# Calculate fan speed

Tags:
1. Nov 21, 2015

### marcusbarnet

Hi to all, this is my first topic and I hope I'm not in the worng section.

As you can see at this link: http://www.skeetty.com/disco.jpg, I have a plate with a diameter of 4 meters which can rotate at 200 RPM.
On this platform, I have four fans with 4 blades where the lenght of each blade is 1 meter and its width is 0.4 meters.

Is it possible to understand if the fans are able to rotate during the disc rotation?
If not, what is the condition which make the fans to rotate? For example, if I increase the RPM of the disc, then the fans can rotate?

What kind of formulas do I have to use in order to better understand the problem?

Thank you and I'm sorry for my english.

2. Nov 21, 2015

### Staff: Mentor

What should prevent them from doing so? They'll need some force on them, of course. Either from the air or from some motor.
Rotation relative to the disk or relative to the air?

3. Nov 21, 2015

### marcusbarnet

Hello and thank you for your answer! :)

I'd like to know if there is any way to calculate the RPM of each fan when the disk is rotating at 200 RPM under the assumption of no friction on the bearings fans (the fans are free to rotate with no friction around their rotational axes).

Is there any equation which let me find out the fans speed depending on the disk speed?

EDIT: I'm asking if they are able to rotate by the air flow produced by the disk during its rotation at 200RPM.

4. Nov 21, 2015

### Staff: Mentor

It depends on the still unclear conditions.
If the fans are free to rotate, do not have a motor and don't interact much with the air, they will probably not rotate relative to the disk. Every tiny asymmetry leads to some side that gets pushed outwards (more than the rest), and friction in the bearings (even if it is small) might be the dominant force.
If the fans interact significantly with the air, things get complicated and you'll probably need a simulation or a test.

5. Nov 21, 2015

### CWatters

Are you sure? At 200rpm the edge of the 4m disc is going quite fast. I make it about 42 meters per second or 93mph. I was going to suggest putting one of the fans out in the wind to test it but you don't get wind that fast very frequently in my part of the world.

The centripetal forces on the fans will also be quite large. For example a 1kg fan would experience 882 Newtons. How heavy are your fans/wind turbines?

I also suspect that as each turbine blade rotates it will experience forces that vary wildly in magnitude and direction causing extreme flexing and or vibration.

Would love to see a video :-)

6. Nov 21, 2015

### CWatters

Just to expand on that.. As the blades rotate their distance from the centre of the rotating plate varies from 1m to 3m. So the tips of the blades experience a head wind that varies from 20 to 62m/s (not counting any apparent wind due to their own rotation). Variable pitch blades?

Clearly this isn't a practical project and I'm off to get something to eat.

7. Dec 3, 2015

### HunterThomson

Onan interesting pointcalculation

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook