EP2229722A2

EP2229722A2 - Commutators

Info

Publication number: EP2229722A2
Application number: EP08806316A
Authority: EP
Inventors: Thomas Mahon Shaw; Jonathan James Bremner
Original assignee: Mechanical Interpole Technologies Ltd
Current assignee: EPSTEIN, JOHN; HYLLOS INVESTMENT LTD.
Priority date: 2007-09-19
Filing date: 2008-09-18
Publication date: 2010-09-22
Also published as: GB0801853D0; WO2009037455A2; GB0718260D0; WO2009037455A3

Abstract

A commutator for an electric motor-generator comprises two opposed commutator elements (3, 4) each providing a continuous circumferential conducting surface (5, 6), to which a DC supply is connected via supply brushes (18, 19). A conductive commutation segment (7, 8) projects axially from each commutator element (3, 4). Three winding brushes (20, 21, 22) are located (120) degrees apart about the commutator. Each brush (20, 21, 22) engages alternately with the commutation segments (7, 8) and freewheeling segments (9, 10) as the commutator rotates. As each brush (20, 21, 22) moves from a commutation segment (7, 8) to a freewheeling segment (9, 10), the presence of the commutation ring (23) connecting the two freewheeling segments serves to inhibit sparking between the brush (20, 21, 22) and the commutation segment (7, 8). Alternatively, a plurality of brushes are provided for each phase, such that the winding current passing through each brush is reduced, thereby to reduce resistive losses.

Description

COMMUTATORS

The present invention relates to commutators for rotating electrical machines that operate as either motors or generators.

Direct current (DC) electrical machines will act as either a motor or a generator. If a DC current is supplied to the stator or field windings the machine acts as a motor. If mechanical power is applied to rotate the machine shaft it will act as a generator. The following discussion relates to an electric motor, but the present invention also applies to generators.

Multiphase permanent magnet brushless motors are extremely versatile and efficient machines offering superior control and efficiency compared with AC synchronous and permanent magnet DC motors in many different applications. In some cases, however, end users require the compactness, high efficiency and long life of a brushless motor but without the high cost of the drive electronics and position-sensing electronics required to operate the brushless motor.

The ability to use a simple, inexpensive mechanical commutation arrangement in place of an expensive electronic drive may allow end users to specify a multiphase PM motor in cost-sensitive applications.

Wound rotor or armature DC "commutator" motors are often used in cases where smooth torque and simple voltage control are prerequisites. However, when the brushes require replacement the motor has to be disassembled which takes significant time. Furthermore, a wound rotor restricts the minimum rotor diameter that is practical to manufacture. A wound rotor also leads to inferior copper slot fill compared with brushless and asynchronous motors. Low-inertia rotors are difficult to achieve because of the inertial mass of the wound armature. The large number of commutator segments needed to achieve smooth torque can lead to increased sparking, which accelerates brush wear and produces high-frequency RF noise.

It would therefore be desirable to provide a rotating electrical machine that overcomes or ameliorates the above problems, or at least provides the public with a useful alternative. Thus, in accordance with a first aspect of the present invention there is provided a commutator for an electric motor-generator, the arrangement, comprising: an electrically conductive commutation surface which comprises, in a cyclic sequence: (a) a first region arranged for permanent connection to the positive rail of a DC supply; (b) a second region; (c) a third region arranged for permanent connection to the negative rail of a DC supply; and (d) a fourth region, the second and fourth regions being electrically connected together via an electrical path; a phased array of electrical connectors arranged for movable contact along the commutation surface, at least some of the connectors being arranged to establish sequential electrical contact with the first, second, third and fourth regions of the commutation surface at different, equally spaced phases, each connector being of a size sufficient to bridge each adjacent pair of regions as it moves from one region to the next and arranged such that, as a first connector bridges the first or third region and the second or fourth region respectively, a second connector bridges the same first or third region and the fourth or second region respectively, the electrical path connecting the second and fourth regions serving substantially to inhibit arcing between the respective first or third region and the first connector as the first connector leaves the said respective first or third region.

Thus the inventors of the present invention have found that when the second and fourth regions of the commutation surface are electrically connected together via a current path, sparking between the first or third regions and the electrical connectors is effectively eliminated. Two possible physical explanations for this are now described.

