EP1888921A1 - Impeller for centrifugal pumps with permanent-magnet synchronous motor - Google Patents

Abstract

An impeller for centrifugal pumps with permanent-magnet synchronous motor, comprising a main blade array (4a-4c) and an additional blade array (6a-6c) of moving blades; in the nominal direction of rotation the moving blades are resting on the main blades, so that they work as a single blading; in the event of reverse rotation the moving blades are separated from the main blades, so that they can act separately on the fluid flow.

Description

"Impeller for centrifugal pumps with permanent-magnet synchronous motor"

DESCRIPTION

The invention relates to the field of centrifugal pumps powered by a permanent-magnet synchronous motor.

The invention relates in particular to low-power pumps, typically ranging from a few dozen watts to about 500 W. These pumps, generally made of plastic, are used e.g. for water recirculation of above-ground swimming pools, irrigation systems, aquariums, household appliances, etc.

Said pumps are usually powered by a bipolar synchronous motor with permanent magnets. This kind of motor is preferred because of certain advantages: it is cheap and simple; it has much higher efficiency - in this power range - than an asynchronous motor; it can easily be provided with a sealed stator for immersed operation.

The synchronous motor with permanent magnets, however, is able to start up indifferently in both directions. At each startup, the rotor may start to rotate in a random and non-predictable direction, depending on its position relative to the stator poles.

This feature is a problem because the pump impeller is normally designed to operate in a determined direction: in most cases the rotor blades are designed with "backward" discharge, being well known that this configuration ensures a better hydraulic performance compared to straight radial or forward discharging blades. In such a case, the impeller may also work in the opposite direction ("reverse" operation), but obviously with much lower performance compared to design conditions.

It is therefore appropriate for these pumps to be provided with a system or constructional arrangement for preventing reverse operation and ensuring that the pump impeller will always work in the right sense. Prior-art systems adopted to solve this problem can generally be classified as mechanical, electronic or hydrodynamic.

Purely mechanical systems simply make use of a mechanical cam or ratchet gear to prevent reverse rotation of the motor shaft: although simple, they are little used because the motor shaft is exposed to blows; vibrations are increased and operation is noisy.

Electronic systems are more sophisticated, using electronics adapted to manage the startup phase. The motor can, for example, be provided with Hall-effect sensors disposed to detect the rotation direction of the shaft, or even with a small inverter that modulates the frequency of the power supply during startup.

These electronic systems can work very well, but are generally uneconomical below a certain pump size. In addition, the cost of the electronics substantially depends on the intensity of the current and for a given power it has a great impact on low-voltage (12 V) pumps, such as smaller pumps and/or pumps for domestic purposes.

Hydrodynamic systems are based on the fact that the impeller is exposed to significantly higher hydrodynamic resistance during reverse rotation than during design operation. Attempts are thus made to intensify this phenomenon, until reverse rotation requires a drive torque greater than the output torque of the motor, thus becoming impossible.

A first known hydrodynamic system consists of modifying the volute of the pump by adopting an almost symmetrical design, without volute teeth, instead of the classical Archimedes spiral. This arrangement enables the impeller to achieve a significant water flow even with reverse rotation.

In the case of reverse rotation, the resistant torque on the impeller grows rapidly with the flow, substantially because the blades operate with forward discharge, and the impeller tends to greatly accelerate the fluid flow (lower degree of reaction); for flows greater than a threshold value (reverse flow) the resistant torque becomes greater than the available drive torque and the motor cannot drive the impeller any longer.

This solution, however, is effective only above the aforesaid reverse flow: at partial loads, for example when the port of the pump is partially closed, the resistant torque may remain lower than drive torque also in the opposite direction, and the motor may still drive the impeller. It is therefore an unreliable solution; it is further necessary to modify the shape of the volute, moving away from the optimal shape with lower hydraulic performance even at normal conditions.

Another known hydrodynamic system modifies the impeller, providing it with a certain number of flexible blades. The latter are disposed to bend outwards in the event of reverse rotation, thus modifying the geometry of the channels between the blades and increasing the blade tip diameter. This modified geometry is designed to cause a noticeable increase in the resistant torque until it becomes greater than the output torque of the motor.

