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.
From 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.