Eric Severson

Magnetic Equivalent Circuit Modeling of the AC Homopolar Machine for Flywheel Energy Storage

This paper develops a magnetic equivalent circuit model suitable to the design and optimization of the synchronous ac homopolar machine. The ac homopolar machine is of particular interest in the application of grid-based flywheel energy storage, where it has the potential to significantly reduce self-discharge associated with magnetic losses. The ac homopolar machine features both axial and radial magnetizing flux paths, which requires finite element analysis to be conducted in 3-D. The computation time associated with 3-D finite element modeling is highly prohibitive in the design process. The magnetic equivalent circuit model developed in this paper is shown to be a viable alternative for calculating several design performance parameters and has a computation time which is orders of magnitude less than that of 3-D finite element analysis. Results obtained from the developed model are shown to be in good agreement with finite element and experimental results for varying levels of saturation.

Dual-Purpose No-Voltage Winding Design for the Bearingless AC Homopolar and Consequent Pole Motors

A winding design approach is proposed for the bearingless ac homopolar and consequent pole motors that utilizes the same coils for both suspension force and torque production. This enables a “pure” bearingless motor design, where the same iron and copper are used for both magnetic bearing operation and torque production, and can result in more optimal machine performance. Separate terminal connections are provided for suspension and torque operation; the suspension terminals experience no motional-emf when the rotor is centered, thereby reducing the required voltage rating of the bearingless drive compared to other “dual purpose” winding designs. It is shown that only certain combinations of stator slots, phases, and poles yield suitable winding designs; most notably, the number of motor pole-pairs must be co-prime with the number of phases. A design procedure is proposed which allows the designer to simply modify end-connections of many conventional armature windings, including both integral- and fractional-slot windings; an example winding design is presented and analyzed using 3-D finite element analysis (FEA); and a hardware prototype has been constructed with experimental results included to validate the proposed design approach.

Practical Implementation of Dual-Purpose No-Voltage Drives for Bearingless Motors

Practical implementations of the power electronics required to operate bearingless machines with the so-called “dual-purpose no-voltage” (DPNV) windings are proposed. These windings enable a bearingless machine to use the same coils to produce both radial suspension forces and torque, which has been shown in the literature to enable better machine performance. In this paper, the use of two-level inverters is investigated for the two configurations of DPNV windings: the “parallel” and “bridge” configurations. The investigation is valid for conventional

Suspension force model for bearingless AC homopolar machines designed for flywheel energy storage

p \pm 1

Practical implementation of dual purpose no voltage drives for bearingless motors

pole-pair bearingless motors (i.e., bearingless permanent magnet and induction motors) and

A compact active filter to eliminate common-mode voltage in a SiC-based motor drive

p=1

Design of dual purpose no voltage combined windings for bearingless motors

bearingless motors (bearingless ac homopolar and consequent-pole motors). The advantages and disadvantages of each configuration are explored in terms of control complexity and required hardware, potential paths for circulating currents are identified along with mitigation strategies, and experimental results are presented to validate the proposed power electronic implementation.

Circuit-level characterization and loss modeling of SiC-based power electronic converters

Bearingless ac homopolar machines combine magnetic bearing and motor/generator functionality into a single electric machine which features variable excitation, high power density at high rotational speed, a simple and robust rotor structure, and magnet-less excitation. These features make the bearingless ac homopolar machine a promising machine for highspeed flywheel energy storage systems (FESS). The variable excitation of the bearingless ac homopolar machine has the potential to increase the FESSs efficiency by allowing for low excitation during periods of free-wheeling and high-speed operation. However, the magnetic suspensions position stiffness and current stiffness depend upon the excitation level. This dependency must be taken into account in the suspension controller or the magnetic suspension may become unstable at certain excitation levels. A technique for modeling this dependence is presented in this paper and explored through 3D finite element simulation. A prototype design is analyzed for two rotor structures: one with a square airgap length profile and one with an inverted sinusoidal airgap length profile.

Mitigation of common-mode noise in wide band gap device based motor drives

Practical implementations of the power electronics required to operate bearingless machines with so-called “dual purpose no voltage” (DPNV) windings are proposed. These windings enable a bearingless machine to use the same coils to produce both radial suspension forces and torque, which has been shown in the literature to enable better machine performance. In this paper, the use of two-level inverters is investigated for the two configurations of DPNV windings: the “parallel” and “bridge” configurations. The investigation is valid for conventional p±1 pole-pair bearingless motors (i.e. bearingless permanent magnet and induction motors) and p = 1 bearingless motors (bearingless ac homopolar and consequent-pole motors). The advantages and disadvantages of each configuration are explored in terms of control complexity and required hardware; potential paths for circulating currents are identified along with mitigation strategies; and experimental results are presented to validate the proposed power electronic implementation.

Design and measurement of a passive thrust magnetic bearing for a bearingless motor

This paper presents an active compensation device for common-mode (CM) voltage elimination in 3-phase space-vector pulse-width-modulated (SVPWM) inverters. The proposed device consists of a single-phase 2-level inverter (H-bridge) which supplies a compensating voltage to the inverter via a step-up common-mode transformer tied to all three phases at the output. The H-bridge active filter is supplied by a low voltage bus and switched several orders of magnitude faster than the inverter switching frequency. This device takes advantage of the direct knowledge of the switching pulses sent to the inverter to predict and generate the compensating voltage. A technique is employed to subtract the low frequency harmonics from the modulation of the H-bridge which allows for the size of the common-mode transformer to be reduced significantly. Small passive components are added to attenuate the active filters PWM frequency content and thus produce an effective compensating voltage. This paper will review existing common-mode voltage compensation techniques and demonstrate that the proposed method is a logical choice for certain drive applications. Design considerations are included to provide understanding and guidance for implementation of the device, as well as MATLAB/Simulink simulation results to demonstrate the operation of the active compensation device. Final validation is presented through experimental results from a hardware prototype.