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Techno Linear Motion Catalog 26 Technical Information Permanent Magnets (Rotor) Brushless Motor Armature (Stator) Coils Commutator Brush Brush Brushed DC Motor Heat Heat high to prevent motor shaft oscillation at resonance points.  Finally, stepper motors may perform poorly in high-speed applications.  The maximum steps/sec rate of the motor and drive system should be considered, carefully. Servo motors are more robust than stepper motors, but pose a more difficult control problem.  They are primarily used in applications where speed, power, noise level as well as velocity and positional accuracy are important.  Servo motors are not functional without sensor feedback;  they are designed and intended to be applied in combination with resolvers, tachometers, or encoders (closed loop).  There are several types of servo motors, and three of the more common types are described as follows. The DC brush type are most commonly found in low-end to mid-range CNC machinery.  The “brush” refers to brushes that pass electric current to the rotor of the rotating core of the motor.  The construction consists of a magnet stator outside and a coil rotor inside.  A brush DC motor has more than one coil.  Each coil is angularly  displaced  from  one  another  so  when  the  torque  from  one  coil  has  dropped  off,  current  is automatically switched to another coil which is properly located to produce maximum torque.  The switching is accomplished mechanically by the brushes and a commutator as shown below. There are distinct advantages to using DC brush servo motors.  They are very inexpensive to apply.  The motor commutates itself with the brushes and it appears as a simple, two-terminal device that is easily controlled.  Among the disadvantages is the fact that they are thermally inefficient, because the heat must dissipate through the external magnets.  This condition reduces the torque to volume ratio, and the motor performance may suffer inefficiencies.  Also, the brushed motor will require maintenance, as the brushes will wear and need replacement.  Brushed servo motors are usually operated under 5000 rpm. The DC brushless type offers a higher level of performance.  They are often referred to as “inside out” DC motors because of their design.  The windings of a brushless motor are located in the outer portion of the motor (stator), and the rotor is constructed from permanent magnets as shown below.  DC brushless motors are typically applied to high-end CNC machinery, but the future may see midrange machinery use brushless technology due to the narrowing cost gap.