| dc.description.abstract | Switched reluctance machines (SRM) are permanent magnet free, and have a simple rotor construction with no current carrying parts. These are particularly suitable for high-temperature and high-speed applications. However, modeling and control of SRM are challenging on account of both phase inductance and back-emf being dependent on phase current as well as rotor position. This thesis addresses modelling and characterization of SRM, current control for low speed operation, single pulse control for high speed operation, and electromagnetic design of SRM.
Further, this thesis is oriented towards providing a solution for a high-speed switched reluctance generation (SRG) system, which is directly coupled with a high-speed thermal turbomachinery. Hence this thesis focuses on modelling of SRG system (inclusive of machine, power converter and control), control techniques for high-speed generation, high-switching frequency SiC MOSFET based power converter, fast fault detection and protection schemes for such converters, and B-H characterization of magnetic materials at high frequencies. On account of direct coupling to the thermal turbine, the generator shaft temperature and consequent expansion are expected to be high. Hence, special rotor constructions utilizing a common material for shaft as well as rotor are considered and evaluated experimentally.
Delta modulation and variable gain PI based current control are well known techniques for current control in an SRM. This thesis proposes and validates a fixed gain PI control with back-emf compensation for current control of SRM. A novel model predictive based current control is also proposed, which has better current tracking ability. Then a novel constant current injection based characterization method is proposed, which can yield the flux-linkage characteristics of the SRM without the requirement of blocking the rotor at known positions. The novel current control and characterization techniques are demonstrated and evaluated on a 4-phase, 8 stator pole, 6 rotor poles, 4 kW, 1500 rpm SRM drive.
The thesis derives a mathematical model of SR generation (SRG) system, and utilizes this model to study voltage build-up during stand-alone operation of the SRG system. A new high-speed optimal single pulse controller for SRG is also reported. Unlike the existing methods, the proposed real-time technique does not require any prior knowledge of the SRM characteristics or any off-line optimization procedure, and would be useful for self-commissioning of SRM drives. These modelling and control techniques are also demonstrated on the 4-phase, 4 kW, 1500 rpm SRM.
High-speed SRM requires high switching frequency power converter for effective control. Hence SiC devices based 50 kHz, 800 Vdc, 50 Arms power converter (asymmetric H-bridge) is developed, which is suitable for 20 kW 3-phase SRM. A fast fault detection and protection technique is part of the gate drive circuit of the above power converter.
Design and performance prediction of high-speed machines require knowledge of magnetic properties of materials over a wide range of frequency and excitation, which are often not available. A novel linear precision power amplifier (PPA) is developed for characterization of magnetic materials, which does not need a coupling transformer. This is a multi-stage, direct-coupled amplifier with low output offset, rated for 70 V peak, 10 A peak, DC-5 kHz frequency range. Using this PPA, the magnetic properties of numerous ferromagnetic alloys are studied experimentally.
Finally, design, fabrication and test results of high-speed SRM prototypes are presented. Two 10,000 rpm, 5 kW prototypes are fabricated, one having a solid rotor and another with a slitted rotor proposed in this thesis. These rotors are machined from single body, and therefore, are suitable for harsh operating conditions such as high temperature and high speeds. The flux-linkage characteristics of the solid-rotor SRM are experimentally investigated. A new, simple yet accurate stator-side model of the same is proposed for the solid-rotor SRM. This model derives unique phase voltage and current relationship from the measured multi-valued flux linkage versus current loops. The proposed slitted rotor structure retains the mechanical integrity of the solid rotor, while having lower iron losses. The measured flux-linkage characteristics and the measured no-load losses at various speeds establish the significant reduction in iron losses. Another 40,000 rpm, 10 kW, liquid-cooled prototype is also designed and fabricated. Measurement of flux-linkage characteristics at different rotor position and no-load losses at different speeds are repeated towards performance evaluation of this prototype. | en_US |
