Tagged With "50hz"

Topic

Motor blower vibration issue

kri ·
Motor directly mounted with blower shows vibration issues mainly at 50hz 1/3 octave. Checked everything rotor balancing, bearings, impeller balance. Motor is having proper mounting on a concrete bench as suggested by the vibration experts and using shock mounts on the bench then a intermediate plate and top of it is motor. we found more vibration on mount areas. Any possible pointers on this issue
Topic

vib.at 50Hz in D.C.motors

Rajj ·
Since few days , In our paper machineries, at some of the D.C. motors, we are getting vibrations at line frequency = 50 HZ (3000 rpm). At one of the D.C. motor (210 KW), the amplitude at this freq. is very high@ 8 -10 mm/sec. and is irrespective of speed of D.C. motor. Vibrations at 50 HZ are also observed while the motor is in decoupled condition, but with lesser amplitude i.e @0.7 mm/sec. What does it indicate? / what may be the cause? / How to find out the cause?
Topic

collector rotor balancing

hamid.mechanic ·
does collector rotor that couples with a flexible rotor 50Hz, is rigid or flexible? and balance process do only mass residual balancing?(iso1940) Keep in mind that collector from one side couple to the main rotor and another side-mounted on journal bearing.
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Re: Motor blower vibration issue

GoodVS ·
Hi KRI. You should check out this technology. https://www.youtube.com/watch?v=23WnFWqKawQ (Best viewed in HD format) It may be able to help.
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Re: Motor blower vibration issue

kri ·
goodvs, thanks for your quick reply but nothing like that happening in our case as shown in the video. Everything is intact
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Re: Motor blower vibration issue

kri ·
this is how my set up looks. The base plate shows 30db and the above the mounts intermediate plate shows 100db and on the motor body its 90 db. I need to control that 100db down to 88. Any suggestions. Please look at the attached pic
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Re: Motor blower vibration issue

John from PA ·
Just to clarify, are you measuring vibration and expressing in units of decibels (VdB and/or AdB)?
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Re: Motor blower vibration issue

kri ·
John thank you for the reply. Yes am talking the vibration measurement in decibels is AdB. Look forward to your response soon
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Re: Motor blower vibration issue

John from PA ·
First of all let me say I hate decibels when applied to vibration! The use of AdB is a bit unusual; to the point I suggest you review the guidelines for measurement and the specification with respect to limits. Also, double check my math as it has been about 35 years since I worked with AdB on a regular basis! Adb = 20 log (A/A 0 ) where A 0 is the reference. In this case I’ll assume you are using the reference as defined by ISO 1683 which is 0 Adb = 1 µg. In other words 0 AdB is 1 µg and a...
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Re: Motor blower vibration issue

kri ·
All what you said is right. but the 100dB what i have reported is after taking the consideration of that conversion factor. so my current is 100db and want it to be 88dB. if you look at the photograph..do you find anywhere i need to do something to reduce this vibration
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Re: Motor blower vibration issue

John from PA ·
Why do you need it that low? 0.025 g is very low as a vibration standard.
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Re: Motor blower vibration issue

ivibr8 ·
It seems strange to me that you have 90 AdB on the motor (source) yet an increase to 100 Adb on the intermediate (green painted) plate. How can that be if you have a tight system? Are you certain that the motor hold down bolts are tightened? Can you get narrow band FFT of both the motor and plate (to ensure all energy is only at running speed)? Jim P
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Re: Motor blower vibration issue

Beatnik ·
I don't like how those isolators are placed. It might help to put them in each corner of the plate. When you mount something with isolators it will always vibrate more than a rigid installation but the vibration will be less damaging. If all you want is reduce the DB number to please your bosses, making it a rigid mount might work. Someone here did something similar with good succes: http://www.machineryanalysis.o...r&pid=1294839068 I have blowers on isolators with vibrations that are...
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Re: Motor blower vibration issue

John from PA ·
See image below. I'm not sure you will ever get this down to what you desire but I suggest you try replacing those distorted rubber washers with steel flat washers.
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Re: Motor blower vibration issue

kri ·
yes its for a special application
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Re: Motor blower vibration issue

kri ·
Everything is tightned properly. Narrow band results are satisfactory, but 1/3 octave band is higher at 50hz only
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Re: Motor blower vibration issue

kri ·
Let me explain you clearly the need of shock mounts. The blower with motor assembly need to be less vibration within the specified limits as a independent machine. Like you said if everything is mounted on the floor securely the vibrations would be veryless, but the application demands that the blower motor wherever it is mounted should not have contact with the floor and its like a hanging position where the vibrations of the unit should be less within specified limits. The idea to mount on...
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Re: Motor blower vibration issue

kri ·
yes those are rubber washers tried to reduce the vibration, we tried nylon, teflon, hylum washers to reduce the vibration as we felt steel flat washers wont dampen the vibration being metal to metal contact. Yes the mounts are with rubber but equipped with metal inside.
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Re: Motor blower vibration issue

