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Tuesday, January 20, 2009

Stepper Motor Data 3

How To Select A Step Motor Driver

A step motor driver provides precisely controllable speed and positioning.
The motor increments a precise amount with each control pulse easily

converting digital information to exact incremental rotation without the
need for feedback devices such as tachometers or encoders. Because
the system is open loop, the problems of feedback loop phase shift and
resultant instability, common with servo drives, are eliminated.
Load characteristics, performance requirements, and mechanical design
including coupling techniques must be thoroughly considered before
the designer can effectively select the most suitable motor and driver
combination for an application.

Torque-angle curves for the stepping motor

(a) permanent-magnet rotor and (b) variable-reluctance rotor.

currents. Reversing the phase currents will cause the rotor to reverse
its orientation. This is in contrast to VRM configuration with
a ferromagnetic rotor, in which two rotor positions are equally stable
for any particular set of phase currents, and hence the rotor position
cannot be determined uniquely. Permanent-magnet stepping motors
are also unlike their VRM counterparts in that torque tending to align
the rotor with the stator poles will be generated even when there is
no excitation applied to the phase windings. Thus the rotor will have
preferred unexcited rest positions, a fact which can be used to advantage
in some applications.