WO2010070285A2 - Axial flux motor and generator assemblies - Google Patents
Axial flux motor and generator assemblies Download PDFInfo
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- WO2010070285A2 WO2010070285A2 PCT/GB2009/002898 GB2009002898W WO2010070285A2 WO 2010070285 A2 WO2010070285 A2 WO 2010070285A2 GB 2009002898 W GB2009002898 W GB 2009002898W WO 2010070285 A2 WO2010070285 A2 WO 2010070285A2
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K3/00—Details of windings
- H02K3/04—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors
- H02K3/18—Windings for salient poles
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K1/00—Details of the magnetic circuit
- H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
- H02K1/12—Stationary parts of the magnetic circuit
- H02K1/14—Stator cores with salient poles
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K1/00—Details of the magnetic circuit
- H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
- H02K1/12—Stationary parts of the magnetic circuit
- H02K1/16—Stator cores with slots for windings
- H02K1/165—Shape, form or location of the slots
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K1/00—Details of the magnetic circuit
- H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
- H02K1/22—Rotating parts of the magnetic circuit
- H02K1/24—Rotor cores with salient poles ; Variable reluctance rotors
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K1/00—Details of the magnetic circuit
- H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
- H02K1/22—Rotating parts of the magnetic circuit
- H02K1/26—Rotor cores with slots for windings
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K1/00—Details of the magnetic circuit
- H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
- H02K1/22—Rotating parts of the magnetic circuit
- H02K1/27—Rotor cores with permanent magnets
- H02K1/2706—Inner rotors
- H02K1/2713—Inner rotors the magnetisation axis of the magnets being axial, e.g. claw-pole type
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K19/00—Synchronous motors or generators
- H02K19/02—Synchronous motors
- H02K19/04—Synchronous motors for single-phase current
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K19/00—Synchronous motors or generators
- H02K19/16—Synchronous generators
- H02K19/22—Synchronous generators having windings each turn of which co-operates alternately with poles of opposite polarity, e.g. heteropolar generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K23/00—DC commutator motors or generators having mechanical commutator; Universal AC/DC commutator motors
- H02K23/54—Disc armature motors or generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K29/00—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices
- H02K29/06—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices with position sensing devices
- H02K29/08—Motors or generators having non-mechanical commutating devices, e.g. discharge tubes or semiconductor devices with position sensing devices using magnetic effect devices, e.g. Hall-plates, magneto-resistors
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K3/00—Details of windings
- H02K3/04—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors
- H02K3/24—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors with channels or ducts for cooling medium between the conductors
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K3/00—Details of windings
- H02K3/04—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors
- H02K3/28—Layout of windings or of connections between windings
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F38/00—Adaptations of transformers or inductances for specific applications or functions
- H01F38/14—Inductive couplings
- H01F2038/143—Inductive couplings for signals
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F38/00—Adaptations of transformers or inductances for specific applications or functions
- H01F38/18—Rotary transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K21/00—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets
- H02K21/12—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with stationary armatures and rotating magnets
- H02K21/24—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with stationary armatures and rotating magnets with magnets axially facing the armatures, e.g. hub-type cycle dynamos
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S310/00—Electrical generator or motor structure
- Y10S310/06—Printed-circuit motors and components
Definitions
- the present invention relates to axial flux motor and generator assemblies, suitable particularly for use with modern electric and hybrid road vehicles.
- Pure electric vehicles and series hybrids need to be able to cope with all inclines with purely electrical motor drive.
- weight can be saved by doing away with as much of the mechanical transmission as possible, and this may advantageously be achieved by having a separate motor driving each driven wheel.
- a given vehicle will have a given total motor mass, and so this is more evenly distributed if all wheels are driven, allowing a further reduction in structural mass.
- fast electronic control such as anti-yaw and anti-spin control to add to ABS and distributed braking force systems that have already demonstrated advantage.
- Road wheel rotational speeds are modest by comparison to the speeds that motors can run at.
