Axial shift problem of variable-frequency pump motor shaft
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There is an imported pump (type BB3, 6 stages, with a rated flow rate of 310 m3/h and a head of 315 m), equipped with a 355 kW Siemens motor featuring sliding bearings. After approximately 3,000 hours of operation, frequency conversion control was added to the pump through modifications. Remove the coupling (length 200 mm) and test the motor individually to locate the magnetic field lines. Measure the installation spacing of the coupling: 199.3 mm at 50 Hz, 210 mm at 38 Hz, and 205 mm at 35 Hz. From these values, it can be seen that there is significant axial movement of the motor shaft during variable frequency operation. Please discuss the following questions: 1. Is such axial movement of the motor shaft normal during variable frequency operation? 2. What impact does this axial movement have on the operation of the pump system? 3. What causes the motor shaft to shift? Note: This pump has been overhauled multiple times due to excessive vibration, and its thrust bearings have been replaced several times (the ball surfaces are worn out, with severe wear and scuffing).
Also, I want to know why the motor has such a large drift!
I’m not an expert on motors, but I’ll still offer my opinion: the pump keeps needing repairs due to high vibration levels, and these repairs haven’t been effective. It’s recommended to hire a different repair team, or invest in finding someone skilled in pump maintenance. I guess the amount of money spent on replacing bearings could be enough to hire a qualified mechanic.
I think the issue with the motor needs to be resolved first; could the problem with the pump be caused by the motor shaft shifting? It’s hard to tell right now! The coupling does not have such a large axial compensation capacity! It is recommended to address the motor issue first before checking the pump’s performance!
I would like to know whether there are any requirements regarding the axial movement of motor shafts during motor design, manufacturing, and operation, and whether there are any standard guidelines to guide this. I searched for some information online; I’m not sure about the source of the standard, as shown in the image below.
Additional note: Earlier, instruments were used for testing: when testing the motor alone at different frequencies, its speed remained stable with no fluctuations. If it is a problem with the inverter, could it be related to issues with the peak voltage of the output voltage? Please ask electrical professionals for clarification.
The coupling’s compensation capacity is insufficient; the axial force of the motor is transmitted to the pump, and the excessive axial force on the pump rotor damages the thrust bearing. It is still necessary to address the issue of high axial force in the motor