This thesis introduces a fault-tolerant control method that can be reduced the torque pulsation component in the event of a failure of a multi-phase motor applied to an electric propulsion system in special applications requiring high reliability, such as ships and submarines.
In the past, the propulsion system of most ships and submarines used diesel engines. However, as environmental regulations were strengthened, the diesel engine driving method was gradually changed to a hybrid method. Recently, the entire drive system is changing to an pure electric drive system. For special applications, it is becoming very important that the drive system continues to operate reliably even under fault conditions. In the case of a multi-phase motor drive system, even if one or more phases fail, it can continue to operate using the other normal phase, so it is more reliable than the existing three-phase motor drive system.
Failure of an electric propulsion system can occur for a variety of reasons. The ripple component of the torque due to the failure of the inverter system, the failure of the shaft sensor and the current sensor, and the failure of the stator or rotor causes vibration and noise in the motor drive system. Such noise has special significance in submarines because it threatens the stealth and survival of the vessel. Therefore, the study on the torque ripple reduction of the motor is an important study in the large-capacity electric propulsion system. Recently, many studies have been conducted on fault-tolerant control in an abnormal state of a polyphase motor.
In this paper, a fault-tolerant control method that can secure the reliability of a multi-phase BLDC motor drive system and reduce torque ripple is proposed. A detailed fault analysis model considering trapezoidal back EMF was applied. The effect of harmonic components of voltage and current of BLDC motor on torque ripple was investigated, and an improved method for calculating the reference value of the current to be controlled in case of failure was proposed. Various fault-tolerant control methods suitable for multi-phase BLDC motor drive systems were proposed and control performance was compared. Each method aims to reduce the torque ripple and total resistance loss in the stator windings by controlling the magnitude and phase angle of the healthy phase current corresponding to the selected winding in the open circuit fault condition of the motor. The fault phase causes torque ripple of even harmonic components. The fault-tolerant control proposed in this paper selects and controls the minimum two-phase current (MDC-FTC), hemispherical phase current (HPC-FTC), or multiple steady-phase current (MPC-FTC) of the motor to compensate for the generated ripple component. way to do it
Torque ripple and winding loss are minimized by controlling the magnitude and phase angle of the target phase current according to the proposed control strategy. The control method was verified through real-time simulators (HILS: Hardware-In-the-Loop) such as Plecs simulation and RT box. Among the proposed control methods, the torque ripple is the smallest in the two-phase current (MDC-FTC) control, and the multi-phase It was confirmed that the winding loss was the smallest in the current (MPC-FTC) control. Quietness is most importantly required according to the operation purpose of the application, and considering the possibility of multiple winding failures, it is judged that it is advantageous to use the MDC-FTC method even if the efficiency decreases due to a slight increase in winding loss.