Trains accelerate incredibly slowly and often have a single gear that is optimised for high speed. Similar to trying to pull away in your car in too high of a gear this makes trains really slow and takes along time for them to hit their high speed.
Most cars will often have 4 (for very old cars) – 6 gears to keep acceleration smooth and fast whilst still being efficient at high speed but trains don’t.
I get that electric motors have all of the torque available at low RPMs whilst ICE only have full power at a high rpm but wouldn’t gears still allow trains to get to higher speeds quicker?
In: Engineering
This is going to be a very basic explanation, I understand I’m leaving out a lot of important details here.
Trains dont actually have any gears. A modern diesel electric train is essentially a generator.
Inside the locomotive, a diesel engine is coupled to a gear box. Let’s say 1:25 ratio (just an example). So for every 1 revolution of the crank (output) shaft of the diesel engine, there are 25 revolutions on the output shaft of the gearbox.
The output of the gearbox is then coupled to a generator. Electric generators and motors are essentially the same thing. Apply mechanical movement to a generator/motor and you will produce electricity. Apply electricity to a motor/generator and you will produce mechanical movement.
The power that is generated from the locomotive is used to power everything on the train. Lights, controls, heating, air compressors, and the motors that move the train.
Those motors are located under the locomotive, between each set of wheels, much the same way electric cars work.
The electricity that was generated in the locomotive is fed to the motors through a variable frequency drive (VFD). The VFD controls motor output speed by changing the frequency of the sine wave (Dont worry about this part). It kind of limits the amount of power applied to the motors.
Those motors are then connected to each set of 2 wheels through another gearbox, this time stepping down, so let’s say 25:1 ratio (again just an example). So for every 25 revolutions of the motor (input of the gearbox), there is 1 revolution of the wheel (output).
Trains are slow because they produce an insane amount of torque, and the process I outlined is what makes that torque. As others have mentioned, slow speed, high torque is exactly what trains need, as they are very heavy, and have little friction between the wheels and the rails to work with.
As they start to gain momentum, the frequency can be turned up on the VFDs to slowly add more speed, and less torque to the wheels, this is essentially what the throttle control on a locomotive does. (Again, very simplified here).
Another benefit of this is the ability for trains to use regenerative braking, the same way electric cars do. (I wont get into this),
Streetcars, subways, LRT vehicles, and some passenger/freight trains use the same process of adjusting frequency to drive motors, but instead of generating the electricity right inside the locomotive, power is fed to the train by an overhead wire, or a third rail.
Latest Answers