This invention relates to an electric machine having an energising winding and a liner therefor.

An electric machine having an energising winding will have an output capacity that is, in part, limited by the ability of the machine to dissipate heat. An example of such a machine is a brushless DC motor having a rotor on which are mounted permanent magnet poles, and a stator defining stator poles and supporting at least one winding for energising the stator poles. The energising current in the winding has a heating effect. This heat has to be dissipated and the greater the output of the motor the more heat must be dissipated. By improving the heat transfer from the winding to atmosphere, the same motor is able to produce a higher maximum output by coping with a larger energising current in the winding. A description of brushless permanent magnet d.c. motors is given in the book "Brushless permanent-magnet and reluctance motor drives" by T.J.E. Miller, 1989, Magna Physics Publishing and Oxford University Press.

A typical stator comprises a stack of laminations which are made of a suitable magnetisable steel. A typical winding for a brushless DC motor is made of copper and in the form of a succession of turns embracing a stator pole . The heat from the copper of the winding has to be conducted through the steel of the stator to an outer case which is commonly formed of extruded aluminium defining an array of fins to dissipate the heat to atmosphere.

The contact between the steel of the stator laminations

and the aluminium case can be made sufficiently intimate that the thermal resistance between the two is small. However, while it is possible to have good thermal conductivity between bare copper and the laminations of the stator, this is not possible in an electric motor in which the winding has to be electrically insulated from the stator to avoid a short circuit. For example, the windings of a brushless _^ DC motor are coated with an insulating lacquer which presents a first thermal barrier to the conduction of heat away from the winding. Furthermore, the laminations are stamped from sheets of steel and, thus, can have sharp edges. These tend to abrade the insulating lacquer both by snagging the coating as the winding is introduced into the stator in the assembly process and/or by chafing in the assembled motor as a result of vibration of the winding relative to the stator as it is energised and de-energised. To address this problem of abrasion, the winding can be further insulated from the stator by means of a mechanical shield in the form of a liner arranged in the winding space between the stator poles in which the winding itself is positioned.

A common liner material used in many different electric machine applications is a fibrous card-like material known by the trade mark NOMEX. While the liner is used as an electrical insulator and is a suitable means of protecting the winding from abrasion, it represents a further significant thermal barrier to the dissipation of heat from the winding. Thus, as a practical matter, electric machines having an energisable winding are significantly limited in their output capacity by the need for electrical insulation and mechanicaJ protection of the winding, both of which impair the dissipation oi

heat from the winding.

Current proposals for legislation in various parts of the world intend stricter controls on the levels of electromagnetic interference (emi) generated by electrically powered machines. In a brushless DC motor, for example, one mode of operation involves chopping the voltage applied across the winding. This is a technique with which the skilled person would be familiar. The abrupt transitions between zero and maximum voltage applied across the winding can cause significant levels of emissions across a broad band of the electromagnetic energy spectrum, causing emi. It is desirable to be able to reduce this emi as much as possible.

It is an object of the present invention to provide an electrically insulating liner for the winding of an electric machine which is also less resistant to the dissipation of heat from the winding.

It is a further object of the invention to provide a liner for the winding in an electric machine that can be used as a screen for emi emissions from the winding.

According to the present invention there is provided an electric machine having: a first part defining a winding space; at least one electrically conductive winding mounted in the winding space of the first part and an electrically insulating liner interposed between the winding and the first part, wherein the liner comprises an oxidisable material having first and second surfaces respectively adjacent the first part and the winding, at least one of the said surfaces being oxidised to provide an electrically insulating layer between the first part

and the winding.

The machine could be an electric motor or generator having relatively movable first and second parts one of which carries the lined winding. Alternatively, the machine could be another form of electrical machine having no moving parts, such as a transformer carrying at least one winding lined according to the invention.