John from PA ·
I think you should do a bump test of your existing arrangement and determine if it has a resonance somewhere in that 50 Hz region that drives the 1/3 octave upward. If it does change the mass of the mounting plate and determine how much of a shift you can obtain. Looking at your arrangement, you might also try what I've sketched below. The intent is to get the motion of the motor/base into the large rubber blocks at the bottom.
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Re: Motor blower vibration issue

kri ·
John Thank you , tried the same as you told, but results are same. See the attachment
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Re: Motor blower vibration issue

John from PA ·
I would say you are down to doing bump test to help define an existing resonance. As a point of interest, the 1/3 octave with a center frequency covers the range from 44.7 Hz to 56.2 Hz. Its an oversimplification, but essentially everything in that range is contributing to your higher than desired amplitude. I don't know the thickness of that plate but if it is 3 cm for instance, can you increase it to 4 cm to see the effect? Have you done anything like that? It would significantly move any...
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Re: Motor blower vibration issue

kri ·
John Please clarify me what is bump test and how to do it. Thickness of the plate is 20mm, there is a limitation of weight to be loaded on the rubber mount,so we need to see is there a possibility of adding a more thicker plate to control the resonance. the acceptance criteria is called 85-110 db line where each frequency has different limit so the 50hz has 88db on that scale and higher the frequency the limit of db increasing gradually. Yeah motor RPM around 2900. Thank you for your time...
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Re: Motor blower vibration issue

Beatnik ·
http://www.zencovibrations.com...forming-a-bump-test/
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Re: Motor blower vibration issue

John from PA ·
The link "BEATNIK" provided describes the process well but I do not think you need to use a 2 x 4 to accomplish the task. A 2 x 4 by the way is a piece of lumber measured in English units and approximately 2 inches x 4 inches. In your case I would mount your transducer on the point of interest and then impact the base with something like a rubber mallet. The idea of the instrumentation is to capture the resulting peaks and see if something in the way of a resonance exists around the 50 Hz...
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Re: Motor blower vibration issue

kri ·
I did the bump test as explained and the measured values at 4 mounts as 105,112,117,106 at 50Hz. So what does this tells us as while testing with motor the values are almost same
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Re: Motor blower vibration issue

John from PA ·
Are there plots you can post? What was the instrument used? I'm suspicious that the test was not done correctly, or at least you are not reporting the needed data. I've attached a random plot obtained off the web. You can see that the person doing the test watched all frequencies out to 200 Hz while he lightly bumped the machine. The machine responded at about 25 Hz, the resonance. This is similar to hitting a bell, it rings at its natural frequency. There might be other frequencies present,...
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Re: Motor blower vibration issue

kri ·
John, I will explain you what I did as part of bump test as u told. The instrument used is B&K handheld anyliser 2250. On the test bench only this motor is mounted nothing else and non of the machines were running while doing this test. I mounted the transeducer on the green plate beside each mount and hit the plate with a nylon hammer and seen the measurement at 50Hz, the reported values are those. I am not able to seen the picture you attached as it shows blank. Now tell me which kind...
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Re: Motor blower vibration issue

John from PA ·
See the attached PDF file.
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Re: Motor blower vibration issue

Beatnik ·
Are you telling us that when you do the bump test you see elevated vibrations at 50hz similar to when the machine is running?? The machine is stopped when you do the bump test right?
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Re: Motor blower vibration issue

John from PA ·
I looked up the B&K 2250 and I do not think it will give you the desired information. Do you have any form of a "spectrum analyzer"? Last resort, maybe an oscilloscope? To be honest, from the questions you have been asking, I get the impression that this task is well outside your area of expertise. I would recommend you get a mechanical engineer involved. Have that person review everything, starting with the specification. As I mentioned quite early, it is very unusual to have a...
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Re: Motor blower vibration issue

kri ·
yes
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Re: Motor blower vibration issue

John from PA ·
The B&K 2250 has an optional frequency analysis software package (see image attached). As you can see it can provide something like a bar chart with the amplitudes in octave or 1/3 octave (user selectable). Are you or can you use it? If so can you post plots?
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Re: Motor blower vibration issue

kri ·
John To clarify to you, unfortunately am a mechanical engineer from past 20 years. Am using this N&V analyser from past 7 years. Yes we do have the software mentioned by you and we measure the same. Find attached sheet tells you what kind of final output is required for us and measurement units. in the second table see the SOR line which is the limit line for vibrations. B&K 2250 with software well able to measure 1/3 octave and narrow band measurements too
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Re: Motor blower vibration issue

