I am using AT PRO 4.41
Instrument ATPOL II

I had to measure, from the MCC, submerged motors of 5 kW.
These are directly mounted on a gearbox with an output speed of 68 rpm driving a 4 blade propeller.
They are used for displacement of sewage water.
The drives are resting on a pedestal but are not attached with bolts.
The problem with these “pumps” is that the blades get dirty or damaged resulting in high vibration. This vibration destroys the foundation were the machines rest upon.
To repair these foundations they have to use a diver and a crane. Which both are expensive.
They asked our company to perform on line measurements in the hope to avoid these costs.

I am not used to analyse this low speed with ESA, so my question is what do I have to look for?
Peaks in the low frequency demod spectra at 68 rpm?, and if so what about the severity.

Regards,
Antoine Horn

Antoine

You will most likely see a demod spectra for the blades at about 4.5 Hz (blade pass) and watch for a frequency at about 1.1 Hz (running speed).

These will normally carry back to the motor, as we observed in Coast Guard bow and stern thrusters.

You will not want to try to set alarms based on the demod spectra, but instead look to the low frequency, high resolution spectra at the center of the screen. Alarms should normally be considered above 65 dB down or 10 dB above the noise floor for the 1.1 Hz and 4.5 Hz sidebands.

If the motor is directly connected to the gearbox, you will also want to watch for the 1x rmp (in Hz) sidebands related to the actual motor speed. At 65 dB down, or worse, the vibration will be relatively high.

Howard