Design optimization of 3-phase converter fed DC series motor

This paper presents the design optimization of a 3-phase, 6-pulse converter fed DC series motor for minimum material cost using non linear programming (NLP) technique. A mathematical model with 16 design variables, some of them representing the physical dimensions of the motor, is formulated. An octagonal frame for the yoke and poles, nonlinearity of the magnetic circuit, compensating winding in the pole faces and inter-pole winding are considered for the design. Some of the important constraints such as maximum reactance voltage of the commutating coil, maximum voltage between commutator segments, minimum clearance between the main field and inter-pole windings, ratio of torque to moment of inertia of armature, pulse duty factor with reference to armature current, efficiency of the motor etc., are imposed. The design analysis program includes armature circuit inductance calculations, harmonic currents due to rectified voltage, additional losses due to flux pulsations in the armature and magnetic circuit, skin effect in armature and inter-pole winding conductors. The design problem is converted into a sequence of unconstrained minimization problems by Zangwill's exterior penalty function method and Powell's minimization technique is applied to minimize the active material cost of a 150 h.p., 550 volts, 4-pole, 1500 r.p.m. DC series motor fed from a 6-pulse, 3-phase bridge converter. The effect of source impedance and of the triggering angle of the converter on the optimal solution is discussed. At the optimal solution, the torque and speed pulsations of the motor are investigated.