### Author Topic: How to figure out stall torque of DC motor?  (Read 5636 times)

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#### El Jefe 1337

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##### How to figure out stall torque of DC motor?
« on: May 02, 2013, 11:56:19 PM »
Hello, Im foraying into my first robotics project, and I have a question I feel should be pretty straightforward, but I cant seem to figure it out.

Is there any way to figure out the stall torque or torque rating of a dc motor if it isnt explicitly stated?

Here is one motor that I'm looking at: http://hobbyking.com/hobbyking/store/__14632__Turnigy_TrackStar_4168_2650KV_1_8th_Brushless_Motor.html

Here are the specs for this motor:

Kv (rpm/v)            2650
Weight (g)              330
Max Current (A)      129
Resistance (mh)          0
Max Voltage (V)        17
Power(W)                    2200

Like many here, I'd like to know which motor to use for my project, but the RMF section of the tutorial assumes that you already know the torque rating of your motor (or I read it wrong, a very likely possibility).  What should I do in this situation?

#### jwatte

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##### Re: How to figure out stall torque of DC motor?
« Reply #1 on: May 03, 2013, 09:12:37 AM »
To get an accurate value, you'll have to measure it. Find someone who has a "bench dyno" and make friends with them :-)

You can estimate if you know the efficiency of the motor. 2200 Watts is about 3 Horsepower, and torque is horsepower divided by rpm times 5252 (yes, that's an arbitrarily chosen constant everybody uses!) The RPM at 17V is 45000.
So, with those numbers, you'll get about 0.35 foot-pounds of torque. Take that number with a significant grain of salt, though.

#### El Jefe 1337

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##### Re: How to figure out stall torque of DC motor?
« Reply #2 on: May 03, 2013, 07:46:49 PM »
Fair enough.  I didn't need to be incredibly accurate anyways.  A ballpark answer is good enough since it gives me someplace to start.  Thanks for your help!  I've seen a lot of people on sites like these who do provide extensive tests and data on many common dc motors, but from what I've seen these are all from airplane enthusiasts who only care about thrust generation of their motors and efficiency with different props.  Do you know if there is anyone who has done extensive torque tests?  I can't seem to find any.

-Jeff

#### jwatte

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##### Re: How to figure out stall torque of DC motor?
« Reply #3 on: May 04, 2013, 12:03:16 AM »
If you buy motors from an actual motion control company (industrial automation, etc,) then the manufacturer's data sheet will contain the diagrams, charts, and numbers you're looking for.
If you buy hobby motors, then you're kind-of SOL. One reason hobby gear is cheaper than pro gear is that the manufacturer doesn't have to qualify, measure, and then stand behind particular ratings :-)
If a "6 kV" motor isn't quite as fast as it's stated, and your racing truck places third instead of second, well, that doesn't matter so much.
If a "32 foot pounds" servo motor fails to hold a 30 pound box at a foot's extent, and it falls on a factory worker's head, that matters a lot more...

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