Loose Rotor Bars Vibration

Hello all,

I took vibration measurement on HV Motor,

RPM: 990
Synch RPM: 1000
Poles: 6
No. Of Rotor Bars: 88
1XRBPF(Rotor Bar Pass Freq)= 88*990=87120 RPM/60= 1452 Hz
2XRBPF= 2904
2xLF= 100Hz
Bearings: 6324 & 6319

I got very less vibration in overall amplitude. But my concern is when took high frequency spectrum, I get 1XRBPF (Rotor Bar Pass Freq) with 2xLF (Line freq) sidebands and 2XRBPF with those sidebands as well.

2XRBPF amplitude is higher than the 1XRBPF.

There is some sort of rattling noise coming from the motor. There is no bearing damage as per the spectrum because of low vibration and less G values as well.

As per my study I found that 2XRBPF with 2xLF sidebands is much more unusual fault and may indicate the looseness of rotor bars.

Anyone has any idea or came across this type of problem before, please share some ideas or vies.

Thank you


The rattling noise is indicative of multiple loose bars. What instrument are you using? Also, Voltage and HP or kW of motor
Regards, Phil Corso
ps: Is it your intention to keep the motor in service?
In a past life I have repaired quite a number of large MV and LV motors with loose rotor bars. Much depends on the construction of the rotor. If there are gaps in the iron stack (for the purpose of ventilation/cooling) you can usually make slot wedging work. Usually requires also fiberglass banding over the end-rings, and VPI helps if wedging was applied. In any case, this will get worse over time and will result in more or less catastrophic failure.