GB2035715A - Asynchronous electrical machines and rotors therefor - Google Patents

Abstract

A rotary asynchronous A.C. machine comprises a stator B adapted to produce a rotating magnetic field in the zone of the rotor A, and is characterised in that the rotor A comprises a magnetic element 1 of cylindrical configuration; and a non-magnetic, electrically conductive element 4 of tubular configuration embraces the magnetic element 1. The magnetic element 1 may be of mild steel and mounted on the shaft 2 by a body 3 of synthetic resin. The conductive element 4 may be of copper, aluminium or zinc and insulated 5 by a layer of synthetic resin from the element 1. The layer 5 may, however, be omitted. <IMAGE>

Description

SPECIFICATION Asynchronous electrical machines This invention relates to rotary asynchronous AC machines.

According to the invention a rotary asynchronous AC machine comprises a rotatably mounted rotor; and a stator embracing the rotor and adapted to produce a rotating magnetic field in the zone af the rotor, characterised in that the rotor comprises a magnetic element of substantially cylindrical configuration; and a non-magnetic, electrically conductive element of tubular configuration embracing the magnetic element.

The magnetic element may comprise a solid cylinder but preferably comprises a hollow cylinder so that it is also of tubular configuration.

Preferably the magnetic element and the non-magnetic element are each of a one piece construction.

Preferably also, the magnetic element and the non-magnetic element are each substantially uniformly radiused.

The magnetic element may comprise any suitable magnetic metal, such as mild steel.

Preferably, the magnetic element is spaced from the stator at a substantial distance to minimize magnetic attraction in a radial direction between the rotor and the stator and permit the rotor to run up towards zero slip under no-load conditions.

The magnetic element may extend longitudinally beyond the magnetic material of the stator at one or both ends ofthe magnetic element.

Preferably the non-magnetic tubular element is located in close proximity to the stator.

The non-magnetic tubular element may be made of any suitable non-magnetic metal which is electrically conductive, such as copper, aluminium or zinc.

The non-magnetic tubular element and the magnetic element may or may not be mounted in electrically conductive connection with each other.

Thus, the hon-magnetic tubular element may intimately embrace the magnetic element in direct electrically conductive contact therewith. Alternatively, a layer of electrically insulating material may be located between the magnetic and non-magnetic elements.

The magnetic and non-magnetic elements may be mounted on a shaft in any suitable manner for rotation therewith. The magnetic element and/or the non-magnetic tubular element may or may not be mounted in electrically conductive connection with the shaft.

Thus, particularly on small motors, a magnetic tubular element may be mounted on the shaft by means of an annular body of electrically insulating synthetic resinous material which is interposed between the magnetic tubular element and the shaft.

Alternatively, a magnetic tubular element may be mounted on the shaft by means of axially spaced, radial supports located towards opposite ends of the magnetic tubular element. The supports may be made of electrically conductive metal.

The machine may be provided with a wound stator which may comprise a multi-phase, preferably a 3-phase, asynchronous generator or induction motor.

With the arrangement according to the invention a conventional laminated wound rotor or squirrel cage rotor which is relatively expensive and complicated to produce may be dispensed with and replaced by a rotor according to the invention which comprises plain, substantially uniformly radiused, one piece cylindrical elements which are without protrusions, grooves, windings, bars or radially disposed laminations and which are relatively simple and inexpensive in construction.

For a clear understanding of the invention a preferred embodiment will now be described, purely by way of example, with reference to the accompanying drawings, in which Figure 1 is a diagrammatic longitudinal seetional view of one embodiment of a rotary asynchronous A C machine according to the invention on the line I - I in Figure 2.

Figure 2 is a diagrammatic cross-sectional view of the machine of Figure 1 on the line ll-ll in Figure 1.

Figure 3 is a diagrammatic longitudinal sectional view of another embodiment of a rotary asychronous A C machine according to the invention on the line Ill-Ill in Figure 4.

Figure 4 is a diagrammatic, cross-sectional view of the machine of Figure 3 on the line IV-IV in Figure 3.

Figure 5 is a graphical representation of the loading of a conventional 3-phase, squirrel cage induction motor between no-load and about 10% overload.

