J&L static phase converters.

[rusted]

I wonder why people are going through the trouble to build a rotary M/G set when these are so cheap?

http://www.jlindustrial.com/catalog/product.jsp?id=PAM-60000K

This one's rated for 3-5 hp and $138

[Franz]

Those work well on things like Bridgeports, but don't stand up well to heavier loads.
At $138 a copy, there looks to be about $100 profit on a couple capacitors and a relay.

[hankj]

Rated for motor loads only.

hankj

[fla jim]

Static phase phase converter suck. They produce a very large imbalance between phases. You will need to "Derate" your motor by about 85% of rated load. Typically the motor will run very hot.
With a rotorary converter phases can be balanced with capacitors, making for a more efficient motor.
I"ve built several of these commericially for three phase pumps for sewage lift stations in single phase residential neighborhoods.
Here's a picture of the one I built for my old southbend lathe in the Magic Garage.
you can see the large start capacitors to start the 5HP idler motor, and the smaller run capicitors to balance the phases.