| PhD Seminar

Name of the Speaker: Mr. Subramanian AN (EE11D012)
Guide: Porf. Krishna Vasudevan
Venue: CSD-308 (Conference Hall)
Date/Time: 20th January 2025 (Monday), 3:00 PM
Title: Sensorless Control of BLDC Motor Driven Thruster for Submersible Systems - A Novel Methodology Using Inductance Variation

Abstract :

Submersibles (both unmanned and manned vehicles) such as Remotely Operated Vehicles (ROV), Autonomous Underwater Vehicles (AUV) and Manned Submersibles are the main tools in exploring and utilizing subsea resources in a sustainable manner. The brushless DC motors (BLDC) motors of various power rating (up to 10 kW) are used in deep work class and shallow water submersibles due to their higher efficiency, silent operation, compact form, reliability, faster dynamic response, wider speed band, high torque to weight ratios and high-power density. The operation of these motors requires rotor angle information which is typically obtained from hall switches. In sub-sea applications, where depth of operation is up to 6000m, reliability/ruggedness of these motors are very critical during the expedition/operation of submersibles, as these are very expensive experiments in deep ocean. Hindrance in the operation of these propulsion motors would cause very serious affect in terms of human and financial resources, research vessel time, and mission of national importance. In the existing BLDC driven propeller systems, hall sensors are placed over the periphery of the stator winding, moulded, entirely immersed in the oil, exposed to high pressure environment up to 600 times greater than atmospheric conditions. These kinds of environment for hall sensors are highly vulnerable, unreliable and hard for maintenance, especially for continuous long period of operation during subsea expeditions using submersibles. Broadly speaking, control methodologies without sensors have become essential needs in the scenario and critical for operational robustness and maintenance. Different kinds of methodologies have been proposed for sensorless control of BLDC motor. From the literature, it is understood that the back-EMF based sensorless control approaches and other methodologies work at higher speeds, whereas sensorless control methods at lower speeds typically make use of stator winding inductance variation with respect to rotor position. Thus, this research work focuses on sensorless control of BLDC motor based on inductance variation of stator winding, for various power rating machines (up to 10 kW) for deep ocean applications in lower speed ranges.

The first part of the research work focusses on the theoretical basis for the proposed approach to sensorless control, supported by Finite Element Analysis (FEA). The theoretical basis is verified with the help of simulation and experimental studies on considered BLDC motor (Driven Thruster) meant for underwater (ocean) applications. The second part presents the development of a simulation model for a three-phase variable inductance brushless DC (BLDC) motor with blade loads (thruster for submersibles). The inverter model including control circuitry and machine model with blade loads and coupling gear of the thruster (BLDC motor- driven thruster) was accomplished in MATLAB/Simulink. The model is validated by comparing the results with those of the physical thruster system by experiments. The results of the examination proved the effectiveness of the proposed model. Differences in performance estimation with a constant inductance model and variable inductance model are brought out through simulation studies. The impact of assuming constant inductance is discussed in detail, which shows the necessity/importance of implementing variable inductance in the model, especially for sensorless control applications. It has also been shown that sensorless approaches described in literature could result in considerable error in detection of commutation instant if inductance varies with rotor position. The proposed model is simple and would be devised as a good tool for multi-thruster submersible applications and further used in variable inductance-based sensorless controller developments. A novel approach for sensorless control of BLDC motor-driven underwater thruster using variable inductance property is presented in this work. The approach is proposed based on peaks and valleys of the stator winding inductance, the occurrence of which is shown to correspond to commutation instants required for operation. A developed model is used to present this observation. A sensorless control scheme is proposed based on these observations. The work conceptualizes the approach based on finite element studies of the motor geometry and presents a simulation exercise to demonstrate that the triggering points correspond to peak or valley points of the rotor position-dependent inductance of a BLDC motor. The measurement of online inductance is further demonstrated with real-time hardware. A sensorless control scheme based on detecting the inductance variation is demonstrated through simulation studies and tested by laboratory hardware with an industrial underwater three-phase BLDC motor-driven thruster meant for deep- water submersibles and underwater systems.