-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
oltmanndTransition, where the motor connections change from parallel to series (actually a series-parallel connection of pairs of motors, each pair in series) is done to match the generator capability to the motor demand. At low speeds, DC traction motors have low voltage drop but high current. At high speed, the voltage drop is high and the current is low. The generator has design limits on it's maximum current and voltage. So at low speeds, the series-parallel conection is used. This effectively reduces the current demand on the generator in half by doubling the voltage. Newer DC locomotives (SD50 vintage) used a generator transition scheme where the generator had two sets of windings that could be connected in series or parallel - making transition much simpler and smoother. AC locomotives do not have a direct generator to motor connection. The generator produces a constant high DC voltage which feed inverters (one per motor on GE, one per truck on EMDs). Control is provided by varying the frequency to the motors instead of by varying the main generator field (indirectly varies voltage).
I'm somewhat confused by this description...
I'm under the impression the the Diesel Engine drives an alternator which produces AC current. This is converted to DC and then back into AC by the inverters. So, in other words modern AC traction diesel locomotives actually use an AC/DC/AC electrical system?
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
carnej1 oltmanndTransition, where the motor connections change from parallel to series (actually a series-parallel connection of pairs of motors, each pair in series) is done to match the generator capability to the motor demand. At low speeds, DC traction motors have low voltage drop but high current. At high speed, the voltage drop is high and the current is low. The generator has design limits on it's maximum current and voltage. So at low speeds, the series-parallel conection is used. This effectively reduces the current demand on the generator in half by doubling the voltage. Newer DC locomotives (SD50 vintage) used a generator transition scheme where the generator had two sets of windings that could be connected in series or parallel - making transition much simpler and smoother. AC locomotives do not have a direct generator to motor connection. The generator produces a constant high DC voltage which feed inverters (one per motor on GE, one per truck on EMDs). Control is provided by varying the frequency to the motors instead of by varying the main generator field (indirectly varies voltage). I'm somewhat confused by this description...
.
carnej1 [I'm somewhat confused by this description... I'm under the impression the the Diesel Engine drives an alternator which produces AC current. This is converted to DC and then back into AC by the inverters. So, in other words modern AC traction diesel locomotives actually use an AC/DC/AC electrical system?
[I'm somewhat confused by this description...
That's pretty much how it works. Variable-frequency AC is used to control the speed of the traction motors so the current can't be drawn directly from the main alternator without the intermediate steps.
BigJim The DC traction motors while turning create a counter voltage and after attaining a certain speed will not spin any faster, hence, the need for transition to attain a higher speed.
The DC traction motors while turning create a counter voltage and after attaining a certain speed will not spin any faster, hence, the need for transition to attain a higher speed.
Close...
The counter voltage (A.K.A. back EMF), is proportional to the product of the motor speed and the magnetic field strength in the motor. Once the motor speed is high enough that the counter voltage equals the maximum generator voltage, the only way to go faster is to reduce the motor current (or fake it by shunting the field) to reduce the magnetic field strength. Without field shunting the reduction in current will result in a reduction in horsepower output from the motor.
The real reason for transition on DC generator equipped locomotives was that it was not feasible to design a generator for both the maximum current draw in starting and maximum voltage when in run 8 at top speed. The series connection allowed more current to be delivered to the motors at starting and low speeds where motor voltage was low, conversely the parallel connection allowed for a higher voltage to be delivered to the motors at high speeds where the motor current is low.
Since the power producing windings on an alternator are on the stator, it is much easier to configure the alternator to be able to supply high current at low voltages and high voltage at low current. This is done by having two sets of windings where the rectifiers outputs can be either connected in series or parallel. The series connection gives twice the current at half the voltage of the parallel connection and the parallel connection gives twice the voltage at half the current of the series connection.
- Erik
I'm glad that I missed the early "manual change" era!
Manual transition provided me with employment. It would take days to fix a flashed over generator. The engineers were usually pretty good making forward transition and go fast.. just forget to make back transition.
BigJim I'm glad that I missed the early "manual change" era!
Come run with us! We can give you some hands-on!
Nothing like trying to keep track of your speed so you can make transition at the right points while keeping the train under control down a hill using the 6 brake...
We're running a former NYC RS-3 (the first one, in fact - 8223).
It's actually kinda fun to run.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
didn't SOU RR Crescent have an accident in Virginia attributed to an engineer trying to get manual transition engaged and was not paying attention to train and over speeded causing a derailment ?
that will definately be added to my bucket list. love from sunny cocoa florida!!
Randy StahlManual transition provided me with employment. It would take days to fix a flashed over generator. The engineers were usually pretty good making forward transition and go fast.. just forget to make back transition.
