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Friday, 5 July 2013

Dynamic Brakes and Dynamic Braking on Diesel Electric Locomotives




Dynamic Brakes and Dynamic Braking on Diesel Electric Locomotives
by
Gyan C A Fernando

The dynamic brake grids and blower cowling on the short hood of No 628 Class M2 “Kankesanthurai” of the Sri Lankan Railway


Diesel Electric hauled trains generally have three types of brakes.
The first is the Train Brake which can be of the air type or the vacuum type. The second is the Locomotive Brake or the Independent Brake, usually an air brake. Then there is the locomotive Dynamic Brake. (There is also a locomotive hand brake which is used only when the loco has been shut down)
This article is about Dynamic Brakes on Diesel Electric locomotives in general and with reference to Sri Lankan Railway locos.

Above: Warning on control panel (Class M6, No 797)


Definition

McGraw-Hill Science & Technology Dictionary:
Dynamic braking (mechanics): A technique of electric braking in which the retarding force is supplied by the same machine that originally was the driving motor.


Basic Principles
Dynamic braking is the use of the electric traction motors of a locomotive as generators when slowing the locomotive. Dynamic braking takes advantage of the fact that an electrical motor and a generator are essentially the same with different electrical connections.

The principle behind Dynamic Brakes is very simple. If you apply an electric current to a motor, the motor turns. If you turn the motor, then it acts as a generator and produces electricity.
To turn a motor, one needs to overcome the resistance offered by the magnetic field. In other words, kinetic energy, when applied to a motor dissipates the energy, offers resistance to rotation and generates electricity.
This principle is elegantly exploited in the Dynamic Braking system of Diesel Electric locomotives.
During dynamic braking the traction motors which are now acting as generators are connected to the braking grids (large resistors), which put a large load on the electrical circuit.
When a generator circuit is loaded down with resistance it causes the generators (motors) to slow their rotation. By varying the amount of excitation in the traction motor fields and the amount of resistance imposed on the circuit by the resistor grids, the traction motors can be slowed down.
The essential components of Dynamic Brakes are the traction motors themselves, switch gear to switch the field currents and connect the grids and the grids to dissipate the generated electricity as heat.
An additional component is a large blower fan to dissipate the heat from the grids.
Dynamic braking takes advantage of the fact that the traction motor armatures are always rotating when the locomotive is in motion and that a motor can be made to act as a generator by separately exciting the field winding. When dynamic braking is utilized, the traction control circuits are configured as follows:
•           The field winding of each traction motor is connected to the main generator.
•           The armature of each traction motor is connected across a forced-air cooled resistance grid (the dynamic braking grid).
•           The prime mover RPM is increased (throttled up) and the main generator field is excited, causing a corresponding excitation of the traction motor fields. (This is how the dynamic brakes are increased or decreased.)


The Pros and Cons
The standard air brakes or the vacuum brakes, although efficient, extract a heavy toll because of the metal on metal contact. There is heavy wear and tear on the brake shoes, the wheels and also on the rails.
With Dynamic braking there is no metal on metal resistance and therefore there is no wear and tear as such.
However, Dynamic Brakes used on their own are not efficient. For example they do not work well at high speeds or in emergency situations. They also do not work well at low speeds. There is also the 10 seconds delay in applying them.
Where they are most useful is on descents, as on the 1 in 44 grades in Sri Lanka..

In Practice
In practice; the application of dynamic brakes needs a fair bit of forethought even if it is on a Microsoft Train Simulator!.
It is definitely not an emergency brake. There is also the 10 second delay before actually applying the brake after moving the throttle to Neutral. The Dynamic Brake won’t engage unless the throttle is in the Neutral position. Once applied, in most situations a touch of the train brakes is also required, the so-called “Blended Braking”


The Load Meter, an ammeter, which normally indicates the current fed into the motors, indicates the dynamic braking effort when in braking mode but with the needle moving in the opposite direction.
  
The Obvious and Audible Features

The most obvious features of a dynamic brake equipped loco are the grids. These can vary from rather subtle “Elephant Ears” on EMD G12 Class M2 locos of Sri Lanka or can be huge and imposing as on American diesel-electrics.
The grids can be found on either the long or the short hoods.
On some locomotives, such as the Hitachi Class M5, they may not be very obvious visually.
At night, the glow from the grids can be visible and there is a peculiar acrid smell associated with them.
The other feature is an audible feature. One cannot miss the whine of the blower fan of a descending heavy train. This is particularly noticeable on the Class M6 locos of Sri Lanka,.

A General Electric unit with a massive dynamic brake grid on the long hood.
 Carmaguey, Cuba. 16th March 2006
The Use of Dynamic Brakes
As mentioned above, dynamic brakes are particularly useful on descents.
Some American railroad companies which only operated on flat terrain, ordered locos without dynamic brakes in the distant past. However, with conglomeration of railroads and to provide more flexibility of locomotive rosters, most US locomotives now have dynamic brakes as standard equipment.
In reality, dynamic brakes are used in conjunction with the air or the vacuum brakes and this is known as “Blended Braking”.

Sri Lankan Diesel Electrics
As far as I understand, of the Sri Lankan Diesel Electrics, the Class M2 locos were all originally equipped with dynamic brakes but my recent research indicate that these are no longer functional on some except on the Class M2c locomotives (No’s 626 “Montreal” and 627 “Vancouver”) specially built for the Up Country line. This line has a maximum gradient of 1:44 in places.
All the class M2 locos still sport the grids and blower fans whether functional or not.
As far as the other locos go the situation changes from day to day. In general, diesel electrics operating on the Up Country gradients have workable dynamic brakes. If not, they are, hopefully, rostered elsewhere.

The Importance of Dynamic Braking
The importance of dynamic braking is illustrated in this accident report involving an American Union Pacific train


Railroad Accident Report
Derailment of Union Pacific Railroad Unit
Freight Train 6205 West Near Kelso, California
January 12, 1997
NTSB Number RAR-98/01
NTIS Number PB98-916301
PDF Document (533K)

Synopsis: On January 12, 1997, about 11:52 a.m. Pacific standard time, the Union Pacific Railroad unit freight train 6205 west derailed 68 cars on the Union Pacific Railroad Los Angeles Subdivision, milepost 238.7, near Kelso, California. The train consisted of 3 locomotive units and 75 loaded covered hopper cars.
While descending Cima Hill, the engineer inadvertently activated the multiple-unit engine shutdown switch, which shut down all the locomotive unit diesel engines and eliminated the train's dynamic braking capability.
The train rapidly accelerated beyond the 20-mph authorized speed limit despite the engineer's efforts to increase the train's air braking, which the engineer placed in emergency 1 minute and 13 seconds after dynamic braking loss.
The train's consist weight was listed at an average of 13 tons per car less than the train actually weighed.
The train eventually reached a speed of 72 mph and derailed 68 of its 75 cars while exiting a siding near Kelso, California. No fatalities, injuries, fires, or hazardous materials releases resulted from the accident. The total damage cost was $4,079,152.
(This report is in the public domain)



It is clear that once the dynamic brakes failed, the train brakes were insufficient to brake the train even in an emergency application. It has to be borne in mind that this was a heavy freight with 75 loaded hopper cars.






References and Further Reading


2.Extended Range Dynamic Braking http://www.trainweb.org/trainmaster/page6.html


Written on the 30th of April, 2012. All photographs are by the author and copyright
First published in the Lanka Railway Digest

Copyright: Gyan C. A. Fernando.

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