The speed control of the DC electric motor 3 hp motors by changing the inductor flux is practiced by varying the field current and leaving the armature voltage constant. Usually, in this speed control mode, we reduce the effective field voltage to up to 80% of its rated value.
For a given torque, the armature current is inversely proportional to the flux and, thus, if the flux is reduced, there will be an increase of the armature current in the same ratio, ie Ia2 = φfg1 / φfg2. Ia1. Consequently, there will also be an increase in the armature reaction, especially if the motor does not have compensating winding and for these reasons field control is more suitable for light loads.
The slope of the static torque-velocity characteristic increases, when the field current is reduced, and therefore a more marked velocity drop occurs, when compared as the armature tension control, and for this reason, this control offers a regulation of velocity worse than that by armature tension. In electric motors with separate excitation we can use trimmers to vary the field voltage.