Working principle
A brushed DC motor produces torque from the force on a current-carrying conductor sitting in a magnetic field. The field comes from permanent magnets or field windings in the stator; the current flows through armature coils on the rotor. A mechanical commutator and a pair of carbon brushes reverse the armature current every half turn, so the torque always pushes the rotor in the same direction. That commutator is what makes the machine self-running on plain direct current.
Back-EMF and speed control
As the armature spins it generates a back-EMF that opposes the applied voltage. The current that actually flows is the difference between supply voltage and back-EMF divided by the armature resistance, which makes the motor naturally self-regulating: load it down, it slows, back-EMF falls, current and torque rise. Speed is very nearly proportional to applied voltage, so a simple voltage or PWM adjustment gives smooth, linear speed control across the whole range.
Starting current and torque
At the instant of switch-on the rotor is stationary, there is no back-EMF, and only the small armature resistance limits the current. Stall or starting current can therefore be many times the rated value, which is also why a brushed DC motor delivers very high torque from zero speed. The same relationship means a mechanically jammed motor sits at stall current indefinitely and will overheat quickly unless a controller limits it.
Advantages and limits
Brushed DC motors are inexpensive, need no electronic commutation and are trivial to drive, which keeps them in toys, automotive actuators, power tools and small hobby projects. The trade-off is the brush gear: brushes wear, generate dust and sparks, limit maximum speed and demand periodic maintenance. Where longer life or higher efficiency matters, a brushless DC motor performs the same commutation electronically instead.