Steve the motor will most likely has a cast rotor as most from
that vendor will have. The problem is normally voids in the
casting and not a RBPF problem. If you don't see harmonics
of RBPF and the peaks are below 2 G-s on new motors and 3 G-s
on a rebuilb motor it probably is more of a power consumption
issue than a mechanical issue.

The only time I have seen this become a issue is in
cases where motors are loaded to 100% of full load
for example on chillers.

Please keep us posted as you get more info.

Barry,

I have attached some data from the new motor that was installed. It is a Marathon as opposed to the Louis Allis. Everything else is identical. Unfortunately the customer took the old motor back, so we are not able to cut open the rotor. We ran the motor in the shop with no load, and it actually sounded worse than when it was in the field. I checked the amperage on the new motor in the field, and all phases were equal and below rated amps. The motor is running pretty smooth, and we believe the problem has been solved. I am going to try to get the rotor back from the customer to see if we can come to a solid conclusion. Please let me know if you have any further input.

Thanks.

Motor_Data5.pdf (25 Kb, 31 downloads)

The key to your description of this problem is that you have been able to accurately identify the tests performed on the stator only. What were the tests performed on the rotor, itself? If they performed a growler test, did they heat the rotor first? Did they wrap a coil around the rotor and look for hotspots with an infrared camera? Was a dye test performed? Did they single phase the motor at 10% voltage and rotate the shaft looking for current not changing by more than 3%?

Two ways that you can check to verify the condition of the rotor include: 1) Using current signature analysis and looking for pole pass frequency sidebands around line frequency; or, 2) Use an analog ampmeter and measure the current at the motor. If there is a repeating 'ticking' of the current needle, then you have bad rotor bars.

Other things that can cause similar issues: Loose coil(s) in the stator; Loose rotor shaft; and/or broken bars and loose bars.