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Brushed DC motors are composed of a rotor with electromagnets and a stator with permanent magnets. The following graphics illustrate a simple, two-pole, brushed, DC motor.

 

When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation. The armature continues to rotate. When the armature becomes horizontally aligned, the torque becomes zero. At this point, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats, causing continous rotation

Commercial motors have many more coils but they follow this same principle. Controlling such a motor simply requires that a DC voltage be applied to the brushes. Voltage polarity sets the direction. Voltage level sets the speed.  

The permanent magnets can be substituted with electromagnets. Motors with electromagnets are called Shunt, Series wound, or Sepex motors depending on their design. Roboteq controllers are best suited for Permanent Magnet DC motors, altough they can be made to work with electromagnet based motors. See this video from Learning Engineering for a visual illustration on how DC Brushed Motors are constructed and work.

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