How to size a VFD for this Antique 5 HP Motor
It can be done with a vfd? Possibly.  I would want to Meg the motor prior spending the $ for a vfd. There are also ways to do it with capacitors. And capacitors plus another 3 phase motor.
If the voltage is the same 220/240v it would be 1.732 (square root of 3) plus the vfd losses (we don’t make 3 phase without losses)
by code you would need to use the book value 3 phase 15.2 amps that x 1.732 = 26.32  and the book value for a 5 Hp 230v single phase is 28 amps so those work out close by the time you add some for the vfd.
So yes it can be done. Yes I have done it.
Yaskawa drives can be purchased from automation direct by the general public
although inexpensive have been good drives.
You might get some sticker shock when you find out you should use a heavy duty 5 or a general purpose 7.5 rated drive.
As for a soft start, don’t plan on doing two much with this old motor, the windings are not insulated for doing two much and you may find the bearings start disintegrating after a short time, this was a big problem in the 70’s &amp; 80’s with VFD’s running anything other than 60 hz.
I mentioned having the motor megged at the start. Before spending anything I would want to check all 3 phases to ground and verify a minimum of 10k at 1000v
Higher is better, then a simple ohm test a-b a-c b-c all 3 resistances should be similar. If the motor megs low I might heat it to 140f or so for 24 hours and see if the values increase but don’t try below 10 meg @1000 or the overloads if set properly will probably kick out in 10 to 30 seconds depending on the class of overload used in the vfd (10 is normal for most equipment like this).
So yes it can be done, however you may find a 5 Hp 240 single phase motor is less expensive.