MILW-RODR: Don’s response is accurate but terse. You say that you are looking for more in-depth knowledge. I did a quick Google search and found only a Wikipedia hit that has some information about locomotive dynamic brakes but it is only about 50% accurate. I will use Don’s response and expand a bit, but to truly understand the full depth of knowledge would require graphs and diagrams in addition to much more text than this forum can support. Maybe I should edit the Wikipedia page. My answers will be for DC locomotives only.
In braking, the motor field coils are connected to the main generator and the armature is connected to the braking resistors.
When the dynamic brake control handle is placed in the SETUP position the motor-brake transfer switch moves to the brake position. The traction motor fields are connected in series across the main generator and the traction motor armatures are connected in pairs to grid resistors; two armature/grid circuits for 4 axle units, three armature /grid circuits for 6 axle units. The braking field contactor (B on EMD) is not closed in setup.
When the dynamic brake control handle is placed in position 1 the braking field contactor is closed and the motor fields receive minimum excitation current from the main generator. If the motor armatures are rotating, generated electrical current will begin to flow from the motor armatures through the resistance grids. Minimum dynamic brake retarding will begin.
You regulate the braking by regulating the excitation of the main generator.
On locomotives that have a modular type traction control system it is the DR Module, on micro-processor control systems it is the DR Function. DR looks at the dynamic brake handle position, the motor field current and armature/grid current and determines the amount of main generator excitation. DR also sets the limits of motor field and armature/grid current and limits the main generator excitation to prevent exceeding the limits. The main generator excitation control circuit is modified to produce very low levels of main generator output current since 4 or 6 motor fields are the only load.
As the dynamic brake control handle is moved from position 1 toward position 8, the dynamic brake control signal will increase from about 8 volts (minimum DB) to 68+ volts (maximum DB). DR will in turn increase main generator excitation and motor field current that along with armature rotation speed will produce a grid current relative to the braking handle position/DB control voltage. This is “grid current” type regulation and is the most common way that DB is regulated.
As speed drops, you have to increase the current through the motor field coils to keep the braking resistance up. At some speed, you've hit the max excitation and braking effort tails as speed drops. The purpose of dynamic brake is to produce retarding force at the traction motors. The retarding force is produced by magnetism and is a combination of field current/magnetism and armature current/magnetism. At track speeds generally above 30 mph, in maximum DB, the armature/grid current is at its regulated maximum and higher speeds will require the motor field current to be reduced which reduces the retarding force accordingly. At track speeds generally below 25 mph the motor field current is at its regulated maximum and if speed continues to drop generated the armature/grid current and the motor’s retarding force reduces accordingly. Between 25 and 30 mph where the field and armature/grid currents are at maximum, the retarding will force be at maximum (for any given DB control handle position). Maximum retarding force for standard dynamic brake is 10,000 lb/ motor, high capacity dynamic brake is 12,500 lb/motor. The most effective retarding range of the dynamic brake is between about 15 to 45 mph, weakened but still good to 65 mph, weakening and not very effective below 15 mph, but DB is a retarding brake not a stopping brake.
Extended range braking shunts some of the braking resistors so that you can keep high braking force at low speeds. Current from each armature pair passes through 4 grid “sections” that convert the electrical energy to heat. All of the resistors are necessary at track speeds above 20 mph to dissipate the full amount of energy generated. Below 20 mph armature rotation has slowed to where the resistance of the grids reduces the ability of the armatures to generate current/magnetism. Extended range braking control shunts (bypasses) one grid section at a time as speed is reduced, lowering the grid resistance. Armature/grid current will increase, restoring the retarding force. The DB regulator will adjust main generator excitation and the resulting motor field current to prevent over-shoot of the armature current which can damage the remaining grids and also surge the retarding force making the wheels slide.
As speed decreases below 20 mph, the extended range control will shunt the grid sections at about 18, 12, and 6 mph leaving one grid section in the circuit. The final increase in retardation, though not enough to completely stop the train itself can bring the speed down to where on a nearly level grade a locomotive independent brake application will bring the train to a final stop. The dynamic brake’s effective retarding range has been extended to nearly a stop.