Stepper motor model for Pull Out Torque generation?

Hello-has anyone been able to use a stepper motor model to generate a pull out torque? Please see the attached model we are using. In order to generate the pull out curve. we are ramping the motor to a constant velocity and then ramping the load up until the motor seems to lose its position. We are getting good correlation of the model to experimental data at lower speeds but struggling at higher speeds. Any thoughts? Thanks!

[Butchi Siddhartha Pattisapu]

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