motors.
Having control over the motor connections is also how CSX turns their road slugs “on and off”. Since the slug is connected in series with the mother, you wouldn’t be able to get above 30 mph or with the slug “turned on” - you run out of main generator voltage. So, they shunt (short out) the traction motor fields on the slug. With no field, there is no back EMF, and without any back EMF, the only voltage drop over the motors is due to the resistance of the armature winding, which is very low. So, nearly full generator voltage is applied to the motors on the mother, and away you go.
Thanks for the great description of HOW dynamic braking is accomplished using DC Traction motors!
Since AC motors work a little different, anybody know how DB’s work on the AC units?[?]
Not radically different, the traction control computer commands the invertor, to produce lower than the matching frequency (Hz) current for the speed the locomovtive is traveling, this causes the traction motors to become generators. The invertors will function as rectifiers as they are “4-quadrant invertors” so they generate DC using energy from the traction motors. this gets diverted to the Dynamic Brake grids just like the DC motored locomotives. The key is manipulating the input frequency to generate the strong back EMF and then resisting that power.
Not the main generator? As I recall field current in DB comes from the auxiliary. SD-40-2s and their ilk had traction motor blowers directly connected to the engine-- so the only way to get maximum cooling was to have the engine at Run-8 speed.
No doubt the traction motors still need cooling, but Geeps and SDs have a separate fan to cool the DB grids - speed of the prime mover is not a factor.
In grid cooling.
For DC traction motors:
An even more important reason for bringing the armature and field leads out separately is to change direction of rotation (i.e. reverse). That’s done by either reversing the current flow in the armature or the field.
Inertia from the train already being in motion either assisted or opposed by gravity, tends to keep the train in motion but the friction & windage losses will slow the train to a stop … eventually. The train has allot of stored energy due to its mass and to extend the life of the friction brakes, electric (Dynamic) braking was invented.
The dynamic brake handle on the control stand is connected to a resistor and a coupla switches. The voltage (24T) from this resistor is trainlined via the MU connector, and the switches, control relays to set up the configuration of the motorized contactors that connect the traction motor fields to the traction generator and the traction motor armatures to the Dynamic brake grids. Each unit in the consist uses the 24T voltage to regulate the amount of current the traction generator field induces (Thanks to the rotating Diesel & Magnetism) into the traction generator to deliver current to the traction motor fields. The Dynamic brake grid cooling fan is connected across a pre determined section of the dynamic brake grid. The speed of the Dynamic brake grid cooling fan is proportional the amount of current passing through the dynamic brake grid section the fan is connected to. Units with extended dynamic braking use contactors (big switches) to short out (Bypass) sections of the grids, in turn increasing the load on the traction motor Armatures. During the dynamic braking operation the increase in engine speed (notch 4 around 450 rpm) will increase generator output power a small amount but primarily traction generator and traction motor cooling is the design intent. Some testing was done to have the engine speed controlled by traction motor temperature.
TM fields are connected to main generator output and DB is controlled by controlling traction alternator excitation. Aux gen is always regulated to 74VDC output for control system, battery charging, cab heat, headlights, etc.
On EMD’s:
DB grid blower is powered off taps on the grids.
Diesel engine has to rev to provide cooling air to traction motors. TM blower is mechanically driven, as you have stated.
Yes… a pre determined section of the dynamic brake grid = Tap … & The Aux generator supplies power for Traction Generator Excitation
With a DC main gen and battery field excitation, yes. But, on EMDs with AC main gens, the field power comes from the D14 companion alt which is chopped by SCRs (gate signal comes from controls system) and rectified into DC.
Yes again … The Aux Gen provides power to the D14 or D18 (depending on the vintage of Choo Choo) Excitation (Rotor). The D14 or D18 stator is located on the back quarter of the Traction Generator. With an unaltered control system, all three generators (aux, D-14/18, AR10 Family) are needed to spin the traction motors.
Whew! We agree. It just took a while for us to find the words!
This is my first post here… I didn’t know how deep to go…
Welcome to the forum, and thanks for the posts.
Any one remember dealing with field loop dynamics ?