The paper focuses on dynamics of an electromechanical system composed of a DC motor and a planar four-bar mechanism. Minimization of mechanism/frame interactions is considered. Simultaneous elimination of frame shaking forces and torques is requested. The employed balancing method is counterweights allocation. Their parameters are found with a numerical modelling and a numerical optimization. They depend on shape of the mechanism’s velocity. Three alternative drives are tested in the paper: constant velocity drive; constant torque drive; and DC motor. The optimal counterweight’s parameters are derived for all these drives. Obtained results approved necessity of precise electromechanical modelling.