I was recently asked by a friend of mine – Buntu Khumalo – how one engages Regenerative Brakes on a train, so I thought I would write a whole blog post about Regenerative/Dynamic Braking. WARNING: We are going to get down and dirty – this is a very technical article!
What are Dynamic Brakes? Basically, it is a system (on diesel or electric trains) that uses the train’s own momentum to stop the train. The traction motors are changed into generators and turn the electric power they produce into heat and dissipate it. Contrary to popular belief, the motors are NOT put into reverse!
Normally, the traction motors are DC motors. The output of the main generator is applied to the traction motor armatures and fields and they “motorvate”. However, when the Driver changes to dynamic braking, heavy duty contacts “re-wire” the motors. The ends of the field windings are connected across the main generator output so that the main generator is applying power only to the fields. The ends of the armature are connected across iron resistance grids. As the train moves down the track, wheels turn the traction motor armature. Since the armature is turning in a magnetic field, created by the field windings powered by the main generator, the armature generates electricity. This electricity flows through cables up to the resistance grids, the grids get hot using up the electricity. Large blower fans cool the grids to keep them from melting. In principle, the whole thing works similarly for AC drives but the electric setup is different.
It takes a lot of power to turn the armatures to generate all the electricity being thrown away as heat. This power comes from the rolling train thus retarding it’s movement. Because the armatures must turn at a minimum speed to generate power, you cannot stop a train with dynamic brakes. You can only control its speed or slow it down. As you near 20km/h, the armatures are turning so slowly that they generate little power and braking effort drops off rapidly. At higher speeds, the amount of braking is controlled by the Driver. The Driver moves the dynamic brake lever and that in turn controls the output of the main generator which is supplying the traction motor field current. The stronger the fields the more power generated by the rotating armature so the more braking effort you get.
It sounds simple, but in reality it is a little more complicated. The Driver doesn’t really control the output of the main generator directly. The lever controls a rheostat that controls transistors that control the field of the exciter generator. The output of the exciter goes to the field of the main generator and that controls the output of the main generator, which goes into the traction motor fields so the rotating armatures can generate the electricity producing braking effort. Also, various sections of the resistance grids are switched in and out of the circuit to provide different amounts of electrical load thus different braking forces.
In an ideal world, the dynamic brake will be used as much as possible to reduce wear on brake pads (or blocks) and there are often circumstances when the dynamic brake will provide all the braking required. However, it is normal to leave a little air in the brake cylinders in case the dynamic brake switches off suddenly or fails. This reduces the time taken for air pressure to restore to the demand level when dynamic braking is lost.
About the only part of dynamic brakes you can see are the resistance grids. These have their own grid cooling blower. You can tell when any loco is in dynamic braking going down hill because these blowers suck a lot of air and whine. Sometimes, but rarely, a grid cooling fan will fail or a grid will short circuit. This results in the iron grids melting, accompanied by molten slag blowing from the unit and all sorts of arcing and sparks.
If you made it thus far – well done! But I’m going to give you a break until next time! I got the above information off of Piers Connor’s Railway Technical Pages Website and edited it a bit. In the next blog post, we will look at regenerative braking and I’ve asked Bennie for an explanation on using dynamic braking in real life situations from his Transnet days.