Figure 6 is a graphical representation of the superimposed power curves of a sinusoidal 3-phase power input system of 3 x 4 = 12 kW and 3 x 5 = 15 kVA in which the phases are electrically spaced apart at 1200.

Referring first to Figures 1 and 2 of the drawings, the electrical machine M, which may comprise a 3-phase asynchronous generator or induction motor, includes a rotor A comprising a one piece inner tubular element 1 which is made of any suitable metal which is magnetic, such as mild steel, and which is substantially uniformly radiused at both its inner and outer peripheries when viewed in cross-section as well as in longitudinal section, so that it is of substantially uniform thickness throughout.

Inner tubular element 1 is firmly mounted in electrically insulated relationship on shaft 2 by means of an annular body 3 of any suitable synthetic resinous material which is electrically insulating and which fills completely the space between the inner periphery of inner tubular element 1 and the outer periphery of shaft 2.

Rotor A further includes a one piece outer tubular element 4 embracing magnetic inner tubular element 1 Outer tubular element 4 is made of any suitable metal which is non-magnetic and electrically conductive, such as copper, aluminium or zinc, and is also substantially uniformly radiused at both its inner and outer peripheries when viewed in cross-section as well as in longitudinal section, so that it is of substantially uniform thickness throughout. An annular layer 5 of electrically insulating synthetic resinous material is interposed between outer tubular element 4 and inner tubular element 1. The inner and outer tubular elements 1 and 4 are fast with each other and with shaft 2 for rotation therewith, Shaft 2 is adapted to be rotatably mounted in suitable bearings (not shown).

It will be seen from Figures 1 and 2 of the drawings that rotor A does not have any grooves, protrusions, windings, bars or radially disposed laminations and that the inner and outer metallic tubular elements 1 and 4 define a plain drum like structure. Rotor A may be regarded as a cage-type rotor with the other non-magnetic, electrically conductive tubular element 4 constituting an infinite number of cage bars.

Rotor A is embraced by a 3-phase wound stator B of conventional design which is well known with asynchronous machines. Stator B is adapted to produce a rotating magnetic field in the zone of rotor A, at least in the peripheral region of the rotor.

It will be seen that the radial spacing a between the inner periphery of stator B and the outer periphery of magnetic inner tubular element 1 of rotor A is substantial. Spacing a is the magnetic gap between rotor A and stator B.

Furthermore, the spacing b between the inner periphery of stator B and the outer periphery of outer, non-magnetic tubular element 4 of rotor A is small so that outer tubular element 4 is located in close proximity to stator B. Preferably, spacing b only constitutes a clearance permitting free rotation of rotor A relative to stator B. Spacing b is the physical airgap between rotor A and stator B and is substantially smaller than the magnetic gap a.

It will also be seen that the inner and outer drums 1 and 4 of rotor A extend beyond the magnetic material of stator B at opposite ends thereof.

As will be explained in greater detail below, the magnetic gap a between magnetic innertubularelement 1 and stator B must be appreciable in order to suitably weaken the magnetic field flowing from stator B to rotor A and thereby to suitably weaken the magnetic attraction in a radial direction between rotor A and stator B which hinders the running up towards zero slip of rotor A under no-load conditions. However, certain characteristics of the machine, such as its power factor and efficiency, is affected adversely by an increase in the size of magnetic gap a. A compromise between conflicting considerations has to be made and the size of magnetic gap a is selected to suit particular requirements.

In order to assist in weakening the radial attractive force between rotor A and stator B, the ends of inner magnetic tubular element 1 projects to a suitable extent beyond the ends of stator B so as to suitably weaken the magnetic field flowing from stator B to rotor A.

In orderto obtain as high a torque as possible with a magnetic gap a of substantial size, outer tubular element 4 of non-magnetic metal is located in close proximity to stator B with as small a clearance or physical airgap 13 as possible in order to increase the outer diameter of the electrically conductive portion of rotor A which cuts magnetic flux lines and thereby to increase the moment of the tangential force exerted on rotor A at the outer periphery of outer non-magnetic, electrically conductive tubular element 4.

