Electric Motor for an Art Project
June 4, 2007 11:05 AM Subscribe
What kind of motor should I be looking at if I need to rotate a 150 pound, 12' diameter spherical shell in the neighborhood of 10-100 rpms?
A mockup of the project is here. The blue and green spheres will each weigh around 150 pounds (made of fiberglass formed into those tubes.) We'll probably put one motor on each pole so each motor drives one of the spheres.
Then we'll also need a 3rd motor in the base to rotate the whole structure (approx. 400-500 pounds) at around 10-20 rpms. The motors should be able to survive near continuous usage for 1 week (Burning Man project...)
A mockup of the project is here. The blue and green spheres will each weigh around 150 pounds (made of fiberglass formed into those tubes.) We'll probably put one motor on each pole so each motor drives one of the spheres.
Then we'll also need a 3rd motor in the base to rotate the whole structure (approx. 400-500 pounds) at around 10-20 rpms. The motors should be able to survive near continuous usage for 1 week (Burning Man project...)
Response by poster: We considered the 1 motor solution but we're uncertain about the linkage through the curved support frame (and would prefer to not go with a rectangular frame.) Could you run cables for pulleys along those curves if you lubed them a lot with graphite or something? But even then, the problem there is, if the cables get lubed, they'll slip on the pulleys...
Yes, we are forming the fiberglass ourselves and are pretty certain we'll be able to form the spokes with fairly decent precision. (Regardless of the precision of the curvature, we're taking advantage of the north-south symmetry and forming the spokes in halves and using the same jig for each one, so even if we miss perfect geometry, we'll still have symmetry.)
The vane idea, not to mention power generation, is a very good one, considering this year's theme. We'd actually originally considered making this hand-cranked (3 big wheels mounted on the ground nearby, so 3 people could come along and drive it), but again ran into linkage issues.
posted by badstone at 11:41 AM on June 4, 2007
Yes, we are forming the fiberglass ourselves and are pretty certain we'll be able to form the spokes with fairly decent precision. (Regardless of the precision of the curvature, we're taking advantage of the north-south symmetry and forming the spokes in halves and using the same jig for each one, so even if we miss perfect geometry, we'll still have symmetry.)
The vane idea, not to mention power generation, is a very good one, considering this year's theme. We'd actually originally considered making this hand-cranked (3 big wheels mounted on the ground nearby, so 3 people could come along and drive it), but again ran into linkage issues.
posted by badstone at 11:41 AM on June 4, 2007
Best answer: I was going to say, "belt driven washing machine motors, geared down and rewired for 220V", but I'm not sure if they can stand up to continuous use, so I think you need to find something that's of industrial quality
happily, there's an excellent industrial surplus place in the San Jose, Triangle Machinery & Tool, that has a large selection of used electric motors. The last time I was there, they easily had several hundred of these, and many of them have tags that indicate their rated RPMs, voltage, amps, etc. I would start there - just bring a flashlight and some gloves, as the wall where they store the motors is poorly lit, and used motors are dirty.
Ideally, some sort of arrangement that used ribbed alternator belts and an adjustable idler pulley would be the easiest to adjust. Let a good set of wheel bearings carry the load -- say a steering knuckle from a 4x4, which will have a jack shaft to which you can affix a pulley -- and use a belt driven motor to rotate the parts that need rotating.
Neat project -- good luck.
posted by mosk at 11:55 AM on June 4, 2007
happily, there's an excellent industrial surplus place in the San Jose, Triangle Machinery & Tool, that has a large selection of used electric motors. The last time I was there, they easily had several hundred of these, and many of them have tags that indicate their rated RPMs, voltage, amps, etc. I would start there - just bring a flashlight and some gloves, as the wall where they store the motors is poorly lit, and used motors are dirty.
Ideally, some sort of arrangement that used ribbed alternator belts and an adjustable idler pulley would be the easiest to adjust. Let a good set of wheel bearings carry the load -- say a steering knuckle from a 4x4, which will have a jack shaft to which you can affix a pulley -- and use a belt driven motor to rotate the parts that need rotating.
