# Guestimating motor torque?



## Woodsmith (Jun 5, 2008)

Are there any 'rules of thumb' for 'guestimating' the approximate torque of a motor?

Maybe something based on voltage, current, comm bars, diameter of armature, or whatever?
Just looking to see if there is a way of determining if a motor is in a range of 10s of ft-lbs or 100s of ft-lbs.

Just in response to this post:


Duncan said:


> Hi Woodsmith
> 
> I had to make the direct drive/gearbox decision for my machine - if you end up with a light weight machine you may find the logic useful
> 
> ...


I could build a test bed with a big spring balance and a band brake but I could just build my EV instead.


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## major (Apr 4, 2008)

Woodsmith said:


> Are there any 'rules of thumb' for 'guestimating' the approximate torque of a motor?
> 
> Maybe something based on voltage, current,...


Hi Wood,

If you have the nameplate data, just figure torque from the power and RPM. That will give you the rated torque at rated current. For overload torque, multiply rated torque by the overload current ratio to rated current (for series motors). This will give you an approximation.

Or get inside the machine and measure/calculate the air gap area and count the number of effective armature conductors and find the flux density of the type of steel used and figure it out from the basic torque equation for the applied current of interest 

major


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## Woodsmith (Jun 5, 2008)

major said:


> Hi Wood,
> 
> If you have the nameplate data, just figure torque from the power and RPM. That will give you the rated torque at rated current. For overload torque, multiply rated torque by the overload current ratio to rated current (for series motors). This will give you an approximation.
> 
> major


That doesn't sound to complex, it will achieve a ball park figure to play with.

Thank you major.

I'll skip the second option, it would be easier to build a dyno bench!


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## Georgia Tech (Dec 5, 2008)

HP = (T*rpm)/5252 convert your watts to HP... watts/746 = hp....from there solv for T..


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## Anaerin (Feb 4, 2009)

Georgia Tech said:


> HP = (T*rpm)/5252 convert your watts to HP... watts/746 = hp....from there solv for T..


That's pretty easy:

Watts/746 = (T*rpm)/5252
(Watts/746)*5252 = T*rpm
((Watts/746)*5252)/rpm = T


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## Duncan (Dec 8, 2008)

Hi Guys

Nice and easy -BUT - my nameplate has power and voltage - no speed!

I made an engineering estimate (WAG Wild Assed Guess) by looking at a similar looking motor and using its torque curve

In my case my motor looks similar to a Warp11 (a bit heavier at 102Kg) so I estimate (hope) that it will give similar torque

model MT4100, 10 kw Hitachi 48v


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