Randy
Your post got me thinking about a GP7 engineer's manual I have. I checked it out. Transition was indeed manual then. Reminds me of a pre-computer control circuit, Square D breakers, mechanical relays and all.One of my uncles was a shop and line-of -road mechanic for CN at the CN London, Ont. running shop when he retired. He was also a RH Foreman at Toronto Spadina . He talked with me,before he died, about different challenges on the road that he was called out to deal with. He's the one who got me interested in the RR in the first place. I'll not forget him anytime soon....
Charlie
Chilliwack, BC
I've been told the Erie's E8s had manual transition and the DL&W's E8s had automatic transition at the time of the merger. Neither side's engineers liked the other's but the El fleet ended up with automatics in the end.
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Are you certain the GP-7 manual you have specifies MANUAL transition. I worked for EMD the summer of 1952 and remember clearly that GP-7's had automatic transition. This started with F-3s and E7's. Possibly the manual tells you what to do if automatic transition doesn't work for some reason? Or did one railroad have a reason to special order manual transition? What in the world for?
Also spent a total of over 100 hours in the cabs of B&M GP-7's 1567 and 1568 winter 1952-1943 in both frieght and passenger service. Definitely automatic transition. They were testing a load-regulator concept for the GP-9, but otherwise were standard.
Assuming that under manual control all units in the consist will transition at the same time, that could be one heck of a lunge as the train is under full load and slowing down going uphill. Even today's SD70's will give a hard lunge at times. Makes for a tight seat of the pants feeling and I know of one engineer who had a SD70 break a knuckle because of it.
On EMD locomotives, transition for the motors on one truck is at a slightly different speed than on the other truck, or at least this was true in 1952. This may have been an option. But it was automatic, not manuel.
daveklepper Are you certain the GP-7 manual you have specifies MANUAL transition. I worked for EMD the summer of 1952 and remember clearly that GP-7's had automatic transition. This started with F-3s and E7's. Possibly the manual tells you what to do if automatic transition doesn't work for some reason? Or did one railroad have a reason to special order manual transition? What in the world for? Also spent a total of over 100 hours in the cabs of B&M GP-7's 1567 and 1568 winter 1952-1943 in both frieght and passenger service. Definitely automatic transition. They were testing a load-regulator concept for the GP-9, but otherwise were standard.
Dave,
I pulled out the manual (EMD Engineman's Operating Manual No 2312, 1st Ed. 1950) again, and realized that I had skimmed though the section on Operating Problems as they call it. I also did not read the 'Description' section as carefully as I might have. Thus I was incomplete in my assertion that GP7's were manual transition.
There were, according to the manual two types, GP7L and GP7R. The R series was equipped with a transition lever 'to make transition manually on other units, when necessary.' (p102) There was also a 'Transition Forestalling Switch installed so that "when forward transition is not desired,.....traction motor connections will stay in series-parallel regardless of locomotive speed, generator voltage, or position of load regulator arm. Fig 1-6' (p103)
Apparently the R's were automatic transition running singly but when run MU with other non automatic transition units had adaptations which form what I can see, after looking more carefully at the Manual, which allowed for some form or other of manual transition from the Engineers operating stand.
The L series appear to have been automatic transition only.
I'm guessing in 1950, these locomotives and their transition systems were still in their developmental infancy and had various 'tweaks', to allow their operation in MU sets with differing (read non-GM) locomotives. Kind of like an override or cutout of some kind, hence the L and R series designation.
Thanks for making me think again, never a bad thing.....
daveklepper On EMD locomotives, transition for the motors on one truck is at a slightly different speed than on the other truck, or at least this was true in 1952. This may have been an option. But it was automatic, not manual.
On EMD locomotives, transition for the motors on one truck is at a slightly different speed than on the other truck, or at least this was true in 1952. This may have been an option. But it was automatic, not manual.
Dave,The following should be of interest to you and others here...
http://www.railroad.net/forums/viewtopic.php?f=6&t=48131
...which in part I quote;
" In older locomotives, it was necessary for the engineer to manually execute transition by use of a separate control. As an aid to performing transition at the right time, the load meter (an indicator that informs the engineer on how much current is being drawn by the traction motors) was calibrated to indicate at which points forward or backward transition should take place. Automatic transition was subsequently developed to produce better operating efficiency, and to protect the MG and traction motors from overloading due to improper transition."
It is of interest. And the B&O was the first RR to apply automatic transition to existing FT locomotives. As far as I know all EMD manufactured product in 1952 had automatic transition, and it was not an option. As far as I know in 1952 you could not buy and EMD locomotive with manual transition. I may be wrong, but to me talk of E-8s with manual transition is sheer nonsense. Unless e railroad did it themselves after some "problem."
daveklepper It is of interest. And the B&O was the first RR to apply automatic transition to existing FT locomotives. As far as I know all EMD manufactured product in 1952 had automatic transition, and it was not an option. As far as I know in 1952 you could not buy and EMD locomotive with manual transition. I may be wrong, but to me talk of E-8s with manual transition is sheer nonsense. Unless e railroad did it themselves after some "problem."
