Understanding Variable Torque Requirements

September 28, 2012

How To: Select a Motor for an Application that has a Variable Torque Requirement

Many applications today require more sophisticated motion than simply turning a fan. Using the example of a conveyor – if the conveyor is unloaded, it will run at a fixed torque equal to the friction of the system, but as you add and remove packages from the conveyor the load constantly changes. In order to accurately select the best motor for variable torque application you should analyze each of the loads to determine the RMS (root mean square) torque required by the motor.

RMS calculations are a good way of determining the magnitude of a varying torque. The standard equation to determine the RMS torque is shown below.

Trms = √ ((T₁²t₁) + (T₂²t₂) + (T₃²t₃) …) / (t₁+t₂+t₃ …)

  • T = required torque
  • t = time at torque

The accuracy of the calculation will depend on the size of the time slices. When searching for a motor for your application the continuous duty torque should match your RMS torque as a starting point. You will still need to verify the motor can meet all of your load points at the desired speeds.

As an example, this torque profile could relate to an application. In order to figure out the RMS torque, we can use the data in the graph shown below.

Root Mean Square torque graphFrom the graph we can see that:

  • T₁ = 1
  • t₁ = 3
  • T₂ = 3
  • t₂ = 2
  • T₃ = 2
  • t₃ = 4
  • T₄ = 1
  • t₄ = 4

So if we insert those values in our equation we can solve for our RMS torque as shown below.

Trms = √ ((1²3) + (3²2) + (2²4) + (1²4)) / (3+2+4+4)

1.78 = √ (3 + 18 + 16 +4) / 13

Want to learn more about torque? Read our article about how to calculate your torque constant.

How to use the Motor Search Tool

Narrow your search by selecting motor type, gearbox, voltage, and phase options for your desired motor.

Select a dominant variable: choose one of the three parameters to narrow your search. The selected variable determines which slider bar you will be able to manually move.

Use the slider corresponding to your dominant variable to further narrow your motor selection. The other sliders will automatically move to show available ranges based on the range of your selected variable.

Results will upload as your search criteria changes. If you have any questions regarding your results or how to use the search tool, you can chat with us using the green tab on the left-hand side of your screen.


Note: Groschopp Universal motors are custom built to fit your application so no additional options are available to narrow the search. Selecting the Universal motor type will prompt a message taking you to the Universal product page.

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One of our team members would be happy to help. Contact us at 800-829-4135 or by email at sales@groschopp.com. You can also chat with us using the green tab on the left side of your screen.

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Universal Motors

Groschopp Universal motors are custom built to fit your application so no additional options are available to narrow the search. Standard frame sizes and motor features can be found on the Universal page.

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