Changing belt position on the step pulleys will transmit the same (full rated) horsepower at all spindle speeds. Using a VFD at less than 60 Hz will lower the horsepower in proportion to the frequency. That is, 30 HZ will cause the motor to run at 1/2 its rated speed, but also 1/2 its rated power.
You can still use the VFD always set at 60 Hz and change belt position to change spindle speed if you do not want to run the 3 phase wiring to the mill.
Larry
