Rotor speed of separately ecxited DC motor with terminal voltage and field current

The operating speed and torque of a motor is always the intersection of the load's torque vs. speed demand curve and the motor's torque vs speed capability curve. Adjusting the armature voltage and field current creates alternative torque vs. speed capability curves as shown below. For a given load curve, increasing the armature voltage increases the operating speed proportionally and reducing the field current increases the speed.