Sizing a motor

[joshuabardwell]

I was pulling a load of firewood in my garden cart today, and I thought to myself, "Wouldn't it be neat if I put an electric motor on this?" I have a zillion projects in the queue at this point, so who knows if I'll ever get to it, but as I was thinking about it, I realized that I didn't have much idea how to select the motor, and I'm a sucker for a good physics problem.

Like, one factor is the motor's available torque. Torque is measured in foot-pounds and the force required to move the cart is measured in pounds. So if I just work the units, then it seems like I would divide the motor's torque by the radius of the wheel to get the pounds of motive force delivered. Is that right? How does that equation change if, for example, there is a drive gear, a gear on the axle, and then the diameter of the wheel? The torque is affected by the relative size of all of those things, right?

The other factor would be the motor's RPM and how that translates to the speed of motion. With no gearing, the formula should be rotations-per-minute * wheel circumference. With gearing, then it should be rotations-per-minute * (drive-gear / driven-gear) * wheel-circumference. Is that right?

 

[Creamer]

All your statements are basically correct. However you did not mention rolling resistance and wheel type and size which are big factors to be considered. A larger wheel rolls easier so while it takes more torque to get it started once you get it moving it will tend to keep going with less pull. Rolling resistance gets into the terrain and type of wheel. A rigid wheel on a smooth surface pulls the easiest but throw a little grit in the mix and the equation changes big time. Throw ruts and gravel in the mix and pneumatic tires become much better.

 

[brokenot]

Check out some of the specs and video demos from the companies that sell power dollies for trailers. Google "trailer mover" and quite a few manufacturers will pop up. Their spec pages usually show what each of their models is equipped with....as far as motor hp/gearing/speed goes.

Use the same keywords and check out YouTube also. In addition to the video clips from manufacturers, there are quite a few homemade ones to see.

There are several people that have made one "on the cheap" by using an inexpensive and readily-available DC winch for a power unit. While such a setup does provide plenty of torque and forward/reverse, those motors aren't really intended to be run for more than a couple of minutes at a time. Additionally, they're noisy and don't provide any speed control....

 

[KYErik]

Are you wanting to run it off a 12 battery or 110 or 220 volts ac (with a long extension cord)? You can move mountains with a tiny motor, but you will do it slowly. If you are using an AC motor with extension cord, figure that a 1hp electric is equivalent to 2-3 hp gas engine rated HP. I put a 1 hp electric 220v motor on a tiller (3hp gas engine went bad) and it did a great job on the garden- plenty of power and nice and quiet. I just always had to keep an eye on where the cord was.

 

[joshuabardwell]

12 volt.