I have a VFD and induction motor on my mill. It is a good quality Mitsubishi drive but just the standard Bridgeport 2HP motor so I dont know if its an inverter duty motor. I have heard that the inverter duty motors dont drop off so much in torque but how good they are I dont know. What I do know is with my VFD/motor combination I lose too much torque if I go down below 50% of the motors rated frequency. I still use the vari speed pulleys on the mill to get to the rough speed range and then fine tune by the VFD (external potentiometer). I was considering hooking the VFD to Mach but after using the mill for a while I decided there was no point as the drop off in torque would be no use for the varying types of work I do.
As Jim has said you could go up a few HP on the motor and that would help things a great deal.
An AC Servo for the spindle however is a different thing alltogether, the torque is almost constant from 0RPM to Full speed, it does drop off slightly but rather than it dropping off at low revs it is high revs it drops off at. This is perfect for both a lathe an mill as it is the lower revs you are wanting the real torque anyway. However when I say the torque drops off at higher revs I am not talking a lot, below is the torque curve of the motor I am about to fit to the new mill I am retrofitting. It is a 5.5KW motor and its continuous torque rating is 18Nm (160lbs in approx), as it goes above 3000RPM it starts to drop off a bit but as said the torque is not so important way up there. One other advantage is no brake is needed, the AC Servo will have full rated torque when at 0 RPM so when you tell it to stop it will go to zero speed at the acelleration you have tuned it and sit there. I have been testing the motor I have and although its not connected to the spindle it still is impressive. I have tested by running the motor at 4000RPM then gone to -4000RPM with the acelleration set to 40,000RPM per second. That means from full speed CW to full speed CCW is only 2/10ths of a second and the motors out of position error doing that is zero counts and that is with the 8000count encoder (2000line) I have not tried the acell higher as it takes a bit to hold the motor down on the floor when it changes direction
Hood