Abstract
The most conventional approach of controlling magnetic forces in active magnetic bearings (AMBs) is through current feedback amplifiers: transconductance. This enables the operation of the AMB to be understood in terms of a relatively simple current-based model as has been widely reported on in the literature. The alternative notion of using transpermeance amplifiers, which approximate the feedback of gap flux rather than current, has been in commercial use in some form for at least thirty years, however is only recently seeing more widespread acceptance as a commercial standard. This study explores how such alternative amplifiers should be modeled and then examines the differences in behavior between AMBs equipped with transconductance and transpermeance amplifiers. The focus of this study is on two aspects. The first is the influence of rotor displacement on AMB force, commonly modeled as a constant negative equivalent mechanical stiffness, and it is shown that either scheme actually leads to a finite bandwidth effect, but that this bandwidth is much lower when transpermeance is employed. The second aspect is the influence of eddy currents. Using a very simple model of eddy currents (a secondary short-circuited coil), it is demonstrated that transpermeance amplifiers can recover significant actuator bandwidth compared with transconductance, but at the cost of needing increased peak current headroom.
Highlights
The use of active magnetic bearings (AMBs) in industrial machinery has seen a significant increase over the past decade due their high speed capabilities and lack of a need for an external lubrication system, resulting in significant improvements in system-level power density, as well as a notable reduction in the equipment package footprint
This paper investigates the performance differences between AMBs equipped with transpermeance amplifiers and those using a more conventional transconductance amplifier architecture
Turning to the required coil currents for transpermeance amplifiers under strong eddy currents, it can be seen from Figure 5c that the required peak current levels are approximately 100× those required for low eddy current operation
Summary
The use of active magnetic bearings (AMBs) in industrial machinery has seen a significant increase over the past decade due their high speed capabilities and lack of a need for an external lubrication system, resulting in significant improvements in system-level power density, as well as a notable reduction in the equipment package footprint. An efficient AMB system presents a consonant operation between the controller and the mechanical system design This system must include an effective power amplifier, which is responsible for supplying the actuator with proper levels of voltage and current in order to produce the desired output force over the entire operating frequency range. The work in [7] develops analytic models, which are shown to accurately predict the dynamic behavior of non-laminated axisymmetric electromagnetic actuators, including eddy current effects, and [8] follows a similar formulation for non-laminated. Detailed models that accurately predict the dynamic behavior of electromagnetic actuators including eddy current effects have been developed. Some work has been done that involves current-mode (transconductance) and voltage-mode (transpermeance) amplifier-controlled magnetic bearing system behavior in the presence of eddy currents. Observations and conclusions related to the performance differences seen between the two amplifier types are drawn
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