Salam and hello everybody,

We are facing a problem in majority of our motors, where Rotor Bar Pass frequency appears in vibration spectra. In some cases, RBP Frequency has harmonics and sidebands as well which are spaced either at 1x line frequency or running speed. Moreover Line Frequency is also present in such cases. Amongst these motors some of them were running at high vibration i.e. 10 -15 mm/sec and main contributing peaks were Line frequency and RBP frequency. Upon our recommendations, the said motors were inspected thoroughly for rotor runout, gap b/w rotor and stator, winding condition, condition of rotor bars, unbalance of rotor etc. and no abnormality was found. In the end, the motors were installed back and are showing same vibration behavior as past. Moreover, current, voltage, line to line current and line to line voltage were also checked at rated load but all the readings were found within specified range. The electrical guys say that the motors are quite fine. But I am convinced that the problem is electrical related. But on the other hand, thorough inspection could not reveal any abnormality.

We have motor overhauling facility at our plant which was established one and a half year ago. I suspect that there is some common mistake that is occurring during overhaul of all motors. Because the history data of same motors does not show any indication of such electrical peaks and these peaks have started to appear after since the motors were overhauled at our plant site facility.

Can anybody suggest further checks?

Regards,

Yaser

RBPF is quite common in motors and are normally seen with 2xLF sidebands. When RBPF is present I always collect data to get the 2xRBPF. If the 2x amplitudes are larger I know there is a problem. I would not remove the motor from service for this. I would trend the results and make my decision for repair based on several sets of data.

I see RBPF more often in motors when operating with a drive. If possible run the motor in ByPass or across the line to get accurate results. Also the motor should be operating at full load.

ESA is also suggested if the equipment is available. This will confirm if there are any electrical problems.

When in the motor shop have a Growler test performed on the rotor to find any broken rotor bars.