Patent Application
20070080769
The present invention relates generally to an inductor and, more particularly, to an inductor with multiple air gaps for thermal management.
High power motor controllers typically require inductors exhibiting stable inductance at both high magnitude currents and at frequencies ranging from DC to tens of kilohertz. Parameters for one such inductor, typical of aerospace applications, operates at: 35 μH rated for 260 A at 1,400 Hz continuous. An inductor designed to these parameters should retain 90% inductance at DC currents up to 880 amps. These inductors, specifically power quality filter inductors, should be lightweight and be configured for conduction cooling. Use in aerospace applications heightens the need for lightweight inductors.
Many conventional inductor permutations attempt to meet desired performance parameters yet minimize inductor weight. One such inductor is a gapped tape-wound cut core inductor. This type of inductor contains a magnetic core and typically exhibits high losses around the air gaps due to magnetic core eddy currents which are caused by flex fringing near the air gaps in the magnetic core. As a result, the heat generated by the inductor may most noticeably increase in the areas adjacent the air gaps. In addition, high temperatures may be realized in inductor portions proximate the air gaps. Air gaps in the magnetic path create a high reluctance path, avoiding saturation of the magnetic field at lower frequencies.
Powder magnetic core materials have been used in an attempt to reduce the high temperatures. The powder core materials inherently contain distributed air gaps, which minimize flux fringing and eddy current losses. However, as the DC magnetizing force of the inductor increases, the effective permeability of the powder core drops significantly which thereby limits the effectiveness of the powder magnetic core material to reduce inductor temperatures, especially in inductors producing high magnetizing forces.
Reducing the number of coil turns increases the current with which the permeability of the powder core drop becomes unacceptable. However, to maintain the desired inductance, the cross-sectional area of the powder core must increase substantially in response to a decrease in the number of coil turns, such that the overall weight of the inductor increases, with disadvantageous results for aerospace applications.
Other attempts to minimize the high temperatures generated by the inductors include eliminating entirely the ferromagnetic core. This approach results in an air core inductor with no air gaps or gap losses but requires a significant number of turns and relatively large diameter inductors coils to generate sufficient inductance. Eliminating the ferromagnetic core also induces high magnetic fields outside of the area enclosed by the coil windings, which may heat metal surfaces near the inductor and may interfere with the fields of other inductors in the area. Thus, the elimination of the ferromagnetic core results in a relatively large mounting footprint and stray magnetic fields, which may have disadvantageous results in aerospace applications.
Accordingly, it is desirable to provide an inductor for aerospace applications that minimizes eddy current losses and effectively facilitates inductor heat conduction.
A cut core inductor assembly having a magnetic core disposed in a winding. An electric current travels through the inductor assembly generating a magnetic field and thermal energy.
The magnetic core includes magnetic core sections on a mounting frame. The winding includes winding sections each encircling one of the magnetic core sections and the mounting frame. Multiple air gap spacers separate adjacent magnetic core sections of the magnetic core. Thermal energy removed from the magnetic core is communicated to the mounting frame.
The magnetic core section includes substantially rectangular profiled magnetic laminations arranged in a stack upon a planar mounting surface of the mounting frame. The stack of magnetic laminations extends from the mounting frame and perpendicular to the planar mounting surface. Upturned flanges on the mounting frame partially secure the magnetic laminations.
The present invention therefore provides a power inductor assembly which efficiently conducts heat from the magnetic core while minimizing eddy current losses and maintaining a desired inductance level.
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:
The inductor assembly 10 may include magnetic core sections 22 of varying sizes. For example, the inductor assembly 10 may include larger magnetic core sections 22 near the ends of the inductor assembly 10. It should be understood that although a rectangular inductor assembly 10 is described, various other geometries or arrangements of magnetic core sections 22 are included within the scope of this invention, including, toroidal or polygonal geometries.
Referring to
The winding section 28 surrounds a segment of the magnetic core section 22 and a portion of the mounting frame 14, further securing the magnetic laminations 34 upon the planer mounting surface 16 of the mounting frame 14. The winding section 28 contacts both the mounting frame 14 and a portion of the magnetic core section 22 to facilitate thermal energy transfer to the mounting frame 14. The coil windings 26 are typically copper or other highly conductive material. In addition, the coil windings 26 and the magnetic core sections 22 may include a thermally conductive encapsulating material for reducing thermal impedance. The coil winding 26 arrangements and the encapsulating material result in reduced operating temperatures of the inductor assembly 10.
The air gap spacer 30 is disposed between adjacent magnetic core sections 22. The winding section 28 encircles the magnetic core section 22 but need not encircle the air gap spacer 30. Segregating the air gap spacer 30 in this manner optimizes the air gaps in the inductor assembly 10. In addition, flux fringe induced eddy current losses typically peak in the central portion of the magnetic core section 22 and at the perimeter of the magnetic core section 22 which may create a build-up of thermal energy in those portions of the magnetic core section 22. The position of the air gap spacer 30 facilitates removal of thermal energy from the perimeter of the magnetic core section 22 while the position of the winding section 28 facilitates removal of thermal energy from the central portion of the magnetic core section 22.
The air gap spacer 30 extends past the stacks of magnetic laminations 34 to contact a mounting foot 50 of the mounting frame 14. The mounting foot 50 provides an attachment surface to secure the inductor assembly 10 to a desired location. Thermal energy is thereby readily transferred from the magnetic core 18 to the mounting frame 14. Preferably, the air gap spacer 30 is made of a material having a high thermal conductivity and high electrical resistivity, such as aluminum nitride.
As the inductor assembly 10 utilizes multiple air gap spacers 30, the eddy current effect is dispersed around the magnetic core 18 such that losses in inductance due to eddy currents in the magnetic core 18 are reduced. The air gap spacer 30 creates a high reluctance path in the magnetic core 18, avoiding saturation at low frequencies. The multiple air gap spacers 30 provide multiple paths for thermal energy from the magnetic core 18, facilitating rapid conduction of thermal energy from the magnetic core 18. It should be understood that an increase in the number of air gap spacers 30 or the thickness of the existing air gap spacer 30 will modify the inductance of the inductor assembly 10.
Referring to
Threaded tie-rods 62, or other such fasteners, extend through endplates 54 on opposing sides of the inductor assembly 10. Tightening the threaded tie-rods 62 draws the end plates 54 together securing the stacks of the magnetic laminations 34 and the air gap spacers 30 between them. The threaded tie-rods 62 and the end plates 54 effectively clamp multiple air gap spacers 30 between multiple magnetic core sections 22.
Referring to
It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.
