Thread: Motor speeds and vibrations

It depends on whether the vibration comes from the lathe or the motor. It sounds like a lot of work for no definite positive benefit. A trick that I do is sandwich motor feet between two rubber washers. That at least seems to cut down the vibration transmitted through the motor mount into the lathe. If the lathe is belt driven I expect that most of the vibration comes through the mount.
This is the motor mounting method used on my lathe -
P1000948 (Medium).JPG
Both the motor and the gearbox are hard mounted on a sub-chassis which is then rubber mounted to the lathe. Power transmission is via flat belt. It seems to work.

Michael

Another reason for using a VFD.

Yes I know the source of the vibration should be hunted down and made an example of, but as you say "The cause of vibrations in Machinery can be hard to track down".
Just changing the frequency by a few Hz can usually move a motor/machine out of an irritating vibe zone and into something that is much quieter.
If you really need that speed then maybe a different belt position and motor speed can be used - if it still vibrates then that's suggesting its more likely to be the machine than the motor

Originally Posted by morrisman

A thought more than anything .

Most of the 3 phase motors I've seen on lathes are 4 pole 1400 rpm . The cause of vibrations in Machinery can be hard to track down , I guess it's a science in its own right with many variables involved .

In the case of a troublesome lathe . Would it be worthwhile changing to a 6 pole 900 rpm 3 phase motor and fitting the motor with a larger diam pulley ?

Single phase motors are very often the source of vibrations. But I have never heard of a 3 phase motor being the cause of vibrations. It is maybe possible if the 3-ph motor is being run at a much higher voltage than it is designed for. Usually people that upgrade a lathe from single phase to 3-phase report a very noticeable reduction in vibration and an improvement in surface finish.

Changing the number of motor poles in an attempt to reduce vibrations in a 3-ph does not make sense to me. 3-pase motors run extremely smooth and free of vibrations no matter the number of poles. This is so, because the stator of these motors produces a true and near perfect rotating field, without any sudden changes as is the case in a single phase motor.

In the case a "troblesome lathe" with regard to vibrations, I would definitely look elsewhere for the source of the trouble.


PS: of course, many lathes vibrate or resonate at particular rpm's. But that has nothing to do with the motor itself. As can be proven by driving the 3-ph motor with a VFD. There will only be very narrow speed bands at which such a resonance occurs - typically just 1 Hz higher or lower and the problem is gone. This would not be so, if the motor was causing the vibrations.

Motor quality is pretty much all it is about...

My 10EE has an AC motor, connected to two DC generators, running at 1400rpm, then a DC motor running at whatever speed and it is not vibration free, but you can barely feel it...

My J&S 540 with the spindle running is virtually vibration free, as is the Macson TC grinder..

All three phase machines as well which I believe does make a difference...

First thing I'd do is take the belt off the motor and see if the vibration is still there.
Next I'd find another mototr which fits isn the space (irrespective of its power, but the same speed and try and fit the same pulley or one the same size to it,l and run that without a belt (just to insure the test motor doesn't have a vibration issue).
Then fit the belt and see if the vibration returns.
If it does, get another belt.
That way you know if it is caused by the motor, the belt or the gearbox.
No change in motor is going to reduce the vibration if its the gearbox - single or 3-phase.
Next I'd change one of the pulleys slightly (say by 1/4" diameter) and see if the vibration changes. I might do that up and down in size.
It may be a resonance frequency in any of the driven components.

As BobL said, if all fails, try a 3-phase motor and VFD. You'd be amazed at the difference a few Hz can make to resonant frequency vibrations and noise of a motor.

Yesterday Dale (Dsel) was here and we set up his Waldown grinders for VFD use. When we tested them, one made a very unpleasant 'dry bearing' noise at exactly 50Hz. At 48 and 55Hz it was almost silent. At 100Hz it was the most silent.
My guess is that the motor bearings had worn a little and started bouncing wihtin the bearing space at a specific rate related to 50 Hz and made indentations in one of the tracks to match. So when we ran it (after years of sitting still and drying the grease), the balls bounced aoround these marks and dragged on the metal bearing cage, causing the noise.
At a 100Hz, centrifugal force was too high to allow them to do that and shut them up. At other than 50Hz, they didn't follow the wearmarks and were quieter.
All just theory of course, but entirely repeatable when we tried.

Give it a go and report - nothing to loose.