Referring now to Figures 3 and 4 of the drawings, the electrical machine M1 includes rotor A1 comprising uniformly radiused, one piece innertubular element la of magnetic metal, such as mild steel. RotorAl further includes uniformly radiused, one piece outer tubular element 4a of non-magnetic, electrically conductive metal, such as copper, aluminium or zinc, which intimately embraces magnetic inner tubular element lain direct electrically conductive contact therewith.

At opposite ends, inner tubular element 1 a is fast, such as by welding, with a pair of axially spaced supports 6 which, in turn, are fast, such as by welding, with shaft 2a which is adapted to be rotatably mounted by means of suitable bearings (not shown). Outer tubular element4a is fast with innertubular element la for rotation with shaft 2a.

Supports 6 may be in the form of disc-like metal plates or may be in the form of metal spiders which electrically connect inner and outer tubular elements 1 a and 4a with shaft 2a.

As can be seen from Figures 3 and 4, rotor Al does not have any grooves, protrusions, windings, bars or radially disposed laminations and the inner and outer metallic tubular elements la and 4a define a plain drum-like structure.

Rotor is embraced by a wound stator B1 with a magnetic gap a between the inner periphery of stator Bl and the outer periphery of magnetic inner tubular element la and with an airgap b between the inner periphery of stator B1 and the outer periphery of outer non-magnetictubular element 4a.

Inner and outer tubular elements 1 a and 4a project beyond the magnetic material of stator B1 at both ends thereof.

It will be appreciated that many variations in detail in the construction of a machine according to the invention are possible without departing from the scope of the appended claims. For example, instead of both the inner and outer tubular elements projecting beyond both ends of the stator, only the magnetic inner tubular element 1 or 1 a of the rotor A or Al of a machine according to the invention may project beyond one or both ends of the stator B or Bl, the non-magnetic outer tubular element4 or 4a being substantially co-extensive with the stator.

The walls of inner tubular elements 1, la and of outer tubular elements 4, 4a may be of any suitable thickness.

The theory of operation of a rotary, asynchronous A C machine according to the invention has not yet been fully investigated and is not yet fully understood. However, the Applicant believes that operation of the machine is based on the following theoretical considerations which will now be discussed in relation to a rotary asynchronous A C motor. Electrical energy cannot be converted directly into mechanical energy, but must be converted into magnetic energy, which, in turn, is converted into mechanical energy. In a rotary A C motor, electrical energy which is applied as an input into a winding on a stator is converted into magnetic energy in the stator where copper and iron (magnetisation) losses occur.The magnetic energy passes from the stator through an airgap which separates the stator from a rotor, and into the zone of the rotor where the magnetic energy is converted into mechanical energy which can be withdrawn from the rotor as a rotary output. The rotor is physically and galvanically separated from the stator so that all energy which is withdrawn from the rotor as mechanical work must therefor pass through the airgap.

The magnetic energy passing through the airgap is equal to the total electrical energy input into the stator less the sum of the copper and the iron losses in the stator. The mechanical energy which is withdrawn from the rotor as useful work must be equal to the magnetic energy passing through the airgap from the stator to the rotor less any losses in the rotor, such as friction and windage losses and possibly also heat loss.

Tests and caldulations have indicated that with a conventional 3-phase, squirrel cage induction motor the mechanical energy withdrawn as work from the rotor is substantially equal to the magnetic energy passing through the airgap from the stator to the rotor (i.e. total electrical energy input into the stator less the sum of the copper and iron losses in the stator) less the friction and windage losses in the rotor. This indicates that there is very little, if any heat loss in the rotor.

Conventionally, the interaction between the stator (primary member) and the rotor (secondary member) of an A C rotary machine is always compared with that of the primary and secondary windings of a transformer. Applicant believes that while this might be the correct approach when there is no relative movement between the rotor and the stator of a motor, it is no longer the case when the rotor starts moving.

When the rotor of a motor moves, mechanical energy becomes available whereas in a transformer where the primary and secondary windings are rigidly fixed relative to each other, no mechanical work can be performed. To satisfy the laws of the conservation of energy it is applicants's theory that provided energy equilibrium is maintained at any instant, the energy input into the stator of a motor less the losses in the stator and the rotor equals the mechanical energy which is actually withdrawn from the rotor without any magnetic energy being returned from the rotor to the stator.

