The most common types rely on the forces produced by magnetic fields. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current flow in part of the motor. DC motors were the first type widely used, since they could be powered from existing direct-current lighting power distribution systems. A DC motor’s speed can be controlled over a wide range, using either a variable supply voltage or by changing the strength of current characteristics of dc shunt generator pdf its field windings.
Small DC motors are used in tools, toys, and appliances. Larger DC motors are used in propulsion of electric vehicles, elevator and hoists, or in drives for steel rolling mills. The direction and magnitude of the magnetic field produced by the coil can be changed with the direction and magnitude of the current flowing through it. The windings usually have multiple turns around the core, and in large motors there can be several parallel current paths. The commutator allows each armature coil to be energized in turn and connects the rotating coils with the external power supply through brushes. Brushless DC motors have electronics that switch the DC current to each coil on and off and have no brushes.
The total amount of current sent to the coil, the coil’s size and what it’s wrapped around dictate the strength of the electromagnetic field created. The sequence of turning a particular coil on or off dictates what direction the effective electromagnetic fields are pointed. By turning on and off coils in sequence a rotating magnetic field can be created. In some DC motor designs the stator fields use electromagnets to create their magnetic fields which allow greater control over the motor.
At high power levels, DC motors are almost always cooled using forced air. The speed of a DC motor can be controlled by changing the voltage applied to the armature. The introduction of variable resistance in the armature circuit or field circuit allowed speed control. DC current into on and off cycles which have an effective lower voltage.
Today DC motors are still found in applications as small as toys and disk drives, or in large sizes to operate steel rolling mills and paper machines. These are now replaced with large AC motors with variable frequency drives. In diesel electric locomotives they also use their DC motors as generators to slow down but dissipate the energy in resistor stacks. Newer designs are adding large battery packs to recapture some of this energy. A brushed DC electric motor generating torque from DC power supply by using an internal mechanical commutation. Stationary permanent magnets form the stator field. Torque is produced by the principle that any current-carrying conductor placed within an external magnetic field experiences a force, known as Lorentz force.