I have a system as follows:

I installed Altivar 31 inverter compatibly with motor's parameters.

Motor works well (current <~ 31A) as frequency <= 45Hz. But when I adjust to increase frequency (~46-50Hz), inverter has fault as OverSpeed (SOF).

Pls help me to solve it!

Thanks.

Sometimes a motor with a high inertial load, suach as the one in the picture, can drive the motor, especially is it unbalanced, resulting in an overspeed.
You might try decreasing the ramp rate (increasing the ramp time) ACC DEC, which I've found to work in the past.
If that doesn't work, I'd suggest you add a braking resistor and enable the dynamic braking function.
Do you have an encoder for speed feedback fitted?