The electrical energy fed to the AC motors is in AC (alternative current) form and to the stator of the motor (no brushes necessary). The electrical energy fed to the DC motors is in DC (direct current) form and to also the rotor of the motor (via brushes).

(8+4)+2=8+(4+2) : Addition is associative. 5+3=3+5   : Addition is commutative. 8(a+b)=8a+8b     : Multiplication with a sum is distributive.