GB2223627A - Electromagnetic coupling supplied from rotary transformer via a rotating rectifier - Google Patents

Abstract

A torque coupling has a first rotatable shaft (48), a rotary transformer comprising a stator (70) surrounding the first shaft and a rotor (66) mounted for rotation with the first shaft; rectifying means (74) arranged to rectify an output of the rotor coil (68) and to provide direct current to an excitation coil (60) mounted for rotation with the first shaft (48); and a second rotatable shaft having a torque member (42) forming part of a magnetic flux circuit generated by the excitation coil (60), when energised. <IMAGE>

Description

ELECTRIC MACHINES The present invention relates to electric machines and particularly although not exclusively to a heteropolar eddy current drive.

A typical eddy current coupling is shown schematically in figure 1. The input side of the coupling comprises a generally cylindrical torque tube or loss drum 10 which is driven at a constant speed by an induction motor 12 via an input shaft 14. The output side comprises a polewheel 18, positioned within the torque tube 10, and coupled to an output shaft 16. A magnetic field 20 is produced on the polewheel by means of an excitation coil or coils (not shown). Coupling between the input and the output occurs by virtue of the interaction of the eddy currents excited within the torque tube 10 by the field 20, and the field itself.

The speed of the output shaft 16 is varied by controlling the field 20 (for example by means of a switching circuit) so as to vary the level of interaction and thereby the amount of "slip" between the rotational speed of the torque tube 10 and that of the polewheel 18.

The type of coupling just described may be split into two basic types: homopolar and heteropolar. In a homopolar coupling the field 20 crosses the air gap in only one direction and accordingly a suitable magnetic circuit (not shown) must be provided to complete the flux path from the polewheel 18, through the torque tube 10, and back through the output shaft 16. With a heteropolar coupling, on the other hand, this additional return path is not required; instead, the polewheel 18 is split into two pole portions (as shown by means of a dotted line in figure 1), and the flux path is from one of these halves, through the air gap to the torque tube, and back through the air gap to the other half. The pole portions can be either two serarate pole structures or one solid pole structure on which indidvidual coils are wound to define the pole portions.

Heteropolar couplings are generally more efficient than homopolar couplings for two basic reasons. Firstly, the magnetic flux path is considerably shorter, so that losses are substantially lower. Secondly, the amplitude of the eddy currents induced in the torque tube 10 can be twice as large since both positive and negative amplitudes of magnetic flux are generated compared with amplitudes of only a single sign with the homopolar arrangement. Thus, for a given size of magnetic field 20 the coupling between the input and output is roughly twice as large.

The greater efficiency of heteropolar couplings has meant that they have become more popular than homopolar couplings. However, heteropolar couplings have one severe disadvantage in that, in order to create a magnetic circuit between the two halves of the polewheel 18 and the torque tube 10, one has to provide a relatively rotating DC excitation arrangement for the two halves of the polewheel. This can be done either by providing a rotating DC excitation coil or by keeping the coil stationary and rotating the pole system. In the first case providing the DC power required to energize this coil has previously required the use of slip rings or brushes applied to the rotating output shaft 16. Such mutually - sliding parts are extremely undesirable in units of this type since they tend to require frequent maintenance and replacement.In the latter case parasitic air gaps are created which have to be excited. This is a source of significant losses. For these reasons, the present applicants have until now preferred the homopolar type of coupling, accepting the lower efficiency as a necessary disadvantage in return for the much greater advantage of not requiring brushes or slip rings. With the homopolar type of coupling, the DC energizing coil can of course be stationary since it can be placed anywhere within the magnetic circuit which includes the stationary frame.

It is one object of the present invention to provide a heteropolar coupling which does not require brushes or slip rings.

It is a more general object of the invention to provide an eddy current coupling, of the type that does not require brushes or slip rings, which is of improved efficiency. In particular, it is an object to provide more output power for the same frame size or, alternatively, a smaller frame size for the same output power.

It is yet another general object of the present invention to provide frictionless means for energizing a DC coil on a rotating shaft.

According to a first aspect of the present invention there is provided an electric machine having a rotatable shaft, a rotary transformer including a stator and a stator coil surrounding the shaft and a rotor and a rotor coil mounted for rotation with the shaft, rectifying means arranged to rectify an output of the rotor coil and to provide direct currant (DC) to an excitation coil mounted for rotation with the shaft.

