# Brushless Field Excitation Scheme for Wound Field Synchronous Machines
## 1 Introduction  
Permanent magnet (PM) machines have a vast range of applications owing to their high efficiency, power density, power factor, and torque density capabilities [1; 2; 3; 4; 5; 6]. Moreover, they do not require any extra exciter, brushes, or slip rings for their field excitation. This makes PM machines compact as compared to the classical synchronous machines which require complex and bulky field excitation mechanisms that involve exciters installed on the shaft of the machine. Besides these advantages, there are a few fundamental deficiencies such as undadjustable flux and risk of loss excitation associated with PM machines. In addition, the cost of rare-earth magnets used in PM machines has increased to a very high level. This motivated the researchers to search for other options such as reluctance machines [7] and brushless wound field synchronous machines (BL-WFSM). Several researchers have presented different field excitation schemes to avoid the use of expensive PM material, brushes and slip rings and to obtain the brushless operation for WFSMs. In general, these schemes involve a distinct harmonic winding placed in the rotor periphery along with the main rotor winding, and a supplementary winding installed in the stator in addition to the armature winding. The key role of the supplementary or auxiliary winding in such arrangements is to achieve an additional MMF in the airgap. This MMF is used to generate a harmonic current in the harmonic winding of the rotor that is later rectified to provide DC current to the main rotor field winding to obtain the rotor field. As the rotor field interacts with the main stator field it develops torque. Several techniques have been proposed by the researchers to modify the conventional WFSM with minimum modifications and realize maximum advantages. In [8; 9; 10], the authors proposed brushless techniques in which a sub-harmonic MMF component is produced besides the fundamental by injecting currents into the two different sections of the armature winding. In one of such studies, two inverters were used for this purpose. These inverters supplied two unique magnitudes of currents to the armature winding, which was installed with a distinct star connection and was symmetrically distributed in two halves of the stator. The higher harmonic MMF generated through this arrangement was generated in the harmonic winding of the rotor and rectified to produce direct current for the rotor field winding. Later on, the same technique was realized by using a single inverter. However, in this case, the number of turns in each portion of the armature winding was different from each other. This technique generates higher harmonics due to the different magnitude of currents in two different parts of the armature winding, which adversely influence the performance of the machine. In [7], the authors proposed a brushless filed excitation technique involving two inverters connected to the armature winding in parallel. The inverters were operating at \(-180\) degrees phase shift and were connected and disconnected alternatively after each cycle using the thyristor switches. The composite output of this arrangement results in a waveform comprising of fundamental and third harmonic components. The fundamental is utilized to generate the main armature field; however, the third harmonic is utilized to generate harmonic current in the harmonic winding of the rotor. The generated harmonic current is rectified for rotor field excitation. Although this technique achieves brushless operation for WFSM, the usage of thyristor switches with both inverters makes the system bulky and costly. In [11], the authors developed a modified inverter topology to supply the same composite first-third harmonic current waveform for the armature winding as discussed in the previous case. However, this technique required a unique inverter design along with a sophisticated control technique.