What are the hazards of frequency conversion for pumps? Is the alignment of the coupling for variable-frequency pumps different?

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Our plant operates long-distance pipelines for transporting gasoline and diesel. To save energy and facilitate process control, variable frequency drives have been installed on several pump units (operating generally in the 35HZ–normal frequency range). However, based on our years of experience with these variable frequency units, we have observed that vibration levels increase at certain frequencies; moreover, these units are more prone to problems such as vibration and seal leakage compared to conventional pump units. When addressing the issue of excessive vibration in a variable-frequency pump, tests on the motor alone revealed suspected axial movement at frequencies of 35HZ–38HZ (the motor’s bearings are of the roller type). It is planned to conduct further tests on the motor to determine the position of the magnetic field lines at various frequencies (this pump is a 6-stage pump). The pump is an imported Luer pump, usually of type BB1 or BB3, and the motor is a Siemens motor. I would like to ask an expert: 1. Is it acceptable for the motor shaft to shift axially while the pump unit is in operation? (This pump has been repaired multiple times due to vibration issues; when testing the motor at power frequency, wood was used to block the motor shaft, and the shaft would shift axially even with minimal force applied.) 2. First purchase the pump unit, and then add a frequency converter for modification. What is the impact of frequency conversion on the operation of the pump unit, especially on the load on the thrust bearings and the mechanical seal? 3. If the filtering in the inverter is not proper, can peak voltages cause the motor shaft to shift? 4. If axial play of the variable-frequency motor shaft is allowed (the position of the magnetic flux lines may vary at different frequencies), how should the coupling be aligned? Can alignment also be performed using the power-frequency magnetic field lines as a reference?