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Timotheos Fetsis
Ansys Employee

Hello, the issue will be fixed in next release (25R1). As a workaround you can File>Open Examples>RMxprt>assm and pick an already defind model. Then change the parameters but do not change the rotor material so that it won't crash. Then you can create the Maxwell design and change manually the material. If you are only interested in acquiring the RMxprt analytical results and not proceeding with FEA, I am afraid that there is no way of modifying to the desired material so the results will be a bit off. 
I hope this helps!