Single Inverter fed Brushless Wound Rotor Synchronous Machine Utilizing Sub-harmonic Excitation

Abstract

The permanent magnet synchronous machines have been widely used for variable speed applications because these machines offered advantages of higher efficiency, simple control, their robust structure and lack of the need for an external excitation system. However, the recent increase in the prices of the Neodymium-Iron-Boron magnets has significantly increased the cost of these machines. Wound rotor synchronous machines (WRSMs), which do not depend on the market forces due to their magnet-less structure, can be an alternative to the permanent magnet synchronous machines. However, these machines require brushless and slip-rings assembly to provide the DC field current to rotor field winding. This brushes and slip-rings assembly causes sparking and requires regular maintenance. Therefore, in the recent years, many brushless wound rotor synchronous machines (BL-WRSMs) have been introduced. These BL-WRSMs either require a complex structure or dual inverter for their brushless operation. Which makes these machines less suitable for practical applications. This thesis focuses on the development of a new brushless topology for WRSM using a specially designed stator winding supplied through a single three-phase inverter. In this topology, a special stator winding, which comprises two sets of series connected windings with an unequal number of turns is used to generate an additional sub-harmonic component in the stator MMF. This additional sub-harmonic component is utilized for exciting the field winding. There are two separate windings placed on the rotor of the proposed machine: 1) the harmonic winding, 2) field winding. The two rotor windings are connected in parallel through a diode bridge rectifier, which is mounted on the rotor periphery. The sub-harmonic component of stator MMF induces the voltage in the harmonic winding, which is rectified through the rectifier and supply a DC current to the rotor field winding to achieve the brushless operation of the proposed machine. An eight-pole, 48-slots WRSM of 1.0 HP was designed to verify the operation principle of the proposed brushless topology. A 2D finite element analysis (FEA) was performed to analyze the performance of the proposed brushless topology. Although, the proposed BL-WRSM achieves the required torque at the rated speed but the torque ripple was quite high. To reduce the torque ripple, the rotor pole shape was optimized to improve the torque performance of the proposed BL-WRSM. The pole span and the pole shoe height were the optimized variables. The optimization was performed using Kriging method based on the Latin Hypercube Sampling and Genetic Algorithm. The performance of the optimal model of the proposed BL-WRSM was compared with the basic model of BL-WRSM. Due to the increased air gap at the ending edge of the pole shoe, the torque of the BL-WRSM optimal model was reduced compared to the basic model. Therefore, the number of turns of harmonic winding were optimized to improve the torque in the basic model. Then the optimal model of BL-WRSM was manufacture and the performance of the optimal BL-WRSM model was verified through an experimental test, which demonstrated good agreement with the simulation results. Due to the brushless operation, the proposed BL-WRSM has low starting torque. Because of this, it is very difficult to start the machine without any external force. To avoid the starting problem, a novel consequent-pole hybrid BL-WRSM (CPH-BL-WRSM) is developed, in which the permanent magnets (PMs) and the field winding poles together is used as excitation source. Due to the consequent-pole structure, the PMs are placed on alternate poles of the salient pole rotor such that the flux generated by the PMs strengthens the flux generated by the field winding poles. The proposed CPH-BL-WRSM offered the advantages of high starting torque, high torque density and better performance under rated load conditions. A 2D model of CPH-BL-WRSM was design and its performance was analyzed by 2D FEA. Then, the performance the proposed CPH-BL-WRSM was compared with already develop single inverter fed BL-WRSM and the regular PM-assisted BL-WRSM. The proposed CPH-BL-WRSM exhibits better performance compared to the BL-WRSM and the regular PM-assisted BL-WRSM. Finally, the CPH-BL-WRSM was manufactured and the experiment was conducted to demonstrate the performance of the proposed CPH-BL-WRSM. Although the problem of starting torque is resolved by the CPH-BL-WRSM, both aforementioned BL-WRSMs cannot maintain the constant torque under base speed. Due to this problem, these machines are not suitable for variable speed applications. To overcome this issue, a novel dual-mode WRSM (DM-WRSM) was proposed. The proposed DM-WRSM combines the advantages of both the conventional brushed WRSM and the BL-WRSM. In mode 1, the machine is operated as the conventional WRSM to achieve the constant torque in the constant torque region, while in mode 2, the proposed machine is operated is single inverter fed BL-WRSM to maintain the constant power in the field weakening region. A 2D FEA was carried out to validate the effectiveness of the proposed machine.