GB2402450A

GB2402450A - Apparatus for transforming a variable rotational speed input from a gas turbine engine into a substantially constant rotational speed output for an alternator

Info

Publication number: GB2402450A
Application number: GB0313117A
Authority: GB
Inventors: Carl Charles Bourne
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2003-06-07
Filing date: 2003-06-07
Publication date: 2004-12-08
Also published as: GB0313117D0

Abstract

The apparatus comprises an epicyclic gear mechanism (121) having a rotatable sun gear (122); a rotatable annulus gear (128); and a rotatable planet carrier (126), carrying a plurality of planet gears (124) located between the sun gear (122) and the annulus gear (128) . The sun gear (122) is used for input and the planet carrier (126) is used for output. A drive mechanism (134) varies the speed of rotation of the annulus gear (128) and varies the input: output gear ratio of the epicyclic gear mechanism (121).

Description

AN APPARATUS FOR TRANSFORMING A VARIABLE INPUT ROTATIONAL

SPEED INTO A SUBSTANTIALLY CONSTANT OUTPUT ROTATIONAL SPEED

Embodiments of the invention relate to an apparatus and method for transforming a variable rotational speed input into a substantially constant rotational speed output. It has particular application in electric power generation, but could also be used in mechanical drive applications.

Gas turbine engines may be used to generate electricity.

Fig. 1 schematically illustrates one type of electricity generation system 2 comprising a gas turbine engine 10, a reduction gear box 4, an alternator 6 and an electrical output 8. The gas turbine engine comprises a compressor stage 12 connected to a turbine stage 15 by a shaft 16. The compressor stage compresses air received via an inlet 14 and provides the compressed air to a combustor 18. The air is mixed with fuel and combusted in the combustor 18. The combustion products expand and drive the turbine stage 15, which through shaft 16 drives the compressor stage 12. The shaft 16 is connected through the reduction gearbox 4 to the alternator 6 which provides the electrical output 8. The gas turbine engine and alternator are driven at a constant speed to deliver an AC electrical output at the correct constant frequency.

A problem with this arrangement is that the gas turbine engine must be run at a fixed speed regardless of the output load on the electrical output. This constraint can have an adverse effect on the engine performance, particularly under part load conditions. It would therefore be desirable to modify the power output from the gas turbine engine while maintaining the output shaft speed substantially constant.

The power output of a gas turbine engine can be varied by changing the gas mass flow through the engine or by changing the combustion temperature. However, changing the temperature reduces the engine's efficiency. The mass flow through a gas turbine engine can be increased by increasing the engine's compressor speed. However, in a single shaft engine this increases the speed of the shaft driving the alternator and changes the frequency of the electrical output which makes the engine unsuitable for constant speed applications such as electricity generation.

The inventor has devised an apparatus suitable for maintaining the output shaft speed substantially constant when the compressor speed of the gas turbine engine is varied.

According to one aspect of the present invention there is provided an apparatus for transforming a variable rotational speed input into a substantially constant rotational speed output, comprising: an epicyclic gear mechanism having a control component, an input component and an output component selected from the components: a rotatable sun gear; a rotatable annulus gear; and a rotatable planet carrier, carrying a plurality of planet gears located between the sun gear and the annulus gear, and a drive mechanism for varying the speed of rotation of the control component.

According to another aspect of the present invention there is provided a method of controlling an epicyclic gear mechanism to maintain a substantially constant output rotational speed while the input rotational speed varies, wherein the epicyclic gear mechanism includes a sun gear, an annulus gear and a planet carrier, carrying a plurality of planet gears located between the sun gear and the annulus gear, and wherein the output rotational speed is the speed of rotation of the planet carrier and the input rotational speed is the speed of rotation of the sun gear, comprising the step of: varying the speed of rotation of the annulus gear as the speed of rotation of the sun gear varies to maintain the speed of rotation of the planet carrier substantially constant.

For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings in which: Fig. 1 schematically illustrates one known type of electricity generation system; Fig. 2 schematically illustrates an electricity generation system according to one embodiment of the invention; Fig. 3 schematically illustrates a first type of gearing apparatus including an epicyclic gear train; and Fig. 4 schematically illustrates a second type of gearing apparatus including an epicyclic gear train.