According to a second aspect of the invention there is provided a torque coupling having a first rotatable shaft, a rotary transformer including a stator and stator coil surrounding the first shaft and a rotor and a rotor coil mounted for rotation with the first shaft, rectifying means arranged to rectify an output of the rotor coil and to provide direct currant to an excitation coil mounted for rotation with the first shaft, and a second rotatable shaft having a torque member forming part of the magnetic flux circuit generated by the excitation coil, when energized. The torque coupling may be of the eddy current type, either an eddy current drive or an eddy current brake.

In one embodiment, the first shaft is an output shaft, and the second shaft an input shaft. The input shaft, and thus the torque member, can be arranged to be rotated at a constant speed by any conventional means, such as an induction motor. A variable output shaft torque and speed can then be provided by controlling the current flowing within the excitation coil. This is most conveniently achieved by providing switching means or chopper means and applying a chopped AC input to the rotary transformer stator coil. The output from the rotor coil is transformed by rectifying means to DC before being applied to the excitation coil; these rectifying means are conveniently mounted upon and preferably surround, the first shaft.

In one embodiment, the rectifying means comprise a full-wave rectifier-bridge, for example comprising four diodes which are equally spaced around the periphery of the first shaft. For reliability, these diodes may be encapsulated and contained within a generally annular casing.

One particular application of the invention is in heteropolar eddy current couplings, either of the type in which the flux is radial or in the type in which it is axial.

The invention also extends to a synchronous machine in which the DC field is provided by means of a rotary transformer. Instead of using magnetic coupling such as the rotary transformer, the invention could utilise capacitative coupling to transmit the energizing power to the excitation coil.

The invention may be carried into practice in a number of ways and one specific embodiment will now be described, by way of example, with reference to the drawings, in which: Figure 1 is the schematic drawing, already mentioned, of an eddy current coupling; Figure 2 is a longitudinal section through an eddy current drive embodying the present invention; and Figure 3 is a cross section through the diode housing.

The eddy current drive shown in figure 2 can conveniently be described in two separate portions, an input portion 22, contained within an input housing 26, and an output portion generally indicated at 24 and mostly contained within an output housing 28. The input and output housings are secured together by means of longtitudinally-extending spigots 30.

Secured to the end of the input housing 26 by means of bolts 32 is a conventional fixed speed motor (not shown) for example an induction motor. The output shaft (not shown) of this motor extends into the input housing 26 and is secured within a central bore 34 of a rotating input member indicated generally at 36. The member 36 comprises an apertured end plate 38 having fan blades 40, and a generally cylindrical torque tube or loss drum 42 of magnetic iron. The torque tube may, but need not, be internally coated with a layer of a conducting material such as copper. The entire input member 36 is supported for rotational motion on the motor output shaft (not shown).

Passing through the output housing 26, and mounted for rotational motion on first and second bearings 44, 46 there is an output shaft 48. On the driven end of this shaft, within the torque tube 42, there is a polewheel 50 comprising first and second facing generally cup shaped members 52, 54, having a plurality of fingers and apertures around their respective circumferences.

The fingers of the two cups interlock with each other (but do not quite touch), so providing a 2-part cylindrical surface, separated by a small zigzag gap, which is closely spaced from the cylindrical interior surface of the torque tube 42. Each of the cup members has a central boss, the two bosses being secured together at 56, so providing an annular interior space 58, within which there is a DC exciting coil 60 as will be evident, when this coil is excited a magnetic flux 62 will be produced which passes from the fingers of the cup 52, into the torque tube 42, and back into the respective adjacent fingers of the other cup 54, closing the circuit via the junction 56. Of course, this path would be reversed for the opposite energization of the coil 60.

DC power for the coil 60 is provided by means of a rotary transformer generally indicated at 64, within the output housing 28. The transformer 64 comprises a 2-flanged laminated rotor 66 mounted to the output shaft 48 which is wound with a secondary coil 68, and a corresponding annular exciter stator wound with a primary coil 72. Since both the stator and the rotor are rotationally symmetric, it will be appreciated that this transformer acts essentially in the same way as a stationary transformer, notwithstanding the fact that one of its parts is rotating with respect to the other.

In use, alternating currant (AC) is applied to the primary coil 72, and a corresponding alternating currant signal is consequently induced in the secondary coil 68. This alternating currant signal needs to be converted to DC before it can be applied to the excitation coil 60 (since otherwise inductance losses would be large), and this is carried out by a rectifier bridge circuit 74 (figure 3) contained within an annular housing 76 surrounding the output shaft 48.

