Subject of the Invention

The subject of the invention is a permanent-magnet rotor as part of a synchronous electric motor, such as an electric motor for a household appliance, for instance a washing or dishwashing machine, the rotor of which has permanent magnets arranged circumferentially at a distance from its main axis and provided with reinforcing elements arranged within a stator of the same electric motor.

Technical Problem

The technical problem solved by the present invention is how to arrange and fasten permanent magnets onto the rotor in order to achieve continuous fixed fastening of magnets on the rotor regardless of the velocity of rotor rotation especially at a high number of revolutions, wherein the dimensions and mass of the rotor will not increase in comparison to those of the currently known rotors at a certain number of revolutions, and the solution will be simple and cost-effective to produce.

Prior Art

In electric motors that use permanent magnets arranged on the rotor to create torque there is a problem of arrangement and fastening of magnets onto a core of the rotor. This problem becomes even more apparent in rotors running at high or very high velocities, such as rotors of electric motors in modern household appliances. The most common solution is gluing of magnets onto the rotor core and subsequent fixing with various clamping elements, such as clamping rings or clamping shells on the circumference of the rotor. Normally, the rotor together with the clamping elements is surrounded by a plastic or another similar material in order to achieve a constant circular cross-section of the rotor along its entire length and a smooth surface of the shell. Use of additional elements on the rotor increases the gap between the stator and rotor or magnets and this reduces magnetic flux and consequently the efficiency of the motor. Clamping elements must be produced from non-magnetic materials, such as stainless steel or reinforcing fibres in order not to have influence on the magnetic flux between the rotor and stator; however, they must be strong enough to provide for a fixed fastening. The gap between the magnets of the rotor and stator should be as small as possible; therefore arc magnets that are coaxial with the rotor are used. Yet these magnets exhibit huge cogging torque, which results in an irregular motor operation and an increased noise level.

Centrifugal force resulting from rotation of rotors at a high number of revolutions is so huge that fastening of a magnet onto the rotor core by way of gluing is far from being sufficient. The magnets are therefore inserted into specially designed cut-outs in laminations or the core of the rotor. Metallic laminations arranged in packages on the rotor axis are provided with cut-outs complementary to the shape of the cross-section of the magnet, into which cut-outs the magnets are inserted. A section of each lamination arranged under a magnet acts against the centrifugal force when the rotor rotates, thus preventing the magnet from getting displaced in radial direction. This type of magnet fastening is efficient, but the laminations made of a metallic material cause magnetic poles of adjacent magnets to come in contact thus reducing density of a magnetic field. As a result bigger and stronger magnets must be used to achieve an identical effect, what is undesirable both in terms of space occupation and cost effectiveness.

US 2008/0307635 Al (Marioni) discloses a permanent-magnet rotor comprising a cylindrical core surrounded by a plurality of permanent magnets positioned with a cup-like body and fixed into a plastic material. The cup-like body is provided on its outer side with a plurality of step-like longitudinal grooves that form segments into the interior of the body for the reception of magnets. The magnets have step-like edges corresponding to step-like grooves of the cup-like body that serve to radially position the magnets on the rotor. To ensure a proper positioning of magnets onto the rotor, the cylindrical core is provided on the circumference with recesses for the housing of magnets, the recesses being separated by longitudinal bulges. After the cylindrical core and the magnets are inserted into the cup-like body, the longitudinal bulges are parallel to the grooves of the cup- like body and the empty spaces between individual elements are filled with a plastic material that fixes the rotor parts to each other. The plastic material surrounding the magnets and partly also the cup-like body fixes individual magnets but fails to provide sufficient counter-force to the centrifugal force exerted on magnets during operation. The magnets are therefore additionally surrounded by a cup-like section that provides structural stiffness during operation. A drawback of the described solution is also a number of moulded elements that are contained in individual elements for positioning purposes and provide for form- locking connections. This means a more demanding and especially less cost-effective production.

Solution to the Technical Problem

The described technical problem is solved by a permanent-magnet rotor as part of a synchronous electric motor, such as an electric motor for a household appliance, for instance a washing or dishwashing machine, said rotor consisting of a shaft, the axis of which coincides with the main axis of the motor, a substantially cylindrical core and a plurality of magnets arranged on the circumference of the core, said magnets being surrounded by at least one reinforcing element. The rotor includes: a core with circumferentially arranged longitudinal dove-tail grooves that are equally spaced to form seats for magnets, wherein the core consists of positioning laminations and anchoring laminations; lens-shaped magnets having a radius of a side facing outwards that is smaller than the rotor radius; and a plastic reinforcing cage as a reinforcing element.

The invention will be explained in more detail in the continuation by way of an embodiment and the enclosed drawings, representing in:

Figure 1 permanent-magnet rotor of the invention in partial cross-section, Figure 2 detail of the rotor of the invention from Figure 1,

Figure 3 permanent-magnet rotor of the invention with positioning reinforcing laminations in partial cross-section,

Figure 4 detail of the rotor of the invention from Figure 1,

Figure 5 permanent-magnet rotor with transversal grooves.

A synchronous electric motor comprises a stator (not shown) and a rotor 100 arranged coaxially within it. The rotor 100 has a shaft 1, the axis of which corresponds to the main motor axis, a core 2 of a substantially cylindrical shape and provided with a central hole for the reception of the shaft 1, and a plurality of permanent magnets 7 arranged on the circumference of the core 2.

