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Danaher Motion Superior Electric
SS2000MD4 18 400030-
043 Rev G
SECTION 5: TORQUE VERSUS SPEED
CHARACTERISTICS
5.1 MOTOR PERFORMANCE
All stepper motors exhibit instability at their natural frequency and harmonics of that frequency.
Typically, this instability occurs at speeds between 50 and 1000 full steps per second and,
depending on the dynamic motor load parameters, cause excessive velocity m
odulation or improper
positioning. This type of instability is represented by the open area at the low end of each Torque
vs. Speed curve.
There are also other instabilities that cause a loss of torque at stepping rates outside the range of
natural resonance frequencies. One such instability is broadly defined as mid-range instability.
Usually, the damping of the system and acceleration/decelera
tion through the resonance areas aid in
reducing instability to a level that provides smooth shaft velocity and accurate positioning. If
instability does cause unacceptable performance under actual operating conditions, use the
following techniques to reduce velocity modulation.
1) Avoid constant speed operation at the motors unstable frequencies. Select a base speed
above the motors resonant frequencies and adjust acceleration and deceleration to move
the motor through unstable regions quickly.
2) The motor winding current can be reduced as discussed in Section 4.5. Lowering the
current reduces torque proportionally. The reduced energy delivered to the motor can
decrease velocity modulation.
3) Use the half-step mode of operation or use microstepping to provide smoother operation
and reduce the effects of mid range instability. Microstepping reduces the shaft speed
for a given pulse input rate.