Thread: Motor speeds and vibrations

Originally Posted by RayG

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?


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:

Ray_Airflow.png

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.

Hi Chris,

No argument generally with what you are saying..

I'll settle for Axial flow AHU with radial impeller

Ray

Originally Posted by RayG

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.

Originally Posted by cba_melbourne

......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

Originally Posted by RayG

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.

Originally Posted by cba_melbourne

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

Originally Posted by cba_melbourne

... 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

Originally Posted by Michael G

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&r...svFKjxmcYMcDHg

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.

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

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.

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.

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.

Originally Posted by cba_melbourne

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

Originally Posted by cba_melbourne

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