Neat project -- good luck.
posted by mosk at 11:55 AM on June 4, 2007
IANAME. but I've seen setups where rotational energy is transferred through a curved tube by means of an internal rotating rod that is flexible laterally but stiff rotationally (I know I've seen other examples, but the first that comes to mind is a screwdriver I have that's designed for tight spaces.) You could put gears on each end of this rod and drive the speheres that way. What the rod is made of I'm afraid I don't know. Carbon fiber?
Here's an example of such a shaft if I'm not explaining it well enough, but I still have no idea how they do it or how well it would scale. It probably works best at high speed/low torque, so I'd get a fast motor and then gear it down to the correct speed up by the axle.
Another option would be hydraulics, but I think that tends to be noisy, expensive and leak-prone.
posted by contraption at 12:15 PM on June 4, 2007
Here's an example of such a shaft if I'm not explaining it well enough, but I still have no idea how they do it or how well it would scale. It probably works best at high speed/low torque, so I'd get a fast motor and then gear it down to the correct speed up by the axle.
Another option would be hydraulics, but I think that tends to be noisy, expensive and leak-prone.
posted by contraption at 12:15 PM on June 4, 2007
A plumber's 'snake' is like a big version of that dremel gizmo contraption (eponysterically) points to.
The drive shaft linkages in an outboard engine leg might be useful spare parts to get your rotation up those curved columns.
Rock-tumbler motors might be useful as well.
posted by Rumple at 12:24 PM on June 4, 2007
The drive shaft linkages in an outboard engine leg might be useful spare parts to get your rotation up those curved columns.
Rock-tumbler motors might be useful as well.
posted by Rumple at 12:24 PM on June 4, 2007
I'd use decent corded electric variable speed drills geared down a bit with a couple pulleys (to keep the fan speed up). Cheapish, readily available and you can control the speed with a light dimmer. I the advent of a failure you can swap them out easy.
Or you could drive the spheres directly with a rubber tire mounted in the chuck of the drill applied to the surface of the sphere.
posted by Mitheral at 12:37 PM on June 4, 2007
Or you could drive the spheres directly with a rubber tire mounted in the chuck of the drill applied to the surface of the sphere.
posted by Mitheral at 12:37 PM on June 4, 2007
I don't think this would require a lot of power, as long as everything was well-balanced and on good bearings.
Ceiling fans run continuously at about the right RPM, and are readily available. They've even got a reverse switch. That might be a good scavenging opportunity. They're obviously not designed for this kind of weight, so they'd have a hard time getting it going, but I don't think they'd have a hard time keeping it going.
Working out the power coupling through the rotating base up the arms will be a little tricky You'd need brush contacts, I guess.
Just to throw another spice into the pot, have you considered pneumatic drive? Cheap, flexible, clean, low-tech. Most pneumatic applications are high RPM (I think), so you might need to gear down if you were adapting existing hardware. You'd obviously need a compressor, but that could be located some distance away, under a baffle.
I agree that this is a cool project.
posted by adamrice at 12:55 PM on June 4, 2007
Ceiling fans run continuously at about the right RPM, and are readily available. They've even got a reverse switch. That might be a good scavenging opportunity. They're obviously not designed for this kind of weight, so they'd have a hard time getting it going, but I don't think they'd have a hard time keeping it going.
Working out the power coupling through the rotating base up the arms will be a little tricky You'd need brush contacts, I guess.
Just to throw another spice into the pot, have you considered pneumatic drive? Cheap, flexible, clean, low-tech. Most pneumatic applications are high RPM (I think), so you might need to gear down if you were adapting existing hardware. You'd obviously need a compressor, but that could be located some distance away, under a baffle.
I agree that this is a cool project.
posted by adamrice at 12:55 PM on June 4, 2007
One thing I've never gotten about rotating electrical anything: how do you manage all the electrical connections between the rotating element and the stationary base? To use Mitheral's suggestion, how do you plug in the drills in a way that keeps their cables from twisting until they break or come unplugged? I suppose you could have springy terminals on the frame that drag around the base on circular strips, but that sounds like it would be dangerous and hard to keep going unless you had it hermetically sealed, especially around playa dust.
This is done so frequently in various applications (carnival rides for instance) that I feel like there must be a standard way of doing it, but I don't know what it is.
