Analysis, modeling and excitation control of three-stage brushless starter-generators for aircraft applications

In the aircraft application, due to space constraints, employing integrated starter-generator is an efficacious solution. Among various electric machines, wound-rotor synchronous machine (WRSM) offers advantages of high reliability, sufficient controllability, the high power factor in a wide range of torque-speed plan, etc. The requirement of supplying rotor of WRSM is its main drawback. Since in aircraft application, safety is a critical demand, brushless type of WRSM is a practical solution. A conventional structure of brushless WRSM consists three electrical machines which are mounted on the same shaft. Therefore, this kind of WRSM is called three-stage brushless synchronous machine. The first stage of the machine is Pre-exciter which is usually a small PMG. The second stage in an inverse synchronous machine with stationary single-phase field and rotating armature which is called Main Exciter (ME). The third stage is the Main Generator (MG). Electrically, ME is connected to the MG by means of a rotating rectifier. The existence of the rotating rectifier connected between the armature windings of ME and field winding of MG significantly increases the nonlinearity of the system. In consequence, investigating the nonlinear behavior of the rotating rectifier plays a key role in studying of the brushless synchronous starter-generator. In this thesis, using analytical, numerical and experimental analysis, different operation modes of the three-stage brushless synchronous machine are investigated. Using revolving field theory, analytical description of the relationship between the stationary pulsating field flux of ME and its armature voltages is presented. Because of single-phase field winding of ME, at standstill and low-speed operations, for exciting field winding of ME, AC excitation is required. Then, by speed increasing, AC excitation must be switched to DC. Since field excitation of ME is variable, induced armature voltages of ME are different in various operating points. In this thesis, it is explained that during the starting process of the starter-generator, armature voltages of ME are severely unbalanced. Therefore, for the sake of modeling and studying, a model of rotating rectifier with an ability to deal with a wide range of unbalanced conditions is required. As the first step, in this thesis, a novel model of the uncontrolled rectifier with a capability of considering unbalanced input voltages and harmonics in AC-side is proposed.