Within the housing 76 there are four encapsulated bridge diodes 78, equally spaced around the circumference for balancing purposes. The output from this bridge is fed to the excitation coil 60.

The operation of the drive will now be described. The torque tube 42 is driven at a constant speed by the input motor (not shown), and the coupling between this tube and the flux 62 (and thus the torque applied to the output shaft 48, and its speed) is controlled by adjusting the current within the excitation coil 60.

This is done, of course, by suitably adjusting the AC input to the primary coil 72 of the transformer 64, for example by means of a conventional switching or chopper circuit. Suitable circuits include those switched by means of a thyristor and, for control over both cycles, those switched by means of a triac. Typically, the transformer 64 will be capable of dealing with 500 VA using switched mains AC input, and developing around 100 volts on the DC side. It will be appreciated, however, that the exact characteristics of the transformer can be varied according to requirements; in some cases a step-up transformer will be required, and in some cases a step-down.

One advantage of the described embodiment is that the output characteristics of the drive do not to any substantial degree depend upon the frequency of the input to the primary coil 72. Thus, any conveniently available frequency can be used, and variations in this frequency, provided that the control circuitry can deal with it, will be of substantially no consequence. The control circuitry required can be low power (for example 1-2 amps at mains voltages) even when the actual power output of the drive is very much greater.

In situations where it is required to control closely the speed of the output shaft 48, the control circuitry is arranged to control the AC input to the primary coil 72 in dependence upon the output of a tachometer 80.

It will be evident to the reader how the basic concepts of this invention could be applied in other devices, for example in eddy current brakes or in synchronous machines. Another possibility would be to use capacitative coupling, rather than the rotary transformer 64.

Finally, one could apply the present invention to couplings in which the flux is axial rather than radial, for example where the flux passes between a polewheel on the output shaft and a similar, closely spaced, loss wheel on the input shaft.

Claims (16)

1. An electric machine having a rotatable shaft, a rotary transformer including a stator and a stator coil surrounding the shaft and a rotor and a rotor coil mounted for rotation with the shaft, rectifying means arranged to rectify an output of the rotor coil and to provide direct current to an excitation coil mounted for rotation with the shaft.

2. A torque coupling having a first rotatable shaft, a rotary transformer including a stator and stator coil surrounding the first shaft and a rotor and a rotor coil mounted for rotation with the first shaft rectifying means arranged to rectify an output of the rotor coil and to provide direct current to an excitation coil mounted for rotation with the first shaft; and a second rotatable shaft having a torque member forming part of a magnetic flux circuit generated by the excitation coil, when energised.

3. A torque coupling as claimed in Claim 2 in which the output of the rectifying means is applied directly to the excitation coil.

4. A torque coupling as claimed in Claim 2 or Claim 3 in which the rectifying means are mounted to the first shaft.

5. A torque coupling as claimed in any one of Claim 2 to 5 in which the rectifying means comprise a full-wave rectifier bridge.

6. A torque coupling as claimed in Claim 4 or Claim 5 in which the rectifying means comprises a plurality of unidirectional devices spaced around the periphery of the first shaft.

7. A torque coupling as claimed in Claim 6 in which the said unidirectional devices are encapsulated and contained within a generally annular casing.

8. A torque coupling as claimed in any one of Claims 2 to 7 including control means arranged to control the current flowing in the excitation coil.

9. A torque coupling as claimed in Claim 8 in which the control means comprises chopper means arranged to apply a chopped AC input to the stator coil.

10. A torque coupling as claimed in any one of Claims 2 to 9 in which the first shaft is an output shaft and the second shaft is an input shaft.

11. A torque coupling as claimed in Claim 10 including means for rotating the input at a substantially constant speed.

12. A torque coupling as claimed in any one of Claims 2 to 10 of the eddy current type.

13. A torque coupling as claimed in any one of Claim 2 to 12 of the type in which the flux passes generally radially across a gap between the torque member and a second torque member on the first shaft.

14. A torque coupling as claimed in any one of Claims 2 to 12 of the type in which the flux passes generally axially across a gap between the torque member and a second torque member on the first shaft.

15. A synchronous machine in which the DC field is provided by means of a rotary transformer.

16. A torque coupling substantially as specifically described with reference to Figures 2 and 3.