John from PA ·
Your image raises another concern which I touched on earlier, and that is the reference level. It isn't real clear in the image, but I think that along the top edge of the plot I see the notation "Adb Zero threshold 1.10 -5 m/sec 2 . " Up until now I have been assuming you have been using a system where the reference is that defined by ISO 1683 which is 0 Adb = 1 µg. I think this means you are off by a factor of 10. You are the metric guy so please double check. Something you will need to do...
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Re: Motor blower vibration issue

kri ·
Thank you for your information John. Yes reference levels differ where B&K meter is set at 10power of -6 so whatever value we measure we minus 20db from it as per the difference between 10 to the power -5 to -6, so the values I have posted are with 10 to the power of -6 so - 20db comes to some where 10 db higher than what is need to be achieved. Yes ours is marine/defence application. Hope things are clear now and look forward to improve upon further about the vibration levels we are...
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Re: Motor blower vibration issue

kri ·
John Find attached graphs taken during bump test for your reference.
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Re: Motor blower vibration issue

John from PA ·
The two plots, PCC4-16594 & PCC4-16600, seem to indicate a resonance at 50 Hz, or just slightly below that frequency. As a mechanical engineer you should know that resonance has two key factors, stiffness and mass. So you need to change one or both of those terms. I had previously suggested increasing the thickness of the plate under the motor so that is one thought, the other would be to experiment with the rubber blocks under the plate. A thought comes to mind, can a pair of blocks be...
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Re: Motor blower vibration issue

kri ·
John Thank you for your ideas. Unfortunately this is customer specified setup and have lot of limitations using different thickness plate and more rubber mounts. Your finding is correct about the resonance, my aim is to see that how and where I can improve upon to cutdown that higher vibration. I am sorry that I cannot change the setup the way I wanted. But anyway I keep your advise in mind and try and speak to the customer about the findings and changes required. Thank verymuch for your...
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Re: Motor blower vibration issue

John from PA ·
I assume that plate under the motor is steel. Have you looked at perhaps aluminum? Some stress analysis might need to be done but that should push the resonance upward considerably.
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Re: Motor blower vibration issue

kri ·
Yes the plate under the motor is of mild steel and I dont have my choice to use a different plate in place of that. Like I told you we have lot of limitations of these things as I know I can reduce the vibrations by using different materials will give less resonance. We are not allowed to use any kind of anti vibration pads below the mounts. The customer always focused on correcting if any issue with the motor or the blower only as the setup is told by them including the dimensions and...
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Re: Motor blower vibration issue

John from PA ·
In the early picture you provided, I don not In the picture you supplied early on I don't see an half-key on the motor shaft extension. Are you using something there when you test or just running the "bare" shaft?
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Re: Motor blower vibration issue

William_C._Foiles ·
John, I found a reference value for displacement in the new 1683-2015 for dB. The reference value is 1pm. Additionally, I see another for velocity, 50 nm/s .
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Re: Motor blower vibration issue

kri ·
We were having full key on the shaft secured with a tape while testing. Motor manufacturer said there is some mismatch in balancing of the rotor and impeller, so sending the rotor and impeller for rebalancing separately now. After it comes back we take the test again
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Re: Motor blower vibration issue

kri ·
We have done a latest test after balancing the impeller and rotor separately where the values at the mounts on the green plate show as 115,112,117,113adb and on the motor body it shows as 93,94,96,92adb respectively. The customer made it clear to us about the purpose of the green plate, it is to get the real vibrations of motor getting transferred onto the plate as the green plate is isolated from the bottom base for the same purpose. Now we are trying to test the same setup mounting the...
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Re: Motor blower vibration issue

John from PA ·
Full key is on the left and half-key on the right. You really should be using a half-key if you are just holding it on with tape. If you are using a dummy hub, that also has a keyseat, then use a full key.
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Re: Motor blower vibration issue

William_C._Foiles ·
ISO spells out what John has said. This includes how to balance the rotor and how to balance the part.
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Re: Motor blower vibration issue

kri ·
John and William Yeah we are using half key as you have shown. Something started working now after we get entire thing get balanced(impeller+shaft rotor+cooling fan). We will get the other capacities also get done same way and look for the results. Thank you verymuch for your time and support guys. Will get back to you once again after finishing everything
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Re: vib.at 50Hz in D.C.motors

John from PA ·
I have very little expertise in DC motors; some possible help at http://www.mobiusinstitute.com...p;sTitle=DC%20motors .
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Re: vib.at 50Hz in D.C.motors

Ralph Stewart ·
It might be coming from possibly a loose connection or bad rectifier in your AC to DC rectifier system, which is allowing the Line Frequency to "leak" passed the rectifiers and into your motor. Unless your motor is running on "real" DC current from a DC generator, but this is highly unlikely since there should be no AC LF current coming from a DC generator. Just my opinion and I could be wrong, since I am not an electrical guru.
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Re: vib.at 50Hz in D.C.motors

jvoitl ·
I used to work on many D.C. motors. Yes an open SCR can cause a line frequency vibration, and normally there is a line frequency noise from the motor also.
 
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