So how did I decide a VFD (Variable Frequency Drive) was even an option for my mill? To be honest, I wasn’t even thinking about a VFD to begin with. In fact, when I first began entertaining the idea of putting an old industrial grade milling machine in my garage, I was all but 100% sure I would be utilizing a phase converter to power it.
However, after finding the spindle motor was the only electrical element of my mill requiring 3-phase power (see: How to Power the Beast?), and just barely knowing what a VFD was, I decided to do some research. To begin with, my knowledge of the VFD was extremely basic. I just knew they were used to drive AC motors, and later learned more specifically, 3-phase AC induction motors. Knowing this, I really needed to take a closer look at the spindle motor of my mill.
The following picture shows the identification plate attached to my mill’s spindle motor. Well would you look there, in huge letters you can see the spindle motor is just what we need, an induction motor. 
All of this just shows you how ignorant I was about all of this, I’ve since learned nearly all mill spindle motors are induction motors, and here I am talking about it like it’s a big deal. Remember, I’m truly learning as I’m going, and soaking up every bit of it. The information on the motor’s ID plate is extremely important in selecting and sizing the proper VFD. Also, I wonder if this spindle motor is the mill’s original? Look at the date field (2000-something) shown in the picture, it doesn’t really jibe with a 1980’s milling machine. …but I digress.
Search the internet for VFD suppliers, and you’ll find an overwhelming number of brands and resellers. Additionally, I had no idea how to size a VFD, or know which brands were good or bad. As a result, and having zero confidence to pick one out on my own, I started making phone calls to several different resellers I found Googling. Eventually I ended up working with the amazing folks at Marshall Wolf Automation, in particular their technical sales guy, Phil Teeter. I have no affiliation with these folks, and no, they haven’t rewarded me in any way for my endorsement. I was just impressed with their willingness to help me without any sales pressure, and how they described helping others do the exact same thing I was trying to do. This wasn’t a one call deal for me, I called them several times over a week or so, and they never became frustrated.
In the end, I purchased a Lenze AC Tech VFD from them. The model number is: ESV222N02YXB, to be exact. This drive is nominally rated at 3HP, has a NEMA 1 enclosure rating, can accept 208-240 AC single or 3-phase power, and outputs the 3-phase power needed to drive the spindle motor. 
On top of what I consider to be a very reasonable price (I payed under $400 for mine delivered), I’m told it’s made in the USA! You can’t beat that with a stick. This is about half the price of what I was going to pay for a phase converter, and I haven’t even gotten to all the good stuff. VFD isn’t just a cool acronym, it means something awesome I haven’t even mentioned yet.
Variable Frequency = variable motor speed.
Using a VFD means I’ll be able to vary the speed of the spindle motor electronically. Having an electronically variable speed spindle motor is something my mill hasn’t had entire life. Previously the spindle motor could only run at full speed in either forward or reverse, and if you wanted to change the RPM of a milling operation, you had to change a mechanical gearbox setting. 
To be clear, using a VFD doesn’t mean a mill’s mechanical gearbox can’t be used or won’t be needed. It just means there are more speed options available. Really for me, the electronically variable speed control provided by the VFD wasn’t critical to me in purchasing it. I just wanted the VFD for it’s efficient and economical way of operating a 3-phase spindle motor.
Well all of this is cool enough, but to see how I actually got the VFD connected to my mill and spindle motor, visit my Harnessing the Variable Frequency Drive page.
This Old Mill 