I have an upcoming project that is going to China which will be using 380Vl volt 50Hz. Will motors that I've used before which are rated for 460V 60Hz, be able to operate at 380V 50Hz?
I've read online that as long as the voltage to frequency ratio is the same (close to the same) that the motors should be able to operate but just at a slower rate (20% less speed.) Is this claim true?
From the context, these sound like 3-phase induction motors. Yes, they will generally operate, but at a lower speed.
The frequency/voltage ratio should not be 'close to', but 'less or equal to'. 380/50 is less than 460/60 (just), so the motors will not be operating closer to saturation than before.
If so, should I spec my 460V 60Hz motors to be 20% larger than what I normally use in order to account for this 20% drop in speed?
If you need the speed, then you need the speed. Having a motor with more torque is not going to solve that. The motor will produce less power at a lower speed, but similar torque. If you are going to make up for the speed drop by having a gearbox, then you will need a larger motor to get your power back.
If the motors are internally ventilated by their own fan, remember that this will be running slower, you may have to derate the torque for cooling alone.
If you haven't specified your motors yet, then why go for a 460 V 60 Hz one, why not specify something appropriate for the destination? You might also consider running it from a VFD (variable frequency drive).
