Slight edit required re the Lemp control.
Here is J. Parker Lamb on the application to motor locomotives.
The list of Lemp’s patents is here, with the relevant one being US1216237A.
OK, I understand the DC motors, very well explained, Now please explain the AC motors and why they can pull at full force for very long times and to not overheat like DC motors.
The first thing to rememberis that the DC motors in most American freight power are not like model-railroad or treadmill motors – they have a wound armature and also a wound (and not permanent-magnet) field. This wound field’s power varies only in strength, with the more power giving the more ‘magnetism’ but also greater heating due to the resistance in the field windings. Think of this as what the rotating armature ‘pulls and pushes against’ to create torque.
Now, more current passes through some of the armature windings, in the correct direction to produce torque in the intended direction of rotation. This would be relatively little rotation indeed before the magnetic fields aligned; instead, the device known as the commutator switches the connection to the windings as the armature rotates, so the armature poles keep having to turn to try to line up with the field and the motor develops torque. Again, there are heat losses as the current passes through the armature winding, and there can be relatively high peak current if the commutator ‘makes’ and ‘breaks’ connections and the magnetic field induced in particular coils lapses (generating high peak current just as opening circuit on a capacitor induces high peak voltage across the gap). Meanwhile, for functional reasons keeping the air gap inside the motor between the field pole faces and the armature pole faces should be kept to a minimum for best magnetic efficiency.
What this implies is that there is a fair amount of heat generated in one of these motors, and when it is to work in a confined space protected from road dirt and water it will need to dissipate that heat reasonably well. This is why most diesel-electrics have capable traction-motor blowers: to air-c
True; however, much of the time freight IS moving under 18mph. In addition, under 18mph is where the most tractive effort is needed. And even if the C4 locomotives do have fancy wheelslip controls, when the computer detects slipping it REDUCES power, which leads us back to my main point.
You have this a bit sideways. Any train climbing the ruling grade with C4 locomotives at less than 14 mph is very likely to stall.
The maximum TE on AC locmotives is adhesion limited. A six motor locomotive will produce about 150,000# TE. A four motor unit, about 100,000# TE. A 4400 HP six motor unit will hit this limit at roughly 9 mph. A four motor unit, at 13.5 mph. (my 12 and 18 mph guessimates were off a bit. A remnant from the days of GP40s and SD40s)
Lets put these locomotives on a 1.5% grade with a train at 1 HP per ton. A 4400 ton train needs 132,000# TE to climb that grade. A six motor unit can do it and will climb the grade at ~10 mph. A four motor unit will stall with that train unless adhesion conditions allow 47% adhesion - not likely.
The C4 locomotives are good matches for the modern DC fleet. You can mix and match them with DC locomotives without the DC unit seriously limiting the performance of the AC locomotive. At the point the DC units are approching “being in the red”, the C4 units are approachi
Please correct me if I am incrrect. The way I understand the expanatiuon of why AC will not overhet like DC units is because they do not have motor windings, but instead have non magnetic bars that spin and creae the torque as they spin.
You have to visualize exactly where the ‘nonmagnetic bars’ are, what they’re hooked up to do, and what the geometry of their ‘spinning’ is.
There is no motor action in these large motors without magnetic attraction. Therefore, the ‘non-magnetic’ bars have to be made “susceptible” to being attracted by the rotating magnetic field before any ‘spinning’ will take place. This is why they are electrically connected as they are. Part of the action involved concerns ‘eddy currents’ which you might want to read about.
The non-magnetic bars that have had magnetism induced in them are arranged around the outside of a rotating armature that performs the same function a wound-rotor armature does. The ‘bars’ replace the pole faces in such an armature. They are very rigidly fixed in slots in the rotor drum, and shouldn’t move at all, let alone ‘spin’, individually. It is the whole armature, with the set of bars fixed around its periphery, that spins.
And of course there is a pinion arrangement on the end of the drum-armature shaft that communicates with a bull gear on an axle in the ‘usual’ way for nose suspended motors, at least in most of these AC freight locomotives.
Oddly, a great many of the Internet resources on induction traction motors either wildly overcomplicate the perceived math or leave out key details of ‘how the trick actually works’ that laymen can understand. Here is a site that contains a number of interesting observations once you get past all those scary formulae. Note i
Perhaps I did not make my point very well (even my wife says I ramble on sometimes). I agree with what you posted. The C4’s might have their place, but a versatile locomotive they are not.
And so it should come as no surprise that the only Class I railroad to buy them was also the largest buyer of the Dash-9 and ES44DC locomotives before them.
