Hi all, I would like to understand the exact reasons behind the use of AC versus DC power in today’s freight locomotives. I have seen other posts that discuss this topic, but that are missing a full and detailed explanation. I’ve heard AC was better for traction control at low speeds, but I have to heard why this is the case. I’ve also heard that AC is less maintenance than DC but am not sure why this is. I have a 100 questions like this and it’s keeping me up at night! Anybody an expert on this type of stuff that can help? If anybody knows of some books or detailed reading material on this subject that would be of great help as well. Thanks in advance!
There was a recent Discussion thread on this very forum which touched on your questions.
You should read through it:
http://cs.trains.com/trn/f/741/p/221349/2449412.aspx#2449412
Yes, read it thoroughly and then ask questions if there is something you still do not understand. Thanks
The vast majority of electric motors, AC or DC, work on this principle. There are two elements to the motor: a magnet and a wire carrying a current inside the magnetic field. That wireexperiences a force according to a physical law that was discovered in the early 1800’s around the same time the steam locomotive was being invented. Between the wire and the magnet, one part is held fixed – the stator – and the second part is allowed to rotate – the rotor. The force between magnet and wire gets applied to the rotor, whether the rotor is the magnet part or the wire part, and this makes the motor go and in turn makes the train go.
In a DC traction motor, the magnet part is an electromagnet and also the outside stationary part or the stator. The wire part is a dense set of windings in the rotor. Electricity has to get into that wire through some kind of sliding electric contact. The direction of the current in the wires in the rotor has to change back and forth as the wires are carried through the “north” and “south” parts of the magnetic field – if it didn’t, the motor would “fight itself” and stall, producing no net torque to make the train go. The sliding electric contact that switches the current direction as the motor rotates is called a commutator.
The commutator is commonly a pair of carbon “brushes” that rub against a metal cylinder with slots in it. The DC traction motor is higher maintenance because of the commutator. Because it involves rubbing surfaces, it can wear out. The commutator also makes carbon dust as it wears, that dust can pile up in the slots between the pieces of metal it makes contact with, that dust can suddenly conduct electricity and catch fire in a “commutator flash-over.” This problem is not insurmountable, but commutators require inspection, cleaning, and periodic replacement of the carbon brushes, and all of this costs hours of your shop workers.
The AC motor used in locomotives also has the magnet pa
As for traction control, it’s pretty simple. The friction coefficient of a steel wheel on steel rail goes down considerably when they break loose and start slipping on each other. A slipping wheel puts a lot less force into the rail than one that is right on the edge of slipping. The highest force into the rail occurs when the locomotive wheels are just on the edge of slipping. The force a drive wheel puts into the rail is what makes the train move.
With a DC traction motor, once the wheel starts slipping the torque on the wheel goes down. This causes the traction motor to speed up, making the slipping worse, and so on in a runaway reaction that will destroy the motor if the engineer or the locomotive’s electronics don’t intervene and reduce the current to the motor. In either case, tractive effort, the measure of how hard the locomotive is pulling, is reduced.
With an AC traction motor, the speed of the motor is primarily determined by the frequency of the AC electricity to the motor. It turns at about the same RPM regardless of load. Thus when a wheel starts slipping the motor doesn’t speed up, it just draws fewer amps. The electronics will drop the motor frequency a little and full traction will resume. This keeps the drive wheels turning at just under the point where they start to slip, maximizing tractive effort.
When an AC locomotive is pulling hard, you will hear the drive wheels constantly chirp as they loose and regain traction.
AC traction motors are easier to cool so they can draw full amps for longer periods without overheating.
The explanaitions above are good, but the following modifications may be important:
-
Railroad and transit dc motors generally have more than just two poles, and the fixed windings (“field coils”) generally run to six, eight, twelve, sixteen, twenty-four. Going around the circle, one finds north south north south, etc. The point is that the brushes and commutator insures that the rotating windings (“armature coils”) receive current flowing in the right direction to insure attraction toward the fixed winding they are approaching and repultion from that they are leaving.
-
Most classic AC electric locomotives, the GG-1 included, used motors much like dc motors. When you reverse the current in the armature and in the field at the same time, the result is like there is no reversal. The problem in running a dc motor on ac is not that it will reverse so many times a second, but that the changing current is so fast that the magnet structure cannot keep up, and there can be lots of power waisted in heat. That is why the classic ac elecrifications had low frequencies, not 60 Hz (Hz = cycles per second) but 16-2/3 Hz in Europe and 25 Hz USA (PRR, GN, N&W, NYNH&H, BM, Virginian, Reading). Modern electric locomotives share the same technology as modern ac diesel-electrics.
-
The squerrel-cage motors originally were constant speed, and those in fans, computer fans, may air-condiditoning devices, etc, still are, indeed both constant speed and constant load. If load is increased, the motor won’t keep up with the rotating magnetic field, and wil coast to a stop. Railway motors employ a hysterises effect in magnet structures, and either the rotating bars are sllightly slanted, or the magentic structure of the field coils around the permeter are slightly slanted. With increasing load, the rotating bars can slip slighly from the computer controlled synchronous speed of the rotating magnetic fie
A few more comments:
There is a tradeoff in the design of squirrel cage induction motors when running off a fixed frequency supply. A low resistance cage will have high running efficiency, but poor starting torque, while a high resistance squirrel cage will have good starting torque but low running efficiency. The torque peak of the induction motor occurs when the current in the bars of the squirrel by the inductive reactance of the the bars, a higher resistance corresponds to a higher frequency where the inductive reactance equals the bar resistance.