In a first explanation, it may be assumed that the second and fourth regions become electrically charged by virtue of two of the electrical connectors simultaneously bridging these regions with either the first or second region, and that, when one of the two connector ceases electrical contact with the first or third region as it moves along the commutation surface, two different transient current paths are established: (a) between the second region and the electrical connector; and (b) between the fourth region and the electrical connector. One of these current paths will be direct, i.e. from whichever of the second and fourth regions is directly connected to the connector, and the other current path will be indirect, i.e. from the other of the second or fourth regions via the current path linking the second and fourth regions.

An alternative explanation is that, at the time when one of the electrical connectors which is bridging e.g. the first and second regions ceases electrical contact with the first region, a reverse EMF is generated and current is shunted from the second region to the fourth region via the current path linking the second and fourth regions and then to the electrical connector which is at that time bridging the first and fourth regions and about to cease electrical contact with the fourth region.

It has been found that the effect of spark inhibition is enhanced when the current path linking the second and fourth regions presents a relatively high resistance.

The electrical connectors may be in the form of brushes or alternatively rollers. Rollers provide the advantage of lower friction than brushes, and hence less wear, but they provide only a line contact. Thus, in cases of high loads, and hence high winding current, brushes are the preferred option.

Where the commutator is to be used for a rotary motor-generator, the commutation surface defines a circular path

The circular path may define either a cylindrical surface, or alternatively a substantially planar surface. However, in the preferred embodiment, a cylindrical surface is used.

The present invention could also be applied to linear motors, in which case the commutation surface would define a linear path.

The commutation surface may comprise a number of cyclic sequences of first, second, third and fourth regions. In this case, the first regions would all be electrically connected together and supplied with current from the positive rail of the DC supply, and the third regions would also all be electrically connected together and supplied with current from the negative rail of the DC supply. The second and fourth regions would also be connected together via the high-resistance conductive path.

Although in such an arrangement it would still be possible to provide only three electrical connectors, it is preferred that a number of separate electrical connectors are provided for each phase, so as to distribute the electric current between them, thereby to reduce the resistive losses.

In the case of a rotary commutator with a single cyclic sequence, there are typically provided three connectors spaced at 120 degrees separation about the commutation surface. However, in the case of multiple cyclic sequences, it is still preferred that the connectors be evenly spaced apart, for the sake of structural simplicity, even though alternative arrangements are possible. Thus, for example, in the case of a rotary commutator with three cyclic sequences, and three connectors, there are 18 possible angular positions for each connector. Referring to these positions as 1 to 18, where a first connector is at position 1 , the three positions can be (a) 1 , 3, 5; (b) 17, 1 , 3; (c) 15, 17, 1 ; (d) 1 , 5, 9; (e) 15, 1 , 5; (f) 11 , 15, 1 ; which define evenly spaced configurations. Alternatively, the three positions could be (g) 1, 3, 11; (h) 1, 9, 11; or (i) 1, 9, 17; which define uneven spacings.

A preferred material for the commutation surface is a graphite shell, since graphite exhibits particularly low friction. The graphite shell is preferably formed on an underlying surface of a high-conductivity metal, preferably copper.

The present invention extends to an electric motor-generator comprising a commutator of the above type.

Such an electric motor-generator would typically comprise a number of windings each connected between a respective pair of the electrical connectors.

Where there are three windings, these can be connected together either in a delta or star configuration.

The electric motor-generator preferably comprises a housing in which the electrical connectors are fixedly arranged, the commutation surface being arranged for movement relative to the housing.

In a preferred embodiment, the motor-generator comprises: a housing; a shaft mounted for rotation within the housing; a rotor fixed to the shaft and arranged to provide a magnetic field; a stator positioned about the rotor within the housing and having at least one winding; two electrically conductive commutator segments arranged for electrical engagement with brushes connected to windings; two second freewheeling segments electrically isolated from the two commutator segments and interspersed at respective positions between the two commutator segments; and a connecting freewheeling element, providing an electrical connection between the two freewheeling segments, and having an higher electrical resistance than the two commutator segments. The motor-generator preferably also comprises a positive slip-ring and a negative slip- ring each having a continuous conducting circular perimeter; wherein one of the two electrically conductive commutator segments is electrically connected to the positive slip- ring and the other electrically conductive commutator segment electrically connected to the negative slip-ring.

Preferably, the circumferential position of the first and second commutation members and the contact brushes are so aligned that the conduction periods of the three phases are symmetrically phased so as to produce a three-phase set of balanced 120 electrical degree current square waves and said conduction midpoints of the current square waves are synchronised with the midpoints of rotor field induced emfs of the same phase in the three phase stator windings. This minimizes sparking at the point of commutation.

