Thanks again instructor37. I was looking at AC VFD, not DC variable speed. We use inverter duty motors at work, 3 phase 480V, and don't run them much lower than 30% because they heat up and don't last very long. The fan doesn't move much air at that speed and the windings get hotter at the lower frequency. In our case, yours and mine, it may not be as much of a problem since the duty shouldn't be as severe. The motors will get hotter.

If you take a standard motor of rated speed at a rated voltage and frequency and maintain a constant voltage to frequency you get normal flux in the stator and can get rated torque at any speed. If you reduce the frequency to say 30hz, you will get the rated torque, but since power is speed X torque and you reduced the speed by 1/2 by reducing the frequency by 1/2, you also reduce the power by 1/2. If you have a 3 hp motor, you now have only 1.5hp at 30 hz. Now above 60 hz which most motors are rated for unless they are european, things change. You can't get more voltage out of the VFD than you put in so you are limited to 120V or 240V which ever you have. If you increase the frequency above 60Hz to 120Hz, the voltage to frequency ratio drops and so does the available torque. You will have your speed going up and your torque coming down. The power will remain relatively constant. Below 60Hz you loose HP and above 60Hz you loose torque.

This is sort of why I was considering going with a 3HP, 3600 RPM motor to keep relatively good HP below 60Hz, and full power and torque at 60Hz and 3600 rpm. If I go with 3600 RPM motors, I'm assured that the bearings and everything is rated for that speed.

I do like the idea of the gear ratios. Even with the 3600 rpm motors, it offers very good speeds on the lathe and mill. I would like to be able to use the drive output as the speed of the lathe and mill. This may be something I can program into the drive though. Have to look into this. If so, I like the ratios you will have a bit better. Puts more power on the tools.

Hey, thanks again for the reply. Makes more sense to me now, and the decisions are coming closer to reality.

Have a great evening, much appreciated. TeaMan