On preview: brush contacts, right. But then you've gotta keep them sealed from the elements, and the seal itself needs to rotate along with the frame, which is a tough problem in it's own right.
posted by contraption at 1:05 PM on June 4, 2007
This is done so frequently in various applications (carnival rides for instance) that I feel like there must be a standard way of doing it, but I don't know what it is.
On preview: brush contacts, right. But then you've gotta keep them sealed from the elements, and the seal itself needs to rotate along with the frame, which is a tough problem in it's own right.
posted by contraption at 1:05 PM on June 4, 2007
man if there is ever a question that needs mefi's own asavage to chime in i think this is it
posted by ShawnString at 1:38 PM on June 4, 2007
posted by ShawnString at 1:38 PM on June 4, 2007
The motors which drive those baby cement mixers seem like they would be up to the dirt, the continuous duty, the heat, and the load.
posted by jamjam at 2:19 PM on June 4, 2007
posted by jamjam at 2:19 PM on June 4, 2007
For some reason, I can't see the video on this computer, but I think contraption has the right idea. Alternatively, you could use pulleys to get around a curve and just use an extra pully where it would otherwise hit. (rather than using a lubricant on the support.)
A linkage is almost certainly going to be easier than using brush contacts for the additional motors. This is especially true if you want to shield them to protect the health/safety of people on the playa who may have, umm, slightly impaired judgement. You can control the size of the drive-wheels to control the various speeds of rotation relative to each other.
posted by JMOZ at 2:19 PM on June 4, 2007
A linkage is almost certainly going to be easier than using brush contacts for the additional motors. This is especially true if you want to shield them to protect the health/safety of people on the playa who may have, umm, slightly impaired judgement. You can control the size of the drive-wheels to control the various speeds of rotation relative to each other.
posted by JMOZ at 2:19 PM on June 4, 2007
I vote for either pneumatics or hydraulics. Put a big fat electric pump in the base and some hydraulic motors and gear trains wherever you need. No problem routing it through curved passages, arbitrary torque based only on how much you're willing to spend. I know at least Northern Tool sells hydraulic components. You certainly shouldn't need a lot of power, the Falkirk Wheel moves 600 tonnes with only 22kW thanks to clever gearing and precise balance.
posted by Skorgu at 2:59 PM on June 4, 2007
posted by Skorgu at 2:59 PM on June 4, 2007
I've used motors from discarded treadmills. Just off the top of my head, this is what I'd try first.
Goodwill?
posted by SlyBevel at 3:25 PM on June 4, 2007
Goodwill?
posted by SlyBevel at 3:25 PM on June 4, 2007
Response by poster: I really like the idea of doing it with hydraulics. We'd have to have the pump rotating with the whole armature, but that really solves the problem of delivering power from base to poles while allowing the poles to remain relatively lightweight.
posted by badstone at 3:42 PM on June 4, 2007
posted by badstone at 3:42 PM on June 4, 2007
Holy cow... do you realize the tangential speed at the rim of this thing would be 1,170 mph? I don't know if there's anything unusual to be considered in accelerating an item past Mach 1 (probably if it's just a disc), but man.
posted by hodyoaten at 3:50 PM on June 4, 2007
posted by hodyoaten at 3:50 PM on June 4, 2007
Also at that kind of speed it might be worth calculating the centrifugal acceleration and load just to make sure it won't fly apart and turn into a giant grenade ... if there's bolted or welded joints this may be an issue.
posted by hodyoaten at 3:52 PM on June 4, 2007
posted by hodyoaten at 3:52 PM on June 4, 2007
Uhhh... 1170 mph? I think you dodged when you should have ducked.
12' x 3.14 x 100rpm x60min/hr / 5280ft/mile = ~ 43mph
which is at the top end of his scale, one tenth would be 4.3 mph...
As to the motor size... calculating...
posted by defcom1 at 4:09 PM on June 4, 2007
12' x 3.14 x 100rpm x60min/hr / 5280ft/mile = ~ 43mph
which is at the top end of his scale, one tenth would be 4.3 mph...
As to the motor size... calculating...
posted by defcom1 at 4:09 PM on June 4, 2007
contraption writes "To use Mitheral's suggestion, how do you plug in the drills in a way that keeps their cables from twisting until they break or come unplugged?"
