Know anything about electric motor ratings??

[Headless Thompson Gunner]

I need to spec out an electric motor for use in a project.  I understand the basic physics of motors, electricity, and power.  But I have no mental picture of "how big" one horsepower or one foot pound are, so I have no idea what motor specs I need to look for.

The motor needs to rotate at ~30 RPM, be bidirectional, and be strong enough to rotate a small canister (6" dia by 12" long, filled with about 1.5 lb of water).  The canister rotates freely on rollers.

Typically motors are selected based on the load the motor must drive.  But in my case, there really isn't much in the way of a load per se, since my canister rotates freely.  But the motor does need to be powerful enough to easily reverse the direction of rotation periodically.  What kind of horsepower/wattage/torque would my motor need to produce?

Any thoughts?

[280plus]

I don't know about HP I wouldn't think much.but 30 rpm and reversing rotation makes it a bit tricky. Got access to a Grainger catalog? Try www.Grainger.com maybe there's something there you can use.

[Brad Johnson]

Sounds like a good candidate for some type of reversable worm gear setup. The container will brake agains the gearset instead of the motor when you cut power. Like 280 said, Grainger is a good place to look for stuff like that.

How compact does it need to be? If size isn't an issue, find an old cement mixer.

Brad

[ilbob]

I need to spec out an electric motor for use in a project.  I understand the basic physics of motors, electricity, and power.  But I have no mental picture of "how big" one horsepower or one foot pound are, so I have no idea what motor specs I need to look for.

The motor needs to rotate at ~30 RPM, be bidirectional, and be strong enough to rotate a small canister (6" dia by 12" long, filled with about 1.5 lb of water).  The canister rotates freely on rollers.

Typically motors are selected based on the load the motor must drive.  But in my case, there really isn't much in the way of a load per se, since my canister rotates freely.  But the motor does need to be powerful enough to easily reverse the direction of rotation periodically.  What kind of horsepower/wattage/torque would my motor need to produce?

Any thoughts?

Warning - wild guess based on incomplete information in OP.

It sounds like just about any motor 1/8HP and up would work, other than the 30 RPM thing. You can get small DC motors with a built in gear box to handle the speed part of the equation. DC motors are also fairly easy to reverse. I might get a DC drive with it to handle to reversing part, and give you some control over accel/decel.

[Brad Johnson]

Parallel shaft 90v DC gearmotor, 32 rpm shaft speed, replaceable motor, modestly priced. And it's even reversable...

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611777458

Click on the "Alternate Parts" tab below the description and there are a couple of other gearmotor options.

Brad

[Headless Thompson Gunner]

The 30 RPM and direction things ins't my problem.  A DC gearmotor and an H-bridge driver handle those requirements, no sweat.

What I need help with is the power and torque requirements.  One motor I have in mind is 1/20 HP and produces about 25kg*cm of torque.  I have no conception of how much power 1/20 HP is, nor how strong 25kg*cm is, so I can't picture whether or not this motor would be strong enough for me.

[Brad Johnson]

The internet is your friend.

http://www.onlineconversion.com/

25 kg/cm converts to 21.7 inch-pounds torque.

It's not the power rating of the motor that matters, it's the torque rating at the output shaft you need to be concerned with.

What is the mass of the cylinder, how much drag is involved, how long will it need to spin, and how fast will it have to reverse rotation?

Brad