This system is also effective at very small flows, and does not have the drawback of the minimum flow reversal. It does, however, entail other drawbacks: the blades have to be very thin and flexible in order to be deformed, so they are not resistant and may break in the event of accidental blows; the blades further have to be fixed in an overhanging manner to the impeller, with the external end free and the fixing zone is subject to fatigue failure; efficiency is penalised by the fact that the volute has to be wider than the optimal hydraulic design to enable the blade extension.

The object of the invention is a new type of hydrodynamic system that overcomes the defects and drawbacks of the prior art hitherto propounded. In particular, the aims of the invention are: correct rotation of the impeller throughout the whole pump's range of application, i.e. both for the maximum design flow and for smaller flows; avoid modifications to the volute and/or weakening the mechanical resistance of the impeller; optimal pump efficiency in design conditions; simple and reliable design.

The objects are achieved with an impeller for centrifugal pumps with permanent-magnet synchronous motor, comprising an impeller body and a main blade array that is integral with said body, characterised by an additional blade array that is movable with respect to said impeller body, in which: the additional blade array is free to move between two stop positions, determined respectively by the pressure induced on the impeller by the design rotation direction and by the opposite direction; in the first stop position, each blade of the additional array rests on a corresponding main blade, and the two arrays can act substantially as a single blade array; in the second stop position, each additional blade is arranged at a distance from the corresponding main blade and the two arrays can act separately.

According to a preferred aspect, the additional blades, in the first stop position, rest on the back of the main blades; in the second stop position, on the other hand, they are positioned in the channels of the main blading. Said second stop position is determined by suitable stopping means, for example stop pins protruding from the channels of the main blading.

The additional blades preferably have the same profile as the main blades, so that in the first stop position there is a virtually perfect rest between the arrays, which act on the fluid as a single blading.

Each array can be formed of any number of blades; it is furthermore not necessary for the additional blades and the main blades to be the same in number. For normal applications, the number of main blades will be calculated according to the best solution; the number of the additional blades will preferably be the same or fewer. The additional array may even consist of a single blade, when this is sufficient for the purpose.

The additional blades, both in the stop and the intermediate positions, are preferably contained in the diameter of the main impeller body. Additional flexible blades can also be adopted, with the effect of increasing the tip diameter in reverse rotation.

According to a particularly preferred embodiment, the additional blading is formed integrally with an additional body, mounted coaxially on the main impeller body and free to rotate by a given angle.

In practice, the invention provides an impeller with a variable number of blades according to the rotation direction. In the right direction, the additional array resting on the main one creates in practice a single blading with a number of blades equal to that of the main array; in the reverse direction the fluid meets also the additional array, distanced from the main array, and the impeller works as if it had a number of blades equivalent to the sum of the two arrays.

The advantages of the invention are multiple: it is not necessary to modify the shape of the volute, which can be designed for maximum performance; the system operates for the entire field of use of the pump, and there is no flow range wherein the pump can operate in the reverse direction; the blades do not have to be fixed in an overhanging manner and so there are no problems of fatigue failure or blows; operation is regular and not noisy. It is furthermore a simple and cheap solution that is particularly suitable for the field of low- power pumps, where electronic systems are too expensive. The features and advantages of the invention will become clearer from the following detailed description of a preferred embodiment, with the help of the drawings, in which:

Fig. 1 is a front view of an impeller according to the invention;

Fig. 2 is a schematic cross section of the impeller of Fig. 1 , assembled on the rotor unit;

Figs 3 and 4 illustrate schematically the operating principle of the impeller of Fig. 1.

With reference to the figures, an impeller with a body 1 is shown, provided with an array of blades, on which an additional body 2 is mounted, provided with an additional blade array. The additional body 2 is mounted coaxially on the body 1, free to rotate by a certain angle.

The body 1 is made entirely of plastic and is a single body, and comprises a disk portion 3 provided with blades 4a, 4b, 4c, which constitute the main array.

The additional body 2 consists of a ring portion 5, from which three further blades 6a, 6b, 6c extend forming the additional array. The additional blades 6a- 6c substantially have the same profile and the same tip diameter as the main blades.

