120 or 150 Hz is the very most you can spin a regular 3 phase 1725 motor. And 7.5 hors is not going to have enough torque at low speeds....
Karl
Karl, this has NOT been my experience.
My '42 EE used to have the Ward-Leonard DC drive and backgear.
I've replaced it with an Allis-Chalmers 1800rpm 480v 3ph 7.5hp motor, driving a Gates Polychain toothed belt at a 3:1 reduction ratio to the spindle.
The motor has been stripped of it's TEFC cooling fan, and a fixed-speed 240vac ball-bearing fan is mounted in it's place. The motor has been fitted with fresh bearings, same type as original when it was built in the mid '50's.
The motor is driven by a first-generation Allen-Bradley 1336 10hp drive programmed to run 3hz to 216hz, yielding a motor speed of basically zero to 6480rpm, and spindle speed of 2160.
The power source for the 1336 is a 10kva single-phase 120/240 - 480 dry transformer wired backwards, so that the primary powers L1 and L3 of the VFD, while my 240v panel powers the input. I've tapped my connections across one leg of the 120/240 side to provide 120v power for my lathe's lights, coolant pump, and accessories, so the whole works is powered by a no-neutral 240v/50A range plug.
I have it set up for dynamic braking with DC injection for about 2.5 seconds, and coast-to-stop on power loss.
The torque available at ANY speed above just slightly started, is ghastly dangerous. Contrary to my initial concerns, the toothed-belt drive is plenty smooth. It may not be as 'silky' as the flat-belt drive was, but it does NOT want for power under any circumstance.
If the motor is a good quality industrial unit, balance will never be an issue. I would recommend that any USED motor be opened up, cleaned out good, fitted with new bearings, and a fixed-speed fan. I've found circumstances (particularly with 'smarter' drives) where some motor designs don't 'like' a given frequency... a result of the inductive reactance of the windings, etc., cause it to have a bit of non-linearity, or sometimes the reactive voltages are high, and the result is an 'overvoltage' fault or something similar pops up. While 'tuning' up the drive, find those points, and adjust the motor parameters a bit, and that clears up.