DC Motor Speed:

The field of a DC motor is varied by using external devices, commonly field resistors.  To a constant applied voltage to the field (E), as the resistance of the field (R_f) is lowered, the amount of current flow by the field (I_f) increases as display through Ohm's law in Equation (6-4).                   

                                               (6-4)

An increase in field current will cause field flux (Φ f) to increase.  Equally a field flux will be decrease if the resistance of the field is increased.  The motor speed will increase if the field flux of a DC motor is decreased.  The reduction of field strength decreases the CEMF of the motor, because fewer lines of flux are being cut by the armature conductors, as display in Equation (6-5).

           (6-5)

A reduction of counter EMF permits an increase in armature current as shown in Equation (6-6).

                     (6-6)

That increase in armature current causes a larger torque to be established; the increase in armature current more than offsets the decrease in field flux as display in Equation (6-7).

                                           (6-7)

That increased torque causes the motor to increase in speed.

This increase in speed will after that proportionately increases the CEMF.  The speed and CEMF will continue to rise until the armature current and torque are decrease to values only large sufficient to supply the load at a new constant speed.