Stepper Motor
Stepper motors offer many advantages. Although feedback is not
usually required, stepper motors are compatible with feedback
signals, either analog or digital. Error is noncumulative as long as
pulse-to-step integrity is maintained by the stepper motor. A stream
of pulses can be counted into stepper motors, and the stepper
motor's final position will be known within a small percentage of
one step.
Since maximum dynamic torque occurs at low pulse rates, stepping
motors can easily accelerate a load. When the desired position is
reached and command pulses cease, the stepper motor shaft stops and
there is no need for clutches or brakes. The stepper motor is
generally left energized at a stop position. Once stopped, the
stepper motor resists dynamic movement up to the value of the
holding torque. An additional feature of the PM stepper motor is
that when all power is removed, it is magnetically detented in the
last position. A wide range of step angles are available -- 1.8 to
80°, for example -- without logic manipulation. Stepper motors have
inherent low velocity without gear reduction. A typical stepper
motor driven at 500 pps turns at 150 rpm. The stepper motor's rotor
inertia is usually low. Multiple stepper motors driven from the same
source maintain perfect synchronization.
But the stepper motor's efficiency is low; much of the input
energy must be dissipated as heat. Load must be analyzed carefully
for optimum stepper motor performance. And inputs must be matched to
the stepper motor and load. Damping may be required when load
inertia is exceptionally high to prevent oscillation.
Stepper Motor - Excitation modes:
Stepper motors can be excited in different modes, depending on
stator winding and desired performance.
Stepper Motor - Two phase: One entire
phase (stator winding) of the stepper motor, end-tap to end-tap is
energized at a given moment in time. Input current and wattage are
halved (compared to four-phase excitation), and heat dissipation is
decreased. Output can be improved by as much as 10%. In the stepper
motor's two-phase modified mode, both windings (end-tap to end-tap)
are energized simultaneously. Energy input in this mode is the same
as four phase, but output performance is increased by about 40%. The
stepper motor control is complex and costly for this
mode.
Stepper Motor: Basics of Stepper Motor Design
Engineering
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