- the torque that is required is defined by the higher of desired acceleration and required hill climbing specification at full vehicle load (which may include towing). Acceleration requirements depend on the class of vehicle being designed, for example in sports cars acceleration requirements will exceed those for hill climbing. However, high acceleration or braking loads can only occur for short periods of time, and generally the requirement to climb hills will be the more onerous design consideration because hill climbing needs to be sustained.
- Electric traction motors are generally of a type that have a capability to deliver nearly constant torque right across their rated speed range. Motors for direct gear-less drive of road wheels will thus have a theoretical ability to deliver peak torque all the way to full speed. However, theoretically such motors would have very high powers at full rotation speed, which would stretch the ability of energy storage and control systems. Thus it is likely that most vehicles will be constrained to an envelope, typified by Fig. 1 , in which the peak torque is available to the point where it intersects the design peak power curve. At higher rotational speeds the motor controllers would then limit the vehicle to some performance envelope, shown in Fig. 1 as a simple constant power curve.
- Gearless drive motors are designed to meet this peak torque requirement, and are thus oversized for all other aspects of motoring. This is an acceptable cost when set against all the other gains, especially as more than compensating weight losses will result from the absence of conventional transmissions. Lighter motors and lighter vehicles can be obtained if a fixed reduction gear is used, however for the purposes of what follows it is to be understood that it is desirable to obtain motors with the highest torque to mass ratio, sufficient to be used as gearless drive motors, but also applicable to geared designs if that is a vehicle design aim.
- a permanent magnetic field B is applied radially between magnetic pole pieces 3 across a gap between a stator 1 and a rotor 2, and conductors 4 running axially along the rotor 2 are fed current I via a commutator (not shown).
- Current and magnetic field are at right angles and so a tangential force is generated.
- the magnetic properties of magnets and air are such that it is relatively simple to get a flux that is close to the saturation flux of the rotor iron across the gap, so there is a practical but easily achieved upper limit to flux density.
- the currents can take up only a small part of the radial depth of the rotor, and heat generation in the windings is the practical limit, so the interaction of current and magnetic field is limited by geometric considerations and material properties.
- PM DC motors have essentially linear relationships between electrical and mechanical properties because the strength of the magnetic field is nearly constant: the motor 'back e.m.f.' is almost directly proportional to rotational speed, and torque is proportional to current, until some limit is reached. Whilst PM motors can have high torque to weight ratios and are simple, these fixed properties put demands on the electronic controllers: they have to produce very high currents at low voltages for starting, acceleration and hill climbing, and high voltages at low current for high speed cruising.
- Fig 3A shows the force on a conductor carrying a current in the field produced by a permanent magnet
- Fig 3B shows the force on a conductor in the field produced by an electro-magnet, which are clearly the same.
- Figure 4 by comparison shows the force between two parallel wires in a medium of relative permeability ⁇ r .
- the expressions below show the relationships between the current and the force per unit length.
- the governing relations for Fig 3A and Fig 3B are as follows. The force on a conductor of length 1 carrying a current I in a magnetic field of flux density B is simply given by their product, ie
- the current that can be carried is a function of the electrical conductivity of the conductor, its cross sectional area, its thermal conductivity and the properties of the design in being able to remove heat.
- the maximum flux density is determined both by the saturation flux density of any soft magnetic materials used to make the magnetic circuit, the length of the air path of the flux, and the strength of the means (permanent magnets or field windings), to generate flux.
- the voltage necessary to turn a motor at a given speed is the sum of the voltage necessary to sustain the current against the winding resistance, plus the motor ' e.m.f. ' given above.
- motors can be understood to be governed by simple proportionality between two pairs of quantities, ie current and torque, and voltage and rotational speed. It can also be understood that
- the basic geometry is of two discs arranged to face one another, one disc carrying conductors extending radially at a constant angular pitch such that the current in the conductors follows the path in Fig 5.
- the other disc has areas of permanent magnet material spaced around its face with alternate polarity and the same number of areas as there are radial "spokes" of the conductors, and magnetic flux perpendicular to the plane of the disc. Rings of soft magnetic material are placed axially outside the pair of discs to return the flux emerging from one axial pathway into the two neighbouring pathways with the opposite polarity.
