Patent Application
20090039723
The object of the present invention is a motor-pump assembly for household appliances such as dishwashers and washing machines, in particular for discharging water, of the type comprising a pump equipped with an impeller and a single-phase synchronous electric motor equipped with a stator and with a rotor mutually separated by a mechanically-formed air gap, the rotor being of the type using permanent magnets, the stator being of the diapason type and equipped with a pack core of laminations and with a pair of pole pieces that wrap coaxially around the rotor.
It is known that there is a need to make motor-pump discharge assemblies for household appliances, such as washing machines and dishwashers, such as to have the best possible yield, by this meaning the best ratio between the hydraulic operating speed available to the pump (fluid pressure or flow rate) and the electrical power absorbed by the motor associated with the pump, and at the same time having the lowest possible cost of the discharge function.
This sort of motor-pump assembly normally comprises a pump with an impeller made to rotate by a shaft, and a permanent-magnet single-phase synchronous electric motor, equipped with a stator and a rotor mutually separated by an air gap. The rotor is of the type using permanent magnets and is able to making the shaft connected to the impeller rotate. The stator is of the diapason type and is equipped with a ferromagnetic pack core of U-shaped laminations so as to form two prongs of the diapason; each of these prongs has a copper reel wrapped around it and has a pole piece that, together with the other pole piece, combines to coaxially wrap around the rotor, beyond the air gap. In order to keep noise down and obtain a start-up step that is as fast as possible, a motor-pump assembly according to the invention has a rotor with so-called “slimline” configuration, the ratio between rotor length and diameter being at least equal to 1.3, so as to have a moment of inertia such as to go into resonance with the alternating stator field during the aforementioned start-up step.
It is known that the distance between the rotor and the stator must be as short as possible, and therefore the volume occupied by the air gap must be as small as possible, to allow the flow to meet the least possible reluctance. For this reason the pole pieces tend to wrap around the rotor: each pole piece takes on a shape with an arc-of-cylinder concave surface that faces the rotor. Moreover, the length of the pole piece, which is the height of the pack of laminations that form such a pole piece, equals the length of the rotor or in any case is substantially not very different from it. In this way, the flux closes as little as possible on the pole pieces, and the condition considered as being optimal is created.
Since the motor-pump assembly is of the mechanical starting type, there are no electronic start-up control apparatuses. In this way it is more cost-effective to make the motor-pump assembly, but it generally requires special provisions, such as devices with hauling mechanism between the shaft and the impeller. Patents EP 1308628 and EP 1372245, to the same Applicant, give some example embodiments of these.
Again because the motor-pump assembly is of the mechanical starting type, it is necessary to oversize the reels and the core, since it is necessary to bear the breakaway starting current, which is notoriously of the impulse type. Such a breakaway starting current is not limited by ramps (which are generally obtained by means of control programs loaded onto electronic control circuitry), and therefore it can greatly exceed the current necessary during normal operating conditions. In this way, disadvantageously, large pole pieces and prongs and spirals having a large area need to be used to obtain a magnetic circuit of lower reluctance to bear a high-intensity flux. Indeed, such a large amount of space has to be used that the windings can only be made from copper, since other materials of higher resistivity would require conductors having larger sections and, in order to be wound around the prongs, would need more space than what is actually available in the space between the prongs.
Patent application EP 1760859 discloses a motor for a pump in which the core has a shorter axial length than the ends of the pole pieces. In practice, the core can be undersized, with the exception of the pole pieces. This embodiment does not worsen the start-up conditions of the motor, not influencing the breakaway starting current.
Nevertheless, it would be preferable to seek a motor-pump assembly that has greater structural simplicity, possibly using different, more cost-effective materials.
The technical problem underlying the present invention is that of devising a motor-pump assembly for household appliances with characteristics of particular structural simplicity
The idea for a solution forming the basis of the present invention is to use a motor with a pack of laminations that is substantially shorter than the rotor.
In accordance with this idea for a solution, the technical problem is solved by a motor-pump assembly defined in claim 1.
In a motor-pump assembly according to the invention, the longitudinal ends of the rotor magnets do not have pole pieces on top of them; in this way, there seems to be a pointless waste of material, backed up by the teachings of the prior art, according to which the pole pieces must wrap around the rotor magnets as much as possible, otherwise there will be greater reluctance in the magnetic circuit. However, the inventor has discovered, through experimental tests and numerical simulations, that, even if the height of the pack of laminations decreases (this means that the pole pieces are reduced in the direction of the rotation axis of the rotor) the concatenated flux decreases in a less than proportional way. The flux, indeed, does not only close on the pole pieces, but can also close on other parts of the core, such as the yoke and the prongs.
In
A pump according to the invention therefore has a shorter pack of laminations than an analogous conventional motor, which drives to a core having a smaller section and into reels with spirals of smaller area. Certainly, such a smaller area must be compensated by a greater number of spirals, but this greater number of spirals is proportional to the decrease in flux and not to the reduction in height of the pack of laminations. If, for example, the height of the pack of laminations is reduced by 20%, the area of the spirals is consequently reduced by 20%, whereas the number of spirals only has to increase by 8.5%, resulting in a smaller bulk in the space between the prongs of the stator. The motor can thus be made with a smaller and more compact ferromagnetic core.
