Patent Description:
An auxiliary power unit (APU) for constant frequency aircraft power systems generally spins a synchronous generator at a constant speed to produce a constant frequency output. Accordingly, APU's are limited to constant speed operation which does not account for the environmental condition, sacrificing efficiency.

Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for APU generator systems. The present disclosure provides a solution for this need. <CIT> describes a DFIG-based UPS system and methods of control. <CIT> describes an electrical system for an aircraft and a method of control. <CIT> describes an electrical distribution system. <CIT> describes a power generation system having variable speed engine and method or cranking the variable speed engine. <CIT> describes engine speed optimization as a method to reduce APU fuel consumption.

An auxiliary power unit (APU) generator system for an APU can include a doubly-fed induction generator (DFIG) configured to be operatively connected to an APU to be turned by an APU and to have an output frequency that is a function of an excitation frequency and an APU speed. The system can include a generator control module configured to control the excitation frequency to the DFIG to output a substantially constant frequency with changing APU speed to supply the substantially constant frequency to a load.

The generator control module can be operatively connected to an output line to sense the output frequency of the DFIG. The substantially constant frequency can be within about <NUM>% of a set frequency.

The system can include an APU control module configured to control the APU speed to obtain an efficiency of the APU. The efficiency of the APU can be an optimum fuel efficiency as a function of one or more environmental conditions, for example. Any other suitable desired efficiency is contemplated herein.

The one or more environmental conditions can include ambient temperature and/or density altitude, for example. Any suitable one or more environmental conditions are contemplated herein.

The APU control module can be configured to control the APU speed by controlling a fuel flow to the APU. In certain embodiments, the system can include the APU.

In accordance with at least one aspect of this disclosure, a method controlling a speed of an auxiliary power unit (APU) system to optimize fuel efficiency as a function of one or more environmental conditions, and controlling an output frequency of a doubly-fed induction generator (DFIG) turned by the APU to be a substantially constant frequency. Controlling the output frequency of the DFIG can include modifying an excitation frequency to maintain the substantially constant frequency. Controlling the output frequency of the DFIG can include sensing the output frequency of the DFIG. In certain embodiments, controlling the speed of the APU can include controlling a fuel flow to the APU.

So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, embodiments thereof will be described in detail herein below with reference to certain figures, wherein:
<FIG> is a schematic diagram of an embodiment of a system in accordance with this disclosure.

Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, an illustrative view of an embodiment of a system in accordance with the disclosure is shown in <FIG> and is designated generally by reference character <NUM>. Certain embodiments described herein can be used to provide more efficient aircraft system, for example. Any other suitable use is contemplated herein.

Referring to <FIG>, an auxiliary power unit (APU) generator system <NUM> for an APU <NUM> can include a doubly-fed induction generator (DFIG) <NUM> configured to be operatively connected to an APU <NUM> to be turned by the APU <NUM>. The DFIG <NUM> can be configured to have an output frequency that is a function of an excitation frequency and an APU speed. For example, the DFIG generator can include a <NUM>-phase winding on a generator rotor and a <NUM>-phase winding on a generator stator. A frequency input to the rotor and the speed of rotation of the rotor can dictate the output frequency of the DFIG <NUM>, for example. Any other suitable configuration for the DFIG <NUM> is contemplated herein.

The system <NUM> can include a generator control module <NUM> configured to control the excitation frequency to the DFIG <NUM> to output a substantially constant frequency with changing APU speed to supply the substantially constant frequency to a load <NUM> (e.g., an aircraft electrical system that may require a constant frequency). In certain embodiments, the generator control module <NUM> can be configured to maintain a substantially constant output voltage as well. The term "substantially constant" herein means a constant value or any value within an acceptable range of the constant value. For example, the substantially constant frequency can be within about <NUM>% of a set frequency (e.g., within <NUM> of <NUM> desired frequency).

The generator control module <NUM> can be operatively connected to an output line <NUM> to sense the output frequency of the DFIG <NUM>. In certain embodiments, the generator control module <NUM> can be configured to control the excitation frequency as a function of speed of the APU <NUM> and/or the rotor of the DFIG <NUM>. For example, the generator control module <NUM> can include a speed/frequency map configured to provide a predetermined frequency as a function of the known or sensed APU speed.

