-I'm sure GK can give the details better, but as I understand it (which is a very vague understanding at best) that's a motor-generator assembly. Something like a 3-phase motor turns a DC generator which then powers a DC motor which actually turns the spindle.
And it's done that way as a speed control- by varying the voltage coming out of the generator, the speed can be varied at the drive motor. But by simultaneously varying the... current? Amperage? You can keep the HP up even at low speeds. So you get a huge range of infinitely variable speed (including the back gear, I think it's something like 30 to 4,000 rpm) but still have significant HP even at the lowest speeds.
Using a modern VFD can, to a degree, emulate that, but generally speaking you'll lose significant HP at the lower speeds- halve the RPM, you get half the HP. Quarter the RPM, you get a quarter the HP, etc. With the 10EE M/G drive, if it's a 3HP motor, you have 3HP at half the RPM, still close to 3HP at 1/4 the RPM, etc.
Again, very roughly speaking, and I probably have many details wrong, but that's basically the idea behind all that stuff in the base.
Doc.
