# How to measure the maximum weight that a DC motor can move?

We are working on a computer-controlled electric vehicle project which is based on an ATV chasis.

While I was looking for a DC motor to move this vehicle (which is estimated to be 350kg), I found an electric wheelchair motor. It is 500 W 108 rpm 24V DC motor and has its own differential. The salesman told me that it can move up to 540 kg. I don't know which fact this number is based on.

I also have a 500 W 1500 rpm 24 V DC motor. If I decrease rpm and increase torque with the same ratio of electric wheelchair motor via reductors, can it move up to 540 kg? Or does the max weight that a motor can move depend on another factors? How can I measure this?

• You don't measure it before you calculate it. It's basic physics. You have an object with a certain weight, to change it's speed you will need an amount of energy. There's also friction to overcome to keep it at a certain speed. The energy needed comes from the motor. Motors with more power can apply the same amount of energy in less time. So: study physics and make calculations. – Bimpelrekkie Jan 28 '16 at 11:21
• Oh, and: never trust a salesman. Of course the motor can move 540 kg, how much it can move does not depend on the motor but on the friction the weight has to it's surroundings. In space, you can move 540 kg with your finger. – Bimpelrekkie Jan 28 '16 at 11:25
• The motor that spins the rotating restaurant on the top of the Space Needle in Seattle is on the order of a one-HP motor. I assure you the restaurant weighs more than 540 kg. Your question is not answerable without knowing how fast you want the thing to spin and what forces are working to slow it back down. – Eric Lippert Jan 28 '16 at 20:22
• To put things in perspective: A Tesla Model S has 270000 W of power, more than 500 times what you have. – MSalters Jan 28 '16 at 20:52
• Give me a lever and a place to stand and I'll move the world. The same can be said about gearing. – Steve Jan 28 '16 at 21:26

"The salesman told me ..." LOL! If physics was his thing, he'd probably not be a salesman. In any case his job is to make a sale, whether you get your device working or not.

The first thing you need to decide is how much power the motor needs to have. You can then worry about torque and speed later. Those can be traded off against each other, but you can't cheat the physics that requires a certain power for certain tasks.

There are two end-user criteria you need to look at to decide the power. These are how fast you want to be able to accelerate, and what minimum speed you want to be able to go up a decent hill. I'll use the hill criterion as example.

Let's say you want to be able to go 20 MPH up a 20% grade. 20% grade means you go up 1 part for every 5 forward. Since the only physics work being done is going up, the problem reduces to raising 350 kg straight up at 4 MPH. 4 MPH is 1.8 m/s, and here on earth 350 kg weighs 3.43 kN. The power expended is therefore:

(1.8 m/s)(3.43 kN) = 6.13 kW

Of course there will be some friction to overcome, so you'd want about 10 kW in this example. Since 500 W isn't even close, you either have to specify much lower performance or get a much bigger motor (and the power source to feed it).

Let's flip this around and see what 500 W can do.

(500 W)/(3.43 kN) = 146 mm/s

That's how fast 500 W can lift the whole unit straight up. Applying that to a 20% grade, for example, it can move along at 5x that, or 730 mm/s, or 1.63 MPH. In reality there will be friction and other losses, so probably not more than 500 mm/s = 1.1 MPH.