Patent Application
20090146769
The invention relates generally to inductors. More specifically, the invention relates to a light-weight inductor used in power filters for multi-function motor controllers in aircraft engines.
When starting a traditional aircraft engine, the engine's shaft is rotated to operating speed by a pneumatic starter. Sparks are subsequently delivered to ignite a fuel/air mixture, which then powers the aircraft engine. This pneumatic starter, however, uses heavy components, which reduces the efficiency of the aircraft.
More recently-designed aircraft replace the pneumatic starter with an electric motor mounted on the shaft of the aircraft engine and a motor controller mounted inside the fuselage of the aircraft. Power is delivered to the electric motor from the motor controller by electric cables, and the electric motor rotates the aircraft engine's shaft up to operating speed. After the engine starting process is completed, the same motor controller is used to operate other motors, such as motors powering the Cabin Air Compressor (CAC) and the landing gear. This multi-function motor controller is called the “common motor starter controller” (CMSC). Included in the CMSC are three identical differential mode inductors. Up to 800 amperes (amps) at 0 hertz (Hz) is conducted through these inductors during the engine starting process, and up to 350 amps at 1450 Hz is conducted through these inductors during other motor applications.
Therefore, there is a need in the art for a differential mode inductor for use in a common motor starter controller that minimizes power loss and maximizes the extraction of heat generated by power loss, thereby keeping operating temperature below required limits. The inductor should also generate less heat than conventional inductors and be able to dissipate the heat that is generated over the high current range in which the inductor must function. Also, the inductor should be light in weight, since weight is often a significant factor in aerospace systems.
The invention is an inductor with a toroidal core divided into multiple segments, which are separated by electrically insulating material. The inductor is encapsulated in an electrically insulating, but thermally conductive, potting compound, and is housed inside an electrically and thermally conducting can. The inductor is lightweight, works over a broad range of frequencies with low power loss, generates less heat than conventional inductors, and effectively dissipates the heat that is generated.
In one embodiment of the invention, inductor core 110 has an outside diameter of about 104 millimeters, an inner diameter of about 52 millimeters and a height of about 76 millimeters. In that same embodiment, gaps 114 are about 1.25 millimeters wide.
In inductors energized with alternating current, the alternating magnetic fields produced by the alternating current tend to induce eddy currents within the inductor core. These electric currents in the inductor core must overcome the electrical resistance offered by the core, and eddy currents thus generate heat. The effect is more pronounced at high frequencies, such as those high frequencies found in electric starter controllers in aircraft. Small, multiple gaps 114, as well as the toroidal shape of inductor core 110, reduce the extent of eddy currents in inductor core 110, and thus reduce the amount of heat generated by inductor 100.
Can 140 is made of a material that has a high thermal and electrical conductivity, such as aluminum. The wound inductor 100 is encapsulated in a thermally conductive, but electrically insulating, potting compound, such as Stycast® 5954. The encapsulated inductor 100 is housed inside of can 140. Can 140 typically exhibits about 25 times the thermal conductivity of inductor core 110, and is thus able to dissipate much of the heat generated by inductor 100. Can 140 is typically mounted to a cold plate (not shown) to facilitate heat dissipation. In one embodiment of the invention, the bottom surface of can 140 is flat, in order to maximize heat dissipation between the bottom of can 140 and the cold plate. Also, a flat-bottomed can allows this inductor to be used with a liquid-cooled cold plate.
In order for the motor controller of an aircraft to function properly, the inductor must maintain high inductance at a high current. Ideally, as current rises from 0 to 400 amps, the inductance should be constant. The graph of
The present invention is a lightweight inductor assembly that may be used in the motor controller of an aircraft starter. The wound inductor core is positioned inside of a thermally conductive, but electrically insulating, container, which acts as a heat sink and EMI shield, while also reducing eddy currents within the inductor core. The aircraft starter is able to function with multiple applications, yet still dissipate the heat of the inductor. The present invention performs better than prior art inductors, while also demonstrating less power loss and greater heat dissipation than prior art inductors. The invention also performs well in extreme conditions. For example, in high current conditions, such as those found when starting an aircraft engine, the gaps in the inductor core prevent the inductor core from becoming saturated. In high frequency conditions, losses due to eddy currents are minimized by the toroidal shape of the inductor core and the use of a can around the inductor.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