When a motor is running up unloaded from slip 1 towards slip 0 (i.e. from standstill towards synchronous speed) excess energy is forced into the rotor, unless the airgap is of suitable size. Such surplus energy causes rapid heating of the rotor while the stator current may well remain below its thermal limits. Once the rotpr approaches synchronous speed (i.e. slip equals or is close to zero) the only mechanical load on the rotor is due to friction and windage and if the rotor keeps in step with the revolving magnetic field, the rotor is inactive because no field is cut. Under these conditions the metal of the rotor will not heat up because the friction and windage losses are equal to the energy input into the rotor.

If now the rotor is loaded on its shaft, causing a drop in speed, the rotor becomes active and exerts on the shaft an equivalent torque which is directly converted into work, thus maintaining the energy equilibrium.

This means that the rotor draws just sufficient energy always to satisfy the momentary condition: Magnetic input into rotor-losses in rotor = mechanical output from rotor.

Figure 5 shows graphically the loading of a conventional 3-shape squirrel cage induction motor between no-load and about 10% overload. The white, unhatched areas represent load and losses, while the cross-hatched areas represent that energy which oscillates. Such oscillating energy is obviously required fqr magnetisation and remains almost constant over the whole load range but is somewhat higher at no-load. It will be seen that with increasing load, it is only the white areas (which also includes stator copper losses) that increase, resulting in increased power-factor and efficiency.

Since, as stated above, tests and calculations have shown that most, if not all, the magnetic energy input into the rotor less friction and windage losses, can be withdrawn from the shaft as useful mechanical work, it becomes evident that very little, if any, energy is left for any electrical activity in the rotor which under normal operating conditions is only influenced by the slip frequency which may be in the region of 1 to 2 Hz.

The foregoing leads one to the conclusion that the magnetisation and energy flow in a rotary A C machine is uni-directional (i.e. only from the stator to the rotor) and that due to the consequent electrical inactivity of the rotor it is not necessary to laminate the rotor core, but that a plain rotor according to the invention may be used.

It is known that an AC power curve has twice mains frequency and is influenced by power factor in that the power curve is positive when the momentary values of current and voltages are of the same polarity and that the power curve becomes negative when current and voltage are of opposite polarity. Such graphically constructed power curves are found in text books which were published before the turn of the century, but at that time the available measuring techniques did not allow the comprehensible analysis of power and energy which is possible today with electronic equipment.

It is common practice to express instantaneous power as P = VI Cos Q where Cos Q is the power factor.

It has been found that the measurement of alternating power with conventional means based on the concept of power factor is relatively easy and reliable at power factors which do not deviate too far from unity, say from 0,6 inductive to unity to 0;6 capacitive. However, at low power factors, power measurement becomes difficult and with doubtful results. This is also the case under tansient and sub-transient conditions.

It is now possible with the power measuring apparatus of South African Patents 7216045 and 77/5263-to create a clear picture of energy flow. The operation of such power measuring apparatus is based on the principle that the instantaneous rate of energy supplied by an alternating power supply equals the algebraic product of the instantaneous values of the voltage and current in the system so that the energy or power is.qf positive polarity throughout each cycle when the voltage and current are in phase but changes from positive to negative polarity during each cycle when the voltage and current are not in phase. The energy or power absorbed or converted into work from the supply system is the difference between the integrated positive values of the energy and the integrated riegative values of the energy.An ideal condition exists when the voltage and current are in phase so that only positive energy is supplied by the system and all the energy drawn from the system is converted into work or absorbed by losses. This is the condition existing at unity power factor. When the voltage and current are out of phase with each other.(i.e. when power factor is less than unity), positive energising power is initially drawn from the system during each cycle, but a stored power condition exists and during the latter part of each cycle energy of negative polarity is returned to thq supply. The integrated values of positive energy supplied by the system are always larger than the integrated values of negative energy subsequently returned to the supply.

It is logical to label the positive component of power as "forward" and the negative component of power as "return" because only the positive component contains active power which can be converted into work, whereas the negative component represents that portion of energy which cannot be converted into work but is stored momentarily and has to be returned to the source.

In order to analyse this concept, it is necessary to assume that both voltage and current appear as sine waves and that therefore the resultant power curve is also sinusoidal.