- Fig. 6 shows the magnetic flux pattern with which the current of Fig. 5 can interact.
- an axial flux motor assembly comprising: a stack of first and second discs arranged alternately such that there is a gap allowing rotation between each disc, there being at least one first disc and at least one second disc in the stack; the or each said first disc being mounted so as to be rotatable, and the or each said second disc being fixed in position, wherein each of the first and second discs comprises sectors of magnetic material arranged on a face of the disc, between each of which sectors is a radially- extending section of a conductive path for conducting electric current, the sectors of magnetic material on the or each first disc being arranged at a first constant pitch, and the sectors of magnetic material on the or each second disc being arranged at a second constant pitch, where the said first and second constant pitches may or may not be equal; and wherein, when electric current flows in the said radially-extending sections of the conductive path, magnetic flux runs per
- a conventional field wound motor in which the control of current to the armature and the field are independent, the field winding current directly controls the flux density, up to the saturation of the soft magnetic material used.
- a disadvantage of the conventional field wound motor is that the electrical power used to sustain current in the field winding is simply lost, and detracts from the power efficiency of the motor.
- An advantage of such an arrangement is that when there is a requirement for a wide motor speed range, and also a power envelope limitation, as shown by Fig. 1 , it is possible to decrease the field strength when the motor is running at high speed and low load. This has the effect of reducing the motor back e.m.f., and allows the controller to have a more limited dynamic range.
- An axial flux motor assembly embodying the present invention offers a significant advantage over the prior art, because the underlying principles of a field wound motor can be delivered in a design in which all currents interact with magnetic fields, and so there is no current simply 'wasted' in generating the field.
- An advantage of the present invention is that the torque is proportional to the product of the two currents, and if they are the same, to the square of this current.
- the back e.m.f. is proportional to the product of rotational speed and the current. This is a good match to a road vehicle drive application because it maximises torque at low speed, and allows lower drive voltages at part torque and high speed, without trading any of the power efficiency at cruise speed which naturally results from building machines capable of producing high torque.
- the simplest machine employing the mutually-coupled principle consists of two discs, one rotating (rotor) and one stationary (stator). Each carries conductors wound around sectors of magnetic material. Since it is the radial elements of the currents between the magnetic sectors that are important, the direction of the current in the windings must be alternating between adjacent radial conductors.
- the simplest representation is of a single serpentine winding as in Fig. 5, in which case it can be seen that windings need not form complete turns.
- Machines can be advantageously built using more than two discs, preferably adding a rotor and a stator each time.
- the discs in a set may be connected in series or parallel, or a combination, to suit the voltage and current specification.
- an axial flux generator assembly comprising: a stack of first and second discs arranged alternately such that there is a gap allowing rotation between each disc, there being at least one first disc and at least one second disc in the stack; the or each said first disc being mounted so as to be rotatable, and the or each said second disc being fixed in position, wherein each of the first and second discs comprises sectors of magnetic material arranged on a face of the disc, between each of which sectors there is a radially-extending section of a conductive path for conducting electric current, the sectors of magnetic material on the or each first disc being arranged at a first constant pitch, and the sectors of magnetic material on the or each second disc being arranged at a second constant pitch, where the said first and second constant pitches may or may not be equal; and wherein, when electric current flows in the said radially- extending sections of the conductive paths, magnetic flux runs perpendicular to the faces of the discs in axially-extending flux paths, such
- An assembly embodying the first aspect of the present invention is preferably also operable to function as a generator.
- the polarities of the currents flowing in adjacent sections of the conductive path when the assembly is in use are mutually opposite for all sections.
- the polarities of the currents flowing in adjacent sections of the conductive path when the assembly is in use are mutually opposite for all sections except for two sections on either side of a current feed point where they are the same.
- the magnitude of the current in the conductive path of the or each first disc is desirably the same as that of the current in the conductive path of the or each second disc.
- the conductive path may be formed of a conductive winding.
- the or each of the first and second discs may be made of electrically- conductive material and the conductive path may be formed by the disc material surrounding the sectors of magnetic material.
