There are two points:
- Feasible motor speed in terms of mechanical strength.
- Intended torque respectively motor torque. Usually the motor will be operated in field weakening at higher frequencies because the VFD can't further increase the motor voltage as required to keep the flux respectively torque constant. An exception is a case where the VFD can supply a higher voltage to the motor, e.g. operating a 230/400V motor in delta circuit with a 400V VFD. It can theoretically achieve full torque up to 85 Hz, if the motor can stand the speed.
Speeds of 60 or even 70 Hz can be achieved with most drives in field weakening with light load without special measures.