Maybe you are not fully grasping what is being said, I'll give it a shot.
A motor is DESIGNED around a voltage and frequency and thereby the RaTIO of the voltage and frequency becomes critical. If you mess with that ratio, you mess with the motor's performance and capabilities.
You said this is going to China, they are 380V 50Hz. No problem.
But you ALSO said that you have a 380V 60Hz motor. That is what we are saying is unlikely. There are only a small number of countries that actually have 380 or 400V 60Hz power; Brazil, parts of South Korea, parts of Tahiti, parts of Suadi Arabia, the Philipines and several of the Leeward Island Nations of the Carribean. With the exception of Brazil and South Korea, most of those countries do not manufacture motors for export, and the two that do make the 380V 60Hz versions pretty much just for domestic use. So if you do indeed have one, it's an oddball.
But if you investigate it, a 460V 60Hz designed motor has a V/Hz ratio of 7.67:1, and a 380V 50Hz motor has a V/Hz ratio of 7.6:1, so close we consider it the same. Those motors will each put out the same torque. So if you have a 380V 50Hz supply, you can use a 460V 60Hz designed motor just fine. But it will spin 17% slower (50/60) and therefore put out 17% less power (same torque, lower speed). BUT THE TORQUE REMAINS THE SAME in both cases.
So if your motor is doing work that is related to torque, you don't need to do anything. If the work is directly related to POWER, as in kW or HP, then you have to do something; it all depends on the type of load you have. Screw Extruders tend to be torque related loads, meaning you are not necessarily sizing the motor to maintain a particular speed, you are more concerned with having the right amount of shaft torque at the opearting end of your gearbox. You may end up extruding a little slower, but you will not stall the motor.
Now a SEPARATE issue brought into the fray by ScottyUK is that, REGARDLESS of the fact that it is an "Inverter Duty" motor, the regulations for "Explosion Proof" applications in some countries require that SPECIAL motors be used for inverter use in explosive areas. We don't have those rules here in the US, so you may not have been aware of that. But ScottyUK, being NOT in the US, is giving you fair warning that this may in fact rear up to bite your backside when it arrives in China, where the solution will cost you FAR more than whatever you have to spend here to deal with it ahead mof time.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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