Hi All,
I was monitoring a Induction Motor  at 2994 RPM & 1700 kw. coupled to Centrifugal pump.
which present high vibration in all directions and the same spectrum, 7 mm/sec O/All.
the spectrum show one peak 1x rpm whith side band of Pole Pass sidebands.
In solo run condition the high vibration desapear.
also the periodic current variation is present ( 1 cycle / 2.66 sec) the variation of current is 5 amps aproximatly.
the motor show beating with high sound.
The pressure is steady at 200 Kg/cm2.
I would like to share this case to look up the cause.

Ramon Ruiz (vibration CAT II)


Photos (2)
Original Post

Hi ramonruiz,

Can you zoom the spectrum and point out the difference of the sidebands? have you know the number of rotor bars? Also, can you take phase data for determine if the bearing is cocked? how long does this pump running since commissioning? Is the current is balance when fluctuating? may we know the data? can you check it through poweranalyzer? i have a lots of queries since you haven't mention what you have done in the equipment. Also, I would not say the bearing is loose in housing since the waveform is telling modulating. 


HI Rotating Guy,  thanks for your interest,

Phase in bearing , OB H-V : 84 grades IB H-V: 57 grades
this pump is only for standby condition, more than 10 years was installed.
 the current readings are:
L1: 115 amp
L2: 116 amp
L3: 115 amp
we can not do power analizer or Motor Current Signature Analysis and other special electrical test, only portable data collector.
we did alignment corrections, bearing inspection of motor, magnetic center.
you can see the Beating from side bands.
the frequency of beating is 2.66 sec per cycle, and also variation of current is the same frequency, and 1/2.66 sec= 0.39 Hz.
FL= 50 Hz (theorical)
 but 2xFL= 2x50.11hz=100.2hz (spectrum), FL real=50.1Hz (substation )
peak 2xrpm= 99.82 hz
slip= 0.18 hz
number of poles: 2
Rotor bars: 38
Slots: 48



Photos (4)

Looks like classical rotor bar defect. Check motor current signature and compare the ppf wrt LF. If less than 45db then that is it. Motor bar is defected.

if Mcsa normal the mostly SOft foot. While motor is running loose one bolt at a time and see the effect.

if MCSA not possible and motor not responding to sf test then better to inspect the rotor at workshop.

yeah looks like a rotor bar issue, some things id like to see also is what does the rotor bar pass look like? does it have 2xlf sidebands? is there a presence of 2x rotor bar pass? if I don't have the ability to use the rotor evaluation test with the PdMA tester id like to also see these before I made a call to shut one down and send it to the shop.


1.-what does the rotor bar pass look like? 

R: rotor bar frequencies with small amplitude  in high frequency spectrum. 

2.-does it have 2xlf sidebands?

R: you can see in picture spectrum zoom2x (high resolution spectrum): 2xrmp and 2xFL are present also slip x # poles.

3.-is there a presence of 2x rotor bar pass? 

can you explain why we need 2x RBfrequiency? 

Im agree with    ¨if MCSA not possible and motor not responding to sf test then better to inspect the rotor at workshop¨.

heres an attachment for one I had. This was a 2 pole motor on 60hz LF. Rotor bar pass is clearly seen here as it had 51 bars, so that put us close to 3000hz for rotor bar pass. In this one there is 120hz sidebands around the RB pass frequency. There is also some low level activity at 6000hz which is 2xRb pass with some sideband action too. it doesn't look like much but its there, I was magnet mounted not stud mounted so the sensitivity at that high of a frequency is bad, but its still there.


Photos (1)

Dear Chirs, all high frequiencies  peaks still are with low amplitude. 

but the question are:

whats means Pole Side Bands  around of 1x, 2x, 3x rpm,   and how can know if they are dangerous or not.. or what problem in motor is the cause of this phenomenon? 

the current variation showed in video is caused by the same cause of Pole Side Bands? 





in sort these, are all indications of open or broken rotor bars, however, severity can only really be determined by current signature analysis. I'm only adding in the rotor bar pass and 2x rotor bar pass with the sidebands as further indications since you do not have access to the current signature analysis. Can a motor run with an open bar? Sure, but it will get to the point that it wont be able to pull the load if its a cast rotor. If its a rotor with solid copper bars...well, a loose bar *could* come out of the rotor cage and damage the stator and windings. Something else you could try for concrete proof before pulling the motor out is perform a single phase rotor test.


If you disconnect the motor from the electrical supply, apply 1/4th of the rated voltage to two leads, ( for example on a 480vac motor apply single phase 120volts to motor lead t-1 and t-3) use a ammeter on the one of the energized leads and SLOWLY rotate the motor shaft. if you see a fluctuation in current on the ammeter of 2-3% you have broken rotor bars.


"high 1x peak with Pass Pole side bands in Induction Motor"

I have found a few instances with motors with both journal and antifriction bearings with this type of vibration. I confirmed that no significant rotor electrical fault was present using motor current spectrum analysis. The cause of the vibrations was loose/worn antifriction bearing and excessive clearance in journal bearing.


Good morning,

if the motor is governed by inverter, check also the setup of inverter because a bad setup of it, can cause hig value of vibration (mesaured in velocity, mils o mm/sec) at current frequency (50Hz in Italy or 60 Hz in USA).
To check this is very simple, you can open  the interruptor with your component completely assembled, at normal operative speed, while you are measuring vibration in real time. If you can, check contemporary both, spectrum of frequencies in velocity and wave form. If the source is the inverter, you will see decrease immediately sinousolidal wave form and 1x peak.

The look at the harmonics of turning speed showing an increased # of side-bands as the harmonics increase, appears to be classical textbook 'cracked rotor bars'. I published an identical case in the vibrations mag many years ago. This data at least is clear justification to spend the necessary $ for a motor current evaluation. If you wait too long and a bar comes loose you are looking at a new motor instead of a rotor rebuild.

Good luck Marco. Let us know how it comes out.

Add Reply