E8 production began in 1949 and continued to 1954, so potentially some of the early units may have had manual transition.
Never too old to have a happy childhood!
BaltACD daveklepper It is of interest. And the B&O was the first RR to apply automatic transition to existing FT locomotives. As far as I know all EMD manufactured product in 1952 had automatic transition, and it was not an option. As far as I know in 1952 you could not buy and EMD locomotive with manual transition. I may be wrong, but to me talk of E-8s with manual transition is sheer nonsense. Unless e railroad did it themselves after some "problem." E8 production began in 1949 and continued to 1954, so potentially some of the early units may have had manual transition.
Does anyone here have a copy of EMD MI 2203?
I suspect MI 5046 and 5086 are too late chronologically to cover any 'factory' mention of manual transition on E8s ... but if I remember correctly, ABT was supposed to be an E9 feature. That might leave room for an 'intermediate' state of automatic transition: where the locomotive makes automatic transition going UP, but the throttle has to be brought back to idle to make transition to series DOWN.
Hopefully we can jog Dave Klepper's memory a little regarding the specific chronology during the time of his experience...
I remember the old engines having a BTR (backwards transition relay) but curiously not and FTR. The BTR was picked up all the time the engine was loading in power (not dynamic) so it was set to make reverse transition. There was a through cable relay that picked up at something like 800 amps and 500 volts. The through cable relay picked up and dropped out the shunt field contactor and picked up the BTRA making reverse transition. I don't remember the exact sequence of the relay logic.
I do remember that the FTR kit included a new load regulator that operated the transition. Aeroquip, Pulse and Oregon technical products started introducing speed controlled solid state transition modules in the mid 60's, I do remember working on lots of those especially when it came to the complex E/I transition with multiple steps on engines like the SD24,GP35 etc.
I should look around for some old schematics to refresh my memory. I also believe that early E-8s had this "half transition" that only made backwards transition.
I did not realize the E-8 replaced the E-7 so early. 1949. I ihad forgotton that fact. I should stick to what I observed in 1952. Apologies. If I recall correctly, GP-7's, SD-7's, and F-7's had forward and backward automatic transition in 1952. I thought E-8's also, but could be mistaken. Again, unless the railroad specified otherwise, the transition was set just slightly differently for the two trucks. t``
But back to E-8's. I think there were E-7's on the B&M with automatic transition!
Another way of thinking of transition is to look at the impedance of a motor and a generator. The motor starts at 0 RPM with high current and low voltage (low impedance) and as the motor speeds up the voltage rises and the current tends to drop raising the impedance of the motor. The generator, on the other hand, runs at nearly a steady speed (admittedly a doubling of speed from idle to 8). It starts at a fairly high impedance and the impedance rises as rpm is raised.
To best transfer power from the generator to the motor, one must match the two impedances. Here is where it gets fuzzy. Since the motors and generators on a DC system are connected together (whether series or series parallel) their impedances match. (Generator current = motor current, generator voltage = motor voltage). Thus at low speeds a motor connected across a generator will not transfer much power because of the inherent impedance mismatch. To counter this we connect motors in series which raises the impedance of the motor system and better transmits power. Once the motors start to turn, their impedances rise considerably and the mismatch starts to occur the other way (series motor impedance exceeds generator impedance). The load regulator partially compensates for this by raising the excitation on the generator to raise its voltage (and thus its impedance). But again, the generator runs at a steady rpm. Thus, the only way to get the generator impedance to match the rising impedances of the motors is to put the motors in parallel (parallel circuits reduce the impedance by the number of parallel components.)
This is nearly the same with AC motors and generators, but with modern power controls we can modify the voltage and frequency to the motors to better match their impedances. The torque on an AC motor is generated by the difference in frequency between the coils and the rotational speed of the motor. Once AC motors start to turn, torque drops so the frequency control system must raise the frequency as the motor (train) moves. Again it is an impedance matching exercise, but this time we have a complex impedance that accounts for voltage and frequency.
As a follow up to your question, something no one touched on. Modern GE locomotives, Dash 8, Dash 9 and DC EVO locomotives do not make transition. The main alternator on these GE locomotives can handle the current demand, so they only have "P" contactor, no "S" contactors.
Also only EMD 6 axle locomotives, Dash 2 and later make motor transition. Four axle EMD GP38-2 and GP40-2 do not make motor transition. As mentioned earlier, SD50 and later DC locomotives make Main Generator transition. EMD calls the package a Main Generator as the rectifier banks are internal to the package. On SD50's and later you have three machines in one package, two Alternators and a companion Alternator, the Aux Gen still a separate machine.
It is interesting to see how the two major builders (GE & EMD) solved the issues they encountered in two completely different ways. Here is another example of how different the two locomotives can be, EMD's have starter motors, either air or electric depending on the model and railroad specification and GE uses the Main Alternator to start the diesel engine.
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