Transient analysis of electromagnets with emphasis on solid components, eddy currents, and driving circuitry

Mark A Batdorff, Purdue University

Abstract

Valves are commonly used in fluid power systems to control pressure and flow. The emerging field of digital hydraulics demands high-speed, low cost, on/off valves with improved performance. Electromagnets, or solenoids, are commonly used to actuate valves due to their low cost, high reliability, and moderate performance. This work develops a dynamic model for a solid steel electromagnet that can be used for design and optimization, and unveils design tradeoffs with geometry and driving circuitry that are often overlooked. This work develops an accurate, computationally efficient, nonlinear, coupled, dynamic, axisymmetric, high fidelity magnetic equivalent circuit (HFMEC) electromagnet model capable of predicting force, inductance, dynamic response, and energy consumption. The model is intended for applications where both accuracy and solution time are critical. Axisymmetric magnetic fringing and leakage permeances were derived in order to capture nonlinear magnetic field phenomena that affect force and inductance. The tradeoffs between solid-center and hollow-center electromagnets were investigated. It was shown with both simulation and measurement that a hollow-center electromagnet has a 37.7% shorter useful stroke due to increased magnetic fringing and leakage (from 4.0mm to 2.5mm). However, it was also shown that the hollow-center electromagnet has a 70% improved turn-off response (from 617ms to 362ms). A single objective optimization study was performed demonstrating that hollow-center electromagnets are advantageous and can up to 204% increased dynamic response for systems where dynamics are dominated by eddy current lag. Electromagnets experience dynamic lag when turning on and off due to inductance and eddy currents. Coil driving methods, such as peak-and-hold, are often used to minimize turn-on lag by using high initial voltages and currents. However, circuits often do not address turn-off lag, which can be significant. This work investigates the effects of using momentary reversed current pulses to increase the decay rate of eddy currents in order to improve electromagnet turnoff speed. It is shown with simulation and measurement that driving circuitry can improve dynamic turn-off response by as much as 944% (from 1556ms to 149ms), but increase turn-off electrical energy consumption and heat generation by as much as 1030% (from 1.0J to 11.4J).

Degree

Ph.D.

Advisors

Lumkes, Purdue University.

Subject Area

Agricultural engineering|Electrical engineering|Electromagnetics

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