BY CONDUCTING LAYER

TECHNICAL FIELD

Embodiments disclosed herein relate to an electrical

machine, such as an electrical motor, with a random-wound stator winding.

BACKGROUND ART

Referring to figure 1, a conventional electrical machine 40, such as a low-voltage electrical motor, comprises a stator frame 21 with a grounding 50, a stator core 45 and a stator winding 10. The electrical machine 40 further comprises a rotor 44 arranged within the stator core 45 in a rotatable manner by means of a rotor shaft 46 and bearings 43a, 43b. The electrical machine 40 may be driven by means of a variable speed drive (VSD) . A stator winding 10 of an electrical machine 40 can be either random-wound or form-wound. Random-wound stator windings 10 are less expensive to produce than form-wound since the former allows a higher degree of automation to be used during manufacturing. The random-wound technology is therefore preferred over the form-wound one. However, a limiting factor for using this technology is the relatively low partial discharge inception voltage (PDIV) of the resulting random-wound stator winding 10. At voltages in the range of approximately 700 to 1000 V the electric field in the air adjacent to the stator winding 10 will no longer hold dielectrically and corona or partial discharges will occur. These discharges will deteriorate the insulation material of the stator winding 10, ultimately leading to insulation failure. Because of the problem with partial discharges, form-wound stator windings 10 with a mica-based insulation material are often used in applications above 1000 V. At least in theory, an alternative solution using random-wound stator windings 10 is to impregnate the stator winding 10 e.g. with a resin to remove substantially all the air surrounding the winding wires. However, the former solution is more expensive, and the latter one does not work in practice in that there is no reliable method for removing the air surrounding tightly- packed winding wires to a sufficient extent. Even if random- wound stator windings 10 are many times impregnated with a resin, this is done for mechanical rather than electrical considerations. Moreover, none of the given solutions addresses the problem with bearing currents explained in the following.

One problem with electrical machines 40, in particular with electrical motors used in combination with a VSD, is the occurrence of bearing currents. The bearing currents may occur due to fast-rising voltage pulses and high switching frequencies of the VSD, and they may cause bearing fluting, a rhythmic erosion pattern on the bearing races, which eventually leads to failure of the bearings 43a, 43b.

Bearing currents originate from a capacitive coupling 48 between the stator winding 10 and the rotor 44 of the electrical machine 40. Figure 1 schematically shows bearing currents that may occur across the bearings 43a, 43b of the electrical machine 40. It is conventionally known to

mitigate the problem with bearing currents e.g. by isolating the bearings 43a, 43b from the stator frame 21 and/or from the rotor shaft 46, or by grounding the rotor shaft 46.

There thereby remains a desire to provide a stator winding with an increased resistance against partial discharges, and at the same time there remains a desire to provide a stator winding preventing bearing currents.

SUMMARY OF THE INVENTION

One object of the invention is to provide an improved electrical machine comprising a random-wound stator winding with an increased resistance against partial discharges and a prevention of bearing currents.

This object is achieved by the device according to appended claim 1. The invention is based on the realization that by providing a winding wire with a conductive layer surrounding a conventional insulation layer, important prerequisites for both partial discharges and bearing currents are removed from the stator winding. This principally simple solution thereby addresses two significant problems with electrical machines, and enables increased voltage levels in random- wound stators with no additional measures required for managing bearing currents.

According to a first aspect of the invention, there is provided an electrical machine comprising at least one stator winding with a winding wire comprising a conductor. An insulation layer surrounds the conductor along a

longitudinal direction thereof, and a conductive layer surrounds the insulation layer along a longitudinal

direction thereof. The conductive layer is grounded.

According to one embodiment of the invention, the stator winding is random-wound.

According to one embodiment of the invention, the conductive layer has a volume resistivity in the range of 0 - 10000 ohm cm, such as in the range of 0 - 1000 ohm cm, 0.1 - 1000 ohm cm, 1 - 1000 ohm cm, or in the range of 10 - 100 ohm cm.

According to one embodiment of the invention, the conductive layer is made of a semi-conductive material comprising a polymer matrix filled with conductive additives.

According to one embodiment of the invention, the conductive additives comprise one or more of: carbon based particles, metal particles and metal oxide particles.

According to one embodiment of the invention, the electrical machine further comprises a slot-liner made of an

electrically conducting material.

According to one embodiment of the invention, the slot-liner material has a volume resistivity within a range of 1- 1000 ohm cm. According to one embodiment of the invention, the at least one stator winding is impregnated with an impregnation resin comprising an electrically conductive additive.

According to one embodiment of the invention, the resin has a volume resistivity in the range of 1 - 10 000 ohm cm. According to one embodiment of the invention, the electrical machine further comprises a variable speed drive.

According to one embodiment of the invention, the winding wire has a diameter or a maximum linear dimension within the range of 0.5 to 10 mm, such as within the range of 0.5 to 6 mm or within the range of 1 to 2 mm.

According to one embodiment of the invention, a wire end is provided with a field grading system. According to one embodiment of the invention, the field grading system comprises one or more of: a shrinkage tube with linear or non-linear field grading properties, a paint with linear or non-linear field grading properties, and a geometric field grading.

