| PhD Viva

Name of the Speaker: Mr. Subramanian AN (EE11D012)
Guide: Dr. Krishna Vasudevan
Co-Guide: Dr. Atmanand M
Online meeting link: https://krishnavasudevan.my.webex.com/meet/krishna
Date/Time: 5th December 2025 (Friday), 3 PM
Title: Sensorless Control of BLDC Motor Driven Thruster for Submersible Systems - A Novel Methodology Using Inductance Variation

Abstract :

KEYWORDS: BLDC motor-driven thruster; variable inductance; sensorless control; subsea applications; valley points. The submersibles (both unmanned and manned vehicles), such as Remotely Operated Vehicles (ROV), Autonomous Underwater Vehicles (AUV), and Manned Submersibles, are the primary tools in exploring and utilizing subsea resources sustainably. The brushless DC (BLDC) motors, available in various power ratings (up to 10 kW), are used in deep work-class and shallow-water submersibles. The operation of these motors requires rotor angle information, which is typically obtained from Hall switches. In sub-sea applications, where the depth of operation can reach up to 6,000 m, the reliability/ruggedness of these motors is critical during the expedition/operation of submersibles, as these are costly experiments in the Deep Ocean. In existing BLDC-driven propeller systems, Hall sensors are placed over the periphery of the stator winding, moulded, and entirely immersed in oil, exposed to a high-pressure environment that is up to 600 times greater than atmospheric conditions. These kinds of environments for Hall sensors are highly vulnerable, unreliable, and difficult to maintain, especially during prolonged periods of continuous operation in subsea expeditions using submersibles.

Broadly speaking, control methodologies without sensors have become essential in this scenario, as they are critical for operational robustness and maintenance. Various methodologies have been proposed for sensorless control of the BLDC motor. According to the literature, back-EMF-based sensorless control approaches and other methodologies work at higher speeds. In contrast, sensorless control methods at lower speeds typically utilize the variation in stator winding inductance with respect to rotor position. Thus, this research focused on sensorless control of BLDC motors based on the inductance variation of the stator winding, for machines with various power ratings (up to 10 kW) used in deep ocean applications at lower speed ranges. The first part of the research work focused on the development of a simulation model for a three-phase variable inductance brushless DC (BLDC) motor with blade loads (thruster for submersibles, rated for 6000m ocean depth). Differences in performance estimation were brought out through simulation studies comparing a constant inductance representation (model) with a variable inductance model. The impact of assuming constant inductance is discussed in detail, highlighting the necessity/importance of implementing variable inductance in the model, particularly for sensorless control applications. It has also been shown that sensorless approaches described in literature could result in considerable error in the detection of the commutation instant if the inductance varies with the rotor position. A novel approach for sensorless control of a BLDC motor-driven underwater thruster using the variable inductance property is presented. This approach is based on the peaks and valleys of the stator winding inductance, which correspond to the commutation instants required for operation. A developed model was used to present this observation through simulation studies. A sensorless control scheme was proposed based on these observations. The theoretical basis (mathematical analysis) for the proposed approach to sensorless control is presented, supported by Finite Element Analysis (FEA), and further verified with the help of experimental studies on the considered BLDC motor. The work conceptualized the approach based on finite element studies of motor geometry and presented 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 was further demonstrated with real-time hardware. A sensorless control scheme based on detecting inductance variation was 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.