AC motor fan design? 1

[310toumad]

I've noticed for several different manufacturers of AC motors, the fan design looks something like this:

https://www.galco.com/buy/Baldor/36...MIyeqA84Oh5AIVAQOGCh0jDASdEAQYBiABEgJ57fD_BwE

https://www.grainger.com/product/24...wcid=AL!2966!3!340055571002!!!g!545329123390!

Usually they are attached to the motor shaft and encased in a shroud. Maybe I'm off-base with this observation, but wouldn't having straight blades on a fan when you want the flow to move axially along the motor shell for cooling be extremely ineffective? All I can see that would happen is the fan accelerates the air radially within the fan shroud, where eventually it would exit but would then continue spiraling 'outward' away from the motor. In my mind you would want to have some degree of twist to the blades. Can anyone see why straight blades would have an advantage here?

 

[btrueblood]

Easier to mold the straight blades. You get spiral flow with any rotating fan, but may get better pressure rise (and resulting flow through the complex internal rotor passages) with a centrifugal fan vs. an axial.

 

[BrianPetersen]

Has to work the same with the motor spinning in either direction, too.

 

[handleman]

The function of these fans is really closer a centrifugal blower. Air is drawn in at the center and accelerated radially outward. The curved shroud then redirects the airflow axially around the outer circumference of the motor. I'm not sure what you mean by "spiraling outward away from the motor"... If the stream changes direction, it doesn't "remember" that it was flowing radially and try to return to that path. If the fan had angled blades like a regular axial fan, it would just deadhead the flow into the endplate of the motor and you'd get bupkis along the outside of the motor. With regular twisted blades, the fan would have to be significantly larger than the motor to create axial flow around the circumference.

 

[JohnRBaker]

Yes, as already alluded to, these are not 'compressor' fans. The purpose is to move as much air as possible, not achieve any sort of pressure. That being said, I would say the biggest reason is as BrianPetersen stated, it has to work the same no matter which direction the motor is turning.

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[handleman]

Also of note... "complex internal rotor passages" are not directly cooled by the fan on a TEFC motor (for which the linked fans are specified)

 

[Compositepro]

As has been noted, the fan must be bidirectional because most motors can run in both directions. It maybe a little misleading to call this a fan, but that is what it is called. Axial fans and centrifugal blowers operate on completely different principles. These are centrifugal fans. Your question shows that you do not really understand how a centrifugal fan works. I suggest that you do some Googling and develop a deeper understanding because that knowledge is key to so many other things. Some clues for your studies are that:

Air and other fluids have inertia.
Moving air has momentum.
Things move (and continue to move) in a straight line unless acted on by an external force.
Air blowers generally have forward curved blades and centrifugal water pumps generally have backward curved blades. This is for practical reasons due to the great difference in density between liquids and gasses, and therefore the velocity at which they flow. The fluid velocity is related to the tip velocity of the blades, and most motors used for gas or liquid operate at the same rpm.

The fans that you pictured are used for TEFC (totally enclosed, fan cooled) motors. The air from this fan does not go through the motor but around the outside of the motor to cool it. The fan shroud changes the radial flow to axial flow. The rotor inside the motor has projections on it to create airflow inside the motor to help move heat to the casing. This keeps dirt outside of the motor.