What are resonances?
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With its rotor inertia and magnetic retaining forces, each stepper motor constitutes a damped oscillating system with low inherent damping. Thus vibrations can be induced that superimpose the actual step movements and a decrease in torque, loss of step or even a change of direction of rotation can occur.
Similar to synchronous motors or as a spring oscillating system, stepper motors can also be envisaged where the rotor follows the exciting field within narrow bounds. The higher the exciting field, the higher the stiffness and the more pronounced the tendency to oscillate and natural resonance frequency.
At the natural resonant frequencies the rotor loses the rotating field of the stator or synchronicity and a more pronounced loss of torque appears so that the motor disengages after a short time, usually can also no longer engage again and comes to a standstill. If a motor is now operated near the natural resonance frequency, the rotor can “shake" or swing up and the motor drops out of synchronism. Resonances are associated with pronounced running noise.
Stepper motors should, therefore, ideally be operated with a flange-mounted load. This base load, that corresponds to damping, is often already sufficient to allow both low resonance running as well as a safe frequency run-up. In extremely rare cases the motor can regain its synchronicity in the next rotary field phase (due to a brief overload or overshoot in the positive or negative direction of rotation, the rotor drops into the next stable rotary field position).
Because the resonance in stepper motors is due to their design, it can only be reduced or eliminated in part by the application of special procedures.
