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Each time
you step the motor, you electronically move the
equilibrium position S radians. This moves the entire
curve illustrated in Figure 2.1 a distance of S radians,
as shown in Figure 2.6:
Figure 2.6 
The first
thing to note about the process of taking one step is
that the maximum available torque is at a minimum when
the rotor is halfway from one step to the next. This
minimum determines the running torque, the
maximum torque the motor can drive as it steps slowly
forward. For common two-winding permanent magnet motors
with ideal sinusoidal torque versus position curves and
holding torque h, this will be h/(20.5). If
the motor is stepped by powering two windings at a time,
the running torque of an ideal two-winding permanent
magnet motor will be the same as the single-winding
holding torque.
It shoud be
noted that at higher stepping speeds, the running torque
is sometimes defined as the pull-out torque. That
is, it is the maximum frictional torque the motor can
overcome on a rotating load before the load is pulled
out of step by the friction. Some motor data sheets
define a second torque figure, the pull-in torque.
This is the maximum frictional torque that the motor can
overcome to accelerate a stopped load to synchronous
speed. The pull-in torques documented on stepping motor
data sheets are of questionable value because the
pull-in torque depends on the moment of inertia of the
load used when they were measured, and few motor data
sheets document this!
In practice,
there is always some friction, so after the equilibrium
position moves one step, the rotor is likely to
oscillate briefly about the new equilibrium position.
The resulting trajectory may resemble the one shown in
Figure 2.7:
Figure 2.7 
Here, the
trajectory of the equilibrium position is shown as a
dotted line, while the solid curve shows the trajectory
of the motor rotor. |