The body 1 also comprises stop pins 7 that protrude from the channels of the main array, approximately in a median position between the blades 4a-4c.

With reference to the section of Fig. 2, the impeller body 1 is made integrally with a bushing 10 mounted on a plastic support 11. The latter is fixed to a permanent magnet 12 with a central hole 13 for the motor shaft (not shown).

The ring 5 of the additional body 2 is fitted on a seat 14, that substantially consists of a tapered annular edge, protruding from the centre of the disk 3. The coupling between the ring 5 and the seat 14 is free, enabling rotation of the additional body 2.

The whole is fixed axially by a stop element 15 with a ceramic bearing 16.

The blades 4a-4c advantageously have a taper 18 on the front (Fig. 1), at the base, to connect the profiles in the support position and reduce effects of detachment of the fluid flow.

It can be seen in Fig. 2 that the impeller is mounted "freely rotating" on the magnet 12: in particular, the body 1 is dragged by the support 11 by means of a key (not shown) integrally formed with the support and cooperating with a key 19 integrally formed with the bush 10.

This free mounting enables a rotation of about 270-300 degrees between the magnet 12 and impeller body 1. The magnet 12 can start to rotate freely, overcoming only the inertia thereof, and only after a few instants, when the contact between the keys occurs, it "hooks" the impeller and starts to work under load.

All this facilitates the startup of the rotor, which for the synchronous motors is rather critical because of the low static torque. Still in order to facilitate the startup, a slight asymmetry of the polar expansions of the stator can be provided to increase the torque arm; these are nevertheless arrangements that are known in the sector of synchronous motors, which are therefore not disclosed further. However, it should be emphasised that the corresponding excursion between the bladed bodies 1 and 2 has an advantageous effect even during the delicate phase of startup of the motor. In fact, the impeller reaches the design load only when the blades 6a-6c rest on the main blades 4a-4c; a transitional phase is thus created, during the corresponding movement of the body 2 with respect to the body 1 , which lightens the load and facilitates impeller pickup.

Impeller operation is illustrated in greater detail in figures 3 and 4. The pressure on the blades is indicated by the symbols "+" and "-" referring respectively to the pressure zone and the suction zone.

During design operation (Fig. 3) the back side of the blades is in overpressure with respect to the front and this pressure arrangement pushes the additional blades 6a-6c to rest on the main blades 4a-4c. Through the effect of the coinciding profiles, the blades are almost perfectly superimposed and the impeller seems to be provided with a single, double-thickness blading. Referring to the example of Fig. 3, there is in all respects a three-blade impeller.

In reverse operation (Fig. 4) pressure is reversed (back under vacuum and front under overpressure); the main body 1 rotates with respect to the additional body 2, until additional blades 6a-6c are stopped against stop pins 7. This creates an additional blade array inside the main channels, and the impeller presents a greater number of blades to the fluid. In the example, there is a six-blade impeller, compared to the three-blade of design conditions.

The resistant torque of the arrangement of Fig. 4 is significantly greater than that of Fig. 3, because of the greater blade surface exposed to the fluid. This effect is added to the natural performance decrease due to the fact that blades in the position of Fig. 4 (reverse rotation) are working with forward rather than backward discharge.

The motor is dimensioned with drive torque that is less than the resistant torque of the impeller in the position in Fig. 4. In the case of motor pickup in the opposite rotation direction, the motor is not able to drag the impeller, it stops and again tries to start. Startup thus proceeds by attempts until the rotor starts in the correct direction; it is in all ways certain that the pump submerged in the water cannot start in the wrong correction.

If the pump is started up in air, it can obviously rotate in both directions, as the fluid-dynamic resistance is very low; but as soon as the pump is immersed the high hydrodynamic resistance of the water permits rotation only in the correct direction.

It should be underlined that the invention does not penalise pump performance in the normal direction (Fig. 3). In fact, the additional blading remains resting on the main blading and does not substantially disturb the fluid flux. The only disturbance effects may possibly occur at the base of the blading, where the taper 18 is obtained, but these are very minor effects and can be further reduced, if necessary, through a careful design of the blades.