- the electrically-conductive material may be an aluminium alloy. Slots may be provided in the disc so as to define the conductive path.
- the sectors of magnetic material may comprise holes in the disc filled with high flux density soft magnetic material. If slots are provided in the disc so as to define the conductive path, the slots may be arranged so as to interconnect with the holes.
- Each of the first and second discs may have a plurality of mutually-independent conductive paths.
- Electrical current may be delivered to the discs, and/or drawn from the discs, by means of inductive coupling.
- first discs and second discs there are two such first discs and two such second discs, forming two sets each containing a first disc and a second disc adjacent to it, each set having its own flux return portions, and one set of the first and second discs is permanently offset from the other set of the first and second discs by an amount equal to half the width of one of the said sectors.
- the number R of sectors of magnetic material on the or each first disc differs from the number S of sectors of magnetic material on the or each second disc, and the current in the conductive path of the or each first disc is reversed when the or each first disc has rotated by an angle equal to 720(1/R-1/S) degrees.
- R may be an even number and S may be a number which is a multiple of four.
- Figure 1 is a graph of hill climbing ability against road speed
- Figure 2 (described above) is a diagram of a prior art two pole permanent magnet DC motor
- Figures 3A and 3B are diagrams showing the force on a conductor in a magnetic field
- Figure 4 is a diagram showing the force between two conductors
- Figure 5 is a diagram showing a conductive path in a prior art axial flux motor assembly
- Figure 6 (described above) is a diagram showing a magnetic flux pattern with which current in the conductive path of Figure 5 can interact;
- Figure 7 is a diagram of an axial flux motor assembly embodying the present invention.
- Figure 8 is a circuit diagram
- Figure 9 is a diagram of another axial flux motor assembly embodying the present invention.
- Figure 10 shows a disc which can be used in the assembly of Fig. 7 or 9;
- Figure 11 shows the disc of Figure 10 with the conductive path of Figure 5 overlaid
- Figure 12A shows a plan view of another disc which can be used in the assembly of Fig. 7 or 9, and Figure 12B shows a sectional view taken along the line Y-Y in Figure 12A;
- Figure 13A shows part of the disc of Figure 10 in more detail
- Figure 13B shows a part of Figure 13A in further detail
- Figure 13C shows a sectional view taken along the line Z-Z in Figure Fig. 13A
- Figures 13D, 13E and 13F show respective views for use in explaining how part of the disc may be made
- Figure 13G illustrates another detail of Figure 13A
- Figure 13H is a sectional view of the disc of Figure 12 corresponding to Figure 13C;
- Figure 14 shows a straightened-out radial view of part of an assembly embodying the present invention
- Figures 15A, 15B and 15C show diagrams for use in explaining the angular relationship between discs;
- Figures 16A to 16 F show respective current diagrams;
- Figures 17A to 17C show switching circuitry corresponding to the current diagrams of Figures 16A to 16F;
- FIGS. 18A to 18D illustrate various winding possibilities
- Figure 19 is a diagram of a device for use in an assembly embodying the present invention.
- Figure 20 shows a circuit for operating the device of Fig. 19
- Figure 21 is a timing diagram of the circuit of Fig. 20.
- Figure 22 is a diagram for use in explaining a method of building a motor assembly embodying the present invention.
- An axial flux motor assembly embodying the first aspect of the present invention will now be described in detail, but it will be understood that an axial flux generator assembly of similar design can be made. Moreover, the axial flux motor assembly described below may be designed so as to be reversible when circumstances require its use as a generator.
- an axial flux motor assembly 10 embodying the present invention comprises a stack of first and second discs 20a, 20b arranged alternately such that there is a gap allowing rotation between each disc 20a, 20b.
- first discs 20a there are only two first discs 20a and only two second discs 20b in the stack, but in a practical motor there may be a set of several first discs 20a and a set of several second discs 20b.
- the first discs 20a are mounted on a rotatable shaft 40, whilst the second discs 20b are fixed in position.
