Broken or cracked rotor bars Symptoms:

Discussion time!

1. Will loose rotor bars be at RBPF or WSPF

My thinking is if you have a loose rotor bar every time it passes a winding slot the magnetic force causes the rotor bar to lift and drop at the rate of the number of slots; therefore a loose rotor bar will be at the WSPF with 2XFline sidebands?

2. Will looseness in the winding slots be at RBPF or WSPF?

Again along the thinking above would this then be at RBPF with 2XFline sidebands as every time a rotor bar passes a loose winding in the slot the magnetic forces cause the winding to lift and drop and the rate of the number of rotor bars?

3. Cracked rotor bars:

To confirm this will be FPole sidebands surrounding running speed harmonics. Advance problems will exhibit a humming or pulsing sound and feel. Significant is the number and size of the sidebands, they increase as the unit deteriorates. Amplitude at 1xrpm is relatively unimportant it will fluctuate greatly as hot spots cause the rotor to bow unpredictably

What do you all think?

Original Post
Good discussion.

#3 – I agree. Although there are other things that cause pulsing and pole pass sidebands, so it's a tricky diagnosis. I would add current siganture to the mix.

#1 and #2 – I agree that IF there were movement of rotor bar each time it passed a stator tooth it would be at stator slot pass frequency, and IF there were movement of stator coil each time it was passed by rotor tooth, it would be at rotor bar pass frequency.

Now let's talk more about #1 – loose rotor bars. The persistent belief I hear that RBPF+/- 2*LF is associated with loose or open rotor bars. But the bar doesn't pass anything at RBPF, it passes at SSPF (stator slot pass frequency) as you deduced. And the frequency of flux seen by the bar (sitting in the rotor reference frame) is not 1*LF or 2*LF, it is pole pass frequency. So if we were expecting symptoms of mechanically loose rotor bars agitated by passing stator slot teeth, we would expect them to occur at SSPF +/- FP, not RBPF +/- 2*LF. Also let's revisit that IF... IF the bars move as they pass every tooth. These bars are also subject to heavy centrifugal force which tends to keep them in place, and the frequency is quite high... so I'm not convinced loose bars show up as anything different than a shift in balance periodically, possibly triggered by shutdown and startup or thermal changes.

#2 – Loose stator coils – I have posted case study where this showed up as 2*LF and harmonics of 2*LF (although there are other possible causes of harmonics of 2*LF). That results from movement of the bars at a frequency 2*LF. Substantial movement at the higher frequency RBPF is less likely simply due to inertia effects. And I would never be inclined to view RBPF +/- 2*LF as symptom of stator coil looseness because there are too many other normal expected electromangetic sources of RBPF+/-2*LF that have nothing to do with loose rotor or stator bars.

Just my opinion fwiw. I'd be interested for others to chime in.
James, Pete
When i first read the statement

"magnetic forces cause the winding to lift and drop and the rate of the number of rotor bars"

My immediate raction was that it would be physically impossible

Pete's response below is what i would've thought too.

"so I'm not convinced loose bars show up as anything different than a shift in balance"

Sorry for all the cutting and guys are more articulate than I

Personally I've become a bit of a sceptic on RBPF and 2 X LF. I usually only report it as an "observation" rather than a fault. If I see it trend undeniably upwards, I suggest motor current analysis as a second opinion.

Hope I didnt get too far off topic
Just a comment or two about rotor bar problems: As a past user of PdMA Emax online test equipment, I can say that it is a very good way to check for this type of problem. I have several case studies of this fault mode along with tear down pictures of the actual reason it was picked up. About the only thing one needed to be sure of was a loading of about 75% or above to get an accurate test.

Generally speaking, rotor bar problems in AC induction motors will run a long time before actually failing, however keep an eye on the rotor bar pass frequency amplitude and if it gets down to 42db, I'll guarentee you have at least one broken bar or severe voids in your moldings, including end rings. Every time we tore one down for inspection I was always amazed at what we found, without fail on all of them.

One again, if one trends the rotor bar pass frequency data over time, it will tell you when its time to remove it from service.

As a matter of fact, due to the fact that others in the plant were starting to call me "Mr. Rotor Bar", I so named my consulting business after it.
however keep an eye on the rotor bar pass frequency amplitude and if it gets down to 42db, I'll guarentee you have at least one broken bar or severe voids in your moldings, including end rings....

