PhD Thesis Colloquium of student, Kamisetti Prasad

PhD Thesis Colloquium

Title: “Modeling, Design and Control of Power-Electronic-Actuated Electromagnetic Bearings”

Speaker: Kamisetti N V Prasad,

Department: Electrical Engineering

Supervisor: Prof. G. Narayanan

Date and Time: 26 July 2023 (Wednesday), 9 am – 10 am

Venue: MMCR, EE Department

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Abstract

Many practical electrical machines, turbines, and compressors operate in speeds, ranging from tens of thousands of rpm to hundreds of thousands of rpm, and also, handling a significant amount of power. While high-speed operation reduces the machine dimensions for a given power rating, its challenges include high bearing loss, reduced bearing life, and high viscous drag. Contactless bearings, such as gas, oil, or electromagnetic bearings (EMB), offer longer life than conventional bearings in high-speed applications. In addition to being contactless, an electromagnetic bearing (EMB) is lubrication-free; hence this is suited for both clean conditions (e.g., food and pharmaceutical industry) and hazardous applications (e.g., petroleum and chemical industry). This thesis presents the modelling, analysis, design and control of power-electronic-actuated EMBs. The scope of thesis includes both radial EMB and axial (or thrust) EMB, which handle the radial and axial forces, respectively, acting on the rotor assembly.

Drawing from the switched reluctance machine (SRM) literature, a flux linkages-based modelling approach is proposed for radial and axial EMB. The flux-linkage characteristics can be obtained through either numerical simulation or measurement, and can be used to generate the force vs current vs displacement characteristics of the bearing. Such modelling includes the effects of magnetic saturation, leakage flux and fringing. An improved design procedure is proposed, which guarantees linear force characteristics along with the desired maximum force. A radial EMB and an axial EMB are designed for load capacities of 180 N and 1600 N, respectively, using the improved design procedure and are validated using finite element analysis tools. Further, a modified geometry of the thrust bearing is proposed to reduce the thrust disc diameter (and thereby, to cater to higher rotational speeds), while maintaining the same load capacity. A systematic PID design procedure is presented for the position control of the EMB, guaranteeing the required stability margins. The performance of this controller is validated through simulations using detailed models of EMB.

Position control of the EMB, which is an unstable system, require high-bandwidth control of the EMB coil currents. This, in turn, requires high-switching-frequency power amplifiers to feed the coils. An SiC device-based asymmetric H-bridge converter of 300V, 10 A, with a switching frequency of 50 kHz, is designed and tested. Further, the current controller is designed, and its reference tracking capability is validated experimentally for different types of current references that are expected during the EMB operation. Further, this thesis proposes a novel test rig for thrust-bearing characterization. This test rig can characterize the given thrust bearing under static and dynamic conditions (under rotation and varying loading).

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