Instantaneous algebraic multiplication of the instantaneous values of voltage and current according to the concepts of S A Patents No 72/6045 and 77/5263 shows a "natu ral axis" with reference to which the power factor can correctly be expressed as: Forward actor = Forward power - return power Powerfactor Forward power + return power.

Since the areas contained by the power curve represent energy, the following expression is also true: Forward actor = Forward energy - return energy Energy factor energy $ return energy.

Forward energy + return energy.

It is to be noted that the energy factor is not equal to the power factor when looking at the power curve of one phase only. This is so because the abscissae of the power curve represent the momentary watts (a watt has no time dimension) while the areas contained by the power curve include the time factor and therefore represents watt-second or Joules as the true physical unit of energy. This will become evident from Table I given below and from Figure 6 of the drawings.

If the curves of a sinusoidal 3-phase AC power system in which the different phases are eletrically spaced apart at 120 , are superimposed on one another as shown in Figure 6, it will be found that all the instantaneous values sum up to an absolutely linear constant as shown in Table I: TABLE I Summing kVa with reference to the artificial axis of Figure 6: PHASE R 0 0.67 2,5 5 7,5 9,33 10 ' 9,33 7,5 5 2,5 0,67 0 PHASE S 7,5 9,33 10 9,33 7,5- 5 2,5 0,67 0 0,67 2,5 5 7,5 PHASE T 7,5 5 2,5 .0,67 0 0,67 2,5 5 7,5 9,33 10 9,33 7,5 R+S+T= z 15 15 15 15 15 15 15 15 15 15 15 15 15 Summing kW with reference to the natural axis of Figure 6:: PHASE R -1 -0,33 1,5 4 6,5 8,33 9 8,33 6,5 4 1,5 -0,33 -1 PHASE S 6,5 8,33 9 8,33 6,5 4 1,5 -0,33 -1 -0,33 1,5 4 6,5 PHASE T 6,5 4 1,5 -0.33 -1 -0.33 1,5 4 6,5 8,33 9 8,33 6,5 R+S+T= E: 12 12 12 12 12 12 12 12 12 12 12 12 12 Table I shows that the total kVA and the total kW which is obtained by adding momentary values of each sinusoidal power wave in an assumed 3-phase power input of 3 x 4 = 12kW and 3 x 5 = 1 SkVA, remain at absolutely constant values which do not oscillate.

The three power curves shown in Figure 6 have a common axis of symmetry, a common natural axis and equal peak to peak values. They therefore represent their combined energy as.an area which is proportional to the magnetic energy flowing from the stator to the rotor of a rotary asynchronous A C machine.

From the areas thus formed, the power factor can truly be determined, because: Total area of active power - power factor.

Total area of apparent power The energy in the 3-phase A C power system is obviously unipolar, i.e. it does not oscillate and is evenly distributed on all points of the periphery of the rotor of an asynchronous A C machine and cannot flow back, which means that it is either converted into mechanical work or partially into heat at sub-synchronous speed when slip is greater than zero.

It will be appreciated that in a rotary asynchronous AC motor, there is a magnetic attraction in a radial direction between the magnetic bodies of the stator and the rotor and that such radial attractive force hinders the rotation of the rotor relative to the stator which is brought about by a driving torque or tangential thrust which is exerted on the rotor. It can be seen from Figure 6 that in spite of the fact that the magnetic field revolves, the radial attractive force does not oscillate and remains constant for given conditions.

It can also be seen from Figure 6 that depending on the power factor, only the stator magnetic field oscillates partially, i.e. changes direction with a frequency of 100Hz per phase or 300 Hz in the 3-phase system. The reaction between the stator and rotor forces thus produces a torque or tangential thrust which clearly depends on the power factor, i.e. the relationship between forward and return power. At a low power factor the proportion of active power is low, yet at the same time sufficient energy must enter the rotor to keep it moving.

With a rotary A C machine according to the invention, the spacing a between the stator B and the magnetic inner tubular element 1 of the rotor A (i.e. the magnetic gap) must be suitably selected so as to adequately.

weaken the radial magnetic attraction between the stator and the rotor but without too drastically affecting the power factor and efficiency characteristics of the machine. If the magnetic gap is too small, the radial magnetic attraction prevents the rotor from coming up to speed at no-load and if it is too big a higher magnetising current is required to pass sufficient magnetic energy into the rotor. A suitable compromise has to be found to suit circumstances.

