Help with Picking the Right Motor

[Help!]

Hi, first of all I know nothing about motors so please bear with me. I have 6 big circular wooden cut outs that I want to rotate but I don't know how to pick the right motor for them. The cut outs made of mdf are 1210mm in diameter. They are about 5 kgs in weight and they will be rotating off center. I have tried to calculate the torque and inertia etc, but I'm not sure if my calculations are right nor am I sure if I have understood the physics of it all. Could someone please help me choose the right motor? Honestly at this point I don't care bout how fast or show the motor rotates. I just want it to be able to rotate my piece 360 degrees.

Any help would be greatly appreciated.

 

[chuckey]

You do not say in which axis the "off centre" discs are rotating, are all six on the same shaft, what sort of bearing have you got. With a suitable gear box any motor will rotate this assembley, albeit very slowly. What have you in mind for your working speed? As a first step get your self a spring balance and pull tangentially on the perifery of the discs to get a handle on what torque is required to get them moving.
Frank

 

[Help!]

Hi,
No they are not all on the same shaft. I want them rotating independently of each other so each wheel has its own motor. I would like them to move slowly. I have designed them in such a way that they have a psychedelic effect on the viewer when rotated. i've tried calculating the torque and i'm pretty sure i did it wrong. But thank you for the suggestion.

 

[FvM]

Inertia won't be involved with slow rotation, just weight force. Respectively spatial orientation of the shaft matters, as well where it's attached to the wooden "discs". Sketiching the setup would be helpful.

 

[BradtheRad]

A windshield wiper motor could have sufficient power to do what you want.

Possibly also a drive motor such as found in the larger-size radio-controlled model trucks, etc.

You may need a gear train to reduce the rpm.

The above are DC motors. You can vary the speed easily.

As regards AC motors, they are likely to be much faster rpm.

 

[Help!]

I want a really simple set up..The wheel mounted on the motor which is on the wall.

How would you suggest I power a windshield wiper motor?

 

[Velkarn]

using torque indicating wrench (rotating by hand) you can approximately measure start torque of disc. for DC motor, for example start torque is 5-15x of nominal. so use DC motor with
start torque>measured start torque
nominal torque = measured start torque/(3 to 10)
and keep in mind that gear changing rpm and torque of motor
it very approximately calculations

 

[D.A.(Tony)Stewart]

IF you want the rotational speed to be relatively constant, the drive torque available at the desired speed, must be much higher than the load torque due to the offset weight and radius otherwise the speed will cycle with the position of the center of mass.

You can compute the equivalent mass and radius from your dimensions to get kg-m of load torque.
Motor torque is increased by the gear ratio, but power is constant minus gear loss and torque is limited by current drive and limit of motor.
Gear ratio converts the motor RPM to your desired RPM. You can adjust speed by reducing applied voltage from 30~100% using a PWM for efficient voltage regulation and speed control, unless you are lucky enough to find the right match.

The limiting current is called the "Stall Current " which is the drive voltage, I_stall= V / ESR (ESR=effective series resistance of the coils & driver.)

Power = Torque * RPM/60.


Since you omitted the details of the hole radius and offset, you can work this out.

 

[BradtheRad]

A windshield wiper motor typically operates on 12 to 14V (vehicle voltage). So you'll need a supply of that voltage, at 1 or 2 A.

Only if you are lucky will the supply be just right, turning the wheels at the right speed. You can PWM a supply that is too strong.

The motor probably will have a gear (or gearbox) attached. You will need to use ingenuity to connect it to your project.

You can use gears or pulleys. Gears are hard to fabricate. Pulleys are easier. You could turn all 6 discs with one long rubber band (bungee, etc.).

You can make each disc out-of-round, but I think each disc will need to be weight-balanced, so that it can be spun by a loose linkage, and by a small motor.

 

[FvM]

I think each disc will need to be weight-balanced, so that it can be spun by a loose linkage, and by a small motor.

Good point, although the bearing load is increased. A balanced design is probably the only way to get the discs rotating at constant speed with a small motor.

A windshield wiper motor is pretty loud, you better look for a geared AC motor. A quad pole-pair motor (750/900 rpm) will be almost silent.