After simulating the configuration, the saturation current of the inductor was measured experimentally using an RLC meter and a simulated ideal current source. The hypothesis was the inductor would saturate at ahigher current when the EPM was added to the inductor. Figure 11 shows the hypothesized shift in the saturation current for the same inductance.

Increasing Inductor Power Density Using Controllable Electropermanent Magnets

Maeve Lawniczak, Dr. Daniel CostinettUniversity of Tennessee, Knoxville

Abstract

Inductive components are used widely in the power grid for the purpose of controlling power flow, regulating fault current, and providing compensation. Magnetics are also a principal component of power electronics circuits. In both cases, inductor design focuses on achieving a desired impedance while maintaining the component within its saturation limits. Recent research has shown that permanent magnets or electromagnets can be used to counteract internal flux, thus altering the saturation characteristics of the inductor. Permanent magnets exhibit a fixed magnetic field, and therefore are not controllable. Electromagnets require current to maintain the field, and therefore generate significant steady-state power loss. Electropermanent magnets differ from electromagnets in that they have zero steady-state power losses and heating issues, while maintaining the ability to electronically control the magnetic field. Through research and analysis of permanent magnetic materials and their operational characteristics, this project will examine the use of electropermanent magnets in power electronics and power systems applications as a more power efficient means of implementing devices currently being used in motors, switched mode power supplies, and the power grid.

How does an Electropermanent magnet work?The magnetic materials, Neodymium Iron Boron (NIB) and Alnico 5 are used in parallel for the EPM. The NIB magnet is classified as a hard magnetic material because of its high coercivity and the Alnico 5 is classified as a soft magnetic material due to the material’s low coercivity.

Intrinsic Coercivity

Remanence

Neodymium Iron Boron

800 kA/m 1.2 T

Alnico 5 50 kA/m 1.2 T

Experiment and Results

The Finite Element Method Magnetic software was used to simulate the distribution of flux from the EPM through the ETDcore. Different geometries were created and compared.

i(t)

B(t)

Bmax

NIB

Alnico

ferrite pieces

Future WorkFuture work includes configuring new EPMs that require less power forthe switching of the magnet. This will include magnets with greater areaand greater window area (for more turns). Other experimental set upswill be hypothesized and tested for greater accuracy. Also, inductors with other geometries such as the toroid will be tested for efficient flux distribution. During this phase of the research the flux created from the current pulse was not considered, however this will also be furtherinvestigated.

InductorInductor

EPM

Figure 2. EPM combined with smaller inductor would have a smaller total area than original inductor.

Figure 1. Inductors take up almost half of the area of this inverter used in power electronics. Decreasing inductor size would decrease size ofpower electronics.

Figure 8. Simulated waveform of internal flux cancelation of the inductor by the EPM

Bmax

i(t)

L(t)

The experimental results showed the inductor saturating quickly. If the current between 0 and .1 [A] were zoomed in on, one may be able to see the knee shape as hypothesized. New experimental setups using transformer theory are currently being explored.

The low coercivity of the Alnico 5 material allows the magnetic field direction to be flipped depending on the polarity of the current pulse applied to the coil. The phases of the EPM are illustrated in Figure 5 . The EPM in Figure 6 is modeled as a magnetic circuit in Figure 7. Each magnetic material consists of an internal mmf and reluctance. The flux from the EPM can be controlled by the inverse relationship between the area of the magnet and the reluctance.

Figure 3. Coercivity values indicate the ability for magnetic field directions to be switched

Figure 7. Magnetic circuit Model of EPM

No EPMEPM AddedFigure 11. Hypothesized shift in

saturation current for the sameinductance.

Figure 9. FEMM simulation of flux distribution through ETD coreFigure 10. EPM attached to inductor

Figure 12

Figure 5. The polarity of the current pulse determinesthe magnetic field direction of the Alnico 5 material. The low coercvity of Alnico 5 enables the magnet to be switched on and off with zero power needed tomaintain the state of theEPM.

Figure 6. Components of the EPM

Figure 4 Hysteresis Loop of Hard vs. Soft magnetic materials

Figure 8 illustrates the EPM turning on when the core is in danger of becoming saturated. The EPM would cancel the internal flux of the inductor and prevent saturation of the core.