What RK and Leigh said...
Vipe- I'll say what RK and Leigh said.
From a purist-scientist standpoint, there's all sorts of high-frequency EMF voltage multiplication/insulation breakdown nightmare consequence ramifications, but in the true scheme of things, you'll be hard-pressed to ever get to that.
EVERY machine I have running on a VFD... has a standard motor.
Leigh's point about heat is really the foremost issue- when you have a motor with internal cooling fan, that fan moves a certain amount of air across the motor. When you slow the fan down, the fan's output is not necessarily linear... but one can guarantee that it's moving LESS air than at full speed. The motor, however, may be generating a whole lotta power, hence, there'll be plenty of waste heat to shed.
My favorite solution (and a common one, too) is to yank the mechanical fan, and install a constant-speed 'muffin' fan on the shroud... that way, regardless of motor speed, the airflow will be continuous, and plentiful. The motor will be happy at 100rpm, or 6,000rpm.