Slip rings with carbon brushes are the standard way of accomplishing this. Any motor rebuilding shop could set you up with a set sized appropriately. They are used in car alternators too so can be quite resistant to damage from atmospheric contaminants.
posted by Mitheral at 4:43 PM on June 4, 2007
Slip rings with carbon brushes are the standard way of accomplishing this. Any motor rebuilding shop could set you up with a set sized appropriately. They are used in car alternators too so can be quite resistant to damage from atmospheric contaminants.
posted by Mitheral at 4:43 PM on June 4, 2007
Best answer: Ok, ran the math... though somebody may want to check it, you will need a min 20HP motor to spin that load at 100rpm.
This is using the formula = Torque = HP * 5252 / rpm, assuming 100rpm.
The torque was calculated T = E/(2pi), E = 1/2 x I x w^2, I = 2 m*r^2 / 3, w = omega (angular velocity) at 2 x PI x 100rpm/60, m = mass, r = radius.
At 10 rpm, you will require about 0.2HP, which is a much more reasonable number. If you undersize your motor, you could burn it out.
You will need a heavy duty, TEFC motor (totally enclosed, fan cooled). I know my dad's old table saw had a good motor, maybe a bit bigger than you're looking for, but in the right range. You could also adapt an old vacuum cleaner motor.
As to getting the rpms you want, I'm not quite sure. I know how I would do it industrially, but that's not the answer we're looking for here. I think a dimmer switch may do the trick, it looks like they work by quickly switching the power on and off, which is the best way to control motor speed, assuming it's the right kind of motor... easiest way is probably to just plug it in and see if it works....
Good luck.
posted by defcom1 at 4:51 PM on June 4, 2007 [1 favorite]
This is using the formula = Torque = HP * 5252 / rpm, assuming 100rpm.
The torque was calculated T = E/(2pi), E = 1/2 x I x w^2, I = 2 m*r^2 / 3, w = omega (angular velocity) at 2 x PI x 100rpm/60, m = mass, r = radius.
At 10 rpm, you will require about 0.2HP, which is a much more reasonable number. If you undersize your motor, you could burn it out.
You will need a heavy duty, TEFC motor (totally enclosed, fan cooled). I know my dad's old table saw had a good motor, maybe a bit bigger than you're looking for, but in the right range. You could also adapt an old vacuum cleaner motor.
As to getting the rpms you want, I'm not quite sure. I know how I would do it industrially, but that's not the answer we're looking for here. I think a dimmer switch may do the trick, it looks like they work by quickly switching the power on and off, which is the best way to control motor speed, assuming it's the right kind of motor... easiest way is probably to just plug it in and see if it works....
Good luck.
posted by defcom1 at 4:51 PM on June 4, 2007 [1 favorite]
If you need some more details, explanations, big messup in my math, my email is in my profile
IAAME
Oh, and on mounting it, I tend to agree with you, putting motors on the poles seems much more straightforward than running linkages, they are more complex. You could human power it with two to four excercise bikes hooked up to a generator if you really wanted to, but that adds an extra level of complexity.
Ok, sorry for the rambling. Good luck!
posted by defcom1 at 4:57 PM on June 4, 2007
IAAME
Oh, and on mounting it, I tend to agree with you, putting motors on the poles seems much more straightforward than running linkages, they are more complex. You could human power it with two to four excercise bikes hooked up to a generator if you really wanted to, but that adds an extra level of complexity.
Ok, sorry for the rambling. Good luck!
posted by defcom1 at 4:57 PM on June 4, 2007
Response by poster: thanks much for the details defcom1, and yes!, I may email you for more. and thanks to everyone else for all the other great ideas in this thread.
posted by badstone at 5:02 PM on June 4, 2007
posted by badstone at 5:02 PM on June 4, 2007
I wouldn't sweat the TEFC requirement. If this was a commercial project that was going to run 24X7 for months it would be a concern. Stuff like angle grinders aren't TEFC and they work for hundreds of hours in some of the most abrasive environments you can imagine. Going TEFC will easily double your costs.
