Electromagnetic variable transmission

An electromagnetically variable transmission includes an outer rotor and an inner rotor. The inner rotor is independently rotatable within a center aperture of the outer rotor. The outer rotor is independently rotatable about the inner rotor. One of the rotors has a plurality of permanent magnets configured in pairs and facing an air gap disposed between the outer rotor and the inner rotor. The other rotor has a plurality of slots spaced about a magnetically permeable core having embedded windings. The outer inner rotors are simultaneously rotatable in one direction. In response to rotation of the outer rotor portion and the inner rotor portion, a magnetic flux path is generated between the permanent magnet pairs, the air gap, the outer rotor core and the inner rotor portion core, to induce electrical power in the windings, which transfers power between the inner rotor portion and the outer rotor portion.

FIELD OF THE INVENTION

The present invention is directed to a method and apparatus for transferring power between rotating shafts of an engine, and more specifically to an Electromagnetic Variable Transmission (EVT) for transferring torque and power directly from one rotating shaft with operating at one speed to another rotating shaft operating at a different speed than the first shaft.

BACKGROUND OF THE INVENTION

A gas turbine engine generally includes one or more compressors followed in turn by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power.

The engine power is transferred either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbofan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers.

It is well understood that individual components of turbofan engines, in operation, require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low turbines with independent power transmitting devices are necessary for the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power.

Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly.

Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than rely solely on the HP engine spool to drive the electrical generators. The LP engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds. The boost cavity of gas turbine engines has available space that is capable of housing an inside out electric generator, however, the boost section rotates at the speed of the LP engine spool.

However, extracting this additional mechanical power from an engine when it is operating at relatively low power levels (e.g., at or near idle descending from altitude, low power for taxi, etc.) may lead to reduced engine operability. Traditionally, this power is extracted from the high-pressure (HP) engine spool. Its relatively high operating speed makes it an ideal source for mechanical power to drive electrical generators that are attached to the engine. However, it is desirable at times to increase the amount of power that is available on this spool, by transferring torque and power to it via some other means.

Another source of power within the engine is the low-pressure (LP) spool, which typically operates at speeds much slower than the HP spool, and over a relatively wider speed range. Tapping this low-speed mechanical power source without transformation result in impractically large generators. Many solutions to this transformation have been proposed, including various types of conventional transmissions, mechanical gearing, and electromechanical configurations. One solution is a turbine engine that utilizes a third, intermediate-pressure (IP) spool to drive a generator independently. However, this third spool is also required at times to couple to the HP spool. The means used to couple the IP and HP spools are mechanical clutch or viscous-type coupling mechanisms.

U.S. Pat. No. 6,895,741, issued May 24, 2005, and entitled “Differential Geared Turbine Engine with Torque Modulation Capacity”, discloses a mechanically geared engine having three shafts. The fan, compressor, and turbine shafts are mechanically coupled by applying additional epicyclic gear arrangements. The effective gear ratio is variable through the use of electromagnetic machines and power conversion equipment.

Unlike the conventional electrical machine having a rotor or rotating portion, and a stator or stationary portion, the present invention includes two rotating portions. Further, in the conventional electrical machine, power is converted either from electrical to mechanical or from mechanical to electrical. By contrast, the present invention is used to transfer mechanical power from one rotating shaft to another without any electrical power output or input. This is also a major distinction between the present invention and previous variable transmissions.

SUMMARY OF THE INVENTION

The present invention is directed to an electromagnetically variable transmission for transferring power between a pair of independently rotating shafts. The electromagnetically variable transmission includes a hollow cylindrical outer rotor portion and a hollow cylindrical inner rotor portion, the inner rotor portion being disposed within a center aperture of the outer rotor portion and independently rotatable within the outer rotor portion. The outer rotor portion is independently rotatable circumferentially about the inner rotor portion. A first one of the outer rotor portion and the inner rotor portion has a plurality of permanent magnets pairs spaced about a first surface. The magnets are configured in pairs and facing an air gap. The air gap is disposed between the outer rotor portion and the inner rotor portion. The other one of the outer and inner rotor portions has a plurality of slots spaced about a magnetically permeable core portion. Some of the slots have windings embedded therein. The outer rotor portion and the inner rotor portion are simultaneously rotatable in one direction. In response to co-rotation of the outer rotor portion and the inner rotor portion, a magnetic flux path is generated between the plurality of permanent magnet pairs, the air gap and the inner rotor portion core. The magnetic flux path induces electrical power in the windings and causes mechanical power to be transferred between the inner rotor portion and the outer rotor portion.

