Mixing Tank Motor and Agitator Structural Resonance
Excessive vibration levels in a mixing tank motor and agitator were caused by resonance at the stirrer blade pitch frequency. A bump test confirmed the resonance, and modifying the stirrer base to increase its natural frequency significantly reduced the vibration levels.
Structural Resonance.
Case of a motor and agitator of a mixing tank.
Technical data:
Motor 1200rpm (6p) full voltage.
Agitator 247rpm
3 stirring blades.
Random vibration with excessive levels reaching up to 1.00 in/s (rms) horizontally in both motor supports, vertically as a maximum level 0.14 in/s (rms), in spectra it is observed dominating the frequency of the stirrer blade pitch (BPF) at 740cpm, according to the difference between horizontal and vertical indicates resonance problems.
To confirm this condition, a bump test was carried out on the base of the motor horizontally in order to excite its natural frequency.
Test result, excitation of 4 natural frequencies:
the first Fn is located at 12.21Hz (732cpm) which coincides with the frequency of the stirrer's blade pitch.
It was recommended to modify the base of the stirrer giving greater rigidity to increase its natural frequency. This decision was made based on the study and the location of its Fn. The result was the following as can be seen in the global vibration trends, significant decrease from having 1.00 in/s (rms) to 0.150 in/s (rms) horizontally and 0.10 in/s (rms) vertically.
Additional Images
