Synchronous motors apply DC to the rotor windings to prevent slip. Wound motors are induction motors that have a wire wound rotor that can we connected to external resistors for speed control.
You have some semantic issues here that may be interfering with your search for information.
BOTH types can be referred to as "slip-ring" motors, but that is like saying that all squares are rectangles, but not all rectangles are squares. That term just means that there are slip rings on one end of the motor shaft that facilitate connection to the rotor, for whatever purpose. It is the purpose of connecting to the rotor that makes the difference.
Synchronous motors use the rotor connection to vary the DC magnetic field strength in the rotor so as to adjust the power factor and lock the rotor in step with the rotating field in the stator, a.k.a. "synchronous rotation". Synchronous motors can rotate much more slowly with higher efficiencies than asynchronous induction or "squirrel-cage" motors. They are also used to correct power factor in an overall plant system since they can be run in a leading power factor, as opposed to induction motors which are always lagging. Not meaning to add to your confusion, but realize that most new synchronous machines are now brushless, meaning that the DC field generator is on the rotor itself, and an induction coil is used to pass power to it without the need for slip rings and brushes. So the term "slip ring synchronous motor" is almost always referring to an older system now.
The other type of slip ring motors are Wound Rotor Induction Motors (WRIM). You used the term "wound" all by itself, which does not clearly differentiate since technically all motors are wound in one way or the other. "Wound Rotor" means that instead of having a system of rotor bars connected at each end with a solid ring to create the necessary (but fixed strength) magnetic filed, the rotor has individual windings brought out to the slip rings. These slip rings are then connected to resistor banks that allow adjustment of the strength of the magnetic field. By selecting different amounts of resistance in the rotor windings, the output torque can be adjusted, and consequently the resulting speed will be lower (if the load is constant). It is actually a misconception that WRIM systems are true speed control, that is just the eventual effect in some applications.
In your search for more information, be clear that you want "synchronous slip ring" or "wound rotor slip ring" motors.
"Venditori de oleum-vipera non vigere excordis populi"
