GB2428138A - Controllable shaft coupling including a rotary transformer - Google Patents

Abstract

Shafts 2, 3; 102, 103, are coupled by the interaction of permanent magnets 5, 105 and windings 4, 104a, controlled switching of electrical resistance into the winding being provided relative rotation between the shafts such that induced current flow in the winding proportionate to the difference in rotational speed between the shafts creates electromagnetic torque tending to bring the shafts towards the same rotational speed. The switching may be provided directly in the relevant rotary part or alternatively in a stationary part 107, coupled via a rotary transformed 104b, 1006 to the winding 104a.The system may be applied to concentric shafts in a gas turbine engine and more than two shafts may be so coupled.

Description

Shaft Coupling The present invention relates to shaft couplings and more

particularly to linking the rotational speed of two concentric shafts by an appropriate shaft coupling.

There are a number of situations where it is desirable to equalise the rotational speed of two concentric rotating shafts. One example is with regard to a gas turbine engine in which there are concentric shafts upon which the compressor and turbine stages are secured. Furthermore, it may be desirable to enable selective equalisation of shaft speed at certain times, whilst at other times divergence between rotational speeds of the shafts may be allowable or desirable. Again, with regard to gas turbine engines, it may be desirable to substantially equalise rotational speeds between concentric shafts at start up or otherwise.

Previously, equalisation of concentric shaft speeds has been achieved through mechanical couplings such as clutches and gear couplings. Inherently, such approaches add drag to the arrangement which may reduce overall machine efficiency and also mean previous couplings between the shafts for rotational speed equalisation tend to wear over time.

In accordance with the present invention there is provided a shaft coupling arrangement for a gas turbine engine, the arrangement comprising an inner shaft and an outer shaft arranged to rotate upon a common axis, one of the inner shaft or the outer shaft has a permanent magnet assembly and the other of the inner shaft or the outer shaft having an electrical winding assembly comprising a plurality of coupling windings (typically three in number), each coupling winding coupled to a controller for specifically varying the electrical resistance of the coupling winding whereby, when there is relative rotation between the inner shaft and the outer shaft, the permanent magnet assembly induces electrical current in the coupling windings dependent upon the voltage generated and the winding resistance (dependent upon the specifically variable electrical resistance switched into the coupling winding by the controller) and the reactance of the winding (dependent upon the relative speed difference between the inner shaft and the outer shaft) such that by electromagnetic torque effort the inner shaft and the outer shaft are drawn towards a similar speed of rotation.

Typically, the permanent magnet assembly is secured upon the inner surface of outer shaft and the electrical winding assembly is secured upon the inner shaft, as in Fig. 1.

Generally, each winding is associated with a variable electrical resistor with a switch for selective electrical connection with the winding to specifically vary electrical resistance of the coupling winding. Normally, the variable electrical resistor comprises a plurality of electrically connectable resistor elements. Generally, each electrically connectable resistor element or groups of such electrical resistor elements are addressable by the controller to vary the electrical resistance of a winding.

Alternatively, the variable electrical resistor is a potentiometer controlled by the controller to vary electrical resistance in a winding.

Possibly, each shaft coupling winding is connected to a transformer comprising a primary winding and a secondary winding with the primary winding connected to the shaft coupling winding and the secondary winding secured upon a stationary frame, the electrical resistance of the secondary winding controlled by the controller to vary the impedance of the combination of the primary winding and the shaft coupling winding.

Also, in accordance with the present invention there is provided a gas turbine engine incorporating shafts rotating concentrically, and having a shaft coupling arrangement as described above.

Typically, an engine as described above is arranged where the controller allows selective coupling between the shafts dependent upon engine operational status.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which; Fig. 1 is a schematic plan cross section illustrating a shaft coupling arrangement; and, Fig. 2 is a schematic half longitudinal cross section of a shaft coupling arrangement, including the transformer and variable resistance in the stationary frame.

As indicated above there are a number of situations where it is desirable to initially bring and then maintain two concentrically rotating shafts at substantially the same rotational speed. The present invention utilises an electrically wound rotor induction approach in order to bring the two concentric rotating shafts into conformity at substantially the same rotational speed.

Permanent magnets (PM) on one rotating member (called the outer rotor or stator) provide the magnetic field excitation within the machine. A conventional induction machine would use electrical current in a stator winding rather than PM to excite the machine. A multi-way switch in the electrical winding of the other rotating member (called the inner rotor) switches resistors of different values, including zero (i.e. short circuit), into the circuit. The magnitude of the resistor in circuit at each instant helps, as described below, to determine the induction machine's electromagnetic torque at that time. A useful instance is when the rotor circuit is open circuit and the electromagnetic torque is essentially zero so that the device is off. Note that this embodiment requires a switch control and activation mechanism to be positioned on the rotor in the rotating frame.