One method of achieving an oxidised surface is anodisation. It has been found that the anodised layer provides an abrasion resistant surface having sufficient electrically insulating properties to be applied only relatively thinly. The anodised layer is also a better conductor of heat than known liner materials . It is only required to be applied relatively thinly to achieve a necessary level of electrical insulation, thus keeping the thermal resistance of the liner low.

Preferably, the anodisable material is aluminium, titanium or, possibly, magnesium. The anodisation process converts the base material into an electrical barrier layer that is thermally more conductive than the electrically insulating liners made of NOMEX.

It will be appreciated that other materials, such as steel, can be oxidised using other processes to provide a suitable electrically insulating layer of oxidisation. While only one surface of the liner need be oxidised, it is sometimes desirable to oxidise both of the first and second surfaces. For example, in the event that one surface is abraded during assembly or use o£ the machine, the other surface maintains the integrity of the liner as an electrical insulator between the winding and the

stator .

It is also found that the anodisation process creates an electrically insulating layer on the edges of the liner to the same depth as the main surfaces. The thermal conductivity of metals, such as aluminium and titanium can be exploited in the liner according to the invention.

The necessary electrical insulation is provided in the form of the anodised layer or layers, but with the further advantage that the electrically insulating layer is now provided in the form of only a thin but tough wear-resistant surface.

In some applications it may be desirable to polymerise the anodised surface of the metal. For example, anodised aluminium has a microporous surface into which the polymer, such as polytetrafluoroethylene (ptfe) or polyester, is impregnated. This need not be in the form of a coating, but can be absorbed in the interstices of the porous structure.

Preferably, the liner is electrically grounded to earth in order to create an electromagnetic cage for inhibiting the emission of electromagnetic interference from the windings.

Also according to the present invention there is provided a method of assembling an energisable part of an electrical machine comprising: forming an energisable body defining an energisable member and a winding space; oxidising a sheet of oxidisable material on at least one side; forming the sheet into the shape of a liner for the

winding space; arranging the liner in the winding space; and arranging the winding in the winding space so that the liner is disposed between the winding and the body.

The energisable part of the machine may be a stator of an electric motor or generator.

Preferably the sheet is formed into the shape of the liner before it is oxidised. This avoids damage to the oxidised layer as the sheet is formed into the shape of the liner.

As with the machine of the invention a preferable form of oxidisation is achieved by anodising an anodisable machine.

The present invention can be put into practice in various ways, some of which will now be described by way of example with reference to the accompanying drawings in which :

Fig.1 is a schematic cross-section of a brushless DC motor;

Fig.2 is a scrap-section of a lamination for the stator of the motor of Fig.1 incorporating the invention; Fig.3 is a cross-section of a stator slot; Fig.4 illustrates a liner and cap for use in the invention; and

Fig.5 is a scrap-section of part of the stator of Fig.1 showing detail of the liner in place.

Referring to Figs. 1 and 2, a brushless DC motor comprises a rotor 10 and d statoi \ 2 ^' . Such a motor is available from Control Techniques Dynamics Limited of

Andover, Hampshire, UK under the trade mark 'Duty Max' . The rotor 10 defines a number of rotor poles 14 that each have permanent magnets (not specifically shown) attached to the pole faces. The stator 12 comprises a stack of lateral laminations 16 which define stator poles 18. Fig.2 illustrates the nature of the profile of each lamination of the stack. It comprises a back iron region 19 from which extend the radially inwardly projecting stator poles 18. Each stator pole 18 comprises a parallel sided column portion 20 and a pole face 22 which has a greater radial extent than the column portion 20.

