# of rotor bars x rpm.

Is it safe to assume that an increase in load and corresponding droop in speed will cause a increase in rotor bar pass. I have some machines with 3600 rpm motors that the RB pass comes and goes, it seems to correlate with speed, I have no current data. I plan to add that parameter. These machines are generally reliable and have no issues.

Alan

Yes, RBPF +/- 2*LF pattern vibration is expected to increase as load increases.

From Siemen's Analytical Approach to Motor Vibration Problems: "High frequency, load-related magnetic vibration at or near rotor slot passing frequency is generated in the motor stator when current is induced into the rotor bars under load. The magnitude of this vibration varies with load, increasing as load increases."

A similar conclusion is given with more supporting details in "The magnetic noise of polyphase induction motors" by Alger, IEEE 1954.

As you point out, speed drops slightly as load increases, so higher RBPF pattern should occur when speed is lower corresponding to higher load.

Alan,

Another factor that can affect bearing housing or motor case vibration measurements is that when the rotor bar or stator slot frequency changes with load, the amplification by the structure can change. Accurate measurement point location and accelerometer mounting can also affect measurements over 1000 Hz.

Walt

Thanks

This one has CTC stud mount pads glued to the motor casing, The accelerometer is attached using the stud, my prefered method for mounting on just about anything that is faster than 2000 rpm and is worth monitoring. On this unit the pads mount on the housing casing are in line with the bearings, only horizontal readings and one axial are collected, Works for me.

Will the quality of the motor manufactures design and materials influence the presence of the RB pass, will it be more prevalent in "economy motors"? As opposed to Cadillacs?

Alan

It will be more prevalent in 2 pole motors than those with more poles. Ecomony motors and also rebuilt motors could be more prone.

Zackman

Just a curiosity...why would a rebuilt motor be more prone? Than the same unit original from the factory.

Alan

fwiw, Here are my beliefs about RBPF pattern.
2 - pole motors definitely are more prone.
High RBPF pattern vib can be inherent in the design of a perfectly healthy motor.
High RBPF pattern vib can be increased by soft foot problem on 2-pole motor as reported in two case studies in P/PDM magazine.
In theory, rbpf pattern vib can also be increased by eccentricity.
As mentioned above can change with load.
As mentioned above, the high frequency content can be sensitive to accel moutning.
Since the vibration originates in the stator core, RBPF pattern vib can be is heavily dependent on the the way the stator core is linked to the stator frame. I think for large sleeve bearing motors, this type of vib will not likely show up on the bearing measurements.
These vibrations generally are not indicative of any destructive condition.

I have heard/read people suggest 2*LF as a test of a quality rebuild (particularly concentricity)). I would be more inclined to agree with that statement than to view RBPF as an indicator of the qualtiy of a rebuild since there are so many things (above) that can affect rbpf. Note 2*LF is not load sensitive. not very sensitive to accel mounting, but is still sensitive to base/foot issues.

TIOMOAICBTWA

Electricpete

I should have mentioned before that this motor is endmount with a flange and is 600HP with ball bearings driving a CENTAC, the readings in question are on the ODE. It is a Toshiba (One of the better brands)it is on it's second rebuild in 2 or 3 years (this is why were are in the picture now!) The last rebuild was in November of 2005, which I am sure was a rewind. For sure they must have changed how the loading is since resinstalled after the last rebuild because the RB pass is a recent thing, the loading and its indication thereof is handled by a PLC, all of this seems to be privledged info that is behind some barriers that we have to break down. The 1st stage suction is apparently from the exterior, and the air is definately getting cooler here as the seasons change, I am sure that would affect the loading.

Alan

After rewind the stator windings are are generally varnished or A resin vip is used, after curing many repair shps may use high speed abrasive wheels to remove the excess varnish/resin.
Some surface metal will be removed in this process; exactly how much depends on how hard the wheel is pressed.
If after numerous rewinds more metal is removed and the air gap will change, maybe this has something to do with it.
I do know that after many rewinds high speed inverter controlled linishers were prone to overheating as the air gap changed and the laminations blended together
just a thought hope this helps and makes sense ( full of falling over liquid at the moment)
Mike

quote:

Originally posted by Alan Branch:
Zackman

Just a curiosity...why would a rebuilt motor be more prone? Than the same unit original from the factory.

Alan

Alan,
Is there a beat noise coming from the compressor or did you just notice the rbpf? Why so frequent rebuilds? What failed each time?

quote:

TIOMOAICBTWA

???

This Is Only My Opinion And I Could Be Totally Wrong Again

Not throwing stones, but Toshiba motors have more often than not shown higher RBPF and +/- 2x line freq. sidebands. Not detrimental to operation, just fact. I believe it has more to do with construction.