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Techno Linear Motion Catalog 25 Technical Information 6)    Motors and Encoders Electric  motors  are  by  far  the  most  common  component  to  supply  mechanical  input  to  a  linear  motion system.  Stepper motors and servo motors are the popular choices in linear motion machinery due to their accuracy  and  controllability.    They  exhibit  favorable  torque-speed  characteristics  and  are  relatively inexpensive.  The term NEMA refers to the physical size of the motor, and has become an industry standard. All motors of the same NEMA frame size should exhibit the same dimensions.  This section will discuss the features associated with each of these types of motors. Stepper motors convert digital pulse and direction signals into rotary motion and are easily controlled. Although  stepper  motors  can  be  used  in  combination  with  analog  or  digital  feedback  signals,  they  are usually  used  without  feedback  (open  loop).    Stepper  motors  require  motor  driving  voltage  and  control electronics. The rotor of a typical hybrid stepper motor has two soft iron cups that surround a permanent magnet which is axially magnetized.  The rotor cups have 50 teeth on their surfaces and guide the flux through the rotor- stator air gap.  In most cases, the teeth of one set are offset from the teeth of the other by one-half tooth pitch for a two phase stepper motor. The stator generally has the same number of teeth as the rotor, but can have two fewer depending upon the motor’s design.  When the teeth on the stator pole are energized with North polarity, the corresponding teeth on the rotor with South polarity align with them.  Similarly, teeth on the stator pole energized with South polarity attract corresponding teeth on the rotor that are energized with North polarity.  By changing the polarity of neighboring stator teeth one after the other in a rotating sequence, the rotor begins to turn correspondingly as its teeth try to align themselves with the stator teeth.  The strength of the magnetic fields can be precisely controlled by the amount of current through the windings, thus the position of the rotor can be precisely controlled by these attractive and repulsive forces. There are many advantages to using stepper motors.  Since maximum dynamic torque occurs at low pulse rates (low speeds), stepper motors can easily accelerate a load.  Stepper motors have large holding torque and stiffness, so there is usually no need for clutches and brakes (unless a large external load is acting, such as gravity).  Stepper motors are inherently digital.  The number of pulses determines position while the pulse  frequency  determines  velocity.   Additional  advantages  are  that  they  are  inexpensive,  easily  and accurately controlled, and there are no brushes to maintain.  Also, they offer excellent heat dissipation, and they are very stiff motors with high holding torques for their size.  The digital nature of stepper motors also eliminates tuning parameters. There are disadvantages associated with stepper motors.  One of the largest disadvantages is that the torque decreases as velocity is increased.  Because most stepper motors operate open loop with no position sensing devices, the motor can stall or lose position if the load torque exceeds the motor’s available torque. Open loop stepper motor systems should not be used for high-performance or high-load applications, unless they are significantly derated.  Another drawback is that damping may be required when load inertia is very Stator Windings Rotor Permanent Magnet