Originally Posted by jhovel

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Yesterday Dale (Dsel) was here and we set up his Waldown grinders for VFD use. When we tested them, one made a very unpleasant 'dry bearing' noise at exactly 50Hz. At 48 and 55Hz it was almost silent. At 100Hz it was the most silent.
My guess is that the motor bearings had worn a little and started bouncing wihtin the bearing space at a specific rate related to 50 Hz and made indentations in one of the tracks to match. So when we ran it (after years of sitting still and drying the grease), the balls bounced aoround these marks and dragged on the metal bearing cage, causing the noise.
At a 100Hz, centrifugal force was too high to allow them to do that and shut them up. At other than 50Hz, they didn't follow the wearmarks and were quieter.
All just theory of course, but entirely repeatable when we tried.

The most common speed at which vibe has been an issue on old motors I have played with is 50Hz and after that, at around 25Hz.

Belt tension plays a part as well ! My Myford has two "V" belts which are tensioned by a lever. The headstock one vibrated and caused an annoying hum at the highest pulley speed. Simply increasing the belt tension slightly stopped the noise.

Originally Posted by jhovel

First thing I'd do is take the belt off the motor and see if the vibration is still there.
Next I'd find another mototr which fits isn the space (irrespective of its power, but the same speed and try and fit the same pulley or one the same size to it,l and run that without a belt (just to insure the test motor doesn't have a vibration issue).
Then fit the belt and see if the vibration returns.
If it does, get another belt.
That way you know if it is caused by the motor, the belt or the gearbox.
No change in motor is going to reduce the vibration if its the gearbox - single or 3-phase.
Next I'd change one of the pulleys slightly (say by 1/4" diameter) and see if the vibration changes. I might do that up and down in size.
It may be a resonance frequency in any of the driven components.

As BobL said, if all fails, try a 3-phase motor and VFD. You'd be amazed at the difference a few Hz can make to resonant frequency vibrations and noise of a motor.

Yesterday Dale (Dsel) was here and we set up his Waldown grinders for VFD use. When we tested them, one made a very unpleasant 'dry bearing' noise at exactly 50Hz. At 48 and 55Hz it was almost silent. At 100Hz it was the most silent.
My guess is that the motor bearings had worn a little and started bouncing wihtin the bearing space at a specific rate related to 50 Hz and made indentations in one of the tracks to match. So when we ran it (after years of sitting still and drying the grease), the balls bounced aoround these marks and dragged on the metal bearing cage, causing the noise.
At a 100Hz, centrifugal force was too high to allow them to do that and shut them up. At other than 50Hz, they didn't follow the wearmarks and were quieter.
All just theory of course, but entirely repeatable when we tried.

Give it a go and report - nothing to loose.

A lot of noise can also come from stator/rotor within the magnetic field which can also sound like bearings...had one the other day...could it be that ??
The older (around 30yrs ago) Weg motors had a unpleasant whine (non vfd)...I replaced 15 of them under warranty back then.

Why do mostly older electric motors make that whining noise while under load?

Originally Posted by .RC.

Why do mostly older electric motors make that whining noise while under load?

Loose laminations.. A bit like transformer hum.

Ray

Some of the hissing noise can come from the fan.
50 Hz x a 20 bladed fan passing vents can produce a 1kHz.
Same goes for the any slight protrusions on the rotator that sweep past the poles.

If the motor is quiet to begin with the fan appears to be the main reason for any increase in noise with increasing speed.
The newer motors seem to have quieter fans but not always.

I wonder why they don't use more of a curved vane type fan which are supposed to be quieter than straight bladed fans.

Always assumed that the straight radial blades on motor fans allowed them to be run in either direction at similar efficiency.
Some fans now seem to have the radial blades spaced in an irregular pattern to avoid resonance, whining etc.
Combustor.

Originally Posted by RayG

Loose laminations.. A bit like transformer hum.

Ray

With some success, my dad used to dip noisy armatures and transformer laminations in shellac or turps diluted Solpah paving paint, then wipe off.

Originally Posted by RayG

Loose laminations.. A bit like transformer hum.

Ray

There can be many reasons for whine noise (magnetic noise).

- a shorted winding (will soon smoke)
- a broken rotor bar (can be caused by age/vibration, rotor touching the stator, corrosion, or someone drilling into the rotor to balance it).
- poor or decayed impregnation of stator laminations, embedded air bubbles/cavities
- lamination stampings are not flat, especially the thinner laminations of high efficiency motors
- windings are not evenly distributed
- rotor not accurately concentric to stator causing irregular air gap.... can also happen if motor is carelessly rebuilt
- rotor harmonics interacting with stator harmonics, depends on slot numbers (some odd numbers are worse), slot depth, airgap length etcetcetc... These are design matters. But one cannot exclude components from different motors may have been mixed when repairing/rebuilding/salvaging a motor.... leading to a noisy motor