GB2408390A - Additional short circuited dem windings to compensate unbalanced magnetic pull (UMP) effects - Google Patents

Abstract

An electromagnetic rotary machine comprising; a rotor and a stator 1, the rotor and stator being mounted in co-axial alignment with respect to each other and arranged such that one is rotatable relative to the other about their common axis and an air gap provided between the rotor and stator; at least one set of primary coils carried by one or both of the rotor and stator such that the net magnetomotive force (MMF) acting to drive magnetic flux across the air gap can be changed by changing the currents flowing in these coils with the result that torque can be produced tending to rotate one of the rotor or stator relative to the other is characterised by a second set of coils 2a, 2b carried by or positioned in close relation to one or both of the rotor and stator 2, the second set of coils comprising individual closed circuits configured such that a net loop electromotive force (EMF) is induced in the circuit by the occurrence of any imbalance in the flux on diametrically opposite sides of the rotary machine.

Description

IMPROVEMENTS IN ROTARY MACHINES

This invention relates to electromagnetic rotary machines. In particular, the invention provides a means for compensating for unbalanced magnetic pull (UMP) effects which may occur during operation of such machines.

The electromagnetic rotary machines to which the invention relates typically comprise a substantially cylindrical rotor and stator, the rotor mounted within the stator and being in co-axial alignment with the stator.

The rotor and stator are arranged such that one is rotatable relative to the other about their common axis. An air gap is provided between the rotor and stator.

At least one of the rotor and stator carries a set of primary coils such that the net magnetomotive force (MMF) acting to drive magnetic flux across the air gap can be changed by changing the currents flowing in these coils with the result that torque can be produced tending to rotate one of the rotor or stator relative to the other.

Rotary machines of this configuration suffer from UMP. UMP is a radial force which results when a rotor and a stator are not concentric.

Eccentricity of the air gap can be easily introduced in the manufacture of a rotary machine through deformation of components or manufacturing tolerances. Eccentricity may also occur during use of a rotary machine, for example by the build up of deposits on the rotary mechanism of a vacuum pump. The UMP tends to close the air gap at a given point by pulling the rotor out of radial alignment with the stator. The occurrence of UMP in a rotary machine can lead to inefficient operation of the machine and in particularly bad cases, jamming of, or damage to the machine.

All rotary machines suffer from vibrations during operation. Considerations relating to these vibrations may limit the working speed range of the machine. All rotary machines have certain speeds called critical speeds at which vibration levels reach a maximum. It is relatively common practice, still, to design machines such that the first significant critical speed lies a. : :e a . . above the top speed of the machine running range. The presence of UMP in a working rotary machine tends to bring the critical speed down (by introducing an effective negative stiffness between the rotor and stator of the machine). The working speed range of the machine is often limited by this. The negative magnetic stiffness within a rotary machine is typically compensated for by the inclusion of large positive stiffness in the shaft and in the bearing supports. Inevitably, the achievement of such positive stiffness via the shaft involves the use of larger quantities of material, adding to the costs of manufacture.

The invention aims to alleviate problems associated with the occurrence of UMP in electromagnetic rotary machines.

In accordance with the present invention there is provided an electromagnetic rotary machine comprising; a rotor and a stator, the rotor and stator being mounted in co-axial alignment with respect to each other and arranged such that one is rotatable relative to the other about their common axis, and an air gap provided between the rotor and stator; at least one set of primary coils carried by one or both of the rotor and stator such that the net magnetomotive force (MMF) acting to drive magnetic flux across the air gap can be changed by changing the currents flowing in these coils with the result that torque can be produced tending to rotate one of the rotor or stator relative to the other; and characterized by a second set of coils carried by or positioned in close relation to one or both of the rotor and stator, the second set of coils comprising individual closed circuits configured such that a net loop electromotive force (EMF) is induced in the circuit by the occurrence of any imbalance in the flux on diametrically opposite sides of the rotary machine.

Optionally an individual closed circuit may comprise a pair of coils, the two coils being disposed diametrically opposite to each other about the axis of relative motion between the rotor and stator and connected in series.

e ce:: e. ee: :: : In another option, where the number of pole pairs of the torque-producing flux is even, an individual closed circuit may comprise a singular fully pitched wound coil in a closed circuit. A fully pitched wound coil is to be understood to mean one in which the two sides of the coil span 180 mechanical degrees.