The system <NUM> includes an APU control module <NUM> configured to control the APU speed to obtain an efficiency of the APU <NUM>. The efficiency of the APU <NUM> is an optimum fuel efficiency as a function of one or more environmental conditions.

The one or more environmental conditions include ambient temperature and/or density altitude.

The APU control module <NUM> can be configured to control the APU speed by controlling a fuel flow to the APU. In certain embodiments, the system can include the APU <NUM>.

The APU control module <NUM> and the generator control module <NUM> can be or include any suitable hardware and/or software module(s). In certain embodiments, the APU control module <NUM> and the generator control module <NUM> can be software modules hosted on the same hardware. In certain embodiments, the APU control module <NUM> and the generator control module <NUM> can communicate with each other such that the generator control module <NUM> can receive a speed setting from the APU control module <NUM> and can modify excitation frequency to the DFIG <NUM> accordingly.

In accordance with at least one aspect of this disclosure, an aircraft auxiliary power unit (APU) system can include an APU, and an auxiliary power unit (APU) generator system connected to the APU. The APU generator system can include any suitable generator system disclosed herein, e.g., as described above.

Many devices on aircraft require constant frequency input to operate. Certain aircraft systems traditionally operate at <NUM>. The APU operation can be more fuel efficient if it could adjust speed based on environmental condition. However, the APU generator is traditionally a direct-drive synchronous type where the output frequency is proportional to the input speed of the generator. Therefore, adjusting the speed of the APU would cause the generator frequency to vary outside of the acceptable range. However, there is no way to modify speed (by using fuel flow) of the APU traditionally without modifying the output of the synchronous generator outside of suitable range. An IDG type generator which takes variable input speed and outputs a constant frequency relies on a hydromechanical device that is not desired due to complexity, efficiency, and reliability issues. A VSCF is another type of generator that uses power electronics to adjust the frequency, e.g., a box that is electrically attached to the generator, but is also very large, heavy, unreliable, and requires more effective cooling.

In accordance with certain embodiments, in order to still produce constant output frequency from a variable speed APU, rather than using an IDG or VSCF system, a doubly-fed induction generator (DFIG) can be used. In the most common implementation of this type of generator, both the rotor and stator have a <NUM>-phase winding, for example. The output frequency of the generator can be based on the sum of the frequency proportional to the rotor speed as well as the frequency applied to the excitation winding of the generator. This way the constant output frequency of the generator can be maintained despite the variable frequency input by applying an appropriate frequency to the exciter winding.

In embodiments, a generator control module can take in a sensed output frequency or rotational speed and modify an excitation input (e.g., increase frequency of excitation for a drop in speed). For example, if rotational frequency drops <NUM>%, a <NUM>% increase can be made to the excitation frequency input to the DFIG <NUM>. Embodiments provide a generator system that is smaller, lighter, and more reliable than other options. Also, control can be all done internally to the generator.

Aspects of the this disclosure may be described above with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of this disclosure. It will be understood that each block of any flowchart illustrations and/or block diagrams, and combinations of blocks in any flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in any flowchart and/or block diagram block or blocks.

Any suitable combination(s) of any disclosed embodiments and/or any suitable portion(s) thereof are contemplated herein as appreciated by those having ordinary skill in the art in view of this disclosure.

Claim 1:
An auxiliary power unit "APU" generator system (<NUM>) for an APU, comprising:
a doubly-fed induction generator "DFIG" (<NUM>) configured to be operatively connected to the APU to be turned by the APU and to have an output frequency that is a function of an excitation frequency and an APU speed; and
a generator control module (<NUM>) configured to control the excitation frequency to the DFIG to output a substantially constant frequency with changing APU speed to supply the substantially constant frequency to a load;
characterized by further comprising an APU control module configured to control the APU speed to obtain an efficiency of the APU, wherein the efficiency of the APU is an optimum fuel efficiency as a function of one or more environmental conditions, and wherein the one or more environmental conditions include ambient temperature and/or density altitude.