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[quote]QUOTE: Originally posted by adrianspeeder Could I please have a detailed description of what happens electrically at transition? Adrianspeeder [/quote] 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[:)]
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