I worked with Dan to convert that Bauer/Eagle to single-phase. This is roughly how we ended up where we did..
We were looking for a cheap solution, and evaluated the static phase converters first. The problem with these as compared to a true rotary-phase converter is that you only get 2/3rds the horsepower out of the motor, so you would need to put a smaller pulley on it (and new belts). When we factored that into the equation, and considered the age of the 15HP 3ph motor we were replacing, we decided to go with a new motor.
Single phase electric motors are common in 7.5HP at 1800 RPM and 3600 RPM. They are available in 10HP at 1800 RPM (you likely have a 15HP 3ph 1800 RPM motor). Since we needed to transport this motor to the third-world, small was as much a factor as cheap. I started by looking for 7.5HP 3600 RPM motors, but realized that we couldn't get a small enough pulley (it would have been around 2.25 inches) to run the compressor at the right speed. Next we looked at 7.5HP 1800 RPM motors, but for the price difference, we decided to go with a 10HP 1800 RPM motor (this is being used in a dive shop high-volume operation, so we were interested in getting everything we could out of the compressor). As long as we were hauling a 10HP (>100 pound) motor to the third world, I wanted to be sure that we were going to USE all 10HP, so we ended up buying an adjustable-diameter pulley for it (more money) so that we could measure the running current of the motor and set the diameter to draw exactly 10HP. The 10HP motor has a different mounting-bolt hole pattern, so we added a set of adapter-rails designed specifically to mount a 10HP 1ph on a 15HP base. The total cost that Dan quoted includes the few hundred dollar pulley, new belts, and the adapter rails. All new, from Grainger. The motor ended up bolting right in, but we needed to, uh, "relieve" the sheet metal belt shield that sat between the motor and belts to allow the new motor to stick through a bit. We did this with third-world tools and gusto.
If you want to do this on the cheap, I'd look for a used 7.5HP 1800 RPM motor (they're cheaper and you're more likely to find one used). At half the horsepower, you need to spin the compressor at half the speed, so buy a pulley which is half the effective diameter (in other words, save money by not getting an adjustable pulley). One thing to remember is that these motors are not internally protected -- if you go 7.5HP, you'll need to replace the heater elements in the external thermal protection device appropriately. If you use a 10HP 1ph motor, that would use the same heaters as the 15HP 3ph motor. Just wire two of the wires straight through and leave the third unused.
About spinning the compressor slower -- If you have less horsepower available, you MUST use a smaller motor pulley to spin the compressor slower. There is, however, a limit on how slow you can go; if you go too slowly, the compressor won't develop sufficient oil pressure. I know the Bauer block is good at 7.5HP, but I wouldn't go slower than that without talking to someone at Bauer about it.
Sell it and buy a smaller compressor? I wouldn't. A de-rated compressor block with a smaller motor will last forever. (Of course, I'm running a 20cfm for "personal use".)