“Squirrel Cage” induction motors have large conducting bars in the rotor, the slip between the rotating field (stator windings) induces currents to flow in the bars which interacts with the rotating field to generate torque. These bars are not embedded in iron slots as in the case of a DC motor, and also do not need insulation. Both of these help with heat transfer and the lack of insulation also allows the bars to operate at higher temperatures. A more subtle issue is that the currents in the bars are at a lower frequency than in a DC motor, which takes care of skin effect and proximity effect.
Back EMF in a motor can be thought of as the equivalent of the back pressure provided by a piston during the power stroke of a piston engine (energy equals force times distance moved). In essence, the back EMF is an inherent part of the motor converting electrical energy to mechanical energy. This simplest in case of a DC motor as there is no phase shift between the voltage and current.
Back EMF for AC motors is a bit more complicated as it provided by the field (from field windings or permanent magnet) in the case of a synchronous motor or the slip between. The proportion of electrical power that is converted to mechanical power is the proportion of the current that is in phase with the applied voltage - hence power engineers reference to real power versus reactive power. An unloaded induction motor will draw current from the lines, but the current is almost 90 desgrees out of phase with the line voltage, hence little real power is transferred to the motor.
FWIW, inverters do have to be designed to handle the reactive power needs of the motors, mostly in a higher current rating than what would be expected from a resistive load of the same power.
I was under the impression the C4’s were considered versatile machines, at least in comparison to modern DC motored C-C power like SD70M-2’s or ES44DC’s. Equivalent or better performance to a ES44DC, yet no concerns about short time ratings and such on a heavy drag freight when mixed in a consist of more capable six motored AC power.
Obviously a less capable machine with less tractive effort, but one that’s still better able to blend in on something like a heavy coal train up a grade in a pinch than an ES44DC or Dash 9 could.
BNSF is apparently pleased with their performance as they have taken delivery on Tier 4 locomotives from GE with the same arrangement. They also have a small batch of SD70ACe-P4’s with a similar setup.
Agreed.
You don’t often see a solid block of C4s on a drag freight of any kind in either case. It’s usually amongst a bunch of 6 axle units. BNSF also put DC traction on all sorts of trains that no other railroad does. So, in the context of BNSF’s trains are they different from what they’re replacing? Sure, they aren’t as versatile as 6-axle AC…but BNSF wasn’t giving you those in the first place. Also worth noting that UP (at least up till now) will regularly throw in SD70Ms on trains going over Donner. (though I almost never seem them on the massive trains headed to Roeper or Bailey) Those DC units mixed with assorted AC units seem odd to me. All I can imagine is that they’ve over powered the train enough that those units won’t limit the train.
No reason why not, assuming the train’s speed won’t drop below … guess a DC SD70 will be perfectly happy at 12 mph, won’t it? I’m guessing most UP trains are doing better than that on the steep part, above Gold Run.
BNSF is also rebuiling its older Dash 9s into AC44C4Ms.
BNSF only did a small batch of AC44CM’s back around the 2014-2016 time frame (There’s no C4 in the BNSF model designation even though they’re rebuilt along the lines of factory built C4’s).
20 of the earliest Santa Fe Dash 9’s from the 600 class went through a pilot program back then. But since then, BNSF Dash 9’s have only been going off lease or been sold, including more of the early 600’s and newer examples of the model.
Almost 600 of their one time fleet of almost 1800 Dash 9’s are gone if my tally off the Diesel Shop roster just now was accurate. And over 500 of the survivors are laid up per the latest Loconotes BNSF update.
No sign of further rebuildings that I’m aware of for these or the SD70MAC’s, despite trialing rebuild programs for both. In fact they don’t seem to be doing much of anything in this area, unlike the current trend for most Class 1’s.
The last two Locomotive special issues from Kalmbach show just two GP23ECO’s in 2019 and nothing for 2020 for major locomotive capital rebuilds for BNSF. And while they’ve been a fairly regular visitor for new C4’s (Including an order for some T4 credit units in 2020), one has to go all the way back to 2009 for the newest GE C-C’s on the roster and 2014 for the newest SD70ACe’s.
So not only do they not do rebuilds, but they’re also not buying any new six motor road power it would seem.
Actually the rebuilt Dash 9s are in fact designated as AC44C4M’s on the BNSF.
They’re marked that way on the frame below the nose as well. Not sure why I thought what I did.
Thanks
http://www.rrpicturearchives.net/showPicture.aspx?id=4411284