Hello,

Our vib tech noticed 2 ac motors that have a high g level that matches up with 6XLF. Motors in question are baldor ac motors, 1.5 or 2 hp. 1785 rpm. When he reviewed the collected data with his software,(CSI) it does line up at 21600cpm. These motors are used to drive a circulation fan on a dryer. There are 10 other motors/circ fan on this dryer, but just these 2 have this high 6X peak. I do not have access to post any plots for all to see since it is a contracted service doing the data collection/analysis. I just was wondering if anyone else has seen a similar high peak for a ac motor.

Thanks

Probably a resonance in accelerometer mounting or with motor structure.

Walt

As you know, 6*LF is often a frequency created in a 3-phase input full wave rectifier used in many electronics including VFD's and dc power supplies.

I assume this is not a vfd.

My guess would be there may be some dc circuit in the area somewhere. It would be interfering either electrically (electromagnetic noise) or more likely by the magnetic field acting on your mounting magnet (try changing mount method to see if it goes away).

We had a similar case. In our case it appeared for one set of readings and was gone the next. Discussion of that case and good suggestions by forum members was here:
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...781072373#9781072373

quote:

Originally posted by electricpete:
As you know, 6*LF is often a frequency created in a 3-phase input full wave rectifier used in many electronics including VFD's and dc power supplies.

I assume this is not a vfd.

My guess would be there may be some dc circuit in the area somewhere. It would be interfering either electrically (electromagnetic noise) or more likely by the magnetic field acting on your mounting magnet (try changing mount method to see if it goes away).

We had a similar case. In our case it appeared for one set of readings and was gone the next. Discussion of that case and good suggestions by forum members was here:
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...781072373#9781072373

Thanks for the reply. Yes, these motors are not connected to a vfd. If my memory is correct, the motor starters are combined into one mcc panel. Ie 4 motors per bucket. The other two that are with in the same mcc bucket are straight ac, ie not on vfds. As for the mounting, all the motors are mounted exactly the same. They are attached to a spring loaded base that is adjustable to maintain belt tension. Fiberglass spacers/ isolators are used under the mounting base of the motors. Another difference is that these two baldor motors are as I would describe them as steel frame and the others are of a cast type ie with raised fins (cooling) on them. I will be able to be a bit more specific, ie cat# when I get back to the plant.

I'll see if he can take another reading while using a probe and on the endbell vers metal housing by stator... But whats odd to is that I have watched while he takes reading on similar style motors, and these 6XLF peaks are not there. Sensor placement is the same as with these two weird motors. But it wont hurt to do a little more investigating...


Ps, for some reason, the link that you posted, brings me back to this thread...

This message has been edited. Last edited by: Xracer,

My bad. The link should be:
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...221095072#6221095072

When I was talking about changing mount method I was talkinga about the accelerometer. If you are using a magnet to attach to the machine, then it may be seeing force from 6*LF magnetic fields from nearby dc circuits with 6*LF ripple.

quote:

Originally posted by electricpete:
My bad. The link should be:
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...221095072#6221095072

When I was talking about changing mount method I was talkinga about the accelerometer. If you are using a magnet to attach to the machine, then it may be seeing force from 6*LF magnetic fields from nearby dc circuits with 6*LF ripple.

My bad too. At first I thought motor mounting change.. but then later on, the light bulb went off in the pee brain, and I caught on to what you were saying about the magnetic fields etc....

How many blades on the fan?

quote:

Originally posted by THMotorMan:
How many blades on the fan?

I believe it is a 7 bladed fan...

Hi
I too have experienced exactly what you are talking about. Same general size motors, fans, and pumps. Moet are not VFD driven and some are. Some time ago I posted about this on this board and a gentleman suggested it may have something to with the coils in the windings (if memory serves me correctly). I suppose the magnet mounting of the transducer could possibly be considered also. On some of mine I have seen this spike for a long time and so far none have failed. Leading me to wonder if it is really a concern or not. I'm still interested if someone can shed some more light on the subject. Lonnie

P.S. It's probably just the cold weather we have to deal with. LOL

quote:

Originally posted by electricpete:
If you are using a magnet to attach to the machine, then it may be seeing force from 6*LF magnetic fields from nearby dc circuits with 6*LF ripple.

The idea sounds plausable from theoretical point of view and I wonder if it has been confirmed.

Although magnetic field at 6xLF may exist in the area of vibration measurement, in the case when a magnet is attached ( magnetically ) to a stationary body, motion caused by a 6xLF cycling force and acting upon the magnet itself will be IMO non-existing unless it shakes the body it is attached to. It will theoretically fluctuate magnetic pull but I am not sure if it will result in motion at this frequency.

Would you have DC drives on the same grid/transformer so you could have one that feed 6*LF back on the grid? Olov

quote:

Originally posted by David_G:

quote:

Originally posted by electricpete:
If you are using a magnet to attach to the machine, then it may be seeing force from 6*LF magnetic fields from nearby dc circuits with 6*LF ripple.

The idea sounds plausable from theoretical point of view and I wonder if it has been confirmed.

No, it has not been confirmed or disproved. Nor have the other theories we have discussed: electromagnetic interference or 6*LF on the ac input (it is a common frequency for dc ripple but not a common frequency to exist on ac circuits...distortion in ac circuits typically shows up at LF harmonics like 3, 5, 11, 13 etc). I am happy to consider them all plausible in the absense of any compelling argument for/against one of them. But discussion is good and each can form any opinion for themselves.

I think you would agree a magnet can move when it is at resonance. Under ideal conditions, a magnet mounted resonance frequency should be far higher than 21000 cpm, perhaps 100kcpm or 1000kcpm. But who said we have ideal conditions.... maybe there is an irregularity at the attachment point that causes the stiffness and resonant frequency to lower. For example maybe it is an irregular surface on the equipment where the magnet is attached. That might be something for Xracer to check out. But even if no such rough surface is evident, we don't really know how strong is the magnetic field we are talking about and I personally would not rule any of the above theories out without further information/investigation.

Whatever the cause, my opinion is that it is not a cause for concern.

This message has been edited. Last edited by: electricpete,

I want to thank you all for your replies. They have been very informational. I started the post due to the fact the these high g spikes fall right on the 6xlf exactly and it had our vib guy scratching his head a bit. So I took it upon myself to bring the question up for discussion here. He did not seem to be too concerned with them since that other like motors did not have the same spikes. We'll know more during the next rounds of data collecting.

quote:

Originally posted by OLI:
Would you have DC drives on the same grid/transformer so you could have one that feed 6*LF back on the grid? Olov

There is a vfd located in a mcc bucket that is close to this motor's mcc bucket. But, there are 4 motor starters within this one mcc bucket, and the other 3 motors, show no sign of this 6XLF spike.