The second set of coils may comprise individual closed circuits such that current within any one of the coils will result directly in a radial MMF producing flux in a pattern which tends to balance the imbalances pattern of flux which causes the UMP. In such an embodiment, the second set of coils are desirably disposed radially about the stator/rotor.

The second set of coils may comprise individual closed circuits such that current within any one of the coils will result directly in a circumferential MMF producing flux in a pattern which tends to balance the imbalanced pattern of flux which causes the UMP. In such an embodiment, the second set of coils are desirably disposed around the back of the rotor or stator, reminiscent of the arrangement of coils on a "gram wound" machine.

The flux pattern produced by the radial or circumferential MMF may be symmetrical or asymmetrical depending on the configuration of the rotary machine. For example, where the machine is a two pole machine (or any machine with an odd number of pairs of poles), the main flux produced by the primary coils is asymmetric, i.e., the pattern of the field may be symmetric about a diameter of the machine, but the direction of the flux is not. The introduction of a small, four pole (or any even number of pole pairs) field results in a symmetric field (in both pattern and direction) which results in an imbalance in the overall flux pattern of the rotary machine and consequent UMP. In such a machine, the second set of coils may be disposed to produce a symmetric flux field having an opposite direction to the four pole field, thereby counteracting the four pole field and restoring balance to the overall flux pattern of the machine.

The opposite case may apply where the machine is a four pole (or any even number of pole pairs) machine having a small, imbalancing two pole ce:. . cec. :. te:e ece.

(or any odd number of pole pairs) field resulting in UMP. In this case, the second set of coils may be configured to produce an asymmetric flux with direction which opposes that of the UMP producing (two pole) field, thereby restoring balance to the overall flux pattern of the machine.

Optionally, any or all of the second set of coils may be mounted in the air gap. Also optionally, any or all of the second set of coils may be wound around protruberances extending radially from the stator or rotor, for example rotor or stator teeth or rotor or stator poles.

Optionally, the second set of coils may include one or more individual closed circuits disposed to result in a radial MMF and one or more individual closed circuits disposed to result in a circumferential MMF.

The closed circuit may, optionally include capacitive and/or inductive elements selected to influence the dynamic characteristics of the UMP.

Where there is a plurality of closed circuits, some or all of the closed circuits may include capacitive and/or inductive elements. The passive components may include variable capacitors and/or variable inductors.

Variable resistors may also, optionally be included in the closed circuits.

The second set of coils may comprise any suitable conductor, for example copper or an alloy thereof.

The invention may have application in rotary machines used for any purpose to increase the working speed range of the machine and/or reduce vibrations occurring in the normal working speed range. Examples of types of rotary machine to which the invention may apply include (but are not strictly limited to); induction machines, hysteresis machines, synchronous reluctance machines, wound rotor machines; stand alone motors and generators, induction/permanent magnet machines, grinding spindles, machines with centrally arranged or overhung rotors. One particularly useful application is in pumps, for example vacuum pumps.

:::: . at: :: . .. ... : For the purposes of exemplification, some embodiments of the invention will now be further described with reference to the following Figures in which; Figure 1 illustrates a first embodiment of a rotary machine in accordance with the invention.

Figure 2 illustrates a second embodiment of a rotary machine in accordance with the invention.

Figure 3 illustrates a third embodiment of a rotary machine in accordance with the invention.

Figure 4 illustrates an embodiment of the second set of coils according to the invention.

Figure shows an oblique view of the stator core 1 of a typical small induction motor. As can be seen the stator includes a surface la for receiving a primary coil (omitted for clarity) arranged circumferentially of the stator. A second set of coils to be associated with the stator in accordance with the invention includes a pair of coils 2a, 2b which, as shown, are connected in series. Each coil in the pair encircles a single stator tooth Ti, TI3, the two teeth being substantially diametrically opposed in the 24 tooth stator illustrated. The two coils have identical numbers of turns. The connection of the coils is such that a pure 2-pole flux field and any flux field component comprising an odd number of pole pairs will never induce a net loop EMF in the circuit formed by the pair of coils. Thus, when the rotor and stator are concentric, no current will flow within the second set of coils and there will be no MMF generated by the coils 2a, 2b.

