The bearings and winding design of a motor intended to operate at 50 HZ does not necessarily are satisfactory for 60 HZ. Some lucky people has connected 50HZ motors in 60 HZ systems with satisfactory results when the supply voltage at 60 HZ is increased proportional to the frequencies.
V60=V50*60/50 e.g. 400*5/6= 480 Volts. This voltage change produces the same magnetic strength as that of the original motor