# How do i figure out the maximum constant torque for a DC motor

Hi, I am new here so here I go.
I need to find the maximum constant torque for a motor i have. (i think that is what it is called) or the maximum rpm and torque with out
slowing down the motor. and i am going to be making gear reductions. so if you know how to do both you help is much appreciated.
here is the info on the motor : Voltage Range: 6V-12V. Nominal Voltage: 12V. Current: 0.14A. RPM: 12,500 max. Torque: 44.2g/cm. Terminal Type: Solder. Shaft Dia.: 2mm. Shaft Length: 10mm. Size: 24mm dia. x 32mm length.
ps could you included the formula as well (or just the formula)
pps the torque givin is the stall torque.
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snipped-for-privacy@gmail.com wrote:

I think the formula is just a straight line (but I could easily be wrong). That is, if you plot RPM against torque on a graph, at one end, you have 0 torque and max RPM (12,500) and at the other end, you have max torque (44.2 g/cm) and 0 RPM. Draw a straight line between those two points and you have the performance curve of your motor. The more load you put on the motor the slower it spins.
So the numbers you have from the spec are the maximum rpm, and the maximum torque. Any load (aka torque) you put on the motor will slow it down some so there's no such thing as "maximum rpm and torque with out slowing down the motor".
Putting gears on the motor just shifts the scale of the graph (or shifts the slope of the line). A 2:1 reduction will cut the RPM in half and double the torque. So just change the end points to account for this and draw a new line to see the performance curve of the system with the gears.
Gears however also waste energy because of friction so the torque delivered to the load won't be double, or the torque reflected back to the motor will be more than half the load - I don't know how much extra load a typical gear train adds but I think it can be substantial - especially for small motors.
I guess max torque (stall torque) will be exactly 2x (the gears add no friction when not spinning I'm thinking) but max RPM will be more than 1/2 the max RPM since the gears will add some load. So I guess you could calculate the effect by letting the system run at max RPM with no load on the gears and then measure the speed then use that as the new MAX RPM output by the motor with the gear and use that point on the graph instead of the 1/2 RPM for the 2:1 gear.
--
Curt Welch http://CurtWelch.Com /
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snipped-for-privacy@gmail.com wrote:

The torque figure you quoted is likely to be the stall torque of the motor at the rated current. This is the maximum torque available at that current. More current means more torque, more voltage means more speed. Too much of either will mean the smoke getting out.
You could have a look at this:
http://micromouse.cannock.ac.uk/motors/motorequations.htm
Please let me know if it is any help.
Pete Harrison
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As others have said, DC permanent magnet motors are almost perfectly linear. (Most other types of motors are not.) However, maximum *constant* torque is generally limited by thermal constraints and cannot be computed from the numbers you have provided.
-chris.
On Nov 27, 6:54 pm, snipped-for-privacy@gmail.com wrote:

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thanks for all your help...ill take your info to my group and see what they think
i think i figured out that the optimal gear ratio is either a little more or a little less than 1:2 (to account for friction.
i will let you know how the project went once it is finished.