In order to assist in weakening the radial magnetic attraction between the stator and the rotor, the ends of the magnetic inner tubular element 1 of rotor A may project at one or both ends beyond the ends of stator B as shown in Figure 1 of the drawings. The extent of the projection may be selected to suit particular circumstances.

As the magnetic gap a is increased with a given inner diameter of stator B, the diameter of the outer periphery of magnetic inner tubular element 1 of rotor A must decrease and therefore the moment of force (i.e. torque) produced by tangential thrust at the outer periphery of magnetic inner tubular element 1 must also decrease.

In order to obtain as high a torque as possible with a magnetic gap a of substantial size, non-magnetic outer tubular element 4 is provided round magnetic inner tubular element 1 on rotor A. Outer tubular element 4 is located in close proximity to stator B with a small clearance or physical airgap b to increase the outer diameter of the electrically conductive portion of rotor A which cuts magnetic flux lines and thereby to increase the moment of force exerted on rotor A at the outer periphery of electrically conductive, non-magnetic outer tubular element 4.

The considerations discussed above with reference to the embodiment of Figures 1 and 2 of the drawings also apply to the embodiment of Figure 3 and 4 of the drawings.

Although it is considered advantageous for the magnetic inner tubular element 1 or la of rotor A orAl of Figures 1 and 2 or Figures 3 and 4 respectively of the accompanying drawings, to project at one or both ends beyond the ends of stator B or B1 respectively, it is contemplated that in particular circumstances the magnetic inner tubular element 1 or 1a need not project and may be schematically co-extensive with the stator B or B1 respectively.

Claims (19)

1. A rotary asynchronous A C machine comprising a rotatably mounted rotor; and a stator embracing the rotor and adapted to produce a rotating magnetic field in the zone of the rotor, characterised in that the rotor comprises a magnetic element of cylindrical configuration; and a non-magnetic, electrically conductive element of tubular configuration embracing the magnetic element.

2. A machine as claimed in claim 1, wherein the magnetic element is also of tubular configuration.

3. A machine as claimed in claim 1 or 2, wherein the magnetic element and the non-magnetic element are each of a one piece construction.

4. A machine as claimed in any one of claims 1 to 3, wherein the magnetic element and the non-magnetic element are each substantially uniformly radiused.

5. A machine as claimed in any one of preceding claims, wherein the magnetic element comprises a magnetic metal.

6. A machine as claimed in claim 5, wherein the magnetic element comprises mild steel.

7. A machine as claimed in any one of the preceding claims, wherein the magnetic element is spaced from the stator at a substantial distance selected to permit the rotor to run up towards zero slip under no-load conditions.

8. A machine as claimed in any one of the preceding claims, wherein the magnetic element extends longitudinally beyond the magnetic material of the stator at one or both ends of the magnetic element.

9. A machine as claimed in any one of the preceding claims, wherein the non-magnetic tubular element is located in close proximity to the stator.

10. A machine as claimed in any one of the preceding claims, wherein the non-magnetic tubular element comprises copper, zinc or aluminium.

11. A machine as claimed in any one of the preceding claims, wherein the non-magnetic tubular element and the magnetic element are mounted in electrically conductive connection with each other.

12. A machine as claimed in any one of claims 1 to 10, wherein the non-magnetic tubular element is electrically insulated from the magnetic element.

13. A machine as claimed in any one of the preceding claims, wherein the magnetic and non-magnetic elements are mounted on a shaft for rotation therewith.

14. A machine as claimed in claim 13, wherein the magnetic element and/or the non-magnetic tubular element is mounted in electrically conductive connection with the shaft.

15. A machine as claimed in claim 13, wherein the magnetic element and/or the non-magnetic tubular element is electrically insulated from the shaft.

16. A machine as claimed in any one of the preceding claims, comprising a multi-phase asynchronous generator or induction motor.

17. A rotary asynchronous A C machine substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings.

18. An asynchronous A C machine rotor as defined in any one of claims 1 to 16.

19. An asynchronous A C machine rotor substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings.