Matt, just trying to visualise the configuration of the motors, do I understand that the field winding of one motor is connected in series with the armature of a companion, and likewise for the second? I can't quite get my head around how it might work, so if you have time, further comment would be appreciated.
I don't have diagrams of a dynamic brake car, but I do have diagrams of Sydney and Melbourne car electrics.
I also can't quite get my head around the psychics involved. (right hand rule, magnetic flux, induced currents, etc), which makes it difficult to explain.
All motors can be generators.
In the 4 motor cars - the motors are always in parallel pairs.
The field coils are always in series with each armature of it's own motor (It is after all a 'series' motor, if the fields were not in series it the motor would have another name!).
Series motors are used as they are simple to control and have great starting torque.
All motors when running are also generators. A series motor reaches it's maximum speed for a given load when the 'Back ElectroMotive Force' it is generating is fully counteracting the input voltage. (Balancing speed).
If the reverser is off, the armature and fields are disconnected from one another. The car will freely roll. The magnetism is still there, but electricity needs to flow.
If the reverser is forward, the motors are connected to one another, but the car will roll forward as the 'back EMF' generated by the motor will counteract and neutralise the fields, the whole generator/motor effect is rendered ineffective.
If the car rolls backwards however, the generated force is no longer 'backwards', but instead re-reinforces the weak residual fields and the motor generates more and more power. This power feeds into the paired motor, which then tries to rotate in the opposite direction. At the same time this other motor is also generating power and trying to drive the first one backwards.
A single motor on it's own can brake - this is called 'plug braking' and is often used on things like electric forklifts and golf carts. They disconnect the supply (the battery) throw the motor into reverse (reverse either the field or armature polarity) and short the output. The generated current is forced straight back into the motor. The motor effectively tries to drive itself in reverse. The motors get pretty hot pretty quickly. Forklift motors are built to take this sort of punishment.
The '4th' emergency as taught to tram drivers is a form of dynamic brake - 'pull the key back'. By putting the tram in reverse, even with out applying any power the motors get into a configuration where the fields reinforce instead of collapse, and the resulting braking effect is savage. If done at any speed, the voltages generated can ruin the motors.
Dynamic braking is just 4th emergency with external resistors switched in to control the amount of force generated. The physics is the same.
But they don't explicitly cross connect the fields and armatures as you suggested. The armature and it's field stay in series with each other. It's just connected to another motor in similar configuration. The key to making it work is the motor has to be 'reversed'.
You reverse a series motor by swapping the polarity of either the armature or the field. Swap both and the motor keeps going in the same direction.
So on all the trams, the field and armature connections all run up to the reverser drum, so while the armature and it's field always stay in series, a mass of wires go to the reverser drum so they can control which polarity is applied the armature and the field together.
I've probably just confused things more
. I really need a refresher on my high school physics.