According to a particularly advantageous embodiment, the windings are made from aluminium, which is more cost-effective than copper even if it has higher resistivity. A motor according to the invention can house aluminium coils, which are larger than copper ones. The aluminium windings extend outside requiring greater volume than the equivalent motor with copper windings, but such a greater volume, due to the volume of the windings, is acceptable since it gives a larger heat exchange surface that means more efficient heat disposal.
Moreover, since it is clear that such a motor-pump assembly has more aluminium and less iron, the motor-pump assembly advantageously weighs less.
Therefore, a motor-pump assembly according to the invention, in particular comprising a high-density single-phase asynchronous electric motor with aluminium reels, solves the typical problems of household appliance components.
The characteristics and advantages of the motor-pump assemblies according to the invention shall become clearer from the following description of an example embodiment thereof, given for indicating and not limiting purposes with reference to the attached drawings.
With reference to
The impeller 2A is connected, through a shaft 2B, to the rotor 3 of the single-phase synchronous electric motor 7. A plastic overmould 3A is fixedly connected to the rotor 3. Impeller 2A, shaft 2B, rotor 3 and overmould 3A are able to rotate coaxially.
Bearings 9, 10, per se known, bear the shaft 2B and, consequently, the rotor 3, in such a way that they can rotate.
The rotor 3 is of the type using permanent magnets. It is separated, through an air gap 4, from a stator 5 of the diapason type (sometimes known as forked), which acts as ferromagnetic core. Such a stator 5 is formed from a pack of U-shaped laminations made from ferromagnetic material, stacked in the longitudinal direction (which is the height of the pack of laminations), so that their surfaces stick together. The stator 5 is therefore formed from the following portions:
The length (meaning in the longitudinal direction, i.e. in the direction of the rotation axis of the rotor and transversally to the laminations) of the pole pieces 5C coincides with the length of the stator and with the height of the pack of laminations.
The length of the rotor 3 is greater than the length of the pole pieces 5C, i.e. the height of the pack of stator laminations 5. The rotor has permanent magnets with equal portions, at the longitudinal ends, free from the winding of said pole pieces, i.e. not wrapped around by the pole pieces.
Aluminium windings 6 are wound around the prongs 5B, so as to totally fill the volume between the prongs 5B and so as to therefore constitute a high-density motor. Different embodiments could provide the use of copper windings, in which case the volume occupied by the reels would be less, whilst not having the advantages that can be obtained by aluminium.
A casing 8, shaped essentially like a tube and formed in one piece with a flange 8B coupled with the volute of the pump (not illustrated since it is conventional), insulates the rotor 3 water-tight with respect to the stator 5, which in turn is housed in a casing 8A coupled with the flange 8B.
The impeller 2A is connected to the shaft 2B through a known kinematic linkage 11.
In the example shown, the rotor has a diameter of 19 mm and a length of 45 mm. The stator pack has a height of 35 mm, i.e. 22% less than the length of the rotor. The height of each of the prongs 5B is therefore about 35 mm. The aluminium windings 6 each consist of 510 spirals wound around such prongs 5B, with conductors having a diameter of 0.5 mm. The total weight of such windings is just 62 g.
A different embodiment could provide reels with 510 copper spirals with conductors having a diameter of 0.40 mm. The total weight of such windings would be 126 g.
According to the prior art, with a rotor having a diameter of 19 mm and a length of 45 mm, copper windings with 470 spirals having a diameter of 0.40 mm would be required, for a weight of 165 g.
The pack of laminations that constitutes the stator 5 has a height s that is substantially less than the length r of the rotor 3 (according to the prior art, it would be r=s or else r≈s). In
The main advantage of the present invention is the particular structural simplicity of the motor-pump assembly, which optimises the spaces using less ferromagnetic material, and therefore a stator having smaller volume, than analogous motors according to the prior art.
Moreover, in embodiments in which the reels are made from copper, they are shorter and use less material than what is normally provided by the prior art. The decrease in overall length of the conductors means smaller losses by the joule effect, optimising the thermal yield of the motor-pump assembly.
Alternatively, it is advantageously possible to use reels made from aluminium, which is a material with greater resistivity and therefore requires conductors with a larger section than copper does. Since the prongs have a smaller section and the space between the prongs is greater, voluminous aluminium reels can be wound around the stator prongs; the greater heat exchange surface with the outside of aluminium reels allows a more optimised disposal of heat.
Advantageously, a motor-pump assembly according to the invention has a motor that works in an area close to saturation of the ferromagnetic material, maximising the electrical and hydraulic efficiency of the motor-pump assembly.
Moreover, as has been seen with the numerical examples, a motor-pump assembly is advantageously obtained that is lighter than current ones, since there is less lead and lighter reels, above all if made from aluminium.
A motor according to the invention can be configured like a high-density motor, i.e. such that the space is totally filled with conducting material.
It is clear that a motor-pump assembly according to the invention is advantageously less expensive than conventional motor-pump assemblies, and allows the cost of the water discharge function to be minimised, so as to limit the assembly costs of household appliances such as dishwashers and washing machines.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07425519.1 | Aug 2007 | EP | regional |
