Dynamic braking - a question

[johnmckenzie]

Hi Guys

I have a question for you - I have seen photos in books and magazines of US diesel locos with comments like "As you can see this loco isn't fitted with dynamic brakes". How can a novice like myself tell the difference?

Thanks in advance!

[Chessie8638]

First dynamic brakes are the locomotives own traction motors spinning in the opposite direction to slow and stop a train. This relieves stress and wear on brake pads of the locomotives and freight cars.

How can you spot the difference? Let's take an SD45 as an example. The Union Pacific SD45's came with dynamic brakes. As you can see from this picture it has extra fans and vents on the long hood (over the word "Union") to cool the traction motor: http://www.rr-fallenflags.org/up/up34ags.jpg

Now looking at a unit without dynamic braking. Chicago & North Western (CNW) ordered their SD45's without dynamics. So they are missing these extra fans and vents (If it had dynamic brakes they would be over the number "906"): http://www.rr-fallenflags.org/cnw/cnw0906dsa.jpg


This is the main spotting difference. All locomotives today have dynamic brakes. If a train is using it's dynamics, you can tell. The locomotives make a loud "Whining" noise: http://www.youtube.com/watch?v=cFfsJS5MMgQ

[johnmckenzie]

Thanks very much for that - I knew the principle but not how to spot the difference! Dynamic braking isn't the norm in the UK, if I remember correctly only the 50 locos of the EE class 50 built in 1968 had a (not successful) system originally (which ended up being removed) and also the 11 new class 70 locos built by GE!

[Kali]

Some electrics, too, not sure how many of the older loco classes ( 87s and 90s do, so I suspect 91s ) but some EMUs have also.

[johnmckenzie]

Yeah, I forgot those. I think all of the WCML AC locos had rheostatic braking and the 91s will do as well!

[Kali]

Not as much fun as the APT's hydrostatic brakes... not quite sure who thought those were really a great idea.

[mapitts]

First dynamic brakes are the locomotives own traction motors spinning in the opposite direction to slow and stop a train. This relieves stress and wear on brake pads of the locomotives and freight cars.

Traction motors spinning in reverse? The "noise" you hear is the tractoin motors being used as generators, and the current made by them being turned into heat in the DB resistor grids and then cooled off by the fan or fans. A traction motor is suspended on its nose in the bolster and rides on the axle of the locomotive with springs to cushion the traction motor from the axle. The traction motor has an out put shaft on one side with a pinion gear. The axle has what is called a bull gear pressed on to it. If the locomotive is running forward and the traction motor is turning in reverse, the wheels are sliding. Dynamic brakes on a DC locomotive are fairly simple. The traction alternator or generator induce a field in the traction motor to cause it to generate a DC current. A DC motor can generate a DC current. This sets up a back EMF that causes the motor to slow it self. The greater the EMF the more DB you have. That is why there are notches in the DB control. Unless you have "extended range" DB, they are only good down to about 15 MPH. AC dynamic brakes are a little different