A DC motor principle is just to convert Electrical energy in Mechanical Energy.
As shown in figure, an armature is kept between magnetic field generated by 2 magnets.An armature is connected to commutator, brushes with DC supply as shown in fig.
Fleming’s Left hand thumb Rul e :-
According to Fleming’s Left hand thumb Rule,when a current carrying conductor is kept between magnetic field,then conductor experience force in the direction perpendicular to the magnetic field.
Thus here in DC motor,an armature experience force perpendicular to magnetic field as soon as current passes through it due to DC supply given as shown in fig.