We want to buy a paper machine from canada, the electrical power used for driving the motors (for pumps, refiners, ...) is 380 vac, 3ph, 60hz; We want to install in country with 380 vac, 3ph, 50hz.
Monophoto has provided a very good answer, but I take exception to one point. When an AC motor is operated at a frequency that is different from the rated frequency, the voltage must be changed in proportion to the frequency change. If that is done (within some limits) the motor can produce rated torque at rated current at the new operating frequency and speed. For a 460 volt 60 Hz motor, 380 volts is the required voltage for operation at 50 Hz. However, the motors will operate at 5/6ths of the rated speed. If pulleys and gears are changed to bring the driven machinery up to the original operating speed, the torque demanded of the motors will be increased. Unless all of the motors were over-sized to begin with, they will not be able to supply sufficient torque without overheating.
What an english! Anyway from what I gather, you have a VFD that as per name plate can operate at 50 hz or 60 hz. So obviously the output speed of the motor is not dependent on the supply frequency. So increasing supply frequency to 60 hz will not change the speed of motor that is driven by this VFD. There must be some setting for frequency of output (yes, output) which in turn will increase speed of motor. Look for that.
The paper machine might become overheat because the AC motor designed for 60Hz has less winding turns than 50Hz. To get arround this problem,you may lower the 3 phases 50Hz voltage into less than 380 volts, like 360 or even 340 volts. If you cannot change the voltage,the AC motor shall run faster than in 60Hz power source and it might shorten its life.