In contrast, any flux field comprising an even number of pole pairs will induce an EMF tending to drive a current in the closed circuit which includes the coils 2a, 2b. The current results in an MMF which tends to oppose the rate of change of the flux field. Thus when the rotor is

eccentric, the field tends to re-centre it. Fir

:::: . t: :: ce I:.. ..e: .

As an example of the nature and size of the second set of coils, let us assume that the 2-pole induction machine shown in Figure 1 has 24 teeth (T1, T2, T3, ... T24) and that it has a length of L. The two diametrically- opposite stator teeth T1 and T13 each have a width W. It will be shown that introducing a set of secondary coils of very small net cross- sectional area has a substantial effect on the unbalanced magnetic pull in this machine.

Flux passes between the rotor and stator at the two diametrically opposed positions of the teeth T1 and T13. Let the root-mean-square flux densities at top (T1) and bottom (T13) be BT and BB respectively.

These two flux densities can be expressed in terms of the mean flux density, BMean, and the difference, AB.

(BT + BB) = 2 BMean (T) (BT - BB) = 2 AB (T) A UMP force component, F. arises from the two teeth T1 and T13 as a result of the imbalance in flux and F can be expressed as: (WL) (BT2- BB2)/(2,UO) = F # (WL) 4 BMeanAB /(2po) From this it can be seen that AB # (21lo)F/(4WL.BMean) = > ^ # (2 po)F/(4 BMean) where 2 = (2WL.AB) is the total flux difference between teeth T1 and T13. Evidently, the force, F. is directly proportional to the flux unbalance, Am, and the constant of proportionality depends on the rms flux density in the air gap. To have a significant effect, the secondary coils must produce a net MIFF which results in a significant reduction in the total flux imbalance. Any reduction in flux imbalance will achieve a proportionate reduction in the corresponding UMP force.

Now consider the introduction of the secondary set of coils 2a, 2b around teeth T1 and T13 respectively in accordance with the described invention.

In this case, each coil 2a, 2b will have a voltage induced in it by the AN terns as that varies with time. This induced voltage will drive currents within the secondary coils 2a, 2b.

Suppose for the present that ^' term varies sinusoidally in time with angular frequency or.

The (peak) voltage induced in coil 2a (2b) on one tooth T1, (T13) will be N ((k)Pk where N IS the number of turns of the coil 2a on tooth TI. Coil 2b has the same number of turns about tooth T13 and it sees a similar voltage induced. The two induced voltages add together if the coils are connected in the correct (intended) way.

The current flowing in coils 2a and 2b is determined as the induced voltage due to the flux imbalance. Let A represent the cross-section of a single conductor of the coils and let P represent the mean length of a turn of the copper. The total resistance of one coil is then NPP/A where p is the resistivity of the material used for the coils.

The peak current flowing in each secondary coil will therefore be [pk = NO()Pk / (NPP/A) = A amp/ (Pp) and the peak MMF arising from each secondary coil will be X = NA c0Aq>/ (PP) This MMF acts to oppose the rate of change of the flux imbalance. As such, it is 90 out of phase with Am. Observe that NA is the total cross- section of coil wrapped around tooth T1 (T13). This formula indicates that the actual number of turns in the coils 2a,2b is irrelevant; only the total r , ' At, ë cross-sectional area of conductors in those coils is important (assuming that no passive components are included in the circuit).

The fundamental question in this case is whether the MMF induced by a given flux imbalance is itself sufficiently large to drive a flux imbalance substantially larger than the one which induced it.

Suppose that the average radial thickness of air gap is h. Ignoring MMF drops in iron, the flux driven back across the air gap by the induced MMF, X, is A(/J(7S(q!- /0 XWL/h = NA WLyirll (hPp) Rearranging this expression provides the criterion for whether the secondary coils will be effective. Defining the dimensionless number, Z as Z=) (hP). p This dimensionless number provides a direct indication of how effective the secondary coils will be. The factor by which the magnitude of flux imbalance will be reduced is determined as A. If Z is substantially greater than 1, this reduction factor approximates closely to Z itself.