One way of getting the best of both worlds is to use a wound rotor induction motor where high resistance is inserted at starting and low resistance when running. The GN 3 phase locomotives as well as the N&W and VGN phase converter locomotives used wound rotor induction motors for that reason, along with variable resistance acting as a form of speed control.
A variable frequency drive allows can be set up where the applied frequency is at or below the frequency where maximum starting torque is provided by a low resistance/high efficiency squirrel cage. There are a couple of advantages to that design besides high efficiency. One is that the low resistance implies that less heat is generated in the motor for a given torque output. The other is that the torque speed characteristic is steeper, so a slight increase in motor speed due to wheel slippage will cause a marked decrease in motor torque - which should put a stop to the slippage.
The big cost reduction in variable frequency drives came from the development of IGBT, which made pulse width modulation possible at all speeds possible for locomotive sized motors. The GTO’s on the Siemens/EMD locomotives used PWM at low speeds, changing over to a fundamental switching frequency at some intermediate speed (switching frequency equals output frequency). The next step is likely to be Silicon Carbide MOSFET’s which will allow for an even more compact inverter due to higher frequenc
Modern locomotives maximize wheel-rail adhesion by regulating wheel “creep”, which is the percentage by which the rotational speed of an axle’s wheels exceeds what it would be if it exactly matched the speed of the locomotive. Adhesion is reduced if the creep rate is either too low or too high; and the optimal creep rate is a function of rail conditions. Because rail conditions are seldom constant, the locomotive’s control system constantly searchs for – and attempts to maintain – the optimum creep rate; and the slower the locomotive is moving, the more difficult that process is. Largely because AC-traction locomotives have inverter-based control systems, they are more capable than DC-traction locomotives of regulating creep.
Not to get away from the original poster’s questions but can anyone give me a succinct explanation in layman’s terms of why it is necessary to have a DC “step” in the electrical system of an AC traction locomotive.
The Diesel engine turns the alternator which generates AC current which is converted to Direct current and then back to AC via the inverters.
Why can’t an all AC system be utilized?
An all AC system could be used, the circuit arrangement is called a cycloconverter. There are a few downsides to this approach, the number of thyristors needed, a higher output frequency from the traction alternator and complexity with dynamic braking.
Assuming one cycloconverter per truck, an all-AC system would require at least 36 thyristors (albeit simple converter thyristors vs GTO’s) along with the firing circuitry. A DC link only requires 12 GTO thryristors with one inverter per truck. The traction alternator frequency would need to be at least three times the highest frequency used by the traction motors, probably requiring a considerable redesign of the alternator. The dynamic braking would require at least three more converters to control the amount of power goint to the braking resistors.
- Erik
And the simple explanation is this: The alternating current frequency produced by the alternator is seldom the same as the alternating current frequency required by the ac motors. The alternator produces a frequency that is optimum for the diesel’s rotational speed to operate efficiently for the specific load the locomotive faces, while the frequency required by the motors is dependent almost completely on speed, with load having a very minor effect. So the added complexity of the all-ac system explained above should be understandable.
Transition! [swg]
Erikem: In reference to the need for an electrical source to activate the dynamic braking. Could a permanent magnetic generator ( PMG ) provide enough power to activate the field or maybe even battery power ? This is in reference to my thread of converting the present electric motors to cabbages.
IIRC, that comment was in respect to an all-AC system, though DC and inverter AC systems typically need some sort of excitation. In the all AC system, there is a need for the alternator to provide a source of reactive power for commutation of the cycloconverters as well as some means of controlling the energy flowing into the dynamic braking resistors.
In a DC locomotive, the standard approach is to have a high current low voltage generator or alternator to excite the fields, turning the traction motors into separately excited DC generators. A cabbage could be hooked up to the HEP to provide a source of power for exciting the motors. Something similar could be done with AC motors, where the HEP would keep the DC link powered when no braking is occurring.
My dream is to stuff a bunch of supercaps in a cabbage, using the motors to charge the caps and the stored energy could be used to provide additional acceleration.
- Erik
One of the side effects of AC-DC-AC drive design is that both dual-mode (diesel-electric/electric) and multi-voltage/frequency electric locomotive design is greatly simplified.
I thought a thyristor effectively converted AC to DC?
To put it more generally, thyristors can be used to convert AC to variable voltage DC and with appropriate topology, that voltage can vary fro positive to negative and vice versa. With a three phase power supply, a thyristor converter can vary the output voltage from positive to negative in about three cycles of the supply voltage, i.e. the converter works as a frequency changer. The Europeans built quite a number of thyristors based frequency changer substation to convert from 50 Hz to 16 2/3 Hz, but 60 Hz to 25 Hz was a bit to small of a transformation ratio to be practical.
One difference between a cycloconverter and a AC/DC to DC/AC link is that reactive power can e transferred through the cycloconverter (though it helps to have a polyphase input and output.
FWIW, I have a copy of the 1949 edition of The Standard Handbook for Electrical Engineers showing a cycloconverter circuit using thyratrons. The thyratron was a gridded mercury arc tube, where current would flow once triggered until it was brought to zero by some means such as the AC input voltage going through zero.
- Erik