According to one embodiment of the invention, the conductor consists of one piece of homogeneous conductor material.

According to one embodiment of the invention, the conductor material is substantially pure single metal such as copper or aluminium.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail with reference to the accompanying drawings, wherein figure 1 shows a conventional electrical machine with an illustration of origin and routes for bearing currents , figure 2 shows a cross-sectional view a winding wire

according to one embodiment of the invention, figure 3 shows a flow chart showing steps of a method for manufacturing a winding wire according to one embodiment of the invention, figure 4 shows a winding wire constituting a part of a

random-wound stator winding, figure 5 shows a part of a random-wound stator winding

according to one embodiment of the invention, figure 6a shows an electric field for a winding wire without a field grading system, and figure 6b shows an electric field for a winding wire with a field grading system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to figure 2, a winding wire 1 according to one embodiment of the invention comprises an electrical

conductor 2 in the form of a metal wire. The conductor 2 may consist e.g. of one piece of homogeneous copper or

aluminium. The winding wire 1 may have a circular cross- section in order to minimize the electric field 6 across the insulation layer, and in order to facilitate the desired manufacturing of a stator winding 10 by using random-wound technology, but it is noted that the cross-section of the winding wire 1 may also have other shapes. In the case of a circular cross-section the cross-section may have a diameter within the range of 0.5 to 10 mm, such as within the range of 0.5 to 6 mm or within the range of 1 to 2 mm. In the case of a cross-section in another shape, the cross-section may have a maximum linear dimension within the range of 0.5 to 10 mm, such as within the range of 0.5 to 6 mm or within the range of 1 to 2 mm.

In the context of the present disclosure, the term "random- wound" refers to a respective winding wire 1 or a stator winding 10 that is suitable for random-wound technology or is a result of the same. Random-wound technology in its turn implies that the winding wire 1 has suitable dimensions, flexibility and other properties such that it can be used in an automated process of manufacturing a stator winding 10 and a respective stator. In practice this means that the maximum diameter of the winding wire 1 is 10 mm or, in the case that the cross section of the winding wire 1 is not circular, the maximum linear dimension of the cross section is 10 mm . The winding wire 1 comprises an insulation layer 3

surrounding the conductor 2 in the longitudinal direction thereof. The thickness of the insulation layer 3 may, for instance, be about 100 - 200 μιη. However, the insulation thickness may be selected and adapted in dependence on the applied voltage during operation. The insulation layer 3 is made of a material with a high dielectric strength, i.e. the material should have a good ability to withstand the maximum electric field 6 (see figures 6a and 6b) within the

insulation layer 3 without failure of its insulating

properties. An insulating material fulfilling these

requirements is high performance thermoplastic, which has high thermo-mechanical properties, such as e.g.

polyetheretherketons (PEEK), polyetherimides (PEI),

polyetherketons (PEK) , polyphenylensulfide (PPS) ,

polyphenylensulphone (PPSU) , polysulphone (PSU) ,

polyethersulphone (PES) , polytetrafloroetylene (PTFE) or polyvinylidenfloride (PVF) . The requirement on high

dielectric strength also enables keeping the insulation layer 3 thin. The dielectric strength of the insulation layer 3 may, for instance, be about 10 kV/mm and its thickness may, for instance, be about 100 μιη.

As mentioned earlier, the winding wire 1 is provided with a conductive layer 4. The conductive layer 4 may comprise a semi-conductive material, such as e.g. a polymer matrix filled with conductive additives, such as carbon based particles (e.g. carbon black, graphene, carbon nanotubes) or metal/metal oxide particles or other conductive materials. Preferably, the polymer matrix consists of the same polymer as the insulation layer 3 enabling good adhesion between the conductive layer 4 and the insulation layer 3. The

conductive layer 4 is arranged such as to surround the insulation layer 3 in the longitudinal direction thereof, the conductive layer 4 hence also surrounding the conductor 2. The conductive layer 4 may, for instance, be in the range of 10-100 μιη thick, preferably about 25 - 50 μιη thick, and have a volume resistivity in the range of 0 - 10 000 ohm cm, such as in the range of 0 - 1000 ohm cm, 0.1 - 1000 ohm cm, 1 - 1000 ohm cm, or 10 - 100 ohm cm. The conductivity of the conductive layer 4 enables proper grounding of the winding wire 1 and enclosure of the electric field 6 inside the insulation layer 3. The electric field 6 is thereby

preferably encapsulated inside the insulation layer 3.

The mechanical properties of the insulation layer 3 should preferably be such that the insulation layer 3 and the conductive layer 4 are kept intact even during mechanical stress e.g. when bending the winding wire 1 to form a stator winding 10. To avoid defects in the winding wire 1 it is necessary to have good adhesion between the conductor 2 and the insulation layer 3 as well as between the insulation layer 3 and the conductive layer 4. This requirement can be achieved, for instance, by using the earlier mentioned materials PEEK and PEI.

