Motor speeds and vibrations

[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 ?

[Michael G]

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 -
334000
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

[BobL]

Another reason for using a VFD. :D

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

[cba_melbourne]

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.

[.RC.]

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...

[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.

[BobL]

.
.
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.

[BaronJ]

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.

[eskimo]

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.

[.RC.]

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

[RayG]

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

Loose laminations.. A bit like transformer hum.

Ray

[BobL]

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.

[Combustor]

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.

[mike48]

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.

[cba_melbourne]

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

[BobL]

Always assumed that the straight radial blades on motor fans allowed them to be run in either direction at similar efficiency..

Good point , maybe they could be made with a fan that could be flipped over?

[cba_melbourne]

Good point , maybe they could be made with a fan that could be flipped over?

A general purpose motor is expected to be reversible on the fly. That is why simple non-directional centrifugal fans are used.

[Machtool]

That is why simple non-directional centrifugal fans are used.
Pretty sure motor fans arn't centrifugal. Wouldn't this be an example of centrifugal? http://b2bimg.bridgat.com/files/Double_Inlet_Centrifugal_Fan.jpg
I think you are looking for the word "Axial" fan.

[BobL]

Pretty sure motor fans arn't centrifugal. Wouldn't this be an example of centrifugal? http://b2bimg.bridgat.com/files/Double_Inlet_Centrifugal_Fan.jpg
I think you are looking for the word "Axial" fan.

Centrifugal fans usual move the majority of their air by forcing air out from their rotation point building up pressure on their perimeter and then a cowling directs the air as required. Axial fans usually have a pitch on the blades and move the air parallel to their axis of rotation like a propellor. But it is possible to put a propeller inside a cowling or up against the blank end of a motor and then direct the air outward and over motor cooling fins on the outside of the motor - so is an "axial" or is it a "centrifugal" fan?

The blades may be axial but because of the configuration of the fan surrounds the fan acts more like as a centrifugal fan.

The same propeller forcing air through a motor end plate with holes in it, and into the inside of a motor and out through holes in the other end is more of an axial fan

[Machtool]

Not sure what you are saying there Bob.

Are the Fans on your typical motor Axial or Centrifugal?

[Machtool]

Not sure what you are saying there Bob.

Are the Fans on your typical motor Axial or Centrifugal?

Pretty sure about post #17 they were claimed as universal direction Centrifugal. 30 years in the caper, I've not once seen that.

[cba_melbourne]

.....

Are the Fans on your typical motor Axial or Centrifugal?

Your typical TEFC Motor (Totally Enclosed Fan Cooled Motor) uses a "centrifugal fan" with an "open radial blade" type impeller.

[Michael G]

I would have thought that the fan on the end of a motor would be classed as axial because the air direction is parallel with the axis of rotation. Centrifugal fans typically push the air at a tangent to the circumfrence of the rotating blade - that is, at right angles to the axis of rotation.
It could be argued that a disc type fan with radial blades is centrifugal and with different ducting that could be the case but the sole purpose of a fan is for air flow, so by asking whether the air flow is coaxial or perpendicular the most appropriate label is apparent.

Michael

[RayG]

Your typical TEFC Motor (Totally Enclosed Fan Cooled Motor) uses a "centrifugal fan" with an "open radial blade" type impeller.

Bzzzt.. sorry wrong answer.. TEFC use axial fans, as does pretty much every electric motor I've ever seen...

Ray

PS.. Centrifugal fans aren't generally reversable either, so wouldn't suit motor cooling, unless driven by a seperate motor.

[cba_melbourne]

I would have thought that the fan on the end of a motor would be classed as axial because the air direction is parallel with the axis of rotation. Centrifugal fans typically push the air at a tangent to the circumfrence of the rotating blade - that is, at right angles to the axis of rotation.
It could be argued that a disc type fan with radial blades is centrifugal and with different ducting that could be the case but the sole purpose of a fan is for air flow, so by asking whether the air flow is coaxial or perpendicular the most appropriate label is apparent.

Michael

Easy. Take the cover that is around the impeller off, and start the motor. You will now see that the airflow is actually radial. The cover only redirects the airflow, just like a duct.

After all, an axial fan does not suddenly become a radial fan either, just because it is installed inside a duct that makes a 90 degree bend. What counts, is how the fan accelerates the air, not in what direction the airflow is afterwards being redirected or ducted.

[jhovel]

CBA is right. THink of an axial fan as a 'propellor' of some sort. In our motors, the fan 'blade' is just a plain open centrifugal fan blade (a disk with radial blades which throw the air outwards by centrifugal or centripedal motion and replace it with new air from the centre. The fan covers are curved in profile so that the air is forced into an axial direction around the motor. This works equally well in either direction.
If they were axial fans, the airflow would be reversed when the motor reverses - not somthing you want on a motor (sucking in dust when going one way and blowing into it the other).
So we need to think separately about airflow versus fan design. I agree the effective airflow is axial but it is produced by a centrifugal fan design to esure the same direction of flow no matter what the motor direction is.

[RayG]

Hi Joe,

I understand your thinking, but it's quite simple... the air flow is axial, therefore it's an axial fan...