One again, if one trends the rotor bar pass frequency data over time, it will tell you when its time to remove it from service

Thanks Jim. As a clarification, you are obviously talking about current signature analysis, not vibration analysis. (I just don't want anyone to think you are saying that RBPF amplitude in vibration is some kind of sensitive ultimate indicator for rotor defects).

And the db difference you are talking about is FP sidebands around RBPF or FP sidebands around LF?

When we do current signature analysis, we compare LF and LF+/-FP (using our normal vib analyser with currrent probe on input) and express as a db difference. We cannot look up at RBPF+/-FP due to dynamnic range difficulties, but I understand some other equipment has special processing to allow that. If you are looking at FP sidebands around RBPF in current, do you find that it has any more diagnostic value than FP sidebands around LF in current?
Hi Pete,

The Emax uses what is called poll pass frequency, which is plus or minus around line frequency. It is figured by taking the running speed of the motor and subtracting this from the synchronous speed of the motor, dividing the result by the synchronous speed to get a per centage, which is multiplied by 2XLF which is then subtracted from the line frequecy.

If one has peaks at these points on either side of line frequency with an amplitude of less (this means the peaks are getting higher or closer to the top of the line frequency peak) than 40db (measured down from the peak of the line frequency spike) then it is considered to have a rotor problem.

With the Baker Instrument Explorer online unit one can look in the "torque spectrum" for pole pass frequencys by multiplying the number of rotor bars times the running speed (much as you can in vibration analysis) to find the peak.

Either one of these online devices is an excellent tool to use and I encourage all online motor testers to work with your vibe guys to cross/double check possible problems you find and help each other out with this verification process. It has been my experience that we did not always agree on severity, which made the difference between a go/no go situation. Very interesting to say the least.

In the above case, my vibe guy said the motor had a rotor bar problem, I disagreed. Turns out it was a vibration problem caused by the sleeve bearings. When we checked out the rotor after removal from the motor, we could find no visible cracks or voids. Then this was verified again, on the dyno, after re-assembly at full load.

This is another reason my boss finally agreed to sending me down to the motor shop after motors were repaired (over 150HP) prior to their return to the plant, again ensuring a good motor in the storeroom, ready to go.
Hi Pete, Jonsey & Jim

Many thanks for your input.

I am glad someone else agrees with loose rotor bars would theoretically give the fault at stator slot pass frequency, and yes that makes sense it would be at FP and not 2LF

In reality yes I agree that loose rotor bars in VA would most likely show up as a shift in balance

Loses stator coils, this is a hard one, and I would not like to call it from VA data alone, as you can’t trend with VA and any movement would only wear the insulation and then blow the coil

Yes as you stated I think I will be reporting these as ‘observations’ of a potential electrical activity and then recommend current analysis

Thanks for your postings about current signature analysis, I have not been involved in this type of testing and am always interested to learn more


In my experience loose rotor bars were represented by pole pass frequency sidebands around the first 4-6 orders of motor turn speed as seen in the FFT, converting to log format really makes them stand out. We were able to verify the condition visually by taking pics through the inspection port while motor was down.

On point two I am curious as almost all of our new toshiba motors have elevated levels at 1x and 2x RBPF with many 2FL sidebands, and their amplitude will vary with load.

I would have to agree with point three, as one of these motors (1000hp 2 pole) was allowed to deteriorate until rotor assembly cracked which allowed us to take frequent readings to trend its breakdown. Luckily it was replaced before any contact was made.

dear james and pete
i am seeing vibration pattern in our boiler feed pump resembles with point no #2. i.e looseness in stator. So, please suggest me what type of electrical check should i do in stand still condition to get more closure to its root cause. This motor is having Humming noise from the supply terminal box. I had taken the vibration at different loading current and i had found 12 mm/sec (peak) vibration at motor body in minimum recirculation mode whereas at rated condition vibration was 5 mm/sec (peak). There was increase in the vibration noticed at rotor bar pass frequency with was having side band of 2 *LF. In addition to this vibration peak was observed at 2*LF. No change in current as well winding temp is noticed and all motor parameters are well with in the acceptable limit. Our pump have been given 1.5 yrs of trouble free operation with this condition.

Add Reply

Likes (0)