The following example shows that the reduction effect can be very large.

In the case of an induction machine case, assume the following parameters: NA: 2E-6 (m2) WL: 300E-6 (m2) hP 30E-6 (m2) a' : 1256 (rad/s) (200Hz) u, : i.256E-6 (Vs/Am) (permeability of free space) p: 17.2E-9 (Vm/A) (for Cu at room temp.) . . . :: .e ë:.

Inserting this into the formula produces Z = 1.8361 and this indicates a reduction factor of 2.1 in all flux imbalances. Note that NA is the total cross-sectional area of copper in one coil of the secondary winding and in the above case, this is only 2mm2.

Thus coils of relatively small cross sectional area can have a useful rebalancing effect on a rotary machine affected by UMP with minimal impact on the size, weight or configuration of the machine.

It is to be appreciated that, whilst the primary coils have been omitted for clarity, any commonly used method for introducing the primary coils to the core in any common configuration can be achieved in the presence of the second set of coils.

Figure 2 illustrates an alternative embodiment to that of Figure 1. The figure shows an oblique view of the stator core 21 of a typical small induction motor. As in the embodiment of Figure 1, a pair of coils 22a, 22b connected in series provides the second set of coils in accordance with the invention. In this embodiment, rather than encircle teeth of the stator, the coils encircle the back of the stator core. As shown in the figure each coil 22a, 22b passes through a slot of the stator, respectively slots S1 and S13, which are diametrically opposed on the 24 tooth stator, and loops around the outside surface (or back) of the stator. The coils 22a, 22b are connected in series and have identical numbers of turns. Again, it will be appreciated that the arrangement of the coils 22a, 22b will not interfere with the introduction of primary coils in accordance with common practices.

As in embodiment 1, the connection of the coils is such that a pure 2pole flux field (or any flux field comprising an odd number of pole pairs) will never induce a net loop EMF in the circuit formed by the pair of coils.

Thus, when the rotor and stator are concentric, there will be no MMF generated by the coils 22a, 22b. In contrast, a four pole flux field (or any flux field comprising an even number of pole pairs) will induce an ELF tending to drive a current in the closed circuit which includes the cons :: . c: . :: :. c.

22a, 22b. The current results in an MMF which tends to oppose the rate of change of the flux field. Thus when the rotor is being pulled off-centre by an imbalance in the flux distribution around it, the presence of the secondary coils is acting strongly to reduce this imbalance in flux.

Using broadly similar principles as set out in the example calculation given above in relation to Figure 1, it can again be shown that the cross sectional area of coil needed to provide a useful rebalancing effect on a rotary machine affected by UMP is small.

Figure 3 shows an oblique view of a stator core of a 4-pole induction rotary machine. Disposed within the stator core 31 is a fully pitched wound coil 32 forming a closed circuit. Any flux field comprising an even number of pole pairs (as might be expected where there is a perfectly concentric rotor in the stator)will never induce a net loop EMF in the circuit formed by the fully pitched wound coil circuit. By contrast, any flux field comprising an odd number of flux pairs (as might be expected to occur when the rotor is not concentric) will induce an EMF tending to drive current in the circuit so as to result in an MMF tending to oppose the rate

of change of flux field.

Again, it will be appreciated that the arrangement of the coil 32 will not interfere with the introduction of primary coils in accordance with common practices.

Again, using principles already set out in the previous example, it can be shown that the cross sectional area of the conductor of the fully pitched wound coil required to provide a useful UMP counterbalancing effect is small.

In the previously described embodiments, it is to be noted that any rotorstatic component of UMP ("rotor-static" assumes a frame of reference in which forces are constant relative to the rotor and they rotate relative to the stator) will be modified strongly by the second set of coils described in the embodiments. However, any stator-static component of UMP ("stator c e a a static" assumes a frame of reference in which forces are constant on the stator and rotate relative to the rotor) will not be modified. These stator- static components may, optionally, be modified by the inclusion of an additional set of coils on the rotor, these coils being arranged so as, in practice, to partially suppress and/or modify those components of the magnetic flux imbalance which vary with time relative to the rotor.

