Hello,

I have a question about a proparly PPF sidebands frequency's around the 1X 220kWatt motor. (Calculation shows out it has to be the PPF.) And the 2X 3X and 4X shows sidebands also nearby. My question is: how does the motorspeed physicly doing? Is it turning constantly or is it not? The motor is for this moment not running for now and there is not yet a physical strobe-light inspection done.

See attechment for more info.

regards M.

220kWatt_motor.doc (112 KB, 74 downloads) sidebands around 1x2x3x4x

Usually when I have seen pole pass sidebands, there is no speed oscillation detectable by strobe. In that case, the pole pass sidebands likely came from amplitude modulattion of vibration.

In theory, if you have a broken rotor bar, it can create torque oscillation which creates speed oscillation on low inertia loads.

I would do a current signature analysis. If strong pole pass sidebands there, might consider calling it a rotor bar problem.

Interesting that in your case in addition to the pole pass sidebands ~ 0.5 hz, you seem to have another set of sidebands ~ 5hz.

What is the running speed of the motor?

Pole pass sidebands will show up with an eccentric rotor, cracked or broken rotor bars and shorted laminations.

collect some additional data with a 300kcpm fmax and at least 1600 lines. You will pick up RBPF (RBPF= RPM x #Rotor bars)with this collection spec. A 300 hp motor will have anywhere from 40 to 80 rotor bars. Presence of 7200cpm (2xline frequency)sidebands around RBPF will help support the theory of broken rotor bars. If the 2xRBPF is greater, there may be a significant problem. Anything greater than .05 ips pk for RBPF is not acceptable. 2xRBPF should not be greater than about .03 ips pk

quote:

Originally posted by jim2f:
What is the running speed of the motor?

Pole pass sidebands will show up with an eccentric rotor, cracked or broken rotor bars and shorted laminations.

collect some additional data with a 300kcpm fmax and at least 1600 lines. You will pick up RBPF (RBPF= RPM x #Rotor bars)with this collection spec. A 300 hp motor will have anywhere from 40 to 80 rotor bars. Presence of 7200cpm (2xline frequency)sidebands around RBPF will help support the theory of broken rotor bars. If the 2xRBPF is greater, there may be a significant problem. Anything greater than .05 ips pk for RBPF is not acceptable. 2xRBPF should not be greater than about .03 ips pk

Can you justify the limits of 0.05 ips and 0.03 ips for the 1xRBPF and 2xRBPF peaks as not acceptable? What does this mean?

I've seen a lot of motors with high RBPF peaks, that don't show any problems in the current spectrums.

technical associates proven method suggest the following for AC Induction motors RBPF:

30kcpm -85kcpm Alarm point .060 ips pk
85kcpm-140kcpm Alarm point .055 ips pk
140kcpm- 195kcpm Alarm point .050 ips pk
195kcpm - 250kcpm Alarm point .040 ips pk
250kcpm - 305kcpm Alarm point .030 ips pk
305kcpm - 360 kcpm Alarm point .030 ips pk

This is assuming a 900-3600 RPM motor
measurment take on the outboard bearing in the horizontal direction.

most motors RBPF will fall around 80 to 120kcpm. as you can see .05 is around the alarm point for that speed.
2xRBPF will come in anywhere between 160 and 240kcpm on most motors. i stated .03 ips in the posting, did not have the standard in front of me at the time. .04 ips is closer to what you should be concerned with.

The 2xRBPF can tell you more than the 1xRBPF. Increased amplitudes in the 2xRBPF can indicate some electrical arcing acrossing broken rotor bars with no increase in the 1xRBPF.

I have yet to do ESA on a motor with big RBPF peaks. I am curious to see if it will show up.
I dont know if i would remove a motor from service for seeing these peaks. I have noticed that these motors tend to run hot. as we all know heat kills winding insulation.

You stated that you had pole pass frequency sidebands around 1x and its harmonics. the 2xRBPF test i described will confirm if there are broken rotor bars

fwiw, I agree with Steve. RBPF +/- 2*LF pattern in vibration will be present on many healthy motors. Sometimes it can be very high simply as a consequence of the motor design. I would not give it any weight in diagnosis unless perhaps if it showed an unusual increasing trend. Current signature analysis is the next logical step for further analysis of this motor.

Pete, you are correct, RBPF is present on many motors. I guess i did not make myself clear. when i see the amplitude of the 2xRBPF increase i know the condition is progressing and further testing such as CSA is warranted.

The original question was regarding PPF sidebands around 1x harmonics. that describes what you see with broken rotor bars. i was suggesting the 2xRBPF to confirm what was found with the 30kcpm fmax collection.

Also, if the motor is on a VFD i would suggest running it in bypass if possible. I have seen drives amplify ppf and rbpf even when running at 60 hz.

When we possible deal with looseness on one or more rotor bars could do a bump test (resonance check) see the looseness of de bars??
Does the rotor bars have there own resonance area? (of corse depending on there mass)

.

Here is a fairly detailed summary of off-line rotor test methods from EASA:
http://www.pumpingmachinery.co..._29/rotortesting.pdf

You notice they describe a "tap test"

quote:

Tap test
Broken fabricated rotor bars may be detected by tapping on the bars from one end ring to the other with a hammer and screwdriver. Loose or broken bars will respond much differently from tight sound bars. This method works best with two people performing it. One person taps the bars and the other monitors bar movement. The bar movement can be sensed by holding a second screwdriver on the bar about 3 to 4” (75 to 100 mm) from the location being tapped. Figure 8 illustrates a rotor bar crack that could have been detected by tap testing.

I imagine that your bump test would work similarly except use the vib analyser to provide a little quantitative feedback about the pitch.

One thing to mention in general: loose rotor bars are not necessarily the same as cracked or open rotor bars. Loose rotor bars on a fabricated rotor are not well swaged, or the swaging has loosened. Loose bars can lead to high 1x and bowed rotor, but won't cause pole pass sidebands in current unless bar vibration leads to cracking of the bar / end-ring joint .

I notice in the link above they suggested broken bar is one of many possible causes of 2*LF. I have never heard that connection before. Has anyone heard or seen a connection between 2*LF and broken bars?

quote:

I notice in the link above they suggested broken bar is one of many possible causes of 2*LF. I have never heard that connection before. Has anyone heard or seen a connection between 2*LF and broken bars?

Yes, I have heard about this too often. IMO: an ineradicable myth.
Arie Mol