When the machine is under load, a phenomenon known as armature reaction occurs. This has the effect of moving the midpoints of the rotor field induced emfs in a direction opposite to the direction of rotation of the machine's shaft. To compensate for this, the second contact brushes are aligned at the point of maximum load for the machine. In applications where the operating load of the machine is small or constant, and the direction of rotation is fixed, this, along with the freewheeling arrangement described above, will be sufficient to minimize sparking.

However, there are a number of applications in which the direction of rotation of the machine needs to be reversed. It would therefore be desirable to provide arrangements which allow for this requirement.

Thus, in a preferred embodiment of the present invention, the motor-generator further comprises: a housing for mounting the contact brushes so that they are rotatable around the stator; first and second electromagnets attached to the stator of the motor-generator and aligned parallel to each other, with their north and south poles facing the same direction, and parallel to the axis of the stator, the electromagnets being supplied with current from the dc supply; first and second permanent magnets attached to the brush housing and arranged for rotation about the stator, and located radially in between the two electromagnets and aligned with their north and south poles facing in opposite directions.

This arrangement will cause the brush housing to be rotated towards one or other of the two electromagnets, according to the polarity of the supply rails, thus moving the brushes into the correct position to compensate for the armature reaction generated by the load in either sense of rotation. In the case of an asymmetric load, the position and strength of the two electromagnets can be varied to compensate for the two different brush positions for each sense of rotation.

With the above arrangement, first and second springs are preferably located between the electromagnets and the permanent magnets.

It is well known to those skilled in the art of spring design and manufacture that springs can be engineered to compress under non-linear loads.

With this arrangement, as the load on the machine increases, the current drawn by the electromagnet increases, in turn increasing the attractive power of the electromagnet. This will compress the spring, to a point where the brush housing is moved to the correct position to compensate for the armature reaction generated by a given load. This system can be described as a 'mechanical inter-pole'.

Thus, in a further aspect of the present invention there is provided an electric motor- generator comprising a commutation arrangement for controlling the voltage applied across a plurality of windings, the motor-generator comprising an array of spaced electrical connectors, each winding being connected between a respective pair of the electrical connectors, the array of connectors being arranged for movable contact along a commutation surface which comprises, in an alternating sequence: (a) a region arranged for permanent connection to the positive rail of a DC supply; and (b) a region arranged for permanent connection to the negative rail of a DC supply; the electrical connectors being so positioned with respect to each other and the regions of the commutation surface that, on relative movement between the electrical connectors and the commutation surface, positive and negative voltages are sequentially applied to different respective pairs of the electrical connectors at different phases, the motor- generator further comprising means for adjusting the position of the electrical connectors by an amount dependent on the magnitude of the electrical current supplied to the motor- generator, thereby to adjust the phase at which the voltage is applied across the windings.

With such an electric motor-generator, the position adjusting means conveniently comprises at least one electromagnet supplied with current from the DC supply and at least one permanent magnet arranged such that current passing through the electromagnet causes the electromagnet to move relative to the permanent magnet against a spring bias, and to transmit the movement to the electrical connectors.

There are some applications, however, in which the load on the machine may vary considerably over the course of operation. In this case it is desirable to be able to fix brush position across a continuum of loads.

A limiting factor in motor-generators of the above type is the current-handling capacity when only three contact brushes connected to the windings. However, in machines with more than 2 poles, the number of brushes connected to the windings can be doubled with each doubling of pole number, i.e. 4-pole motors can employ six brushes, 8-pole motors can employ twelve brushes and so forth.

The use of more than three brushes has a number of advantages. Doubling the number of brushes halves the maximum current density for a given size of commutator. This reduces I²R losses by a factor of two. The effective current-handling capacity of the commutator is thus doubled, greatly enhancing brush life, or alternatively the commutator size is reduced to a half of what it would otherwise be, thus reducing the overall size, weight and cost of the machine. This novel feature can be applied to any mechanically commutated DC machine of similar topology to the invention described herein, and does not require either the freewheeling element or the mechanical inter-pole arrangement described above.