Be aware you can't use a dimmer on an A/C motor. It works for an VS electric drill because they use DC motors. You can only vary the speed on a A/C motor by varying the frequency.
posted by Mitheral at 5:06 PM on June 4, 2007
Be aware you can't use a dimmer on an A/C motor. It works for an VS electric drill because they use DC motors. You can only vary the speed on a A/C motor by varying the frequency.
posted by Mitheral at 5:06 PM on June 4, 2007
Response by poster: while variability would be ideal, we're prepared to go with a fixed speed so long as it looks good. thing is, we'll need to be able to vary the speed initially to find out what does indeed look good, but we can then lock it down when we bring it to the playa. so, I guess what I'm saying is, we'll want to vary speed somehow, but it doesn't necessarily have to be as convenient as a dimmer....
posted by badstone at 5:35 PM on June 4, 2007
posted by badstone at 5:35 PM on June 4, 2007
You could go direct drive. Fisher & Paykel Smartdrive washing machine motors are essentially just great big permanent magnet stepper motors. If they can agitate my full load, or spin it at 1000rpm, it seems to me they ought to be able to turn your gizmo's bits at 10-100.
The power control module in these machines consists of a switching power supply, some heavy duty motor drive electronics, and a microcontroller. It shouldn't take an enormous amount of hackery to disconnect the existing microcontroller from the control FETs and drive them from your own electronics; maybe a laptop parallel port? Though if your controller is outside the main support frame, you're going to need some fancy slip rings to make all the motor phases accessible; it might be better to put a microcontroller in the thing's guts and just feed power through a couple of slip rings.
I wouldn't run the thing anywhere near 100rpm until you've got a handle on how it's going to be affected by precessional (gyro) forces. I think you'll find that Things Will Bend.
posted by flabdablet at 5:53 PM on June 4, 2007
The power control module in these machines consists of a switching power supply, some heavy duty motor drive electronics, and a microcontroller. It shouldn't take an enormous amount of hackery to disconnect the existing microcontroller from the control FETs and drive them from your own electronics; maybe a laptop parallel port? Though if your controller is outside the main support frame, you're going to need some fancy slip rings to make all the motor phases accessible; it might be better to put a microcontroller in the thing's guts and just feed power through a couple of slip rings.
I wouldn't run the thing anywhere near 100rpm until you've got a handle on how it's going to be affected by precessional (gyro) forces. I think you'll find that Things Will Bend.
posted by flabdablet at 5:53 PM on June 4, 2007
Ah yes Mitheral, you're right, on both the TEFC and the dimmer switch, I misread voltage for frequency.
posted by defcom1 at 6:07 PM on June 4, 2007
posted by defcom1 at 6:07 PM on June 4, 2007
T = E/(2pi)
Err...
There is conservation of angular momentum, so the only steady state energy you have to put in is the energy required to oppose friction (and whatever wind you are moving, 'cause it is kind of a big fan :P). With proper bearing design, that will be pretty small, and it has nothing to do with the moment of inertia of the spheres.
There will be some extra forces experienced by the base, due to cross product terms - you are rotating a rotating system, so think gyroscopic effects.
The real issue for sizing the motor is spin up time, and the possibility of over stressing the motor during that interval.
Err...
There is conservation of angular momentum, so the only steady state energy you have to put in is the energy required to oppose friction (and whatever wind you are moving, 'cause it is kind of a big fan :P). With proper bearing design, that will be pretty small, and it has nothing to do with the moment of inertia of the spheres.
There will be some extra forces experienced by the base, due to cross product terms - you are rotating a rotating system, so think gyroscopic effects.
The real issue for sizing the motor is spin up time, and the possibility of over stressing the motor during that interval.
torque = moment of inertia x angular accelerationSo, say you want the thing to accelerate to 10 rpm in a minute,
τ=Iα
α = π/180 rad/s^2To find the power
power = torque x speedwhich reaches its maximum just before top speed is reached
power = π/180 rad/s^2 x I x 10rpmI'm not 100% sure, but presumably α in radians and I in kgm^2 becomes Nm.. So, using 150lbs and 12':
power = 220 Nm x pi x 10 rpm/30000 (wik) = 0.23kW (or about 1/2hp)posted by Chuckles at 7:17 PM on June 4, 2007
Presumably alpha in rad/s^2 and I in kgm^2 becomes Nm, that is..
posted by Chuckles at 7:20 PM on June 4, 2007
posted by Chuckles at 7:20 PM on June 4, 2007
and..
which reaches its maximum just before top speed is reached
Something was bothering me about this. In practice, it won't look the way I described. The motor is going to output a fairly constant power, and the sphere's acceleration will be high at at first, but trail off as it gets to top speed..
posted by Chuckles at 7:31 PM on June 4, 2007
which reaches its maximum just before top speed is reached
Something was bothering me about this. In practice, it won't look the way I described. The motor is going to output a fairly constant power, and the sphere's acceleration will be high at at first, but trail off as it gets to top speed..
posted by Chuckles at 7:31 PM on June 4, 2007
Actually, a better way to think of your motor sizing problem..
