DC locomotives mostly had to make transition which could cause problems such as not making transition. Do you have to make transition with AC motors?
No. The two types of motor run very differently. "Transition" is more properly described as series-parallel transition. In a typical DC traction motor, the speed is voltage-dependent, and the back electromotive force (or EMF) in the armature windings induced by rotation in the field winding's magnetic field increases with speed. In part as a result of these things, the motors are series-connected at starting, to maximize torque, switching to parallel connection to limit armature current as the locomotive gets up to speed. A typical inverter locomotive is actually a frequency and waveform-synthesis locomotive; it actually calculates the external fields needed to induce magnetism in and rotate the armature at a particular speed, and feeds that directly to the motor.
Early AC EMDs used one inverter per truck, with all the motors being 'induced' at the same rate even though their armatures might be turning at slightly different speeds due to differential wheel wear; more modern locomotives have one inverter per axle (which simplifies both antislip and 'creep control' implementation). Some of the evident drawbacks of 'hunting season' contactors making transition can be eliminated by using semiconductor devices of proper design and capacity to 'soft switch' the TM connections. As I recall many modern locomotives are in 'permanent parallel' regardless of speed, with the computer system on the locomotive doing all necessary regulation.
Contact erik-who-used-to-be-erikem, or some of the railroaders with experience, for more details. In particular you might see if you can lure Randy Stahl out from under the wharf with this kind of question.
The fill out on what Overmod wrote, the original DC generator/DC series motors needed transition because it wasn't feasible to design a DC generator with voltage and current ratings to handle all four motors in parallel (limitation imposed by commutator). Borrowing a trick from electric locomotives, the motors were connected in series at low speeds to reduce current draw on the generator, as locomotive speed went up, the back emf on the motors increased and current draw decreased, at which point the motors were connected in series-parallel and finally full parallel. Traction generators for more than 2500 to 3000hp would have been too big to fit in a locomotive.
Next generation was Alternators (AC generators) with silicon rectifiers, which allowed for a much higher current for a given voltage rating. There were still limits and often transition would occur on the alternator, where two windings would be connected in parallel at low speeds for high current and low voltage, and connected in series at high speeds for low current and high voltage.
For AC traction motors, there is a potential benefit using transition with the motors, running two windings in series to halve the current at low speeds and windings in parallel to halve the voltage at high speeds. It's probably cheaper to put more devices in parallel to handle the low speed current than to put the switches in for transition, so that's why tansition isn't done for AC traction motors.
- Erik (formerly erikem)
Erik_MagFor AC traction motors, there is a potential benefit using transition with the motors, running two windings in series to halve the current at low speeds and windings in parallel to halve the voltage at high speeds.
Going up our heaviest grade, 1.25%, with all DC power. Train is maxed out on tonnage and it's raining to beat the band. Speed is dropping and then you reach that magic number, 25 to 28 mph and the engines make transition. Nothing beats that feeling as they momentarily drop their load and then pick it up again.
I don't miss transition on AC power.
Jeff
jeffhergert Going up our heaviest grade, 1.25%, with all DC power. Train is maxed out on tonnage and it's raining to beat the band. Speed is dropping and then you reach that magic number, 25 to 28 mph and the engines make transition. Nothing beats that feeling as they momentarily drop their load and then pick it up again. Jeff
One of the more frequent problems we had on suburban power (F7 & E8) was the locomotives not making transition. Lots of fun spending the entire trip with one's face in the electrical cabinet, manually making transition after every station stop, hoping a flashover doesn't happen.
The F40PH is in permanent parallel.
zardoz One of the more frequent problems we had on suburban power (F7 & E8) was the locomotives not making transition. Lots of fun spending the entire trip with one's face in the electrical cabinet, manually making transition after every station stop, hoping a flashover doesn't happen.
SOU RR's North bound Crescent in Va. had an "E" unit that would not transition. Evidently on a down grade the engineer was trying to get the unit(s) to transition. The train over sped and derailed on a curve killing the chief chef of SOU in the ensuing pile up!
blue streak 1 zardoz One of the more frequent problems we had on suburban power (F7 & E8) was the locomotives not making transition. Lots of fun spending the entire trip with one's face in the electrical cabinet, manually making transition after every station stop, hoping a flashover doesn't happen. SOU RR's North bound Crescent in Va. had an "E" unit that would not transition. Evidently on a down grade the engineer was trying to get the unit(s) to transition. The train over sped and derailed on a curve killing the chief chef of SOU in the ensuing pile up!
NTSB Report
https://dotlibrary.specialcollection.net/Document?db=DOT-RAILROAD&query=(select+4025)
Never too old to have a happy childhood!
The original EMD Super Series locomotives, in particular the early SD50s did not transition. The AR16 alternator was capable of providing all the current at low speed and all the voltage at high speed to six D77 motors always in series. I believe this was also the case with the GP50 and its AR15 alternator but I'm not certain.
However, EMD introduced a new AR11 alternator which consisted of two machines, in parallel at low speed and in series at higher speed. So there was transition, but inside the alternator, and the traction motors stayed in parallel at all times.
I belive some GE alternators on later DC locomotives also had internal transition.
The AR11 was limited to 3800 HP and was replaced after the SD60 with a more powerful machine to allow 4000HP.
But in Australia we obtained 97 locomotives with AR16 alternators, loco model JT26C-2SS of only 3000HP. Those originally belonging to the NSW State Rail Authority, known as 81 class of which there were 84 (until one was lost in a fire) have worked heavy grain trains on steep grades for some 37 years and they are thought highly of by crews. They have had two major overhauls in that time.
Peter
blue streak 1SOU RR's North bound Crescent in Va. had an "E" unit that would not transition. Evidently on a down grade the engineer was trying to get the unit(s) to transition.
BaltACDNTSB Report https://dotlibrary.specialcollection.net/Document?db=DOT-RAILROAD&query=(select+4025)
zardoz BaltACD NTSB Report https://dotlibrary.specialcollection.net/Document?db=DOT-RAILROAD&query=(select+4025) Could not read without a login and password.
BaltACD NTSB Report https://dotlibrary.specialcollection.net/Document?db=DOT-RAILROAD&query=(select+4025) Could not read without a login and password.
Login and password are of no cost.
BaltACD zardoz BaltACD NTSB Report https://dotlibrary.specialcollection.net/Document?db=DOT-RAILROAD&query=(select+4025) Could not read without a login and password. Login and password are of no cost.
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