- the first and second discs 20a, 20b each comprise sectors 200 of magnetic material arranged within the disc 20a, 20b, between each of which sectors 200 is a radially-extending section 202 (hereafter termed a "conductor” or “spoke") of a conductive path 201 for conducting electric current.
- the polarity of the current flowing in adjacent conductors 202 when the assembly is in use is mutually opposite.
- the sectors 200 of magnetic material on the first and second discs 20a, 20b are arranged at a constant angular pitch, but the pitch of the sectors of magnetic material on the first disc may or may not be the same as those on the second disc.
- magnetic flux runs perpendicular to the faces of the discs 20a, 20b in axially-extending flux paths, such that, considering the first disc(s) independently of the second disc(s), the magnetic flux in one axially-extending flux path runs in an opposite direction to that in the immediately-adjacent flux paths on each side of it, and is returned by flux return portions 30 of magnetic material provided at each end of the assembly 10.
- the total flux is the super-position of the flux of the first disc(s) and the second disc(s).
- the assembly 10 further comprises means 56 for sensing relative angular position between the first and second discs 20a, 20b and switching circuitry 50 for reversing the direction of current flowing in the conductive path 201 in one of the first disc 20a or the second disc 20b in correspondence to rotation thereof relative to the other of the first disc or the second disc in such a way as to effect continuous rotation of the first disc.
- Angular position sensing may be achieved by conventional means, such as optical sensing or Hall Effect magnetic sensing (not shown).
- Means 55 are provided for supplying DC to the first discs 20a.
- Switching circuitry 50 comprises a conventional bridge drive circuit comprising semiconductor switches S1 to S4 operable to drive the stator windings with electrical current of either polarity by closing either S1 and S4 together, or S2 and S3 together.
- the output of conventional angular position sensing means 56a are fed to control and drive electronics 56b which also accept external commands in respect of stop-start and direction signals and produces drive signals to S1 to S4 accordingly.
- the flux return portions 30 need to be of low loss steel (so that magnetic cycling does not engender heating). They are preferably constructed with conventional motor steels in laminated form such as to suppress eddy currents, such as the spiral arrangement disclosed in FR2639486, or of composite iron powder or iron wire materials capable of supporting magnetic flux and suppressing eddy current generation.
- the flux return rings 30 are mounted on each of the first and second discs 20a, 20b, but there may be lower losses if the flux return rings 30 rotate (or stay still) with the field that is not switched.
- disc 20b there may be one disc 2Oa 1 and the function of disc 20b may be split into two discs 20c provided at respective ends of the stack, which discs 20c are of half the thickness of each of the first and second discs 20a, 20b and carry sectors 205 of magnetic material of half the thickness of those on the first and second discs 20a, 20b .
- the arrangement may also be reversed in that the fixed disc 20b may be of full thickness and placed centrally between two half thickness discs 2Od mounted to the rotating shaft.
- the arrangement may be used with multiple discs, in the general form that if there are N full thickness discs of one kind 20a or 20b then there are N-1 full thickness discs and two half thickness discs of the other kind 20c or 2Od at each end of the stack.
- the first and second discs 20a, 20b are substantially identical.
- the discs 20a, 20b may take the form of the disc shown in Fig. 10, in which the main structure of the disc 250 is aluminium alloy. All alloys of aluminium conduct electricity well, and have a low density and can have considerable mechanical strength. Considering only the aluminium part, it can be seen that cut-outs 260 and slotted gaps 270 form a conductive path 201 which has the form of Fig. 5.
- Fig. 11 shows the assembly of Fig. 10 with the conductive path 201 of Fig. 5 overlaid.
- cutting out the conducting shape from the aluminium alloy disc will reduce its mechanical strength and stiffness considerably, this can be restored by adding composite material and using of non-conducting high strength resins.
- the outer circumference of the discs can be considerably strengthened by machining a slot and winding a high strength fibre into it, bonded by resin.
- Such a disc is particularly suitable for slow speed, high torque motors.
- the magnetic sectors 200 may be formed of a soft magnetic material capable of sustaining high flux density, such as laminated motor steel or resin bonded iron filings. Between the magnetic sectors 200 and the windings 230 are layers 236 of material provided for added structural strength and protection.