Despite the different number of active blades, the blade obstruction coefficient does not change passing from the geometry of Fig. 3 to that of Fig. 4. The invention can have numerous embodiments that are equivalent to the one illustrated and are easily recognisable by the person skilled in the art. Some embodiments thereof are illustrated by way of example.

There may be any number of blades; the additional array and the main array may have the same or different number of blades; the additional array may also comprise one blade only. Preferably, for common low-powered pumps, the two arrays comprise 3-4 blades each.

It can be noted that the stop pins 7 disturb, albeit slightly, the flow through the main channels. This effect is reduced with the pins 7 orientated according to the flow, or still more preferably, provided with shaped profiles (Fig. 1). Alternatively, the pins 7 can be replaced by equivalent stopping means: for example, stop abutments can be provided between the ring 5 and the seat 14, or in the hub itself.

The illustrated embodiment, wherein the bodies 1 and 2 are made of plastic and in a single piece with blades integrally formed, is preferred for obvious reasons of cost, bearing in mind that the invention relates to low-power pumps that have to be simple and cheap. Nevertheless, other embodiments can be conceived which are more complex from the constructional point of view but still equivalent: the blades 6a-6c can be hinged individually on the body 1 , and the body 1 can consist of several pieces; another embodiment can be obtained with the blades 6a-6c mounted on a second blade-holding disk, parallel to the disk 3 ("closed" impeller). In the preferred embodiment, the additional and main blades have the same profile, and in the position in Fig. 3 there is a virtually perfect support; nevertheless, the additional blades can be designed with a different profile from the main ones and with only a partial support.

The additional blades 6a-6c can also be designed as flexible blades, so as to have a further effect of variation of the geometry of the impeller, going from the correct operation to the reverse operation, due exactly to flexure of the blades.

Also the material can obviously be different, depending on needs.

Normally, blades will be used that have backward discharge, which provides the best hydraulic performance, but the invention is equally applicable to an impeller with radial or anyway configured blades.

Claims

1. Impeller for centrifugal pumps with permanent-magnet synchronous motor, comprising an impeller body (1) and a main blade array (4a-4c) that is integral with said body (1), characterised by an additional blade array (6a-6c), that is movable with respect to said impeller body (1), wherein: the additional blade array (6a-6c) is free to move between two stop positions, determined respectively by the pressure caused in the impeller by the design rotation direction and by the opposite direction; in the first stop position, each blade of the additional array (6a-6c) rests on a corresponding main blade (4a-4c), and the two arrays can act substantially as a single blade array; in the second stop position, each additional blade (6a-6c) is arranged distanced from the corresponding main blade (4a- 4c), and the two arrays can act separately.

2. Impeller according to claim 1 , characterised in that in said first stop position, the two arrays are arranged with the front of each additional blade (6a-6c) resting on the back of the corresponding main blade (4a-4c).

3. Impeller according to claim 1 or 2, characterised in that each additional blade (6a-6c) substantially has the same profile as the corresponding main blade (4a-4c).

4. Impeller according to any one of claims 1 to 3, characterised in that in said second stop position each additional blade (6a-6c) moves to the centre of one of the channels defined by the carrying blading.

5. Impeller according to any preceding claim, characterised in that the main array and the additional array comprise the same number of blades.

6. Impeller according to any preceding claim, characterised in that the impeller body (1) comprises a disk portion (3) that carries the main blading (4a-4c), and an additional body is provided (2) that carries the additional blading (6a-6c); said additional body (2) is mounted coaxially to the centre of the disk portion (3) of the impeller body (1), and is free to rotate by a certain angle.

7. Impeller according to claim 6, characterised in that the additional body (2) consists of a portion central to the ring (5) from which the array of additional blades (6a-6c) extends.

8. Impeller according to claim 7, characterised in that said ring portion (5) is mounted freely on an annular seat (14) of the impeller body (1).

9. Impeller according to claim 6, characterised in that the disk portion (3) of the impeller body (1) is provided with stop pegs (7) protruding from the channels of the main blading, to define said second stop position of the additional blade array (6a-6c).

10. Centrifugal pump with permanent magnet motor, particularly for low-power applications, characterised by an impeller according to any preceding claim.