Abstract Power losses in the elements of the supply system (transformer, line, compensator) at the minimum losses control of the DC motor drive are discussed. The separate components of the losses are described. It is examplified that the supply system losses can be notable in the group drive with the 18 kw motors. They have effect on the course of the optimum motor field current as a function of operating point (torque or armature current, rotational speed). This dependence is determined for the investigated case, and then it is reduced to the form of approximating polynomial by means of the mathematical apparatus of the experiment design theory. Finally, the possibility of the inclusion of the supply system losses in the known minimum losses controllers is being suggested.