Ray

[BaronJ]

Hi Guys,

I'm with Joe on this one.:D:D:D

[jhovel]

Hi Joe,

I understand your thinking, but it's quite simple... the air flow is axial, therefore it's an axial fan...

Ray

Nah, Ray, not thinking at all. I may not have mentioned that I worked in airconditioning and ventilation design in another lifetime. I know stuff about fan designs. The air goes in in the centre and comes out at the periphery.... that makes it a radial fan. SInce the blades are straight, that makes it a centrifugal one - there are several others (airfoil, reverse airfoil, tangential etc..).
But I'm very happy to agree that we have axial airflow over the motor.

and of course always happy to agree to disagree :U

[RayG]

Nah, Ray, not thinking at all. I may not have mentioned that I worked in airconditioning and ventilation design in another lifetime. I know stuff about fan designs. The air goes in in the centre and comes out at the periphery.... that makes it a radial fan. SInce the blades are straight, that makes it a centrifugal one - there are several others (airfoil, reverse airfoil, tangential etc..).
But I'm very happy to agree that we have axial airflow over the motor.

and of course always happy to agree to disagree :U

I like a good semantic discussion... I also worked in airconditioning ( well BMS mostly ) and we never called them fans anyway... they were always AHU's

With a flat bladed impeller like a motor cooling fan, the shroud forms a vital part of the design, without the shroud it doesn't do much cooling if any... so the shroud changes the airflow flow from radial to axial.... ergo it's an axial fan.. :)


Ray

[Stustoys]

Its an axial flow AHU..... but its still a centrifugal fan :D

Have a look at Whittles jet ;)

[cba_melbourne]

I like a good semantic discussion... :)

Ray


Ray, you surely agree that a propeller aircraft like a Cessna 150 is essentially an axial fan.

Using your semantics, that same aircraft would suddenly and unexpectedly become a radial fan, if the aircraft was parked with the tail to a wall. Because the wall would deflect the airflow sideways. So, depending on where in space you place that Cessna, it would change its operating principle. Uuh... that can't be right, can it? :D


Another example are aircraft jet turbines. There are those with axial compressor, as can be seen on most passenger jets. But remember the old British Venom and Vampire fighters? They used a radial compressor jet engine, very similar to the compressor in a Turbocharger, but larger. The air still enters the airplane at the front and exits at the rear, but inside the radial compressor the air follow a Z-path.


And here a pic I just found on the net to illustrate the point:

334475

Green shows the airflow in a typical TEFC motor.

It does not always has to be black and white. Some fan designs are actually a combination or radial and axial, as shown in the blue example. I have however never seen this used on a TEFC motor.

[RayG]

Hi Chris,

No argument generally with what you are saying..

I'll settle for Axial flow AHU with radial impeller

Ray

[cba_melbourne]

Hi Chris,

No argument generally with what you are saying..

I'll settle for Axial flow AHU with radial impeller

Ray

......it must be the magic of semantics.... I thought that the air flow direction of all axial fan designs depended on the rotation direction.

[RayG]

......it must be the magic of semantics.... I thought that the air flow direction of all axial fan designs depended on the rotation direction.

The magic of semantics, axial means parallel to the axis of rotation, nothing more nothing less...

Ray

[cba_melbourne]

The magic of semantics, axial means parallel to the axis of rotation, nothing more nothing less...

Ray

If you reverse the sense of rotation in any axial fan, the direction of airflow reverses too.

If you reverse the sense of rotation of a radial/centrifugal fan, the airflow will not be reversed.

Therefore the fan in a TEFC motor cannot be described as a radial fan. No semantics can change that.

[RayG]

If you reverse the sense of rotation in any axial fan, the direction of airflow reverses too.

If you reverse the sense of rotation of a radial/centrifugal fan, the airflow will not be reversed.

Therefore the fan in a TEFC motor cannot be described as a radial fan. No semantics can change that.


I think you got that back to front. Axial flow refers to the fact that air flow is parallel to the axis , nothing to do with direction. and centrifugal/tangential fans only work one way they aren't reversible. Radial impellers of the type used in TEFC motors are reversible, that's the whole point of the impeller design.

Ray

[Michael G]

... sense of rotation of a radial/centrifugal fan, the airflow will ...

Can you please work out what you are going to call it as you seem to be changing names depending on what point you are trying to make. I'm quite happy as are many others I would guess to call it an axial flow fan and be done with it, but if you are determined to prove to the world that it isn't please do so. So far I haven't read anything that changes my view that it is an axial (flow) fan.

Michael

[cba_melbourne]

Can you please work out what you are going to call it as you seem to be changing names depending on what point you are trying to make. I'm quite happy as are many others I would guess to call it an axial flow fan and be done with it, but if you are determined to prove to the world that it isn't please do so. So far I haven't read anything that changes my view that it is an axial (flow) fan.

Michael

Michael,

- Centrifugal fans and radial fans are obviously two words for the same thing.
- Fans are classed by the method used to accelerate the air. NOT by the direction of exit flow, which in both cases can easily be redirected at will by a ducting enclosure. Rays mistake is to confuse these two.

