Selecting a small DC motor?

[binfordw]

Hello,

I am working on another project, and once again am a bit stumped on how to pick a dc motor for my application..

I am making a RC tank, 40" by 26", weighing about 140 lbs complete. It will use 2 motors, one to drive each track.. I have found various small gearmotors, but am not sure how much power my tank would have with them. The specs on the motor i am favoring so far are-- 12vdc, 110 rpms at output, 1.7 amps at no load, 36 amps at stall, and 160 in-lbs of torque.. Can someone show me the proper way to pick a motor for this application? What about figuring things like battery run-times? How much power would this little tank have??

Thanks in advance,

Ben W

 

[binfordw]

Anybody? Hmm. Maybe I should revise my question a bit. What I need to know is how much work the above gearmotor is capable of doing. How do I figure how much weight can be moved with this motor(actually 2 of them), and is there a way to formulate current draw at a specified load?

IF the stall amperage is said to be 36, (and im assuming that means the motor will stall completely with that much load..) how do I figure how much of a load will 100lbs be?,, 200lbs?? etc..


Sorry if I seem so ignorant on the subject, gotta learn somehow!

 

[dangnm]

Let's start by assuming that you want your tank to be able to climb trees, or at least very steep hills. Ignoring the problem of adhesion, it could climb straight up if each drive sprocket could produce a linear force on the tread equal to half the tank's weight, e.g. 70 lbs.

Then assume an achievable radius for the drive sprocket. Let's guess 1 inch. A sprocket driving a track works like a continuous lever, so the torque required to drive each drive sprocket is the force at the lever tip times the length of the lever, i.e. the sprocket radius. Okay, 70 lb x 1 inch = 70 inch-lbs of torque. Ignoring inefficiences and dirt in the sprockets, you shouldn't need more torque than that.

If the stall torque is 160 in-lbs and the stall current is 36 amps, then by proportion, to get 70 in-lbs, you'd need

70 / 160 x 36 = 15.75

amps at most per motor to drive the tank.

So, if you could find a way to make the tank stick to the tree, the tank could climb it.

If you had a 10Ah battery for each motor, the tank could climb that way for a time equal to:

10 / 15.75 = 0.635 hours, * 60 = 38 minutes

Of course the motor would get real hot real fast at that current level, increasing the motor resistance and driving the current down, and the battery's internal resistance would become a big factor, and, and, well the problem gets more complicated, but maybe you can see where I'm going with this really crude estimation.

If the tank only has to climb a hill instead of a tree, you can reduce the force needed, and the current, by multiplying the track force by the sine of the hill's angle from horizontal.

See if you can find a catalog and some application notes from a DC motor manufacturer and a battery manufacturer. That should help you make some progress.

 

[MikeHalloran]

Now, the other part of the problem is how fast would it go?

At 110 rpm, with a 1" radius sprocket, each track would travel a distance equal to the sprocket's circumference for each revolution of the output shaft, so the tracks would travel at:

110 X 1 X 2 x pi = 1068 inches per minute,

/ 12 = 89 feet per minute,

/ 60 = 1.48 feet per second,

* 60 / 88 = 1.01 miles per hour

... which is not a real speedy tank, and that's with no load. It wouldn't climb trees that fast. On the other hand, you wouldn't have to run to keep up with it.


[ I spent too long composing my previous reply, and got timed out just as I posted it. ]




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[binfordw]

Thanks for your help, I do have a question tho, "110 X 1 X 2 x pi = 1068 inches per minute," I cannot get these numbers to work.. Is there a typo? I keep getting 110 x 1 x 2 x pi = 690.8.

The Idea so far is to use 6" drive sprockets. I'd like to acheive around 5 to 10mph max speed with a gearmotor straight to each drive sprocket, to simplify the setup.

Now, using your caculations,, with a 6" drive sprocket, I came up with this. 3" * 70 = 210 inch-lbs.. Then 210/160*36 = 47.25 amps.. Now since this is above the amp stall of the motor, wouldnt that mean it wouldnt work, or that it wouldn't be capable of a straight climb with justs its own weight, right?

Thanks SO much for your quick reply, I was having no luck looking online for formulas and such.

 

[MikeHalloran]

Yep, that's a math error, all right. That's why I do just about everything in Excel, usually with numbers in the left column, units next, and an explanation of what each line is for in the third column. I think I started with 170 rpm, did the calculations, then realized my error, and went back and fixed it, but not in all the right places. That's the other virtue of doing it that way; you enter every independent number only in one place, and you change them all just by editing that one place.

Your calculation means it wouldn't be able to climb a tree. Put a grade into your spreadsheet, and factor that in.

If you keep track of the units, and what each number represents, you'll be okay.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[binfordw]

Oh good. I was confused over that one. I did some calculations with different grades and different rpms, that worked very well, and is a great help. If I need it to be able to climb a 45 deg. grade, and the amps at stall is 36, is there some kind of limit I should consider? Like a percentage not to exceed- say if it calculates that it will pull 30 amps at a 45 deg. grade, is that too close to the stall amperage?

There are bigger motors I can use to be safe, but I want to be able to get a good idea of what I can expect before I plan on a certain one.

Thank you very much for your help. I am starting to feel pretty confident about this.

 

[binfordw]

I just remembered my other question..

How would upping the voltage, say from 12 to 24 affect these formulas- if at all? A certain motor I was checking into has a 12v and a 24 volt model,, and about the only difference is the 24v model has a LOWER stall amperage... Almost half the amps at stall actually. If I use its specs in the above caculations it would seem alot weaker than the 12v version.. What is the deal with that?

 

[xnuke]

Since power is the product of current and voltage, when you double the voltage you halve the current to deliver the same power. That's why you're seeing a stall current of about half for the 24 V motor.

xnuke

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