Transition

[adrianspeeder]

Could I please have a detailed description of what happens electrically at transition?

Adrianspeeder

[csxengineer98]

the engin dumps its electrical load... and then laungs forward as it picks it load back up.... uselay happens around 25mph... and only takes about a second or 2....

[Anonymous]

Electricaly the traction motors are re-aranged from series to series/parallel then to parallel. It is kind of like changing gears only it's done electricaly.

[Anonymous]

QUOTE: Originally posted by adrianspeeder

Could I please have a detailed description of what happens electrically at transition?

Adrianspeeder

Adrianspeeder--

I'll try not to be too complicated, but if I am, please answer back and I'll try and fill in the gaps.[%-)]

The idea of transitioning occurred about 100 years ago, with its earliest common use on the old type "K", "L" and "B" controllers for streetcars and interurbans. Here's an attempt at a simplified explanation.

A DC traction motor has a fixed (stator) winding and a rotating (rotor) winding, that are electrically connected by a set of brushes and a slotted commutator, sort of like on a standard DC model or toy motor. Each traction motor has its individual winding set connected internally to provide maximum torque (that translates to pulling force) at 0 rpm rotational speed, and from there the torque decreases as the rotor (and, hopefully, the locomotive) speeds up. This arrangement is ideal for pulling a train, where the rolling resistance is very low but the inertia of the stopped train is very high and requires considerably more force to overcome it than is required once the train is rolling.

A locomotive unit generally has 4-6 traction motors, and most light rail vehicles have 2 per vehicle.

DC traction motors are by nature variable speed devices. Speed is controlled by varying the voltage across the motor (usually by running the electricity through a set of resistor grids, where the controller inserts a large resistance to start, allows the motor to run up to maximum speed at the lower voltage caused by the resistor grid, then decreases the resistance somewhat and repeats the process, sometimes switching the grids several times as the motor is increasing speed), until there are no resistors in the circuit and the motor is at maximum possible voltage available. This has to be done because a dc traction motor will burn out if you apply too high a voltage at too low a rotation speed. The higher the voltage, the faster the motor tries to run, once it is running, but if it is stopped, or on low speed, it overheats and fails, permanently. So you increase the available voltage in steps as the motor speeds up and it won't burn up. Think of it as the electrical equivalent of the transmission in your car--sort of like shifting gears.

The problem with doing only this for the set of traction motors is that, among other technical issues that I won't bother you with here, you lose some of your ability to do fine control at low speeds, which any of these locomotive engineers on the Forum will tell you is not good. So to get finer and better control (and solve several other problems as well), the electrical designer sets the controls up so that the motors are connected in series (electrically end-to-end, like the Christmas lights that go out when one bulb burns out) for starting, dividing the total voltage among them so that each motor sees a lower voltage for every step in resistance the controller takes. But this arrangement, while great at low train speeds, can't take the motors up to full running speed because the maximum voltage is too low (ex. 2 traction motors in series--each sees 1/2 the full available voltage). So at some point in the sequence, when the motors are giving all they can in series, the controller throws some REALLY BIG switches called relays and changes the traction motor hookup to parallel (each motor connected across the line voltage, like the Christmas lights where if one goes out the rest stay on) while starting the resistor sequence all over again by inserting a large resistance. Then, the grid resistance is reduced in steps until it is gone and the motors are running at full speed. When connected in parallel, each traction motor ultimately can see full voltage and so it can reach full rated speed safely.

The point where the changeover from series to parallel, or vice versa, occurs is called "transition". And it is like a HUGE gear shift. The controller does other things to the diesel and generator to make it happen safely (so electrical parts don't fail and go flying), resulting in the stuff that CSXengineer98 is talking about.

All of this is done automatically now, but on the first generation diesels, the engineer had to do it manually. In that era, there were some other quirks, like some E-unit types whose controls wouldn't take the lead on making transition when the unit operated backwards. Weird, but that's how technology evolves.

Now, we can do the whole thing with solid state on an ac traction motor, but a lot of RRs still like the dc units.

As I hope you can tell from this, the whole process is rather complicated, and has to be engineered when it is designed. Hope I simplified it enough and not too much. Great question[:)]

[Kozzie]

Thanks drephpe [:)] I'll be reading that lot through a few times - but it is interesting. Appreciate the explanation! good Q and good answer - a good Forum in action [;)][:P][:)]

Dave

(Kozzie)

[Randy Stahl]

I think I can really simplify this, first lets establi***hat current(amps) = torque. Volts = speed. You are limited by the output of the main generator. lets use an SD35 as an example: the gen is capable of a max of 3500 amps and 900 volts. In series the current available per motor is in fact 3500 amps, lots of torque.. too bad the motor would burn up. the voltage in series is divided by 6 so 900 divided by six =150 volts per traction motor, remember : series voltage divides, parellell: current divideds. you can see from our example that at 150 volts per traction motor this engine just is'nt going to go fast. the other thing we need to do in series is get rid of some of that current, ultimatly the high current will saturate the motor fields causing the motor to slow down, this is called Counter Electro Motive Force CEMF , so on our SD-35 we are going to do a few steps of field shunting to bleed off that excess current and speed the motor up a bit.
Now the motor is going as fast as 150 volts will push it , we want to go faster than 10 mph , we have an FTR ( forward transition relay) that is sensing the current and voltage and will make a switch from series to series/ parellell. Now you have 3 sets of two motors in parellell with the Main gen, now instead of dividing the voltage by 6 you can divide by 3. or 300 volts per motor. your current dividedes also now, you can divide your main gen amps by 3. We'll do a couple more field shunts to speed the motors up a bit but at 300 volts per motor were still not going to go very fast.
Now we'll switch to parellell, we now have 900 volts on each traction motor, the current divides by 6, we don't need that current anyhow, it will only slow the motors down at higher speeds .
The locomotive main gens have been evolving, now alot of locomotives don't need to make transition, a GP 40 starts and stays in parellel . The SD50 SD60 SD70 etc don't make motor transition at all ,they use generator transition,changing the config of the main gen, The big GEs start and stay in parellell, these machines are impressive considering that the earliest generators didn't have enough power to start 4 motors in series/parellell
Randy

[Randy Stahl]

Diesel locomotives have no grid resistors to control speed, power is controlled by main gen exitation, each notch on the contoller is a KW reference. Talking about straight electric cars brings back memories of growing up in East Troy and playing with North Shore cars.
Randy

[adrianspeeder]

Very nice work. As I have an intrest in electrical engineering, this is my kind of stuff.

Questions:
Does each traction motor have seperate leads going up to the electricals? I cant picture a common ground setup if it switches between series and paralell.

What type of wire feeds the traction motors?

What is the phase of the power in AC circuits?

Adrianspeeder

[Randy Stahl]

Forget about ground circuits, there are none: in other words nothing on a locomotives power curcuit should be grounded. Each traction motor has 4 cables, two armature leads and two field leads.
Randy

[Randy Stahl]

The AC power comes from the D14 companion alternator at 3 phase, frequency varys with engine rpm.
Randy

[Modelcar]

...Knowlegeable info from all directions...Very interesting. Sounds like our first generation diesels had pretty crude controls.

[Randy Stahl]

The neatest thing about the EARLY engines like the E-3 FT etc. is that ALL of the fans are belt driven . kind of scary walking around in the engine room with all those shafts and pulleys. The engineer had to open and close the radiator shutters manually so he had to monitor the engine temp constantly, like the ALCO S-6 I'm trying to sell, hint hint....
Randy