Failure to balance the V/Hz ratio with 400V/50Hz motors in the US, to accommodate for the widely varying voltage levels in all of the sectional grids across the country results in significant starting torque and HP losses. Variable Frequency Drives (VFDs) capable of full motor voltage/Hz/RPM/FLA levels are the very best option for use of these motor types.
The overall power grid across the US was created over 100 years ago, and though it has of course been expanded across the entire country, improvement beyond the original structure has been well below the pace of expansion. There are power supply types in the US that need to be recognized and understood in motor control as well. Typically Wye/Delta is the norm, but there are also large pockets of corner grounded delta existing. Voltage levels in the ideal 460 VAC range, can in fact run from 440 VAC in facilities in full operation, to as high as 525 VAC at idle. This range is not equal in all areas, with levels that can vary as often as municipality borders are crossed. Full Motor Regulation capable Variable Frequency Drives programmable to motor output, and 400 V/50 Hz motors need to have a 20% boost in both the V/Hz/RPM settings to maintain peak efficiency. In other words, ideally, these conversion settings would be at 480/60Hz. Even if the incoming line levels are lower than 480 VAC, the VFD regulates the V/Hz ratio accordingly. Some VFD's can, with the help of capacitors, boost the motor output to maintain the 480 V output, further improving motor efficiency. For example, a 400V/50 Hz motor @ 65 Kw, (app. 0.75 HP) when coupled to a VFD set with the according 480/60Hz ratio, and with the motor data plate RPM settings also increased by the 20% ratio, will improve to an app. 0.9 HP output, with a matching increase in starting torque. Many European systems designers don't take the poor balance of voltages in the US grid (basically because many are unaware of it) and claim merely applying 460v/60Hz to a 400v/50Hz motor will make no difference, and will just increase motor speed. In an ideal realm where a fully balanced grid of 480/60 is maintained, this can be the case. But, in reality, you won't find that balance in the US.
I am in the overhead hoist and crane industry, have been for 36 years, with all but 8 years of that spent in control systems design and engineering. Preservation of the HP/Torque ratio is critical, as high starting torque is a necessity in both lifting and traversal applications. Motor failure with imported motors not having the V/Hz ratio preserved is fairly common. Some offshore manufacturers have taken the US grid variances into consideration and designed motors that will in fact operate on both 50Hz/60Hz, but with Kw (Hp) outputs listed on the motor data plates that reflect the actual shaft output at the 60 Hz line feeds. However, not all have done so, and this must be taken into consideration by whatever design criteria is applied with regards to the usage of these motors.