In another aspect, the present invention is directed to a gas turbine engine. The gas turbine engine includes at least one compressor, a combustor, a high pressure turbine and a low pressure turbines arranged in serial flow communication and disposed about a longitudinal shaft of the engine within an annular outer casing. The compressor is driven by the high pressure and low pressure turbines and compressor air during operation. An electrical generator is disposed within the annular outer casing; and an electromagnetically variable transmission is provided for transferring power between a pair of independently rotating shafts, one of the independent rotating shafts being attached to the HP turbine, and the other independently rotating shaft being attached to the LP turbine. The electromagnetically variable transmission includes a hollow cylindrical outer rotor portion and a hollow cylindrical inner rotor portion, the inner rotor portion being disposed within a center aperture of the outer rotor portion and independently rotatable within the outer rotor portion. The outer rotor portion is independently rotatable circumferentially about the inner rotor portion. A first one of the outer rotor portion and the inner rotor portion has a plurality of permanent magnets pairs spaced about a first surface. The magnets are configured in pairs and facing an air gap. The air gap is disposed between the outer rotor portion and the inner rotor portion. The other one of the outer and inner rotor portions has a plurality of slots spaced about a magnetically permeable core portion. Some of the slots have windings embedded therein. The outer rotor portion and the inner rotor portion are simultaneously rotatable in one direction. In response to co-rotation of the outer rotor portion and the inner rotor portion, a magnetic flux path is generated between the plurality of permanent magnet pairs, the air gap, the outer rotor core and the inner rotor portion core. The magnetic flux path induces electrical power in the windings and causes mechanical power to be transferred between the inner rotor portion and the outer rotor portion.

An advantage of the present invention is torque transfer between concurrently rotating shafts is achieved through a rotating electromagnetic field without any mechanical connection between the two shafts. Induced field current in the winding is all that is required to generate electromagnetic fields in the air gap to interact with electromagnetic fields driven by the permanent magnets on the other rotor to transfer torque and power from the PM rotor, on which the permanent magnets are fixed to, to the induction rotor, the rotor with windings. Since no electric power flow occurs into or out of the EVT, there is no requirement for a power converter and associated control that are typically provided in conventional power transfer devices, e.g. electrical motors and generators.

Another advantage of the present invention is increased fuel efficiency, reliability and fault tolerance.

A further advantage of the present invention is the ability to transfer power from low speed LP turbine shaft to the high speed turbine shaft, with a variable speed ratio for transferring power over the entire speed range of the engine. Using electromagnetic techniques mechanical power is transferred without creating a mechanical linkage between the LP turbine shaft and the HP turbine shaft. Also there is no audible noise related to a mechanical gear due to its absence.

Yet another advantage of the present invention is that an external electrical power source is not required, and the control circuit for the internally-generated field currents is uncomplicated.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, an electromagnetically variable transmission (EVT)10includes two rotating components, an inner rotor12and an outer rotor14. Both the inner rotor12and the outer rotor14rotate in the same direction around a common axis16. The outer rotor14has multiple permanent magnet pole pairs18facing the outer surface34of the inner rotor12. The magnets of the pole-pairs18are oriented in alternating fashion, such that one magnet of the pair has its north pole directed radially outwards and the adjacent magnet has its south pole directed radially inwards. An optional pole cap24may be attached on the top of each magnet segment22to reduce losses induced in the magnets due to flux slot harmonics inside the magnets22when there is a large difference between the rotational velocity of the inner rotor12and the outer rotor14. The pole caps24may be laminated stack, soft magnetic composite material, or other magnetically permeable material suitable to form a magnetic path. Claps26are positioned between the magnets22to secure the magnets22and pole caps24to the solid rotor core28. The rotor core28is preferably made of solid steel or a laminated stack of steel plates. The rotor core28is similar in construction to a permanent magnet (PM) rotor in an inside-out PM electrical machine.

