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I need help determining the necessary specs of a DC motor I need for a project? v/amps/hp/torque, etc? Answered

I'm a pure novice and know next to nothing about motors. I want to build a motorized platform cart to move some large items around the shop / yard. I want it to be capable of hauling around 800 lbs up a slight incline (10-15 deg.). speed is unimportant (in fact, slower the better probably). I'd like to do it as cheaply as possible with parts I already have or can easily acquire at salvage yards or yard sales. This will only be used on rare occasion and for very short durations so cheap and dirty is fine with me.

I have the frame (cart) including some 8" pneumatic wheels if I choose to use them). The part I'm stumped on is the motor, controller and whatever I need for transmission/gears.

Here's a couple of very expensive commercial options which are overkill for what I need (and far too pricey!), but at least it gives some idea of what I have in mind.


This link includes some detailed specs on the motor they chose to use....

I'm not sure what the relevant specs I need to be concerned with are. For example, I have a bunch of 18v dewalt motors which I understand can be over-volted but does that get me the hp and torque I need? If it's more amps I need than what the little dewalt motors will give me, could I use a 12v car battery and a starter motor? If so, what would I need to use for a controller and drivetrain?

I think what I need is a very low rpm/high-torque motor, but don't know for sure. And if that's what I need, what are some common applications that they were used in that I might find such a motor?

any help or ideas would be appreciated



9 years ago

It is kind of complicated compare the two applications side by side, because the haulzall has already a buil-in reduction gear.

in other words the dewalt may work but you need to throw in a reduction box in order to be able to move the dolly with 800lb on top.

you must then first determine the travel speed of the dolly and the calculate back to the motor or viceversa example:
you have motor X and 2 reduction gear boxes a) 1:100 and b) 1:30
Lets assume that you calculate that the 1:100 gear box with the x motor is able to move the 800lbs at 30 ft/min. So you may think that the 1:30 will be able to do the same job but faster at 100 ft/min, but here you need to remember that you trade speed for torque. So it is possible that the motor will stall trying to move the load


9 years ago

ok so i know this is old but let me give some input and see if it helps you or not...

  Your concerned with HP but Hp comes from ( torque X RPM ) over 5252.  There fore you should be more concerned with torque and rpm.  Just about any motor your gonna find is going to spin way to fast to use as a direct drive, so depending on what you have to gear it with...i guess would b a decent spot to start ur search for motors...

Also very important electric motors produce the most torque at a very low rpm ( basicly 0).  Also most of the posted hp ratings are at there continous rate and electric motor will usually produce 8 to 10 times there continous rating at that low rpm, grant it, it will be for only a few seconds before its possable to hurt the motor...  just an example to show you is the electric motors they use for the GEM electric car is 5.5 hp motor  

so 800lbs and that 500watt scooter motor plus some gearing to make it go slow (its inportant that the motor speed is up because of high gearing) and you should have no problem.....if you'd like id b happy to look at some of this stuff to help you out...im in school for engineering and EV's are a big hobby of mine


10 years ago

It is going to be extremely hard to find motors that can move 800 pounds around at a low price. When you say 800 pounds magmotors come to mind, but they are about 360 dollars per motor and you would probably want to a a gear box just to be sure it can carry the load bringing each motor to 835 dollars. These are available at http://www.robotmarketplace.com/products/MAG-S28-400.html. You might be able to find some motors some where else, because the magmotor probably supplies more power than you need. But I doubt that you will be able to get it to work with dewalt motors unless you use a bunch of them.


Answer 10 years ago

Thanks for the help. When you say it's going to be extremely hard to find a motor that can move 800 lbs, I guess it leads me back to my question of required specs. what are the neccessary specs of a motor that will do what I want it to do? You may be right that such a motor is hard to find, but I don't even know what "such a motor" is?

I'm not an engineer, but presumably, the engineers that designed these motorized carts had the same question I have when they were in design--what type of motor is needed to do the job? They could have just thrown in a big ol' motor and counted on it to get the job done, but I suppose they probably identified how much hp and torque would be required and then went out a found a motor that could perform to those specs.

It doesn't appear as though the commercial carts are using such an expensive motor to get the job done, -- in fact it appears quite small, but I could be wrong given my lack of knowledge of such things

Here's the specs from the commercial unit mentioned in the above post:

Motor: 4" dia PMDC 4 pole, 24V, TENV
Drive: Single Reduction Open Differential Transaxle
Power: 400 watts (0.536HP) at 24V

Batteries: (2) 12V, 12AH Sealed Lead Acid
Controller Capacity: 70 Amps at 24V

The motor used in the commercial unit is only .5 hp and yet it appears as though they've determined that such a motor is sufficient to move up to 1000 lbs.

Here's their heaviest duty cart with specs:

My thought was to use the specs in the commercial unit as a base minimum and then find something slightly better. From what I understand, Dewalt motors when over-volted can produce up to 1.5 hp (but maybe hp is irrelavant and it's only torque that I need to be concerned with?)

Still confused....


Answer 10 years ago

thanks again for the legwork on finding those motors. As much as I appreciate the help, I think we're going at this backwards--finding a motor (and a very expensive motor at that) before we know what kind of motor is needed.

What I'd still like to figure out is how many hp and how much torque I'll need in order to power the cart. Any engineers out there? Then knowing that, I can determine whether any of the motors I have will work, and if not, my next choice would be to find something with a suitable motor in it that I could salvage (old golf cart, wheelchair, mobility scooter, winch, etc.) Only as a last possible resort would I ever consider paying $500+ for a motor for a project that is more whimsical and fun than actually neccessary. Isn't that half the fun of making things--finding some old junk and re-purposing it to make something cool?

From what little I understand about picking the right motor for an application, HP (or watts) is only one part of he equation. those scooter motors do put out 500w (which is more than the haulzall) but I'm assuming that in addition to knowing that I have enough HP, I also need a certain amount of torque. Which is probably somehow related to the RPM's that a motor will produce (lower RPM would mean higher torque). And I'm assuming torque can be manipulated either by picking a high-torque motor or by converting the output of whatever motor is used through gearing to produce more torque. It seems like the easiest solution would be to find a very high-torque motor which I wouldn't have to gear lower. But how much torque do I need?

Here's the specs on a Dewalt motor

- Mass = 470g (18.8oz (1.175lb))
- Terminal resistance = 0.072 ohms
- Stall Current = 250 amps
- Stall torque = 2.1Nm
- No load current draw = 2.6 amps
- Peak Power = 1100w
- Peak Efficiency = 81%
- DeWalt Part # = 393111-01
- Voltage = 18vdc (can be overvolted)
- Nominal Voltage = 24vdc
- RPM = 22,000 ungeared
- Length = 78mm (3.07")
- Diameter = 46mm (1.8")
- Output pinion = 16 tooth gear
- Shaft = 5mm dia x 7mm
- Torque Constant = 1.2 ozf-in/Amp
- Stall Torque = 300 ozf-in
- Angular-velocity Constant =1100 RPM/v
- Peak Power = 1.5 HP

If we're strictly measuring the watts, then at 1100 watts and 1.5 hp, the Dewalt is a much more powerful motor (and it's FREE because I have a bunch lying around). but again, I don't believe that hp is the sole determinant of the right motor for the application.


Answer 10 years ago

From my calculations the full-load torque of the motor on the haulzall is 402.1531 lb-ft and it is .3580 for a dewalt motor, this could be wrong but it kind of make since when you compare the sizes of the motors.