There are fundamental differences between axial and centrifugal impellers regarding pressure and airflow, which makes the one or the other better suited to a given task. Download this small .pdf,
Chapter 10.3.1 will explain what a centrifugal impeller is
Chapter 10.3.2 will explain what an axial impeller is

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0CEEQFjAF&url=https%3A%2F%2Fwww.mvsengineering.com%2Ffiles%2FSubsurface-Book%2FMVS-SVE_Chapter10.pdf&ei=lfSRVIXzCYiI8QWZioLACA&usg=AFQjCNFQvnv752d-zZ2OsvFKjxmcYMcDHg

[eskimo]

I too work in the AC industry...and I say motor fans are an open radial blade type

The blades are off a paddle type rather than forward or backward curved so as to allow for motor direction change.
The fan housing only directs the airflow

An axial fan is looks like a crude propeller


AHU stands for Air Handling Unit and has no bearing on what type of fan wheel or blade is used to deliver the air.

[eskimo]

http://www.cincinnatifan.com/blower-wheels.htm

make up your own minds but I reckon Cinncinatti may know a little more than us

I'll still go with Type 2 in the link

[Ashore]

Your vibration may be due to natural Harmonics
If the mechanical resonate frequency of the lathe matches the vibrational frequency of the motor or gets close you will get vibration.

Changing either on will reduce this, as has been suggested by changing the frequency even slightly will alter the motors driving vibrational frequency enough to stop the vibration, or more difficulty change the lathes mass, longer bed different size chuck etc, can change the natural frequency of the lathe.

[RayG]

Thanks Eskimo.. type 2 or 3 impellor.. :) timely as always... Like I already said it's an "Axial flow AHU with radial impellor" you can't take away the shroud and expect it to provide cooling air flow over the motor. The shroud is an essential part of the design.

Chris, please note for your benefit, I didn't mention the word fan.

Ray

PS.. Actually I can't think of a single radial/centrifugal impellor based system that works without a shroud or duct of some sort.

[BobL]

In the late 80s I was working in a lab in the US and we were using mass spectrometers to measure the intensity of ion beam currents in the sub 10-14A range.
10-14A was the electronic noise level or background of the Peltier cooled amps we were using at the time, so long (many minutes) of repeated integration times of the background were needed to resolve signals at that level.
During a typical analysis time of 4-5 hours the background would be measured approx every 10 minutes so we had a reasonable time based record of the background.
Over a period of a a few days we observed that the background would sporadically increase for periods of minutes to hours by up to a factor of two which really messed up our results.

After spending some time eliminating the usual contamination and other sources, there was no change.
One of the things we knew about was that mechanical vibrations could easily increase the background.
The ion bean counter was a small thin walled SS box (about the size of a 50c piece) suspended inside a 60 mm long SS tube with a wall thickness of 3mm, all under ultra high vacuum. The SS tube itself was sticking out into the lab about 400 mm and if it was knocked or the lab door slammed one could easily see the background jump. However, because these were all short term effects they did not impact on final results, only a consistent ongoing mechanical vibe would do that.

We tried a number of ways to dampen this microphonic effect on the ion counter background and eventually someone discovered if a tool box was placed in a certain spot of the mass spectrometer bench the background was significantly reduced. By experimenting with the position of a aluminium foil covered 20kg lead brick on the bench we were further able to reduce the background, but we could still see it was above spec.

To cut a long story short, after a mind numbing series of eliminations the source of the mechanical vibrations was eventually traced to be a badly out of balance extraction fan in a fume hood two floors below our lab. The minute to hour long periods of vibrations was being cause by users turning the fume hood fan on/off.

The lead brick remained in place on the mass spectrometer bench to help dampen other vibrations and I always found it curious that one of the elements that was being measured by the mass spectrometer was lead, and the difference between the amount of lead in the sample and the brick was about a factor of 1015.

[Michael G]

- Centrifugal fans and radial fans are obviously two words for the same thing.


Therefore the fan in a TEFC motor cannot be described as a radial fan. No semantics can change that.


Therefore the fan in a TEFC motor can not be a centrifugal fan. You win.

Michael

[RayG]

Therefore the fan in a TEFC motor can not be a centrifugal fan. You win.

Michael

Hi Michael, In fairness to Chris, I think that was just a typo, as he has been saying the opposite all along..

I'm willing to concede that a motor cooling fan uses a radial/centrifugal impellor. and leave it at that... :D

Ray

[PDW]

What does it matter if the cat is black or white, as long as it catches mice......

PDW

[shedeng]

Hello all, first post.

Two things cause vibration from electric motors, first is out of balance of the rotating mass, unlikely at the normal speed of the motor,second is magnetic imbalance between the windings.
Magnetic imbalance is more likely as there are many things that can cause it.
A very simple way of finding out if this is likely and is only observable with belt drives is to watch the behavior of the belts when the motor is running. if the belt or belts are vibrating but stop vibrating when you switch off the motor and the revs drop then it indicates a high frequency speed variation per rev. Indication of magnetic imbalance.

A site to read more: http://www.pdma.com/pdfs/Articles/Identifying_Motor_Defects_Through_Fault_Zone_Analysis.pdf