The outer rotor14and the inner rotor12are separated by an air gap30. The inner rotor12is constructed of steel laminations and windings similar to a conventional induction machine rotor. Slots32are located on the outer surface34of the inner rotor lamination36. The slots32may be open, half-closed, or closed. Multiple-phase windings38(See, e.g.,FIGS. 2-5) are disposed within the slots32. The multiple-phase windings38form multiple pole pairs of the inner rotor12. The number of pole pairs18on the outer rotor14is the same as the number of pole pairs of the inner rotor12.

Referring toFIGS. 2 through 5, several exemplary interconnections for the rotor windings38[either the inner rotor winding in the embodiment ofFIG. 1, or the outer rotor winding in the embodiment of FIG.1A.] are shown.FIG. 2shows the rotor windings38as three single-phase connections with switches40wired in series with each phase winding38. The rotor windings inFIG. 3are configured in a wye connection with switches40in two of the three legs of the wye connection, which is all that are required to switch off the current flowing in the wye circuit, although another switch40could be connected in the third phase. InFIG. 4, a delta-connected configuration is used for the rotor windings38, and a single switch40is used to switch off current flowing in the delta circuit arrangement. InFIG. 5, there are multiple parallel circuits shown, which are multiple parallel combinations of the delta circuit42and the wye-connections44ofFIGS. 3 and 4. Note that other interconnection configurations may also be employed within the scope of the present invention, as the configurations shown inFIGS. 2 through 5are intended as examples and not to limit the various configurations that will be readily understood by those persons skilled in the art. Although the circuits shown inFIGS. 2 through 5are preferably configured as 3-phase windings, any number of electrical phases can be used. The switches40are preferably a pair of silicon-controlled rectifiers (SCRs) connected in reverse parallel, or any other power devices having current control capability in both directions.

FIG. 6shows an exemplary control circuit for controlling the torque and power transferred between the inner rotor12and the outer rotor14. The circuit ofFIG. 6controls how much torque and power is transferred and when to transfer torque and power between the two rotors to satisfy the engine power requirement. An exemplary rotor coil46could represent a single coil38, as inFIG. 2, or the rotor coil46could represent multiple coils38. e.g. three series connected coils38, as inFIG. 4. Also, switch40inFIGS. 2 through 5could be either switch40inFIG. 6or the switch40inFIG. 6A. The V, L, and R are a net effect of the coil(s) in the circuits embodied inFIGS. 2 through 5. An exemplary rotor coil38is represented as an inductance46a(L) and resistance46b(R). A voltage V is induced in the winding38due to the variation of flux linked by the winding. The flux is driven by the magnets on the opposite rotor, while the flux variation is due to the relative speed of the two rotors. A pair of power devices40aand40bis arranged in reverse parallel. Preferably the power devices are silicon-controlled rectifiers (SCRs). Switch40can also consist of two insulated-gate bipolar transistor (IGBT) units41a,41binFIG. 6A. The IGBT units41a,41bare connected in reverse series. Each of the IGBT units41a,41bconsists of at least one IGBT and at least one diode that is in reverse parallel with the IGBT(s).

Referring toFIG. 7, power for the torque control circuit inFIG. 6may be provided through a power supply50that is energized by an auxiliary coil52on the rotor14driven by the HP spool54. The whole control system48may be located on the HP spool54, and powered by the HP spool54, or alternately, may be powered by the LP spool. The control circuit48controls the current in the windings38. Control circuit48must be located on the same induction rotor on which the windings38are located, to avoid wire connections between two rotating parts. For the same reason, the control circuit48must be powered by the same induction rotor. The induction rotor could be configured on either the inside or outside, as shown inFIGS. 1 and 1A. A control signal indicated by a bi-directional arrow56may be transmitted wirelessly to the control unit58by an external stationary control unit60.