- the magnetic sectors 200 of the present invention comprise magnetic material 265 filling the cut-outs 260 in the aluminium alloy disc 250.
- the magnetic material 265 is a soft magnetic material capable of sustaining high flux density, such as laminated motor steel or resin bonded iron filings, and it is convenient that the laminations are as shown in the detail of Fig. 13A.
- an electrically-insulating barrier 266 is provided around the edges of the magnetic material 265, so that the electric current is constrained to the intended path and does not short into and across the magnetic material 265.
- This barrier 266 is conveniently combined with the means of fixing, using a glass tape around the boundary, which is impregnated with a resin, such as epoxy.
- the conductors 202 which can conveniently be regarded as radial conducting spokes, have a section with two curved end portions and a substantially straight middle portion. This allows the laminations to be cut into short and long segments 265a, 265b with 'butt' joins 265c where the flat and curved sections of the conductor 202 join. It is important that any gap at the 'butt' joint 265c is small compared to the gap between discs 20a, 20b, so that the magnetic reluctance of the path is little changed.
- the magnetic portions of the discs may be formed using a composite 265d of iron filings and resin, filling the space completely as shown in Fig 13F: where the composite is not electrically conductive the electrical insulating barrier shown in Figures 13B and 13C may be omitted.
- Fig 13G shows two cooling channels 202a in the centre of the conductor 202 which can be used for liquid cooling of the conductors by conventional means.
- Fig 13H shows the conductors formed with a bundle of wires, representing a sectional detail corresponding to Fig 12. The gaps between wires may also be advantageously used to provide liquid cooling channels in this form of construction.
- a small gap in the magnetic medium means that the effective permeability remains high, it is the case that this gap is constant in the axial direction, and is the design parameter that is used to control the strength of magnetic field that occurs for given currents (see below).
- the gaps represent the predominant magnetic reluctance in any magnetic flux path. This has the effect of spreading the flux out evenly across the gap (considered in respect of the field from each set of currents separately). The consequence of this is that the force generated by interaction with a current now becomes essentially linear across most of the face of the magnetic sectors, and the 1/r dependency (where r is separation as per Fig 4) that is expected for the interaction between two parallel currents is linearised.
- Each conductive path 201 can be allowed to generate a flux density only half that of saturation, since it is necessary that where fields from the two sets of discs reinforce they do not saturate the magnetic material.
- the gaps between the rotating discs have to be set such that the required field will be generated across them by the currents in the conductors, and to obtain maximum torque this field should be at half saturation flux density at maximum operating current.
- the ability of a conducting "spoke” to conduct electricity is something that scales with the cross section of a spoke, which scales as the square of the linear dimensions.
- H is the H field A very simple relationship then emerges, which is that the Amp Turns necessary can be expressed simply per millimetre of gap for a given flux, thus
- the gap is g and the current density is J
- the cross sectional area is d 2 and the current is J . d 2 .
- the table below shows the gap for a current density of 5 Amps per square mm and a material saturation flux density of 2 Tesla.
- each axial flux path (neglecting the curvature resulting from the interleaving of first and second discs) will be separated into a zone between like currents where the flux is essentially zero, and a zone between unlike currents in which the flux is essentially saturated (assuming limiting currents). It is then possible to make an interpretation that the forces generated are such that the flux lines in high flux density spread out and for all areas of zero flux to reduce in area.
- Non-ferrous conducting material exhibits a very low magnetic permeability, and this can be treated as being that of free space, or equivalently of the air gaps.
- Figs. 13C to 13H show a conductor 202 with a cross section which is curved in the axial direction towards each disc face, with a finite width at the point where it comes to the edge of the disc and the air gap, and which may have a straight section 202b in the middle of the disc thickness to aid construction with lamina of motor steel.
- Figure 15A shows that the angular relationship between the first and second discs 20a, 20b can be considered as the general case, ie no particular relationship, with conductors 202 of the cross section of Fig 13C.
- Fig 15B shows the same first and second disc set in a position in which they are just beginning to separate (or equivalently are getting close together).
