I have a 3 phase wound rotor induction motor which I would like to convert into a generator. I was wondering if it is possible to excite the rotor windings using a DC current. I attempted to do this, but I don't think the rotor windings were wired for that. The rotor windings are three phases with a common neutral inside the motor. I'm not sure if the approach I'm suggesting is possible. But I would think that applying a DC current through the rotor windings would produce a magnetic field. But it only worked as a "generator" when I applied an AC current
Induction generators don't work very well unless they are being used to supply additional energy into an active power source.
The method involving connecting capacitors to the generator output is used in some do-it-yourself projects, but there are several disadvantages.
The generator may need to be started using a manual procedure each time it is used.
The required capacitor value varies with load.
The output voltage varies with load.
If the load varies widely during use the generator may cease generating.
The load may need to be connected a little at a time.
The maximum load is likely to be considerably less than the capability of the machine when used with an active power source.
A wound-rotor motor can be operated as an induction generator by shorting the slip-rings together and operated using the capacitor method or or by connecting the stator to an active power source.
A wound-rotor motor can also be used as a generator by connecting AC to the rotor. When used in this way, a wound-rotor motor is called a doubly-fed induction generator. Doubly-fed induction generators are commonly used as wind-turbine generators. In that case, both stator and rotor are connected to active sources of power.
If you search the internet using some of the terms used above, you should be able to find additional information. The available information may be difficult to understand without a pretty good understanding of induction motor theory.
For all induction motors, (you would have to short circuit the rotor's slip rings):
Normaly this is done by adding capacitors at the output. The rotor has to have the remanence of magnetic field, if it doesn't have then you have to magnetize it by short current pulse. This is done with 12V battery, you connect the battery on two output leads by very short time, approx 1s (in your case you choose the slip ring instead).
When you spin the motor, the remanence induces voltage in windings, but in induction motor the excitation current is 90 degrees of, so when installing capacitors, the excitation current begins to flow 90 degrees of phase and the generator voltage builds up.
Such generator is not suitable to power a starting induction motor with close power rating to the generator's rating, becuase it will demagnetize the remanent magnetization of the rotor and you'll have to magnetize it again.
If you are using a wound rotor motor as a generator, could you not "excite" the slip rings with a pure sine invertor? In an offgrid system, you would have a DC battery bank anyways for back-up power so using a transfer switch from the invertor to start the system to the generator would "kick-start" the wound rotor motor and then the transfer switch would switch from invertor supply to generator supply.