If its a wound rotor motor you can excite two legs of the three and make some three phase power but it won't be as efficnet as one would like....
There are so many cool things one can do with a wound rotor motor if you do a little research....
Here is one of those places where my "old think shows" ( by that I mean I still think and work in 1950 even though I ma not that old, just a casue of being trained by very old men ).
A frequency changer:
By increasing or decreasing the speed of rotation of the rotor and drawing power from the slip rings you generate any frequency you like with one of these, as low as 25 and as high as 400 hz ( some restrictions apply based on rotor diameter and rim speed fo the rotor)
A variable speed drive with slip:
By adding resistance to the rotor circut with a resisitor bank you can slow the shaft speed, but at an efficiency loss.
Torque control:
You can change the torque curve of the motor so it produces its max shaft snaping torque at a stand still or gently rolls a load up to speed.
Two step speed control:
In some cases depending on the design of the rotor and itr limmitations you can connect it directly to shared load with an induction motor and by clever connections between the motors make the two behave as one with two speeds, with much better efficiency than the straight resisitor banks in the torque control method.
Transformer:
A little clever connections between the rotor and stator can turn a wound rotor motor into a transformer who's voltage increases or decreases with the possition of the rotor with respect to the stator poles.
Selsyn control:
Bu connecting two of these together in the right combination when you energise them turning the shaft on one by hand makes the other one's shaft turn and move in the exact same motion with the exact same amount of torque aplied
Schrague system:
I get excited over this, by combining a syncronous converter and a DC motor you can create a combination of machines that will run on AC but give you smooth stepless variable speed like a modern electronic drive just by adjusting field current on the DC motor. This is so cool 1930 technology that I've only seen done once on a decommisioned grinding mill.
Mind blowing stuff I could spend hours talking about the system....
In college to the horror of my instructor I wired three Labvolt trainers into a Shrague system. Took me hours to configuere and got me an A in machine theory....
I was once told lazy people rely on electronics clever people adapt machines. This isn't quite true but in the hands of smart man you can do a lot with a wound rotor machine.
Doug