For independent excitation electric motors, 3 hp 182t motors speed control by varying the armature voltage can be accomplished by keeping the constant excitation flow, usually at its nominal value. The armature voltage can admit positive or negative values of up to 100% of its nominal value. Although we have the variation of the armature tension, the slope of the static torque-velocity characteristic is stable, because it will only depend on the total armor resistance.
In this way, when we vary the terminal armature voltage, the velocity-torque characteristics compose a family of parallel lines with each other. The velocity can be changed within the range -ωn ≤ ωm ≤ ωn, where ωn denotes the nominal velocity, which is the speed of the rotor when the armature voltage and field current are nominal (plate values). The variation of the armature terminal voltage in the parallel excitation electric motor is not indicated because of the field current that would also be changed, in this case Vta is maintained and the sequential switching of resistors connected in series with the armature circuit .