Adjacent stator pole column portions 20, and the back iron 19 of the stator joining the columns, define a winding slot 24 which carries one part of a winding 26. The winding comprises a number of turns of lacquer coated copper wire which are looped around a stator pole. The turns run inside the winding space to either side of the stator pole. This is illustrated in more detail in Fig.3. A liner 28 is disposed between the winding 26 and the material of the stator defining the slot 24. The liner 28 is made of a sheet of aluminium of 0.25 mm thickness. It conforms to the shape of the winding slot 24 defined by the column portions 20 and the connecting back iron 19. The sheet is anodised on both surfaces to a thickness of 30 microns making a total of 60 microns anodization in all. The liner 28 extends to the level of the winding 26 at the open end of the slot 24 on both sides of the winding. An elongate gate cap 30, having a main part 32 disposed in the open end of the slot 24 and a pair of laterally disposed side members 34 depending from the main part 32, closes the opened end of the liner. It is made of the same material as the liner and is anodised in the same manner. Each of the side members

34 is inserted between the corresponding column portion 20 and the adjacent part of the liner 28. The enclosed winding lies in the slot beneath the ledges defined by the stator pole faces 22.

The thickness of the anodised layer will depend, at least in part, on the required breakdown voltage in a given application. Thus, the thickness of the aluminium will be determined, to some extent, by the required depth of anodisation.

Referring to Figs.4 and 5, each liner 28 has a cuff 36 formed on each longitudinal end. The winding 26 embracing a pole may bear against the end of the liner 28 and the slightly radiused end defined by the cuffs 36 is less wearing on the lacquer coating on the copper wires of the winding than the relatively sharper end of the edge of the aluminium sheet would be without the cuff.

To assemble the stator, sheets of aluminium are folded into the shapes shown m Fig.4 to form the liner and its cap. These are then anodised on both sides to the depth of about 30 microns. The depth of anodization has a significant effect on the brittleness of the aluminium. Polytetrafluoroethylene is applied to the anodised surfaces. It is found that anodised aluminium can be brittle and the impregnation of ptfe serves to retain the microporous structure in place. The depth of anodization as a proportion of the total thickness of the aluminium may also make the sheet as a whole brittle. By forming the liner and then anodising, the risk of failure due to cold working the sheet of aluminium is reduced

Titanium is an alternative to aluminium. It is also non-porous. Another anodisable metal is magnesium.

The anodisation process found to be of particular benefit is the same as that used in the production of electrolytic capacitors from aluminium sheet. In this embodiment 99.5% pure aluminium is used which is an SIC or 1000 series alloy.

The formed liner 28 is then slid along the stator slot so that the cuffs protrude from each end of the stator stack of laminations. The winding is then placed around a stator pole to embrace it, and lies within a pair of adjacent liners. Thereafter, the cap 30 is slid beneath the ledges of the adjacent stator poles in a winding slot to close the liner. The slots are then impregnated with a settable resin material in known manner. Thereafter, the motor is assembled in conventional manner.

The depth of anodization provides an acceptable electrical resistance at a breakdown voltage of 1.5 kilovolts. It is wear-resistant and has a low thermal conductivity. The winding is arranged in close contact with the liner and the liner is in close contact with the stator. It has been found that the use of anodised liner according to the invention improves the temperature drop between the winding and the stator from around 40°C for the prior art NOMEX liner to around 10°C for a winding temperature of 150°C at 20°C ambient temperature. This has made possible a 20% increase in the output power available from the Duty Max motor.

The aluminium of the closed liner is available to be used as a screen to electromagnetic emissions from the

winding. This is done by earthing the liner and the cap closing it. In the case where adequate electrical contact can be achieved between the liner and the cap, only one connection to ground is necessary.

Another form of the liner comprises a sleeve wrapped around the winding. This form of liner provides an emi screen without the need of an end cap. However, it has to be assembled on the winding before the winding is introduced into the winding space. The sleeve can be formed from an anodised sheet of aluminium wrapped around the winding.

It will be apparent to the skilled person that other forms of electric machine can be lined in a similar way and to the same advantage. For example, reluctance motors and generators, conventional DC machines and other motors and generators having windings electrically isolated from their housing are susceptible to the improvements possible by the use of this invention. Furthermore, although the invention has been described in relation to the windings on a stator, the invention is equally applicable to the windings found on some forms of rotor, such as those used in commutated motors.