The core 2 has circumferentially arranged longitudinal dove-tail grooves 21, at least two longitudinal grooves that are mutually equally spaced. The distance between two adjacent longitudinal grooves 21 is defined by the dimension of the permanent magnet 7; a surface between the grooves 21 forms a seat 22 of the magnet 7 that is adapted to the lower surface of the magnet 7. The core 2 is formed of metallic disc-shaped laminations 3 interconnected in a known way to a lamination package to form the so- called lamination core 2. The lamination core 2 consists of two types of laminations, namely of positioning laminations 4 and anchoring laminations 5, wherein the number of positioning laminations 4 is smaller than the number of anchoring laminations 5.

The positioning lamination 4 has circumferentially arranged webs 41 extending outwards in radial direction. The number of webs 41 on the circumference of each positioning lamination 4 is the same as the number of magnets 7, and the width of the web 41 depends on the dimension of said magnets 7 or the required distance between them. All positioning laminations 4 have the same number, shape and arrangement of webs 41. The anchoring lamination 5 has circumferentially arranged cut-outs 51 , the side walls 52 of which are formed in the shape of a dove tail with rounded edges. The arrangement of cut-outs 51 on the anchoring lamination 5 corresponds to the arrangement of webs 41 on the positioning lamination 4. The positioning laminations 4 and the anchoring laminations 5 are joined to a lamination package and form the lamination core 2. The cut-outs 51 of the anchoring laminations 5 form longitudinal dove-tail grooves 21 on the circumference of the lamination core 2, wherein each groove 21 is interrupted at a certain distance with webs 41 of the positioning laminations 4.

On the circumference of the lamination core 2 in each section between two adjacent grooves 21 or between each longitudinal set of adjacent webs 41 there is arranged a permanent magnet 7, the lower side of which corresponds to a seat 22 on the lamination core 2. For cost-effectiveness reasons, the embodiment shows the lower side as a flat surface and the opposite, upper side is shaped like a lens with a radius smaller than the radius of the rotor, wherein the other two opposite sides are parallel to each other. The lens-like shape of the magnet allows smoother transitions between individual magnets and this considerably contributes to reduced clogging torque and consequently less noise.

The magnets so positioned are surrounded by a plastic material possessing required characteristics. The plastic material which fills the gaps between individual magnets covers the magnets and forms a reinforcing cage 8 consisting of a cylindrical rigid outer wall 81 and complementary reinforcing rings 82 on each end of magnets. The thickness of the rigid outer wall 81 varies along the circumference due to the lens-like cross-section of the magnets. In the area above the dove tail between the two magnets the plastic material gets fixed in each longitudinal groove 21 and forms a form- locking connection therewith. Thus the force of the plastic material that counteracts the centrifugal force of the magnets increases during the rotor operation to such an extent that additional reinforcing by means of an external reinforcing is no longer needed. The plastic material filling the area above the groove 22 expands in radial direction and forms a fixing wedge that extends over each magnet 7 and thus prevents the latter from slipping through the plastic material in radial direction due to centrifugal force. The plastic shell is minimal in the area above the top of the magnet, which guarantees the minimal air gap between the magnets and the stator. The thickness of the stiffness shell 81 is proportional to the velocity of rotation and hence the strength of the centrifugal force during the operation, and inversely proportional to the required distance between the rotor and stator. Various types of thermoplastic and duroplasts can be used as plastic material and they must have sufficient stiffness and temperature resistance to be capable of resisting high loads during operation. Such type of rotors is intended for medium to high velocities of rotation.

In cases of high velocities when huge centrifugal forces are exerted on the magnets, the form-locking connection between the plastic material and the groove 21 of the lamination core 2, and the reinforcing plastic wedge are no longer sufficient. The magnets need to be additionally fixed in such cases. Each web 41 of the positioning lamination 4 therefore has two complementary extensions 42 at its free end, said extensions being directed in opposite directions and extending circumferentially in a way that each projects at its side across the magnet towards the magnet's centre. The magnet is thus additionally fixed in a certain position and prevented from getting displaced in radial direction. As the extensions 42 do not extend beyond the centre of the magnet, preferably not beyond one third of the magnet width, the magnet poles of adjacent magnets cannot get in contact. The extensions 42 are shaped in a way that the total height of the web 41 and the extensions 42 does not go beyond the height h of the lens-like magnet 7. The stiffening shell 81 can therefore have minimal thickness, wherein the cross-section of the rotor remains constant and rounded. Due to the fact that the extensions 42 are arranged one on each web 41 of the positioning lamination 4 and not on each lamination, the influence on the magnetic field strength is negligible, whereas contribution to fixing of the magnets 7 onto the core 2 of the rotor is huge. The magnets fixed on the rotor in this way do not need any additional fixation in the form of additional external reinforcing elements. In order to achieve a better interconnectivity and better stiffness of the plastic material the webs 41 are provided with reinforcing holes 43 that are intended to be filled with a plastic material and thus additionally reinforce the whole assembly.

If the magnets 7 need additional fixing, a known way of reinforcing with reinforcing fibres 72 is applied, wherein the magnets 7 are directly wrapped in reinforcing fibres along their entire length and the wrapped magnets are then moulded in plastic. The reinforcing fibres 72 can be wrapped around the magnets 7 only at certain preselected spots formed as transversal grooves 71 on the outer, lens-shaped surface of each magnet 7 and the wrapped magnets then moulded in plastic. The reinforcing fibres may be selected from various types of fibres, such as Kevlar, carbon, glass or a fibre possessing adequate characteristics. An advantage of this type of reinforcing over classic reinforcing on the shell of the rotor is a constant cross-section of the rotor both in size and shape.

The magnets circumferentially arranged on the rotor are completely covered in plastic which forms the shell of a constant diameter both at the magnets' front and rear sides. A plastic cage 9 is thus formed that firmly holds the permanent magnets 7 in their places during the entire rotor operation.

It is understood that a person skilled in the art can carry out different embodiments based on being acquainted with the description of the invention without circumventing the essence of the invention claimed in the appended claims.