Motor Type and Torque Needed to Spin This Wheel.

Hey everyone,

I'm wondering what kind of motor/voltage would be best to spin this wheel. It's 12.5" in diameter and 1/4" thick and it's made of MDF board. I'd guess that it's about 1lb but I still have to weigh it.

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I'm building an acoustic effect sort of like a mechanical tremolo. My goal is to spin this wheel and control the speed with a button or pedal. I'd love it to go as fast as possible to moderate speed. I know that's vague, but I'm not sure what exact RPM's that would be.

I plan on using the Arduino Motorshield R3 hooked up to an UNO R3 board. I know there's many things that go into picking the right motor for your project-like how you have your setup configured, the friction between materials, the strain if the load is not perfectly balanced, etc. But I'd love any suggestions or thoughts about what type of motor set-up you'd use.

Would a 12V standard gearmotor do the job? I'm thinking more voltage might be required to move a wheel like this at higher speeds, but I could be wrong. I've worked with solenoids and servos, but don't have any experience with DC motors, so I appreciate any help!

Thank you!

Pat

In my Thread stepper motor basics there is a suggestion for measuring torque. A lot will depend on the bearings it is rotating on - how much friction there is. You won't have to read the whole Thread - start at the beginning and you should find it quickly.

If you want to change the speed quickly (i.e accelerate and decelerate) you will have to get into calculations of the moment of rotational inertia (hope I have the term correct).

...R

I’m thinking you’re looking for a peak of 10Hz or 10 warbles (?) per second. You have five holes in your disc so you need the disc to rotate at 2Hz, or 120RPM maximum.

I’d suggest avoiding gearmotors and using a pulley system to drive the wheel. Gearmotors are quite noisy, plus if you want to mount the wheel in a sturdy manner then you don’t want it directly on the motor shaft anyway. If you use a motor as used with RC planes/cars/etc. you’re looking for one with a low “kV” (RPM per volt). I don’t know what your budget is but a brushless motor and ESC would be around $40.

Moment of Inertia of a disc is (mass * r2) / 2
Angular acceleration, how quickly the disc changes speed, is up to you to decide.
Torque required = Angular Acceleration * Moment of Inertia.

If the wheel is well balanced with quality bearings you won't need much torque, as you'll mainly have to overcome air-drag.

This is the sort of situation where its best to measure. Get the mechanics all set up so that it can be spun with an electric drill or something like that via a rubber disc.

Find a way of measuring the spin speed and spin it up to the desired speed, remove drill, then note the speed every few seconds as it spins down due to friction and drag.

You then use the MoI of the disc to directly calculate the torque.

torque = MoI x angular-acceleration

(get the acceleration from the initial rate of decay of angular velocity)

MoI is easy to calculate for regular shapes, wikipedia will sort you out. Use SI units throughout if you don't want an easy life. Thats N-m for torque, rad/s for angular velocity, rad/s/s for angular accelerations and kg m^2 for MoI.

Thanks, everyone.

Chagrin, I'm going to take your advice and try a pulley system with a brushless motor/ESC. I was worried about the gearmotor's noise, especially since this is an acoustic effect. However, I have one question.

I understand how Hz converts to RPM, but do you mind explaining how the number of holes in the disc affects its maximum RPM? Based on this, I'm going to experiment with more or less holes to achieve greater RPM. 120RPM is definitely too slow for my purpose.

MarkT, great idea to use a drill to calculate the torque. Definitely going to try that.

Thanks again,

A Hertz is a thing-per-second. E.g. mains power is 60Hz, 60 cycles per second ... or something different depending on your side of the planet.

I'm guessing you're looking for a peak rate around 10Hz (10 volume fluctuations) per second for your tremolo effect. You have 5 holes in your disc, so you need to spin it 2 times per second to get 10Hz (2 * 5 = 10). 2 times per second times 60 seconds = 120 rotations per minute.

Maybe my assumption is wrong but I was thinking that you're going to spin the disc in front of a speaker so that the sound passes through the hole (or is blocked) as the disc spins.