You need a converter that can start a 7 1/2 HP motor. This is not a hard starting ap[plication.
If building Your own rotary converter You need a 7 1/2 HP or larger idler motor, the RPM isn't critical. To get full output, You will need to add run capacitors in at least one of the generated legs to get the voltage ballanced. There are a lot of plans on the net for building Your own.