- the optimal shape for a conductor 202 can be imagined as somewhat equivalent to a bridge pier in a river: it needs maximised cross section to carry current, but if it is also relatively narrow at the edges on the disc face (such that it is also compatible with the need for w > g) then torque will be nearly constant and continuous until the direct flux path across the gap is closed off by the overlap of the conductors.
- first and second discs 20a, 20b and flux return rings 30 can be mounted on a single shaft, with a phase separation of half a sector between each set. In the general case both will be able to provide torque, and when one is passing through the alignment position, the other will have currents in the centre of the fields and be able to produce maximum torque, so starting is assured. If the conductors 202 are shaped, with the 'bridge pier' effect, so as to have a minimum angle of alignment when there is no torque, then for a high proportion of the time both sets will be able to produce full torque. The worst case with this solution is when starting or rotating very slowly when one set of windings is going through alignment.
- the second approach is to have two windings, with a half sector positional difference between them, in either the first or second discs. There is no advantage in having two such winding sets in both the first and second discs, and for simplicity it is preferable that it is the second discs which have two such sets. It is advantageous that the two windings share the current when both can generate torque, with one winding carrying all the current close to alignment of the other. Thus, torque is continuous and the saturation problem does not exist. There is a cost of double dissipation when only one winding is acting on its own.
- Another advantage of this option is that only one set of flux return rings is needed, and this is a considerable saving in mass.
- the third option is the most complex but also possibly the most advantageous, and it might be likened in principle to the Vernier scale for measurement.
- a set of rotors with 18 sectors of magnetic material and a set of stators with 20 sectors of magnetic material, as shown in Figs 16A to 16F.
- This number of sectors is low by comparison to actual designs, but is representative, and the chosen number of sectors makes the pitches in degrees easy, ie 18 sectors at a pitch of 20 degrees, and 20 sectors at a pitch of 18 degrees.
- Figures 16A through 16F show the radial currents in the first discs (rotors) in dashed lines and those in the second discs (stators) in dotted lines.
- each radial current represents the centre of a conductor which has the essential properties described as like a pier of a bridge, ie an angular width on the face of a disc that is large by comparison to the air gap, and relatively small by comparison to the width of the sector, thus allowing most of the face of a sector to be occupied by magnetic material in which flux can be generated.
- the stator conductor path is shown as a continuous serpentine conductor path, with each outside arc path connected to terminals TO through T9 .
- the winding is a single conductor and also that access in a practical stator is only available to the outside portions, however this should not be considered restrictively.
- the drive voltage is shown as applied between terminals TO (pos) and T5 (neg), and it should be noted that the radial currents in the two conductors either side of the terminal point in use are in the same direction, which means that the field in that sector from those two conductors will be zero.
- the arrows on the conductors show the direction of current flow resulting from this connection, in conventional terms, ie positive to negative.
- the arrows around the periphery show the force on each conductor.
- Figure 16B shows the same system, with the rotor rotated 1 degree in the sense of motion that the torque would generate (ie as a motor), clockwise. Since, as noted, the sector widths are 18 and 20 degrees, this naturally represents a rotation of half of a sector difference angle.
- the arrows showing the direction of torque now show that all currents contribute, and this will be the case if the angular width of the conductors is small. It can be seen that no rotor currents are in the magnetic sector at the feed point of the stator, but that two stator currents are in a single magnetic sector in two places in the rotor. So the general condition is that torque is 38/38ths (ie 100%) with narrow conductors. However if the conductors are wider then there are four places where no torque will be produced, ie the torque is 30/38 th .
- Figure 16C shows a further rotation of one degree, and since this is now a full pitch difference there is now alignment of two conductors half a pitch anti-clockwise from the feed point. The alignment is now of like currents, but otherwise the torque generation is as per Figure 16A.