The Figures illustrate an apparatus 120 for transforming a variable rotational speed input into a substantially constant rotational speed output. The apparatus comprises: an epicyclic gear mechanism 121 having a control component, an input component and an output component, selected from the components: a rotatable sun gear 122; a rotatable annulus gear 128; and a rotatable planet carrier 126, carrying a plurality of planet gears 124 located between the sun gear 122 and the annulus gear 128, and a drive mechanism 134 for varying the speed of rotation of the control component.

Fig. 2 schematically illustrates a 'constant speed' system 102 according to one embodiment of the invention. In this example it is an electricity generation system. The electricity generation system 102 comprises a gas turbine engine 110, a gearing apparatus 120, an alternator 106 and an electrical output 108. The gas turbine engine 110 comprises a compressor stage 112 connected to a turbine stage 115 by a shaft 116. The compressor stage 112 compresses air received via an inlet 114 and provides the compressed air to a combustor 118. The air is mixed with fuel and combusted in the combustor 118. The combustion products expand and drive the turbine stage 115, which through shaft 116 drives the compressor stage 112. The shaft 116 is connected to the gearing apparatus 120. The gearing apparatus 120 is connected to the alternator 106 via a constant low speed shaft 117. The alternator 106 provides an AC electrical output 108 at a constant frequency. However, the gas turbine engine 102 does not need to be operated at a constant speed.

The gearing apparatus 120 operates as a continuously variable transmission and also as a reduction gearbox. It is schematically illustrated in more detail in Fig. 3. The gearing apparatus 120 includes an epicyclic gear train 121 (also known as a co-axial planetary gear train). The epicyclic gear train 121 has a central axis 123. A circular sun gear 122 of radius Rs and with Ns teeth on its exterior circumference is mounted for independent rotation about the central axis 123. An annulus ring gear 128 of internal radius Ra (> Rs) and with Na teeth on its interior surface is mounted for independent rotation about the central axis 123.

A plurality of circular planetary gears 1241, 1242, 1243 are interposed between the sun gear 122 and the annulus ring gear 128. The planetary gears 124 are separately mounted for independent rotation on an annular planet carrier 126 of radius Rc. The annular planet carrier 126 is itself mounted for independent rotation about the central axis 123. Each circular planetary gear 124 has a radius Rp and Np teeth on its exterior circumference. For each planetary gear 124, the teeth on the side of the planetary gear 124 nearest the central axis 123 mesh with the teeth on the circumference of the sun gear 122 and the teeth on the side of the planetary gear 124 furthest from the central axis 123 mesh with the teeth on the interior surface of the annulus ring gear 128.

Thus Rc =Rs + Rp and Ra= Rc + Rp.

The epicyclic gear train 121 is arranged so that the sun gear 122 is driven by the shaft 116 of the gas turbine engine to 110 and so that the planet carrier 126 drives the alternator 106 via shaft 117. Thus input is provided to the gearing train 121 via the sun gear 122 and output is provided by the gearing train 121 via the planet carrier 126.

For example, in one type of gear train 121, the relative IS sizes of the components of the epicyclic gear train are: sun gear radius Rs Rs planet gear radius Rp 0.75Rs planet carrier radius Rc 1.75Rs Annulus radius Ra 2. 50Rs This gives a gear input: output ratio (sun: carrier) of 17:4. Thus if the sun gear 122 is rotated at a speed of 15000 rpm the speed of rotation of the planet carrier 126 is approximately 3600 rpm.

The gearing apparatus 120 additionally includes a speed detector 130, a controller 132 and a driving mechanism 134.

The speed detector 130 detects the speed of the output from the gear train 121 i.e. the speed of rotation of the planet carrier 126. The controller 132 receives the detected speed as an input and provides a controlling output to the driving mechanism 134. The driving mechanism 134 includes a motor 136 that drives a drive gear 138 which meshes with the annulus ring gear 128. The motor 136 can be used to rotate the annulus ring gear 128 and to vary the speed at which it is rotated. The rotation of the annulus ring gear 128 varies the input: output gear ratio of the epicyclic gear train 121.

S If the drive gear 138 is rotated in the opposite direction to the sun gear 122, the annulus ring gear 128 rotates in the same direction as the sun gear 122 and the input: output ratio of the epicyclic gear train 121 is increased. The greater the rotational speed of the drive gear 138, the greater the increase in the gear ratio. If the drive gear 138 is rotated in the same direction as the sun gear 122, the annulus ring gear 128 rotates in the opposite direction to the sun gear 122 and the input: output gear ratio of the epicyclic gear train 121 is decreased. The greater the rotational speed of the drive gear 138, the greater the decrease in the gear ratio.