It is noted that more than one insulation layer 3 may be applied, i.e. the winding wire 1 may comprise an insulation system comprising two or more insulation layers 3. However, material costs and the geometric dimensions of the winding wire 1 in use (e.g. as a stator winding 10) can be kept down when using a single insulation layer 3. In the various described embodiments, an insulation system comprising a single insulation layer 3 is used as an illustrative

example . Figure 3 shows a flow chart with steps of a method for manufacturing a winding wire 1 for a random-wound stator. In step 31, a conductor 2 is obtained. The conductor 2 may, for instance, be produced by wire drawing, wherein a metal material is drawn through a series of dies of decreasing size. In other embodiments, the metal wire is produced by extrusion. It is noted that other metalworking processes may be used or the conductor 2 wire may be purchased from an external source.

In step 32, the insulation layer 3 is applied on the

conductor 2. In contrast to the conventionally used coating baths for applying the insulation layer 3, the method 30 for manufacturing the winding wire 1 according to the present invention preferably utilizes extrusion. This is preferred in order to ensure that the insulation layer 3 is free from defects. However, the insulation layer 3 may be applied in other manners as well, e.g. by powder coating, provided that the requirement of a substantially defect free insulation layer 3 can be fulfilled.

In step 33, the conductive layer 4 having a volume

resistivity in the range of 0 - 10000 ohm cm is applied. In various other embodiments, the outer conductive layer 4 may be applied by coating, by painting or by spraying.

Figure 4 shows a winding wire 1 in the shape of a random- wound stator winding 10 according to one embodiment of the invention. A stator winding 10 according to figure 3 can be used in many types of electrical machines 40, such as in traction motors. By grounding the outer conductive layer 4 of the winding wire 1 constituting the stator winding 10, a capacitive coupling 48 (see figure 1) between the stator winding 10 and the rotor 44 is effectively reduced to a high extent leading to reduced or omitted bearing currents.

Figure 5 illustrates a part of a random-wound stator core 45 comprising a stator winding 10 with a number of turns of the winding wire 1. The individual turns of the winding wire 1 have random locations in respective stator slots 22. The stator slots 22 are provided with slot liners 23 that mechanically protect the winding wire 1 from being damaged from contact with stator laminations. In contrast to

conventional slot liners 23 that in addition to functioning as a mechanical protection also form a part of an insulation system of the random-wound stator, the slot liners 23 according to one embodiment of the present invention can be much thinner than conventional slot liners 23, and do not need to be electrically insulating. Instead, the slot liners 23 according to the present invention may be made conductive to ensure appropriate grounding of the winding wire 1 to the stator core 45. The volume resistivity of the slot liners 23 may e.g. be within the range of 1 to 1000 ohm cm in order to ensure appropriate grounding of the winding wire 1 without short circuiting the stator laminates.

The stator slots 22 may be impregnated with an electrically conductive resin after inserting the stator winding 10. This is done for securing the grounding and mechanical stability of the winding wire 1. For example, a resin with a

conductive additive such as carbon based particles (carbon black, graphene, carbon nanotubes) or metal/metal oxide particles or other conductive materials may be used for this purpose. Further, the electrically conductive resin also promotes the thermal conductivity of the resin and hence a better cooling of the winding can be achieved. The resin may have a volume resistivity in the range of 1 - 10 000 ohm cm.

Referring to figures 6a and 6b, the conductive layer 4 of a winding wire 1 acts as a screen, encapsulating an electric field 6 (illustrated by means of equipotential lines) , and prevents in that way partial discharges in the winding wire 1. However, even if the winding wire 1 itself can be made partial discharge-free, wire ends 7 of the winding wire 1 need to be connected to respective electrical terminals in order to form an electrical circuit. To enable this, a certain length of the conductive layer 4 and the insulation layer 3 need to be removed at the wire ends 7. If the conductive layer 4 is removed without further measures taken, as illustrated in figure 6a, a strong electric field 6 will be pushed out into the surrounding air close to the edge of the conductive layer 4, and partial discharges will occur at relatively low voltages in this region. To address this, each wire end 7 is provided with a field grading system 5.

Figure 6b illustrates an electric field 6 in the case where the wire end 7 is provided with a field grading system 5. The field grading system 5 is applied on the insulation layer 3 at a section where the conductive layer 4 is

removed. The equipotential lines are more evenly spread out by the field grading system 5, and a strong electric field 6 concentration is thereby avoided. The wire end 7 may

furthermore comprise a section where both the conductive layer 4 and the insulation layer 3 are removed in order to enable a good electrical connection between the conductor 2 and a respective electrical terminal.

The field grading system 5 may comprise a shrinkage tube or a paint with field grading properties at the wire ends 7. The shrinkage tube and the paint may have either linear or non-linear field grading properties. Alternatively,

geometric field grading may be used for example by

increasing the thickness of the insulation layer 3 at the wire ends 7. Different combinations of the mentioned field grading methods may also be used. For instance, a field grading paint may be combined with a geometric field

grading . The invention is not limited to the embodiments shown above, but the person skilled in the art may modify them in a plurality of ways within the scope of the invention as defined by the claims. For example, even if the embodiments of the description make reference to random-wound stator windings 10, it is not excluded that the invention can also be applied to form-wound stator windings.