Damping Effect of D-C Marine Propulsion Motors on Vibrations Produced in Drive Shafts by Large Propellers

D-c machines often are employed in ship-propulsion drives where maneuverability and a wide speed range are required. The d-c machine also has the advantage of serving as a source of damping if propeller pulsation should coincide with the natural frequency at some speed. This damping force arises because of the fact that any change in speed of the motor will cause a corresponding change in load current and torque. This torque change acts to oppose the original speed change, thus providing an effective damping force. The necessary equations and circuits are developed for calculating this damping effect and an illustrative calculation is included with actual test values for comparison.