When there is relative rotation between the two shafts the magnetic field of the permanent magnet induces a voltage in the conductors of the inner rotor. The magnitude of that voltage is proportional to the rotational speed difference. The magnitude of the electrical current that flows in the inner rotor conductors depends upon the voltage and upon the impedance of the inner rotor circuit.

This impedance depends upon (i) the total resistance of the inner rotor winding plus the resistance switched into circuit and (ii) the reactance of the inner rotor winding which in turn is proportional to the speed difference. The net effect is that the magnetic fields of the permanent magnet and the inner rotor current interact to create a torque on the rotors which tends to bring them towards the same speed. To a good first approximation the electromagnetic torque is proportional to the speed difference (other things being equal) when the difference in shaft speeds is relatively small.

Fig. 1 illustrates as a plan schematic cross section of a front end of two rotors. Thus, an outer rotor 2 is arranged to rotate concentrically with an inner rotor 3 in the shaft coupling arrangement 1. Both the inner 3 and outer 2 rotors are secured to shafts typically concentrically rotating within a gas turbine engine as part of the compressor or turbine stages. The rotors 2, 3 are formed from a soft magnetic material. The inner rotor 3 incorporates electrical windings 4 as indicated above connected to switching mechanisms to vary the electrical resistance in each winding 4. The outer rotor 2 incorporates in the schematic depiction, two pairs of two permanent magnetic pole elements 5a, 5b. It is by interaction between these poles 5 and the winding 4 that the electromagnetic torque effect described above brings shafts secured respectively to rotors 2, 3 into substantial conformity with respect to rotational speed.

As indicated above, the windings 4 are connected to a multi switch arrangement which in turn is controlled by a controller (not shown) which allows the windings to assume different electrical resistance values including zero.

This controller has means for activating the resistive switching values in the windings 4. Relative rotation between the rotors 2, 3 and therefore the shafts associated with these rotors 2, 3 induces voltage and hence electrical current and power loss if the windings 4 are short circuited.

It will be understood that there will generally be some speed difference called slip between the rotors 2, 3 but this can be controlled by choice of appropriate switching of the electrical resistance within the windings 4. In any event, high accuracy with respect to coincidence of rotational speed between the rotors 2, 3 and therefore the associated shafts is not important but nearer coincidence within an acceptable slip range is desirable.

It will be understood that the number of poles 5, windings 4 as well as the spacing between the rotors 2, 3 particularly the shape of the poles 5 and the windings 4 will all be chosen for operational reasons. As indicated above, there are particular problems with respect to mounting appropriate switch mechanisms and a controller within the inner rotor 3 as it is a rotating member. Thus, there are packaging as well as considerations with respect to centrifugal forces, etc. In order to avoid the switch and resistors being in the rotating frame of an inner rotor as described above, a further embodiment is for each inner rotor winding to be connected to a primary winding of a rotating transformer rather than to the switch and resistors. The primary winding of the rotating transformer is thus on the inner rotor in the rotating frame. The secondary of the rotating transformer is in the stationary frame of the coupling arrangement and can then be connected to the appropriate switch and resistors for the windings. Electromagnetic torque in the induction machine is controlled by the value of the resistance in circuit at each instant as described above. The switch control and activation mechanism are therefore now positioned in the stationary frame. Such stationary form is usually preferred on the grounds of simplicity without the need for slip ring electrical couplings, etc. In summary, by the use of a rotating transformer the resistors can be moved into a stationary frame with a consequent gain in simplicity and reliability of operation.

Fig. 2 illustrates this alternative rotating transformer embodiment of a coupling arrangement as a longitudinal half cross section. Thus, rotating shafts 102, 103 respectively incorporate a number of permanent magnetic poles 105 and windings 104. The shafts 102, 103 are concentric and rotate about an axis X-X. The winding 104 in the area 104a acts as part of the permanent magnet induction coupling as described previously with regard to Fig. 1 in order to create electromagnetic torque which brings the shafts 102, 103 into approximately consistent rotational speed with each other.

In the alternative embodiment depicted in Fig. 2, transformer windings 104b and 106 act respectively as the primary winding 104b and secondary winding 106 of a rotating transformer coupling formed between that winding 104 and the winding 106 in a stationary frame 107. A switch arrangement 108 is utilised in order to allow variation in the secondary winding 106 in terms of electrical resistance 109. Thus, in effect the transformer coupling between the windings 104b and winding 106, varies the electrical resistance and reactance in the winding 104, and in particular in the winding part 104a in the permanent magnet coupling described previously with regard to Fig. 1.

