Speed regulation is accomplished by sensing the motor current with R17 and using it as positive feedback to compensate for motor resistance loss. The gain pot should be set to a point just below the point where the motor speed oscillates. After finding this point, you may want to change the value of R11 to get better speed control range. Q3 limits
motor current. D4 and C8 capture some of L1`s inductive kick to produce a loosely regulated -12 volts for U3. SCR1 acts like the usual flyback diode once C8 has stored the needed energy, preventing significant power losses.