Thus, in accordance with a further aspect of the present invention there is provided an electric motor-generator comprising a commutation arrangement in which an array of spaced electrical connectors are arranged for movable contact along a commutation surface which comprises, in an alternating sequence: (a) a region arranged for permanent connection to the positive rail of a DC supply; and (b) a region arranged for permanent connection to the negative rail of a DC supply; the electrical connectors being so positioned with respect to each other and the regions of the commutation surface that, on relative movement between the electrical connectors and the commutation surface, positive and negative voltages are sequentially applied to different respective pairs of the electrical connectors at n different phases, wherein the number of electrical connectors is a multiple of 2/7.

Further aspects and features of the present invention will become apparent from the following description, which is given by way of example only.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is an exploded view of a commutator according to an embodiment of the present invention;

Figures 2(a) to (d) illustrate of the operation of the freewheeling segments and freewheeling element of the commutator of Figure 1 ;

Figure 3 illustrates a commutator having four poles;

Figure 4 is an exploded view of the mechanical inter-pole arrangement according to a preferred embodiment of the present invention; and

Figure 5 illustrates the mechanical inter-pole arrangement shown in Figure 7 when assembled.

Referring to Figure 1 , a commutator consists of a circular moulded base 1 with a central hub 2 for supporting commutator components. Mounted on the hub 2 are first and second complementarily opposed commutator elements 3, 4 each made from graphite- coated copper and comprising a respective ring-shaped plate having a continuous conducting circular periphery 5, 6 respectively. A conductive commutation segment 7, 8 projects axially from adjacent each periphery 5, 6. On the hub 2 interspersed between the commutation segments 7, 8 are two neutral, freewheeling segments 9, 10. The commutation elements and freewheeling segments 9, 10 are separated by insulation rings 11. The commutator components are fixed to the hub 2 by two fixing screws 12, 13 located within matching threaded bores 16, 17 in the base 1. The fixing screws 12, 13 are located within electrically insulating sleeves 14, 15 to prevent a short-circuit of the commutator components.

The commutator is coupled to, or disposed on, a motor shaft with electrical connectors in the form of brushes which are arranged for slidable engagement with the commutator surfaces to connect a supply of DC electrical current to motor windings. A first supply brush 18 is connected to the positive side of the DC supply and positioned for continuous contact with the conductive slip-ring surface 5 of the first commutator element 3. A second supply brush 19 is connected to the negative side of the DC supply and positioned in continuous contact with the conductive slip-ring surface 6 of the second commutator element 4. By this arrangement, the first commutation segment 7 is a positive segment and the second commutation segment 8 is a negative segment.

In the case of a three-phase motor, there are provided three phase or winding brushes 20, 21 , 22, which are spatially located 120 mechanical degrees apart about the commutator. The phase brushes 20, 21 , 22 engage alternately with the commutation and freewheeling segments in the rotational order 7, 10, 8, 9 as the commutator rotates.

For the purposes of the present description, the direction of rotation of the commutator is clockwise as viewed in the drawings. A disc-shaped high-resistance freewheeling element 23 is electrically connected between the freewheeling segments 9, 10. The freewheeling element 23 is made from a metal selected from: stainless steel, mild steel and copper, or is alternatively made from ferrite.

The function of the freewheeling element 23 will now be described, in accordance with the first possible physical explanation for the inhibition of arcing described above. As each phase brush 21 , 22, 23 bridges one, e.g. 7, of the two commutator segments 7, 8, and one, e.g. 9, of the two freewheeling segments 9, 10, one of the other one two phase brushes 21 , 22, 23 at the same time also bridges the same commutator segment 7 and the other, e.g. 10, of the two freewheeling segments 9, 10. As a result, the two freewheeling segments 9, 10 become charged in accordance with the polarity of the commutator segment 7. As the phase brush 21 , 22, 23 passes out of contact with the commutator segments 7, a first transient current passes between the freewheeling segments 9 and the phase brush in a direction such that the charge stored on the freewheeling segment is neutralised. However, the other freewheeling segment 10 is also connected to the one freewheeling segment 9 via the high-resistance freewheeling element 23, a second transient current is caused to pass from the other freewheeling segment 10 via the freewheeling element 23 to the phase brush. The high-resistance of the freewheeling element 23 causes the second transient current to be longer-lasting than the first transient current. The effect of this is that sparking between the commutating element 7 and the phase brush which would otherwise occur is substantially inhibited. However, there would still be an appreciable reduction in sparking even if the freewheeling element 23 were made from a low-resistance conductor, such as metal.

The workings of the freewheeling segments 9, 10 and the freewheeling element 23 will now be explained with reference to Figures 2(a) to (d), in which the positive and negative supply brushes have been omitted for the sake of clarity. The arrangement illustrated is electrically equivalent to that described in the commutator described above. The commutator rotates clockwise as viewed in the drawings.

