Thread: Motor speeds and vibrations

Originally Posted by BobL

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.

Originally Posted by cba_melbourne

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/Doub...ifugal_Fan.jpg
I think you are looking for the word "Axial" fan.

Originally Posted by Machtool

Pretty sure motor fans arn't centrifugal. Wouldn't this be an example of centrifugal? http://b2bimg.bridgat.com/files/Doub...ifugal_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

Not sure what you are saying there Bob.

Are the Fans on your typical motor Axial or Centrifugal?

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.

Originally Posted by Machtool

.....

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.

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

Originally Posted by cba_melbourne

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.

Originally Posted by 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

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.

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.

Hi Joe,

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

Ray

Hi Guys,

I'm with Joe on this one.

Originally Posted by RayG

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

Originally Posted by jhovel

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

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