JValdes so perhaps he can buy a VFD for approximately the same price as the motor in question, (but I doubt it). What you have not discussed is the time to locate and size a VFD, the labor and materials to install it. The permitting required to do a new installation as opposed to the non-permitted motor swap,(in theory). Whether the person in question is capable of doing the math to size the VFD or if he needs to pay an engineer to do it for him. Also probably a few more items I have not thought of....
You are talking to people you do not know on the internet, please give the simplest and most correct answer possible. Neither I, this site, nor you I am sure wants to be sued for giving bad or hard to follow advice. In this case the answer is simple and obvious, unless the motor is part of a larger and listed assembly then junk and replace it. If it is part of a listed assembly then he needs to get engineering supervision and alter the listing. Adding a VFD to control a motor within a listed assembly will violate and obliviate it's listing as surely as replaceing the motor will.
You are placeing yourself on thin ice by giving the kind of advice you just gave and a lawyer will have a feild-day with it. Don't put yourself or the site in that position. If you are confident that you are talking to a professional then offer the full range of your experience and knowledge, but be aware that they still might blame/sue you. If you are talking to an unknown???....well use your best judgement but keep it as simple as you can.....
