I wish to design and build a belt grinder. For those of you who aren't familiar with them (they're also called belt sanders), the design I like a lot is the KMG design: http://beaumontmetalworks.com/. Click on the belt grinder and look at the videos on how they operate. You'll see that changing the tool bar can quickly give you a variety of different grinding behaviors. The main attraction of the tool for me is twofold: a) the ability to change the belt grit quickly and b) the ability to substitute a variety of platens and contact wheels to change the nature of the grind. My personal challenge is to build such a grinder for as low a cost as possible (I'm retired and have more time than toy money). I'll be writing up my experiences, frustrations, successes, etc. eventually and sharing them on the web. While I do both woodworking and metalworking in my shop, I mostly use a grinder for maintenance tasks, sharpening lathe tools, and sharpening drills. I have a hand-held belt sander that works well for the occasional woodworking task, so making this belt grinder suitable for woodworking isn't a priority. OK, here's where I need electrical help. From what I've read (and my own experience), I definitely wish this grinder to have a variable speed motor. I also know from experience that I want a motor rated around 1.5 to 2 hp (roughly 1 to 1.5 kW) and my preference would be the 2 hp motor. This desire partly comes from numerous posts on the web concerning knife grinders, for which this design is often used with 2 inch wide belts that are 72 inches long. If you watched the KMG videos referenced above, you'll see that a serious motor can do some serious hogging off of metal. My first question is: 1. To satisfy this variable speed requirement, should I get a DC motor or an AC motor? My preference would be a DC motor rated to around 200-250 VDC. I could then build a PWM circuit to control the speed and run it from the numerous 240 VAC outlets I put in my shop. From what I've read and my own experience as a machinist, I will design this grinder to have belt speeds of between 15 and 30 m/s. Most operation will probably take place between 20 and 25 m/s. I also like the idea of the DC motor since it's easy to change direction -- this grinder will be used with the belt running mostly in one direction, but I'm considering designing and building a stand-alone "rotary power module" that I can wheel around the shop to power different tools (a future project is a fairly large bandsaw made from wood that can be used to cut both metal and wood). Note: I will likely have to buy the motor. It will be surplus or from a junkyard, so if I have to wait a year to find the right motor, so be it. One likely source is an old exercise treadmill (which I've read sometimes use DC motors). 2. Should I design in closed-loop control of the speed? My intuition says yes. I would probably use a home-built tachometer that uses a Hall effect sensor and a magnet to count spindle rotations. 3. If you feel I should be using an AC motor, should I look for a single phase or three phase motor? 4. If an AC motor, how should I control the speed? My electronic skills are not beginner, but not terribly sophisticated (I'm a physicist, not an EE). If you recommend an AC control, remember it has to be something I build, so should be within my capabilities: I can build most things (not surface mount stuff because of my chronologically-gifted vision ) but I'd prefer to work with an experienced person's design, as I don't feel comfortable designing something from scratch.