The equipment: personality versus sterility. The crew: “ man, this air conditioned cab is great compared to the old days!”
A few years ago, when air conditioned cabs begin to appear, I remember remarking to a CSX engineer, it must be great having that air conditioning. His response was, they aren’t doing that for us, they’re doing it for those 
computers.”
Always learning something new from you. Thanks for the post.
Why do AC locos not have a 10 second rule? They must be gentler somehow?
While a bit of a tangent, this kinda fits in with the dynamic brakes segue…I’ve always been surprised by the mountain roads who had a lot of high horsepower B-B locomotives. Two of the best examples are the C&O and DRG&W.
Modern AC locomotives synthesize three phases of variable-frequency AC from the DC-Link supply for the three induction-motor windings that generate the ‘rotating’ field in each motor (which both induces current and hence magnetism in the rotor bars and then applies magnetic force on the magnetized rotor). The inverter does this repeatedly through zero, with no residual current when the rotor field collapses as there can be in a powered armature like that in a DC traction motor.
“Dynamic braking” is essentially reversing the direction of the rotating field, like running the motor in reverse. This likewise takes the current through zero without concerns about induced current. There will obviously be a reason you don’t try to slam the motors from one direction to the other quickly (you will almost certainly cause a skid or worse!) but there is no real likelihood of flashover or setting something on fire with high current.
To be a bit more accurate, dynamic braking reduces the speed of the rotating field to less than the rotor speed (for induction motors). Think of it being like an automatic transmission with plain torque converters, when applying power the engine speed is higher than the converter output speed and when engine braking the engine speed s lower than the converter output.
Yes, that’s better. But remember the rotating field is still inducing magnetism in the bars and then pulling on the magnetized bars… it can be trouble visualizing why ‘slip’ is a necessary part of the result.
No slip no force right?
Sort of like a jet engine. Exhaust velocity has to be higher than the plane speed to generate thrust.
The Milwaukee Road’s mountainous trackage was largely electrified, and the electric engines had regenerative braking, essentially dynamic braking.
For an induction motor, no slip no force is correct, though the reason is that the slip induces currents to flow in the rotor which then interacts with the rotating field to generate torque. Slip is not needed in a synchronous motor. The upshot is that tractive effort can be controlled by varying the frequency generated by the inverter, increasing frequency for a constant road speed will increase tractive effort, decreasing the frequency will reduce tractive effort and when the frequency is reduced enough the motors will provide braking effort.
Merci’!
AC motors are fantastic pieces. No wonder they’re everywhere now.
Regenerative braking in an EV means you scarcely use the regular disc or drum braking systems. Works well.
The experience with the Milwaukee Road’s electrification was that the value of regenerative braking came more from the large reduction in brake wear than from the savings in energy. There was savings from reduction in accidents as well. The Milwaukee set up a stationary “dynamic braking resistor/fan” to handle the regenerative power that couldn’t be handled by the substations.
The Sprinter service between Oceanside and Escondido had problems with excess brake wear - this could have been avoided with a hybrid drive train on the DMU’s.
Thus the Class 1’s in the 21st century are training engineers that the extended range dynamic braking is to be the primary braking force on train and air brakes are only to be used when dynamic brakes don’t have sufficient power to control the train. Such teaching was not done for the first generation of diesels. With the first generation that had dynamic braking, use of it was a learning exercise for every engineer that used it and how it got used on their specific territory.
I don’t have any figures, but I suspect car brake shoe expenses are much cheaper than they were when air braking was the primary form of braking.
In older power, all of the switching that takes place to change the circuitry from power to braking mode is done electromechanically, with a series of relays and contactors being put into their proper state in the proper sequence. Powering up the brakes too quickly can cause arcing and potentially welding the relay/ contactor contacts. Then, once in dynamic, the prime mover speed is increased to (typically) the equivalent of notch 3 to increase the excitation of the generator fields. This also takes time. The reverse also has to happen when coming out of dynamics - the prime mover needs to slow from notch 3 to idle, relays/ contactors need to return to power mode, etc. Thus, the 10 second rule was created.