Figure 4 illustrates in more detail an embodiment of the individual closed circuit of a second set of coils in accordance with the invention. As can be seen from the Figure, a pair of coils 42a, 42b are connected in series in the circuit generally designated 40. In practice of the invention, the coils are magnetically linked by a flux imbalance across the machine. The circuit includes a variable capacitor 43, a variable inductor 44, and a variable resistor 45. It will be appreciated that not all these components need be variable. By adjusting the values of capacitance and/or resistance and/or inductance, the UMP characteristics of the machine can be modified as explained below.

It is to be appreciated that it is not essential that the inductor, capacitor and resistor components are variable. Indeed it is probable that in many cases the values of inductance, capacitance and resistance provided by these components will be fixed for any given rotor application.

In the absence of capacitance or inductance, the MMF resulting in the secondary coils from induced currents caused by changing flux imbalance in the machine will lag behind the original flux imbalance by 90 degrees.

By introducing capacitance by means of the variable capacitor 43, the impedance of the circuit is increased and the lag is reduced for any given frequency. By introducing inductance by means of the variable inductor 44, the opposite can be achieved, i.e., the lag can be increased. Thus the UMP characteristic can be shaped by applying different combinations of inductance and capacitance.

. . . * a t . It is to be understood that the foregoing embodiments are merely exemplary of some embodiments of the invention and are not intended to restrict the true scope of the invention as defined in the appended claims. . : o

Claims (14)

1. An electromagnetic rotary machine comprising; a rotor and a stator, the rotor and stator being mounted in co-axial alignment with respect to each other and arranged such that one is rotatable relative to the other about their common axis and an air gap provided between the rotor and stator; at least one set of primary coils carried by one or both of the rotor and stator such that the net magnetomotive force (MMF) acting to drive magnetic flux across the air gap can be changed by changing the currents flowing in these coils with the result that torque can be produced tending to rotate one of the rotor or stator relative to the other; and characterized by a second set of coils carried by or positioned in close relation to one or both of the rotor and stator, the second set of coils comprising individual closed circuits configured such that a net loop electromotive force (EMF) is induced in the circuit by the occurrence of any imbalance in the flux on diametrically opposite sides of the rotary machine.

2. A rotary machine as claimed in claim 1 wherein one or more individual closed circuits comprises a pair of coils, the two coils being disposed diametrically opposite to each other about the axis of relative motion between the rotor and stator and connected in series.

3. A rotary machine as claimed in claim 1 or 2 wherein one or more individual closed circuits comprises a singular fully pitched wound coil in a closed circuit.

4. A rotary machine as claimed in any preceding claim wherein the individual closed circuits are associated exclusively with the stator.

5. A rotary machine as claimed in any of claims 1 to 3 wherein the individual closed circuits are associated exclusively with the rotor.

6. A rotary machine as claimed in any preceding claim wherein the individual closed circuits are mounted in the air gap.

:: . : be: : he: : :: .. ..

7. A rotary machine as claimed in any preceding claim wherein the second set of coils includes one or more individual closed circuits arranged such that current within any one of the coils will result directly in a radial MMF producing flux in a pattern which tends to balance the imbalanced pattern of flux which causes the UMP.

8. A rotary machine as claimed in any preceding claim wherein the second set of coils includes one or more individual closed circuits arranged such that current within any one of the coils will result directly in a circumferential MMF producing flux in a pattern which tends to balance the imbalances pattern of flux which causes the UMP.

9. A rotary machine as claimed in any of claims 2 to 8 wherein one or more of the individual closed circuits include pairs of coils wherein each coil is respectively arranged around diametrically opposed teeth or poles of the stator or rotor.

10. A rotary machine as claimed in any of claims 2 to 9 wherein one or more of the individual closed circuits include pairs of coils wherein each coil is respectively arranged to pass through diametrically opposed slots of the stator.

11. A rotary machine as claimed in any preceding claim wherein one or more of the individual closed circuits is arranged to pass circumferentially around the back of the stator or rotor.

12. A rotary machine as claimed in any preceding claim wherein the second set of coils comprises copper or an alloy thereof.

13. A rotary machine substantially as described herein and with reference to the accompanying Figures 1, 2, 3 and/or 4.

14. A rotary machine as claimed in any preceding claim embodied in a pump.