The controller 132 is arranged to provide a negative feedback loop that maintains the output speed of the planet carrier 126 constant. 'Constant' in this context typically means within +/- 1% of a predetermined value. As the output speed of the planet carrier 126 increases, the controller 132 decreases the input: output ratio of the epicyclic gear train 121 by controlling the motor 136 to rotate the drive gear 138 in the same direction as the sun gear 122. As the output speed of the planet carrier 126 decreases, the controller 132 increases the input: output ratio of the epicyclic gear train 121 by controlling the motor 136 to rotate the drive gear 138 in the opposite direction to the sun gear 122.

Thus as the input speed of the sun gear increases, when the gas turbine engine is operated at higher power, the input: output gear ratio is controlled (decreased) to maintain the output speed of the planet carrier, which drives the alternator, constant. As the input speed of the sun gear decreases, when the gas turbine engine is operated at lower power, the input: output gear ratio is controlled (increased) to maintain the output speed of the planet carrier, which drives the alternator, constant. The epicyclic gear train is designed so that at the design speed of the gas turbine engine the drive gear 138 and the annulus gear ring 128 are stationary.

Fig. 4 illustrates an alternative gearing apparatus 120.

IO Where similar reference numerals are used to Fig. 3, they designate similar components. The gearing apparatus of Fig. 4 differs from the apparatus of Fig. 3 in that it uses a feed forward control system instead of a feedback control system.

The speed detector 130 detects the speed of the input to the epicyclic gear train 121 i.e. the speed of rotation of the sun gear 122.

The controller 132 is arranged to provide a positive feed forward path that maintains the output speed of the planet carrier 126 constant. 'Constant' in this context typically means within +/- 1% of a predetermined value. As the input speed of the sun gear 122 increases the controller decreases the input: output ratio of the epicyclic gear train 121 by controlling the motor 136 to rotate the drive gear 138 in the same direction as the sun gear 122. As the input speed of the sun gear 122 decreases the controller increases the input: output ratio of the gear train 121 by controlling the motor 136 to rotate drive gear 138 in the opposite direction to the sun gear 122. This causes the annulus ring gear 128 to rotate in the same direction as the sun gear 122.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

Claims 1. An apparatus for transforming a variable rotational speed input

into a substantially constant rotational speed output, comprising an epicyclic gear mechanism having a control component, an input component and an output component selected from the components: a rotatable sun gear; a rotatable annulus gear; and a rotatable planet carrier, carrying a plurality of planet gears located between the sun gear and the annulus gear and a drive mechanism for varying the speed of rotation of the control component.
2. An apparatus as claimed in claim 1, wherein the drive mechanism is arranged to maintain the speed of rotation of the output component substantial' v constant.

IS
3. An apparatus as claimed in claim 1 or 2, wherein the control component is the annulus gear, the input component is the sun gear, and the output component is the planet carrier.
4. An apparatus as claimed in claim 1, 2 or 3 further comprising a speed detector and a controller, responsive to the speed detector, for contra Sing the drive mechanism to vary the speed of rotation of the control component.
5. An apparatus as claimed in claim 4, wherein the speed detector detects the speed of the input component.
6. An apparatus as claimed it claim 4, wherein the speed detector detects the speed of the output component.
7. An apparatus as claimed in any preceding claim, wherein the drive mechanism varies the speed of rotation of the control component to maintain the speed of rotation of the output component substantially constant.
8. A reduction gearbox compris ng the apparatus of any one of claims 1 to 7.
9. An electricity generation system comprising the apparatus of any one of claims 1 to 7.
10. A method of controlling an epicyclic gear mechanism to maintain a substantially constant output rotational speed while the input rotational speed varies, wherein the epicyclic gear mechanism includes a sun gear, an annulus gear and a planet carrier, carrying a plurality of planet gears located between the sun gear and the annulus gear, and wherein the output rotational speed is the speed of rotation of the planet carrier and the input rotational speed is the speed of rotation of the sun gear, comprising the step of: varying the speed of rotation of the annulus gear as the speed of rotation of the sun gear varies to maintain the speed of rotation of the planet carrier substantially constant.
11. An apparatus or method substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.
12. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as the preceding claims.