Characterization of Near-Propeller Bubbles and Fluctuation in Thrust Force in a Constant Torque Control Motor Propulsion System

Abstract

In recent years, power trains that consist of propeller propulsion and motor drive systems have been widely used for battery electric boats, which are gaining popularity as a type of small boats, as well as for diesel electric ships that are renowned for their good maneuverability. The AC motor drive system used for these boats has a response time of just a few milliseconds — much quicker than that of the internal combustion engine. In addition, it is technically possible for the system to accurately monitor motor control signals (current, voltage, torque and rotational speed) in real time. In our previous report, we measured (torque signals, rotational speed signals and bubble behavior around the propeller) in the motor propulsion system under constant torque control in a circulating water channel through such means as analyzing high-speed video images. By analyzing rotational speed signals, we confirmed ahead of other researchers around the world, that it is possible to detect generated bubbles, measure its quantity, monitor the resulting thrust reduction and accurately gauge its magnitude. This report, has verified the results of our experiments theoretically by using the propeller characteristic equation. It has also demonstrated the possibility of improving propulsion efficiency enabled by suppressing the generation of bubbles and stabilizing the motor power output.