How to lower the RPMs in small a DC motor

Question

I make Christmas ornaments for a living.

I have an idea that is a tube that is 18" tall, 6" diameter. I need it to slowly spin at around 5-10 RPM. Please keep in mind that I need this to be as cheap as possible. This design is versatile and can accommodate any method. It will spin easily because it will be spinning on a needle point, and there will be a 2" high hollow base for any parts. I have plans for using gears or pulleys, but I was hoping that I could alter the speed of the motor somehow instead.

Thanks for any advice and please keep in mind that I need to do this as cheap as possible.

Answer 1

Theoretically, it's possible, but in reality, you won't be able to without your own control circuit, or doing something that might damage your motor.

The reason for this is because standard DC motors will require a minimum amount of power to operate.

Here's a section of a datasheet for a 3V-4.5V DC motor:

At the graph in the bottom, you see that the motor speed (in RPM, symbol N), there's pretty much no way you're getting it down to 5-10RPM, unless you can change physics.

A couple of things you can do:

1. Use gears to change ratio of speed, which is what you're going to do. This is probably the most simple, cheapest, and safest option, in my opinion
2. Use a stepper motor, which are commonly used for high-torque, low RPM applications
3. Find some sort of PWM control circuit to slow it down, although you probably won't be able to get it down to 5-10RPM

Things you shouldn't do (you probably already know this, just leaving this here for others):

• Use some sort of physical restriction to slow it down (i.e. weights). This is bad. It could burn out your motor, and put a lot of stress on your power supply (if it's not mains), and even if it does work, you're going to be running very close to stall current for a long duration of time, which is wasting A LOT of power.

Answer 2

Goofy suggestion; if it is that tall with that large of a diameter, and it's rotating on a pinpoint, could you just put a light inside it with radial airfoils at the top of the cylinder and let it rotate solely powered by convection currents of the air heated by the lightbulb? (In the late 70's when I was a kid, I had a Star Wars light in my bedroom that operated this way. It was soooo awesome...)

Answer 3

As you mentioned 5-10 RPM, I advise to go for a small gear-set rather than any control circuit (I can't guarantee but it might not be possible at least for a low price). Gear-set will be inexpensive and will be easily available at robotics or hobby shop.