In Figure 2(a), the first phase brush 20 is fully in contact with the negative commutation segment 8. The other two phase brushes 21 , 22 are positioned so as to bridge the other commutation segment 7, and the two freewheeling segments 9, 10. It should be noted that the arc angles of the freewheeling segments 9, 10 are such that a phase brush can never bridge both commutation segments 7, 8 as this would cause a supply short-circuit.

In Figure 2(b), the commutator has rotated 60 degrees clockwise until first winding brush 20 is just in contact with freewheeling segment 9 and about to break contact with the negative commutation segment 8. In accordance with the second, alternative possible physical explanation for the inhibition of sparking described above, as brush 20 breaks contact with the negative commutation segment 8, a reverse EMF is produced and current is shunted via the freewheeling element 23 from freewheeling segment 9 to the second brush 22. This helps suppress any voltage spike between the brush 20 and commutation segment 8.

In Figure 2(c), the commutator has rotated a further 120 degrees clockwise until third winding brush 22 is in contact with freewheeling segment 9 and about to break contact with the commutation segment 8. Again, as brush 22 breaks contact with the commutation segment 8, it is thought that the current may be shunted via the freewheeling element 23 from freewheeling segment 9 to the second brush 21. This helps suppress any voltage spike between the brush 22 and commutation segment 8. In Figure 2(d), the commutator has rotated a further 120 degrees clockwise until second winding brush 21 is in contact with freewheeling segment 9 and about to break contact with the commutation segment 8. Again, as brush 21 breaks contact with the commutation segment 8, it is thought that the current may be shunted the via freewheeling element 23 from freewheeling segment 9 to the second brush 20. This helps to suppress any voltage spike between the brush 21 and commutation segment 8.

Figure 3 illustrates the arrangement of four commutating segments 7, T, 8, 8', four neutral, freewheeling segments 9, 9\ 10, 10", the freewheeling element 23 and two sets of three brushes 20, 21 , 22 and 20', 21', 22" in a 4-pole electric-motor generator. Each of the three pairs of brushes labelled 20, 20" and 21 , 21' and 22, 22" are positioned in relation to the commutation surface at the same phase and can therefore share the current load, resulting in reduced resistive losses. In this arrangement, all of the four neutral, freewheeling segments 9, 9', 10, 10' are connected to the high-resistance freewheeling element 23.

Figures 4 and 5 illustrate an arrangement for automatically adjusting the phase of the voltage waveform applied to the windings of the motor-generator when operating as a motor. The three phase brushes 20, 21 22 are located within corresponding rectangular apertures within an annular housing 24. The housing 24 is arranged for rotation about a stationary support element 31 so as to permit limited rotation of the phase brushes 20, 21 , 22. Two electromagnets 25, 26 are connected to the DC power supply and mounted parallel to each other and also parallel to the axis of the motor-generator on the support element 31. Between the two electromagnets 25, 26 are arranged two cylindrical permanent magnets 27, 28, which are mounted in two corresponding apertures in the housing 24 and arranged so that their magnetic poles face in opposite directions. Two springs, 29, 30 are arranged between the permanent magnets 27, 28 and the electromagnets 25, 26.

The function of this arrangement will now be described. When operating as a motor, the electric current in the windings is determined to a large extent by the load applied to the motor. It is this fact which enables the phase of the applied voltage waveform to be varied in dependence on the load. The current is supplied to the windings of two electromagnets 25, 26 which are electrically connected in series. The associated magnetic field interacts with the magnetic field from the two permanent magnets 27, 28 to cause the annular housing 24 to rotate about the axis of the motor against the biasing return force of two coil springs 29, 30. The extent, and sense, of the rotation is directly related to the magnitude, and direction, of the current in the motor windings. As a result, the phase brushes 20, 21 , 22 are caused to contact the commutation surface of the motor at respective positions such that the desired phase of voltage waveform is applied to the motor windings.

Where in the above description reference has been made to integers or elements having known equivalents, then it is intended that such equivalents are included within the scope of the present invention.

Although preferred embodiments of the invention have been described above, it is nevertheless understood that variations, improvement or modifications can take place. For example, in alternative embodiments, different topological commutator arrangements may be used. For example, the brush assembly could be mounted inside the circumference of the commutator, rather than outside, thus reducing the overall volume of the device. Alternatively, the entire device could be configured as a disc, with the brushes being arranged perpendicular to the plane of the disc.

