AFAIK most (all ?) universal motors have their rotor and stator windings in series (check with Wikipedia). I'm pretty sure my mains-powered drill has a universal motor, and its speed control is a simple phase-angle control (similar to a light dimmer).
It is not the actual circuit I am looking for it is just what I have to do. Is it a frequency change, a voltage change or what?
Also does the rotor get wired in series to the stator?
Average of the absolute voltage.. or RMS voltage. If you run it from AC mains power, you can try to connect a diode in series with the motor. The diode cuts away half the power (half wave rectification) and the speed should be about half also. You can google for dimmer circuits that can control the average voltage more precisely. http://www.techlib.com/electronics/power_control.htm
Universal motors can run on DC (the stator is connected in series with the rotor). Actually they have better efficiency when driven with DC because AC causes eddy currents in the stator. Stalling the motor can blow it up, but that is a problem with all DC motors.