Scenery Motor Questions

[wavemaster447]

I'm currently trying to create a turning windmill (for a production of Oklahoma) and our set designer is looking for ~5rpm for a 5' diameter windmill he built out of wood. The only specs he gave me is that it weighs ~80lbs, and will be 18 feet up in the air on a structure. I was browsing some industrial catalogs looking at motors, and found some that look good (nice and cheap, set RPM motors w/ 50 p/i torque), but I had a couple questions you guys might be best suited to answer.

1) How do I calculate the torque/HP required to turn the windmill?

2) Is there any to vary the speed on a motor, without finding a specifically designed variable speed motor? (Our shop foreman suggested dimming with SCR lighting dimmers, but I'm very hesitant about that).

3) Since there will be some fun times had balancing this rig on top of a 18' windmill structure, has anybody had more success with a base mount motor versus face mount?

And finally, anybody have recommendations for this in general? We were trying to use some ancient (1991) 230v DC turntable motors, and unsurprisingly they didn't work in even the slightest, not to mention they weighed a good 80lbs on their own.

Thank you all!

 

[Les]

Ah, the ever-revolving windmill thread

Sounds like you're on the right track so far. You are correct about the fact that an SCR dimmer (or any dimmer really) can not dim [slow] a motor. They can however stall and burn out a motor.

Let me see if I can call up a few threads here.

Here's a "how would YOU do it" thread:
http://www.controlbooth.com/forums/question-day/21235-operational-windmill.html

Here's a great thread about why you shouldn't invert a ceiling fan in to a windmill. Luckily, it culminates in to a couple of really good alternatives:
http://www.controlbooth.com/forums/scenery-props-rigging/16068-ceiling-fan-windmill.html
Unfortunately for our case, the thread has been merged with a "why can't I run a ceiling fan on a dimmer" question about halfway through. Further down though, there is mention of a rotisserie motor. Might be of interest.

 

[kicknargel]

The book we recommend in every post:

http://www.amazon.com/dp/024080354X/?tag=controlbooth-20

will help you figure out the math. Or, you could test it. Assemble the rig, sans motor, put a 1' handle perpendicular to the shaft, and see how much weight it takes to get it turning. That's your torque in ft-lbs.

 

[wavemaster447]

Thanks for the advice guys. I actually already had that book on order from a library, but doing a test on the windmill sounds like the way to go. What should I do as far as putting it in a ball-bearing unit, or counterbalancing it (to get it to stand) for when I test it? I'm assuming the weight of the motor will counterbalance the rig eventually, but should I do anything as far as the test? And I'm assuming this will give me torque as lb/foot, not lb/inch?

 

[kicknargel]

You don't want to rely on the motor to provide any structural support to the rig. You'll need a shaft on the "fan" going through two bearings to hold it rigid. I would get all that built, so your test is under actual conditions.

Yes, if your test weight is 12" out from the shaft, then you're getting ft-lbs. Pretty easy to convert.