Given AC power is controlled by solid state electronics and not electromechanical, the concerns with arcing disappear.
I work on a tourist RR that uses Alcos exclusively (right now), but had 1st generation EMDs in the past. I was told by the mechanical supervisor that the 10 second rule was not necessary on the Alcos because of the way their dynamic brakes were designed, and I have observed him quickly going in and out of dynamics when he ran (he is also a qualified engineer). I still use the 10 count, even in the Alcos. Some habits die hard.
Very interesting. I didn’t know that.
I’ve read that other RRs said they couldn’t use regeneration because the substations couldn’t handle it. I guess they didn’t ask the Milwaukee!
The more I learn about the Milwaukee the more I learn how innovative they were. Sad to see them go.
I’ve read that too. Makes a lot of sense. Other than maybe the fans needing a little maintenance now and then it’s a free way of braking .
The CTA L trains have gone all dynamic braking since the 50s. A parking break is applied via a battery to hold the stop. No air at all on the trains.
Does anyone know if any other train system in the US is all electric?
Thanks! Old habits are hard to brake! (Pun).
I still occasionally jab a non existent clutch pedal on my automatic.
BART had regenerative/dynamic braking from the start. The substations were not set up to accept regenerated power, so the cars were equipped with resistors that would be switched in when the third rail voltage went above the normal 1,000VDC. Problems with the traction circuitry in the early years led to the friction brakes being used more than expected with the resultant excess brake wear.
The main reason dynamics are preferred is fuel conservation. The preferred methods to control speed, slow down, etc. is first throttle modulation. Then dynamics, then dynamics with air brakes. Using exclusively air is a last resort. It might reduce brake shoe wear, but it’s saving fuel that they want.
Using air while using throttle is (for us) considered, depending on the throttle notch, either stretch braking or power braking. The demarcation has changed, currently notch 6 is allowed. (The notch position is important when releasing the air, not when first setting it.) It’s been as low as notch 2. (I think the change to that and some other train handling instructions is because there are still people in authority who have experience running trains in the real world, not just on a simulator.) Stretch braking is OK, power braking is not.
The early dynamics were considered a holding brake, from an article I once read in a period magazine. Being able to use dynamics allowed them to reduce or do away with the need to set-up/turn-down retainers and stop to cool wheels at the bottom of heavy grades.
To an extent, that still is true. Only with the better dynamics, it’s sometimes the difference between having to use air or just dynamics. On some grades if you get into dynamics early enough and at a lower speed, you may not need to use air. If you wait until speed is too high, or over an approaching restriction, dynamics alone won’t be enough.
With the better dynamics, especially on AC power, some trains can be brought to a stop with dynamics alone. I’ve done that a few times when we knew we had a dynamiter (car’s control valve that goes to “emergency” instead of service braking.) in the train.
Jeff
Yes, that’s the thing. Operating a diesel locomotive - making it move - isn’t any harder than driving a car. You release the brake, put it in ‘forward’, and move the throttle up to notch 1, and it moves forward. Apply the brake to stop, put it back to neutral and back off the throttle. I learned enough to run one (under ‘adult supervision’) in about 10 minutes, and (except for a tractor as a teenager) have never even driven a car with a stick shift. So it’s really not that hard, easier than a steam engine, so easy for an engineer to learn.
What is hard - what only comes with years of experience - are the things that made those ‘old head’ engineers so valuable. Knowing that at this station, you had to apply sand when you started because, although not visible to the eye, the track was on a slight upgrade in the direction you’re facing. Or knowing which signal is which as you speed past them - when you’re running a train in a blizzard or dense fog - so you know when to cut your speed down to go around the sharp curve that’s coming up. Or when to question a train order because you know your train is too long to fit in the sidetrack it’s telling you to take. Or that you have to go slower than usual approaching a particular station during this month each year, because the Mayflies are so plentiful that your train will crush so many of them on the rails that it will make the rails slick and it will be harder to stop.