I finished my motor upgrade to my little mill and it's working pretty darn good. I removed the puny ½HP DC motor and replaced it with a ¾HP 3 phase AC motor using a Variable Frequency Drive. The new motor is about 5 times heavier than the original so I beefed up the motor mount casting with stiffeners and added a support leg because I didn't trust the casting to take the weight.
I selected the KB Electronics KBMA Hybrid VFD because it doesn't require night classes in programming to get it working. The VFD is entirely configured via trimmers and jumpers for operating parameters and it works flawlessly. I had it working in 10 minutes time but the manual was written by someone who spoke American English as a first language and all settings are clearly explained and shown in it.
I used a TEFC motor because it's much cheaper than the usual TENV motor and I added an external cooling fan for operation at low rpms. I made a motor pulley for the XL timing drive the same size as the spindle pulley, but I'll probably make another with half the teeth so that I can get really low spindle rpms and add a switch to put the VFD in frequency doubling mode for extended top end rpms.
The motor assembly was designed so that I can still use my convertible mill in horizontal mode as well but due to the heavier weight it's more of a chore to change over than it was before. That's a deficit that I'll live with.
I made a motor adapter to mount the motor onto the mill mount.
And I made this XL timing pulley to drive the spindle. I used a boring bar with a bit ground to the tooth profile and a rotary table to cut the aluminum pulley and the second one turned out perfect. I broached a keyway into the pulley using another ground tool on my lathe and it fit perfectly. Keeper collars above and below the pulley hold it in position on the motor shaft and I can add flanges for the belt if it becomes necessary but it doesn't appear that it will be. I'm still waiting for the pulley I ordered from China when I started to arrive by rowboat (he may have been eaten by sharks).
A very time saving technique is to install a bolt into the chuck with the bolt head behind the chuck jaws and center it one time using an indicator. Then parts can be securely mounted on the bolt with a nut and spacers and will be perfectly on center if the through hole in the part is sized closely to the smooth bolt shank. Leave the bolt in the chuck ready to go for next time. I use this same technique frequently on my lathe because I can securely mount pieces larger in diameter than my chucks will hold and I can turn to diameter without flipping the part around after turning it part way. I've settled on using 7/16" bolts.
Useful info from the KBMA manual:
Inverter duty and most totally enclosed non-ventilated (TENV) motors can provide full rated torque over an extended speed range without overheating. If external fan cooling is provided, open ventilated motors (TEFC) can also achieve an extended speed range at full rated torque. A box fan or blower with a minimum of 100 CFM per HP is recommended. Mount the fan or blower so the motor is surrounded by the airflow.