Referring next toFIG. 8, there is another exemplary control circuit for torque control. In this embodiment a switch62controlled by centrifugal force of the rotors12and14acontrols the speed at which to transfer torque between LP and HP spools. Switch62must be on the same induction rotor as the windings38, and the induction rotor could be located on either the inside or the outside as shown inFIGS. 1and1A. Rotor coil46has a characteristic inductance L and resistance R, and an induced voltage V. Centrifugal switch62is closed when the rotor12or14ais rotating at low speed and opens when the rotor12or14aexceeds a predetermined speed, in response to the centrifugal force applied by the rotational speed. Optionally, if necessary, a resistor having a resistance RNTCwith negative temperature coefficient may be included. The resistor RNTClimits the current pulse that occurs when the centrifugal switch62closes. Transfer of torque occurs between the LP spool64and the HP spool54, when the centrifugal switch62is in the closed position, and the HP and LP spools64,54are disengaged when the centrifugal switch is open.

InFIG. 9, there is another exemplary control circuit option for torque control. In the configuration shown inFIG. 9, a control switch is not required. Torque transfer is controlled by the slip frequency, or the speed difference between two shafts. During aircraft cruise or taking off, the HP and LP spools rotate at higher speed and the speed difference between two spools or between the two rotors of the EVT is smaller. Therefore the slip frequency or the frequency of the current induced in the windings is lower. During aircraft landing or idle descending, the LP spool speed is reduced more than HP spool speed change and the slip frequency is larger. Based on induction machine principle, there will be more torque and power transferred between the two EVT rotors at a larger slip frequency than at a small slip frequency. The gear ratios of gearboxes66and68inFIG. 10are selected such that the desired slip frequency is achieved at both the high and low speed range. The characteristic inductance L and resistance R of the rotor coil46are designed, e.g. by adjusting turns and conductor cross-section area, using skin effect for high slip frequency, or by selecting material in such a way that the desired torque can be transferred from LP spool to HP spool when it is needed at or near idle descent, while the torque transferring is minimized when it is not required during cruise and taking off.

According to another embodiment of the present invention, there are one or more auxiliary coils52in the rotor slots shown inFIG. 1. The induced electrical power in the coil or coils52can be used to supply power for the control circuit of the switch or switches in the circuits that are located on the rotor.

The SCR switches40, control unit58and power supply50are preferably mounted on the same rotor as the main rotor windings38, so slip rings are not required to electrically connect a stationary portion to the rotating windings38. Signals required to control the SCR switches40can be transmitted wirelessly to the inner rotor12(SeeFIG. 7).

FIG. 1Ashows an alternate embodiment of the present invention. In this configuration, the rotors are arranged as the reverse of FIG.1—the outer rotor14ahas slots32awith windings is now outside, and the inner rotor12ahas permanent magnet PM poles22a.

In either of the configurations inFIG. 1orFIG. 1A, the magnetic flux is driven by the permanent magnets and link the rotor winding38across the air gap30, as with conventional PM machines. When the PM rotor14,12arotates, a rotating flux field (not shown) is induced in the air gap30. Based on conventional induction machine principles, voltage and current is induced in the coils when the PM rotor14or12ais rotating at a different speed than the induction rotor12or14a. When the PM rotor14or12ais rotating faster than the induction rotor12or14a, torque is transferred from the PM rotor14or12ato the induction rotor12or14a. When the PM rotor14or12ais rotating lower than the induction rotor12or14a, torque is transferred from the induction rotor12or14ato the PM rotor14or12a.

