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This paper presents a novel design by integrating a planetary gear train (PGT) within a DC commutator motor as a compact structural assembly. It provides functional and structural integrations to overcome inherent drawbacks of traditional designs. By sharing a designated part, the electric motor and the gear reducer are combined without extra transmitting elements. The rotor integrated with the gear teeth, which is the interface between the motor and the gear reducer, is considered as the input sun gear of the PGT to provide the transmission function. By applying the equivalent magnetic circuit method, important parameters related to the magnetic field and motor performance of the DC commutator motor are derived in terms of magnetic material properties and motor dimensions, which are also verified by the finite element analysis. Two different numbers of gear teeth are integrated on the rotor of an existing DC commutator motor to reduce the cogging torque and torque ripple successfully. By applying the analytical model on the optimal design, the main dimensions of this integrated device are obtained. As a result, the proposed integrated device performs better than the existing design on the cogging torque with 92.02% decreasing and torque ripple with 50.14% decreasing.