In such circumstances, there is a contact-less inductive coupling association between the windings lO4b, 106 in order to vary the reactance in the winding 104. This avoids necessary complications created by having the resistors 109 and switching mechanism 108 along with any associated controller in a rotating part by accommodating them in the stationary frame 107. The shaft coupling arrangement in terms of permanent magnet induction between the magnet poles 105 and winding portion 104 is still achieved and results in the drawing of the concentrically rotating shafts 102, 103 into rotational speed consistency.

Additionally the switch 108 may switch between a plurality of resistors 109, 109B which have different value of resistance.

A further modification to the embodiment depicted in Fig. 2 would be to replace the switch 108 and resistors 109 with an electronic switch circuit operated in the form of a potentiometer arrangement.

As indicated above, the present invention has particular application with regard to gas turbine engines where concentric shafts are utilised in the compressor and turbine stages. However, the shaft coupling arrangement may be utilised in a range of situations where it is desirable to bring any concentric shafts into approximately the same concentrically rotating speed with each other.

The present coupling arrangement has been described above with regard to two concentrically rotating shafts, one having permanent magnet poles and the other windings which are arranged through an appropriate switching mechanism to have varying electrical impedance in order to create the electromagnetic torque drawing the shafts into substantially the same rotational speed. However, where possible, more than two concentric rotating shafts may be brought into substantially the same concentric speed where required. In such circumstances, as described previously, it will be understood that relative differences in rotational speed will again be utilised such that the differences in shaft rotational speeds induce a voltage in the electrical conductors of the windings, and as indicated above, the induced electrical current is then controlled by varying the electrical resistance switched into the winding circuit such that the net effect is that the magnetic fields of the permanent magnet and the rotor incorporating the windings interact to create a torque on the rotors which tends to bring them into approximately the same rotational speed. In such circumstances, provided shaft coupling arrangements are provided between adjacent or appropriate pairs of rotating concentric shafts, it will be understood that these shafts can be brought into conformity at substantially the same rotational speed.

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 (11)

1. A shaft coupling arrangement for a gas turbine engine, the arrangement comprising an inner shaft and an outer shaft arranged to rotate upon a common axis, one of the inner shaft or the outer shaft has a permanent magnet assembly and the other of the inner shaft or the outer shaft having an electrical winding assembly comprising a plurality of coupling windings, each coupling winding coupled to a controller for specifically varying the electrical resistance of the coupling winding whereby when there is relative rotation between the inner shaft and the outer shaft the permanent magnet assembly induces electrical current in the coupling windings dependent upon the voltage generated and the winding impedance dependent upon the specifically variable electrical resistance switched into the coupling winding by the controller and the reactance of the winding dependent upon the relative speed difference between the inner shaft and the outer shaft such that by torque effort the inner shaft and the outer shaft are drawn towards a similar speed of rotation.

2. An arrangement as claimed in claim 1 wherein the permanent magnet assembly is secured upon the outer shaft and the electrical winding assembly is secured upon the inner shaft.

3. An arrangement as claimed in claim 1 or claim 2 wherein each winding is associated with a variable electrical resistor with a switch for selective electrical connection with the winding to specifically vary electrical resistance of the coupling winding.

4. An arrangement as claimed in claim 3 wherein the variable electrical resistor comprises a plurality of electrically connectable resistor elements.

5. An arrangement as claimed in claim 4 wherein each electrically connectable resistor element or groups of such electrical resistor elements are addressable by the controller to vary the electrical resistance of a winding.

6. An arrangement as claimed in claim 3 wherein the variable electrical resistor is a potentiometer controlled by the controller to vary electrical resistance in a winding.

7. An arrangement as claimed in any preceding claim wherein each winding is coupled to a transformer coupling comprising a primary winding and a secondary winding with the primary winding coupled to the shaft coupling winding and the secondary winding secured upon a stationary frame, the electrical resistance of the secondary winding controlled by the controller to vary the impedance of the combination of the primary winding and the coupling winding.

8. A shaft coupling arrangement for a gas turbine engine substantially as hereinbefore described with reference to the accompanying drawings.

9. A gas turbine engine incorporating a shaft coupling arrangement as claimed in any preceding claim wherein the shafts are rotating concentrically and have the shaft coupling arrangement secured respectively across the shafts.

10. An engine as claimed in claim 9 wherein the controller allows selective coupling between the shafts of the engine dependent upon operational status.

11. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.