The armature series resistor starts a circuit that changes the excitation current speed regulation.

When the armature voltage of a DC motor remains unchanged, reducing the excitation current can increase the speed; increasing the excitation current
can reduce the speed.
    The circuit layout is shown in Figure 3-192. This circuit still uses the starting method of stringing resistors in the armature loop, and the speed adjustment
uses the method of changing the excitation current.
    In the figure, R is the starting resistor, which is also used as a current-limiting resistor during braking; the porcelain rheostat RP is used for speed regulation, which can be adjusted to
change the excitation current , thereby changing the speed of the motor; KMi is the energy consumption braking contactor; KM2 is the working contactor;
KM3 is the contactor for short-circuiting the starting resistor , that is, the accelerating contactor.