Speed is proportional to voltage, torque is proportional to current. The primary reasons for failure are spinning too fast for the mechanics of the motor (brushes bouncing, bearings overheating, pieces flying off), and overheating due to too much current in the coils. In this way, the power output of the motor doesn't matter, you want a motor that can produce enough torque for your application, and then just run it at whatever speed you need.
With more speed, more power is transmitted through the motor, but that doesn't primarily effect the amount of power dissipated in the motor. Power dissipated in the motor is mostly i2R losses in the windings, aka torque. There is a point when you exceed the safe operating speed, but up to that point speed is more or less irrelevant to the motor.
This can get complicated quickly, the variables aren't really independent of one another, and then there are synchronous AC motors..
What you really want is any DC motor that can produce enough torque to get a sphere moving (220Nm?). Also, avoid stepper and brushless motors, unless they come with the appropriate controller, because you don't need the added complication. Mitheral's idea of looking for drill motors looks good, this Rigid 12V drill has more than twice the torque capability you need, so I bet any but the cheapest drill motor will be in the neighbourhood.
Obviously though, all of this is conditioned on what's available at your budget. Surely lots of people are trashing old cordless drills with dead battery packs?!?
Then there is the belt drive vs. gearing vs. direct drive question, and your bearing and axle arrangement..
posted by Chuckles at 12:24 PM on June 5, 2007
Speed is proportional to voltage, torque is proportional to current. The primary reasons for failure are spinning too fast for the mechanics of the motor (brushes bouncing, bearings overheating, pieces flying off), and overheating due to too much current in the coils. In this way, the power output of the motor doesn't matter, you want a motor that can produce enough torque for your application, and then just run it at whatever speed you need.
With more speed, more power is transmitted through the motor, but that doesn't primarily effect the amount of power dissipated in the motor. Power dissipated in the motor is mostly i2R losses in the windings, aka torque. There is a point when you exceed the safe operating speed, but up to that point speed is more or less irrelevant to the motor.
This can get complicated quickly, the variables aren't really independent of one another, and then there are synchronous AC motors..
What you really want is any DC motor that can produce enough torque to get a sphere moving (220Nm?). Also, avoid stepper and brushless motors, unless they come with the appropriate controller, because you don't need the added complication. Mitheral's idea of looking for drill motors looks good, this Rigid 12V drill has more than twice the torque capability you need, so I bet any but the cheapest drill motor will be in the neighbourhood.
Obviously though, all of this is conditioned on what's available at your budget. Surely lots of people are trashing old cordless drills with dead battery packs?!?
Then there is the belt drive vs. gearing vs. direct drive question, and your bearing and axle arrangement..
posted by Chuckles at 12:24 PM on June 5, 2007
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This looks like a very ambitious project, but if I were attempting it I'd probably go for a single large motor in the base with a mechanical linkage to drive the spheres (though this would sacrifice the independent speed control you'd get with 3 separate motors.) Putting a big motor on each end of that rotating frame adds a lot of weight that needs to be supported and spun around.
If you're forming the fiberglass yourself, have you considered making flattened vanes rather than tubes and letting them spin using wind power? You could angle the blue and green in opposing directions to get the counter-rotation. You could even add some cups at the ends of the axle (like an anemometer) to make the whole thing spin around on the base, and then you don't even need to worry about supplying power at all.
In fact, you could even use the rotation of the thing to generate a bit of power for nighttime illumination. EL wire is low-power and would look pretty awesome routed along those vanes, and it would be fairly simple to flash it in varying patterns depending on the strength of wind, time of day, seconds left before the Burn, etc.
posted by contraption at 11:28 AM on June 4, 2007