It should also be noted that, while the above description and drawings relate to a 2-pole commutator, a commutator of n poles where n = 2, 4, 6, 8 etc. can be constructed by increasing the number of positive, negative and freewheeling segments.

Claims

CLAIMS:

1. A commutator for an electric motor-generator, the commutator comprising: an electrically conductive commutation surface which comprises, in a cyclic sequence: (a) a first region arranged for permanent connection to the positive rail of a DC supply; (b) a second region; (c) a third region arranged for permanent connection to the negative rail of a DC supply; and (d) a fourth region, the second and fourth regions being electrically connected together via an electrical path; a phased array of electrical connectors arranged for movable contact along the commutation surface, at least some of the connectors being arranged to establish sequential electrical contact with the first, second, third and fourth regions of the commutation surface at different, equally spaced phases, each connector being of a size sufficient to bridge each adjacent pair of regions as it moves from one region to the next and arranged such that, as a first connector bridges the first or third region and the second or fourth region respectively, a second connector bridges the same first or third region and the fourth or second region respectively; the electrical path connecting the second and fourth regions serving substantially to inhibit arcing between the respective first or third region and the first connector as the first connector leaves the said respective first or third region.

2. A commutator as claimed in claim 1 , wherein the commutation surface defines a circular path.

3. A commutator as claimed in claim 2, wherein the circular path defines a cylindrical surface.

4. A commutator as claimed in claim 2, wherein the circular path is substantially planar.

5. A commutator as claimed in any one of claims 1 to 4, wherein the commutation surface comprises a plurality of cyclic sequences of first, second, third and fourth regions.

6. A commutator as claimed in claim 5, and comprising 3n electrical connectors where π is a positive integer from 1 to the number of cyclic sequences.

7. A commutator as claimed in claim 6, wherein the electrical connectors are evenly spaced.

8. A commutator as claimed in any one of claims 1 to 7, wherein the commutation surface comprises a graphite shell.

9. An electric motor-generator comprising a commutator as claimed in any one of claims 1 to 8.

10. An electric motor-generator as claimed in claim 9, further comprising a plurality of windings each winding being connected between a respective pair of the electrical connectors.

11. An electric motor-generator as claimed in claim 10, and comprising three windings connected to the electrical connectors in a delta configuration.

12. An electric motor-generator as claimed in claim 10, and comprising three windings connected to the electrical connectors in a star configuration.

13. An electric motor-generator as claimed in any one of claims 9 to 12, comprising a housing, wherein the electrical connectors are fixedly arranged with respect to the housing and the commutation surface is arranged to move relative to the housing.

14. An electric motor-generator comprising a commutator for controlling the voltage applied across a plurality of windings, the motor-generator comprising an array of spaced electrical connectors, each winding being connected between a respective pair of the electrical connectors, the array of connectors being arranged for movable contact along a commutation surface which comprises, in an alternating sequence: (a) a region arranged for permanent connection to the positive rail of a DC supply; and (b) a region arranged for permanent connection to the negative rail of a DC supply; the electrical connectors being so positioned with respect to each other and the regions of the commutation surface that, on relative movement between the electrical connectors and the commutation surface, positive and negative voltages are sequentially applied to different respective pairs of the electrical connectors at different phases, the motor- generator further comprising means for adjusting the position of the electrical connectors by an amount dependent on the magnitude of the electrical current supplied to the motor-generator, thereby to adjust the phase at which the voltage is applied across the windings.

15. An electric motor-generator as claimed in claim 14, wherein the position adjusting means comprises at least one electromagnet supplied with current from the DC supply and at least one permanent magnet arranged such that current passing through the electromagnet causes the electromagnet to move relative to the permanent magnet against a spring bias, and to transmit the movement to the electrical connectors.

16. An electric motor-generator comprising a commutator in which an array of spaced electrical connectors are arranged for movable contact along a commutation surface which comprises, in an alternating sequence: (a) a region arranged for permanent connection to the positive rail of a DC supply; and (b) a region arranged for permanent connection to the negative rail of a DC supply; the electrical connectors being so positioned with respect to each other and the regions of the commutation surface that, on relative movement between the electrical connectors and the commutation surface, positive and negative voltages are sequentially applied to different respective pairs of the electrical connectors at n different phases, wherein the number of electrical connectors is a multiple of 2/1.