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

Posted by on November 27, 2006, 6:54 pm

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.

Posted by Curt Welch on November 28, 2006, 1:36 am

t.squire@gmail.com wrote:

> 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.

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/
curt@kcwc.com http://NewsReader.Com/

Posted by Peter Harrison on November 28, 2006, 3:17 am

t.squire@gmail.com wrote:

> 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.
>

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

Posted by e c kern on November 28, 2006, 9:57 am

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, t.squ...@gmail.com wrote:

Posted by on November 28, 2006, 11:43 pm

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.

Subject
Posted In
Date

dc motor control for a balancing platform (segway)
Industrial Control Group
2009-08-28

dc motor control for a balancing platform (segway)
General Robotics Forum
2009-08-28

rating of the Futaba S3104 High Torque motors
General Robotics Forum
2005-11-01

More RPM = more windings or stronger magnets ?
General Robotics Forum
2008-09-09

Torque needed to generate lateral force with eccentric
Industrial Control Group
2010-02-20