Festo MTRE-ST42-48S Serie Instrucciones De Utilizacion página 29

MTRE−ST42−48S−..
Basic information
Despite the simplicity of the stepping motors, it should be mentioned that it is not
a controller. The control system does not receive any kind of acknowledgement
whether the motor has, in fact, performed its step.
And so it is extremely important to dimension the motor using its speed−torque
characteristic curve. This ensures that the applied load can, in fact, be moved by
the motor with the specified acceleration and speed.
If the motor loses a step, that is, if the applied load is greater than the force the
motor can produce, the part to be positioned is no longer in the desired position.
Errors that must be monitored in the overall system, such as through limit
switches, can thus occur.
Start−stop frequency
Another important variable in the use of stepping motors is the so−called start−stop
frequency. It is the frequency or speed at which the stepping motor can safely, that
is, without loss of step, start and also stop again under the torque to be provided.
Losses of step can occur when decelerating (stopping) just as much as when accel
erating!
Resonance frequencies
Impressing of a fixed current for the maximum holding torque of a stepping motor
can cause the motor to tend to oscillate. Since the stepping motor is a synchron
ous motor, it follows the set field within very narrow limits. If no load is applied,
the rotor oscillates beyond the targeted step due to the very low inertia. If excita
tion of the next step takes place at an unfavourable time, the system resonates.
The following possibilities exist to reduce the incidence of this resonance.
In practice, one tries to put the start−stop frequency into a higher area (ca. 200
Hz and higher), since stepping motors in the lower frequency areas have more
resonance frequencies.
One can combat it by increasing the start−stop frequency or through lower
phase current.
Conversion from full−step to half−step or quarter−step operation can also pro
vide help.
It is helpful to always operate the motor with its nominal load. If that is not
possible, the phase current should be reduced.
Festo MTRE−ST42−48S−.. 0407a English
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