FIG. 10shows a preferred embodiment of the present invention corresponding to the EVT arrangement ofFIG. 1, in which an EVT10includes the outer PM-type rotor14connected to the LP spool64, and the inner induction-type rotor12connected to the HP spool54. The interconnected windings of the inner rotor12are indicated by loops37. The LP spool speed N4is stepped up by gearbox 1:Y to speed N3. In order to transfer torque from the LP spool64to the HP spool54, the rotational speed N3of the outer or PM rotor14has to be higher than the speed N2of the inner or induction rotor12. A first gearbox68having a gear ratio of 1:X is used to couple the HP spool54to the inner rotor12, and a second gearbox66having a gear ration of 1:Y is used to couple the LP spool64to the outer rotor14. The gearboxes66,68are used to match the speed N1of the HP spool54and the speed N4of the LP spool64to correspond to the desired EVT rotor speeds, N2and N3, respectively. Depending on the engine spool operating speeds and EVT rotor speeds, one of the gearboxes66,68may be omitted. Outer rotor speed N3is greater than the speed N2of the inner rotor12so that torque and power will be transferred to the shaft55at speed N2based on the induction principle. A variable gear ratio may be employed on one or both of the gearboxes66,68. In the configuration ofFIG. 1, with LP spool rotating at speed N4and the HP spool rotating at speed N1, the speed conversions are related by Equation 1:
N1*X=N2<N3=N4/YEquation 1where N4<N1

FIG. 11shows an alternate embodiment of the present invention corresponding to the EVT arrangement ofFIG. 1A, in which an EVT10is connected to the LP spool64and the HP spool54of an aircraft engine through gearboxes66,68. In order to transfer torque from the LP spool64to the HP spool54, the rotational speed N3of the outer induction-type rotor14a(with winding interconnections indicated by loops37) has to be lower than the speed N2of the inner PM-type rotor12a. A first gearbox68having a gear ratio of 1:X is used to couple the LP spool64to the inner rotor12a, and a second gearbox66having a gear ratio of 1:Y is used to couple the HP spool54to the outer rotor14a. The gearbox66is used to match the outer rotor speed N3of the EVT10to speed N4of the HP spool54. Depending on the engine spool operating speeds and EVT rotor speeds, one of the gearboxes66,68may be omitted. In the configuration ofFIG. 11, with HP spool rotating at N4and the LP spool rotating at N1, the speed conversions are related by Equation 2:
N1*X=N2>N3=N4/YEquation 2wherein N4>N1; andthe gear ratio X>=1
Optionally, a variable gear ratio may be applied.

As an option, either of the gearboxes66,68described above could have variable gear ratio to reduce the speed range of two rotors in the EVT, therefore reducing the maximum speed and associated mechanical stresses when either of the LP spool or HP spool speed range is too great.

Referring next toFIG. 12, an exemplary arrangement of the PM induction EVT in an aircraft engine110has a core engine138including in serial, axial flow relationship, a low pressure compressor or booster compressor120, a high pressure compressor114, a combustor or burner124, a high pressure turbine116and a low pressure turbine118. Core engine138is downstream from an inlet122and a fan112. Fan112is in serial, axial flow relationship with core engine138and a bypass duct and a bypass nozzle (not shown). Fan112, compressor114, and low pressure turbine118are coupled by a first shaft64, and compressor114and turbine116are coupled with a second shaft54. A portion of airflow entering inlet122is channeled through the bypass duct and exhausted through bypass nozzle, and remaining airflow passes through core engine138and is exhausted through a core engine nozzle. The EVT10is coupled to LP shaft or spool64through gearbox66and shaft57. The output shaft57of the EVT10is connected to the HP shaft or spool54through gearbox68. A pair of starter/generators130is coupled to HP spool54through a primary gearbox, to receive power from or to provide power to the HP spool, depending whether the pair of starter/generators130is operated as starters or generators.FIG. 12is just one example out of many possible starter/generator130and primary gearbox132configurations that may be used with the present invention for sharing torque between the LP shaft64and the HP shaft54, as will be readily appreciated by those skilled in the art. Also, the EVT10may be located either internally or externally of the engine envelope.