That's why an unloaded motor takes little current, while a fully loaded one takes it's full current.
As the motor speed ramps up, the back EMF increases as well. The effective power source for the motor is the difference between the supply and back EMF; with an unloaded motor these are (almost) equal, so the current is very low.
Load the motor, the speed (and back EMF) drop so the current increases.
If you run the motor _faster_ than it's offload speed, the back EMF becomes greater than the supply and the current is reversed - it returns power to the source.
If you disconnect the supply, the back EMF is still there, at a voltage proportional to motor speed.
Putting a resistive load on it dumps however many kilowatts or horsepower out of the system, but as braking force rather than accelerating force.
With a shunt wound motor, the back EMF for a given speed can be varied by changing the field current.
[Electronics designer & programmer for 30+ years]
When the braking system is activated, the motor becomes a generator that is being driven by the transmission that connects it to the train wheels, and when the resistive load is connected it draws current from the generator. The current flowing in the motor armature generates a magnetic field that interacts with the field of the stator, producing a torque that is in opposition to the motion.
IT IS THIS RETARDING TORQUE IN THE MOTOR THAT CAUSES THE TRAIN TO DECELERATE.
The kinetic energy that is given up by the moving mass of the train is first converted into electrical energy then into heat in the braking resistors and dissipated to atmosphere. Hence the requirement for braking resistors to be well ventilated to carry away the heat.
The field supply is not reversed and the armature is not opened. The motor turns into a generator when it is commanded to reverse the torque to slow down the train. The generated energy is pump back into the DC buss and if there is nothing on the buss to absorb the energy the voltage will go to high and cause damage. Therefore, resistors are switched in across the bus when the voltage rises to a certain point.
To generate power with a permanent magnet generator or motor, work must be done. This means that if a rotating shaft (whether powered by a steam or water turbine or a train wheel) moves a magnet through a cage of wires, electricity is generated. This flows to the resistor bank and that heats up. So kinetic energy of the train is converted to heat. This is the same as if a mechanical brake's pads were rubbing on a rotor. Heat will be produced from rotary motion, and the vehicle will slow. Electric motor/generator and resistor bank braking is the same. This has the advantage that there is no surface-to-surface friction and wear.
Try "Electric Braking" or " Controlled Rheostatic Braking".
I'm presently looking at a very old book describing it, but would take far too long to put on here.
I did find the site below, the first page or so are blank, so just carry on with download.
How does rheostatic breaking slow a train down ?
H I'm a rolling stock technician and have a fairly good understanding of this subject , but I require a little more help off an Electrical Engineer to join all the dots so to speak . My understanding of this topic is put simply as follows , as the Train Driver selects braking , the supply to the motor armature is disconnected , but the field supply is STILL connected, now as the axle turns ,which is connected to the gearbox, which in turn is connected to the motor , the motor will start to generate electricity abit like a dynamo, so far so good , then resistor banks are switched in to dissipate the current that's generated/ produced, as heat . But how does this affect the speed at which the axle is turning? how does this change in electrical energy get converted back to mechanical energy or mechanical resistance to slow the Train , is it to do with an opposing magnetic field ? if so how is this generated and why ? Obviously I suspect friction with the rail and the weight of the train and maybe friction in the gearbox (maybe) will help to slow the Train ? but what I can't seem to grasp or get a definitive answer oe explanation to, is how the motor becoming a generator will slow the train , unless the field supply is reversed??? when the brakes are selected. However the stock I maintain the field is not reversed in the dc motor when braking is selected , the motor contactor that supplies the armature is opened and at the other side of the d.c motor armature output (in series with the GTO thyristor ,which gives a path to earth when fired in motoring) a contactor closes making a series parallel circuit through the resistors , at the same time the GTO thyristor is off in braking so no current will flow through it , so it takes the route of the resistors which are of course connected to a earth. Any guidance would be very very much appreciated
