Motor torque/impeller RPM question...

[jon_denmark]

Hi,
I am building my own wavemaker. The basic concepts is much like in tunze's stream pumps. I have ordered a very small 50W brushless DC-motor from maxon. When the shaft is not loaded it will deliver 6800RPM and the maximum continious torque is 90M[mNm]. I haven't been able to find any website releating the requered torque to the rotating velocity for a specific impeller/screw.
I am going to use an impeller as those used in RC boats. So my question is:
- Does you know a manufactor of small impellers (im not sure if you call it impellers, screws og propellers)?
- Does you know where i can find examples or a mathematical expression dealing with the needed torque to rotate a given impeller in water?

The goal of my project is to create an ultra quite and very efficient pump/wavemaker. The use of brushless dc-motor together with a suited amplifier will make it possible to oscillate from close to 0 RPM up to 6000-10000 RPM.

 

[Anonymous]

The motor going to be outside the water, right?

If it is pitched, I think it is called propeller.... unfortunately, I don't know anybody that sell propeller. Most are made in-house or custom-build for small quality, but someone family with RC boat maybe able to help you out on it...

Most low-cost design use empirical (engineering) principle when finding propeller/motor match. High quality pump maker use sophicated physic-based model for simulation in their design (CAD/CAM). I suggest you go with the empirical method.

Sorry that I can't be much help.

 

[jon_denmark]

Thanks for your repply. Actually i was thinking of building the motor into a water-tight container of some sort. I am also looking for a reliable seal for the motorshaft. The datasheet states thermal resistances from wiring to ambient. From this it should be possible to calculate the temperature in the container and thereby the pressure. But ofcourse it all depends on the reliability of the seals offered in the industry.
I've been trhu the same thoughts as you regarding the way to calculate the requried torque. I was just hopping maybe of some "rule of thumbs".
Does you have any ideas reagarding the optimal seal type to use?
Best regards here from Denmark

 

[Anonymous]

Any seal you use would have to be checked on a regular basis. there will be friction between the shaft and the bushing/seal of the box. Over time either the seal or the shaft of the motor would wear away.

B

 

[jon_denmark]

Thank you, for good answering. It's very usefull to hear oppinions and knowledge from others.
I have thought of something like this. The reliability of the seal must depend on the shaft material and the part of the seal tightning around the shaft. I have found some seals concisting of a spring (giving almost constant force to the shaft) and ptfe plastic wich has benefits of high chemical resistance and very low friction. But ofcourse still there is need for replacement from time to time.
I nother solution for transfering the power could be magnetical transfering. Then there would be no use for dynamic seals wich would make the system much more reliabel. This will just increase costs and make the system more advanced (hence less reliabel).[/img]

 

[Anonymous]

Shaft can be marine-grade stainless steel.

Check the following thread:
http://reefs.org/phpBB2/viewtopic.php?t=47004

 

[rae gordon]

Here is a link for some props.
http://www.dumasproducts.com/cgi-bin/dumas.storefront

Be carefull here, you might buy some stuff you didn't need.

 

[Shinken]

it might be easier just to buy stream pumps! But i know what you mean, they are far far too expensive!

 

[gnx87]

Being in the pump business i can give you a few tips on seal selection. You would need viton elastomers and more than likely carbon vs. ceramic seal seats.

 

[Anonymous]

See if there is any useful formula for your application:

http://www.engineeringtoolbox.com/34_408.html