The control of the speed of the electric motor of direct current by variation of the inductive flux is realized, when modifying the field current and granting the constant tension of armature. Normally, in this principle of speed control, the effective field voltage is reduced to up to 80% of its effective value.
For an informed torque, the armature current is oppositely proportional to the flux, and from this, if the flux is reduced, we will have an increase in the armature current of the same equivalence, that is Ia2 = φfg1 / φfg2. Ia1 and, consequently, also an increase of the armature reaction, especially if the motor does not have compensating winding. It is for these causes that field control is more appropriate for light loads.
The slope of the static torque-velocity property accentuates, when the field current is decreased, however, a more strict speed reduction occurs, when associated as the control by armature tension, and, by this logic, this control has a regulation of Velocity compared to that by armature tension. In separate excitation motors trimmer can be used to change the field voltage.