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STEP MOTORS

  A step motor is a device that converts electrical pulses into mechanical movements. Conventional motors rotate continuously, but a step motor, when pulsed, rotates (steps) in fixed angular increments. Step size, or step angle, is determined by the construction of the motor and the type of drive scheme used to control it. Traditionally, step resolution has ranged from 90 degrees (four steps per rev) to a fraction of a degree, though 15 degrees (12 steps

per rev), to 1.8 degrees (200 steps per rev) has been most common. More recently, however, microstep motors have been introduced that are capable of .0144 degree steps (25,000 steps per rev). Microstep motors are hybrid 200 step per rev motors that are electrically controlled to produce 25,000 steps per rev. Step motors are usually used in open loop control systems, Figure 10, though an encoder may be used to confirm positioning accuracy. 

  There are many types of step-motor construction. However, permanent magnet (PM) and variable reluctance (VR) are the most common types.


 PM step motors
  The permanent magnet step motor is also referred to as a synchronous inductor motor. It moves in steps when its windings are sequentially energized, or it can operate as a low speed synchronous motor when operated from a two-phase ac power source. Figure 11 illustrates a permanent magnet rotor surrounded by a twophase stator. Two rotor sections (N and S) are offset by one half tooth pitch to each other. As energy is switched from phase 2 to phase 1, a
set of rotor magnets will align with phase 1, and the rotor will turn one step. If both phases are energized simultaneously, the rotor will establish its equilibrium midway between steps. Thus, the motor is said to be half-stepping.

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