- Figure 16D shows a further 1 degree rotation, and the situation is a little different. If the angular width of the spokes is considered to be narrow, then the torques generated by the spokes half a pitch anti-clockwise of the stator feed points now oppose all the other torques: this however is using the interpretation of currents. It can however be seen that one of the rotor currents is now in a sector in which the stator currents oppose. If, however, the conductor conductors are wide, there will be no contribution from the eight currents which are close to each other, and the situation becomes that of Fig 16B. Note that if it were possible to access the inner arc sections of the stator winding, it would be possible to add more switching and avoid this state, so the existence of these opposing forces is due to the limitation of the access to outside connection points.
- Fig 16E shows the same angular alignment as Fig 16D (ie no rotation has taken place), but the connection point has now been moved round by two sectors (because only outside connection points can be accessed).
- connection point 36 degrees away has happened after 4 degrees of rotation of the physical rotor. Whilst the commutation proposed here is not physical, the apparent rotation of switching is happening at a multiple of the physical rotational speed.
- Fig 16F shows a further 1 degree rotation of the rotor with the new connection point.
- the pitch angles in degrees are 360/R and 360/S.
- the angle of rotation of the rotor between switching is twice the difference of pitch angles, ie
- each switch can be a simple semi-conductor switch since the polarity remains constant. If they are to be implemented as motor generators then bi-directional semiconductor switches are needed. Reversing can be done either by changing the polarity of the rotor connection, or the order of connection of the terminal points (ie swapping the connections between TO and T5, T9 and T4 etc).
- Fig. 17A shows the switch positions for the arrangements of Figures 16A, 16B, 16C and 16D.
- Figure 17B shows "make before break” switching
- Figure 17C shows the switch positions for Figure 16E and 16F, with the intermediate position applying a bit before and a bit after the angular position of Figs. 16D and 16E.
- FIGS 18Ato 18D Examples of one, two, three and four windings are shown in Figures 18Ato 18D respectively. It should be noted that a single serpentine winding is a special case, and that any multiple winding results in a reversal of current direction in respect of the polarity of the voltage applied; to retain compatibility with the description in Fig 16 the polarity of feed is shown as negative in Figs 18B, 18C and 18D.
- the first option of two rotor and stator sets on the same shaft, suffers from the excess mass of one extra pair of flux control rings.
- the second option is of medium complexity and will be efficient in terms of material used and performance, but suffers from thermal trade-offs around torque continuity.
- the third option is the most complex, but can be designed to provide truly continuous torque, and will utilise material as well as the second option.
- FIG 19 shows such a device, consisting of a two part circular ferrite "pot core' transformer T1.
- One half T1 a is attached to the rotor 20a and the other half T1 b to an end plate of the motor.
- the mechanical bearings, shaft, 'pot core' housing and mounting have to be sufficiently rigid and of sufficiently tight tolerance in terms of end-float that the two halves T1a, T1b are retained on a common centre line, and with a constant air gap.
- switches S1 to S4 are operated in a conventional non-overlapping square-wave bridge drive (the signals are not allowed to overlap, i.e. to have two switches in a vertical pair closed at the same time, so as to avoid current flowing directly from the supply rail to ground).
- the voltage waveforms are shown in Fig 21 : lines 1 and 2 show the voltages at points A and B of Fig 20 which put a square wave drive into the primary side of the main inductive element T1.
- T1 is entirely conventional, and thus a square-wave voltage appears on the output terminals at points C and D.
- T2 is a very much smaller transformer used for synchronising the action of the inductive coupler. It is of similar form to T1, mounted, in non-conductive, non-magnetic material (for example, resin or plastic), so as to be rotatably connected between the frame and the rotor through the centre hole of T1.
- T2 is driven by a square-wave signal from a fixed standard voltage, for instance 12 Volts, and D1 to D4 form a bridge rectifier producing an internal power rail at points G and H.
- the signal across the input and output of T2 is shown in line 3 of Fig 21.
- the following detector circuit also recovers the switching clock signal to synchronise operation of the inductive coupler.
- the power circuit is then reversible when the motor is running as a generator. Power is developed in the rotor, switches S5 to S8 form the drive circuit, and switches S1 to S4 form the synchronous rectifier. Timing of the drives to switches S1 to S4 can be adapted in this mode so that they turn on after and turn off before switches S5 to S8.
- the motor may be used as a generator.
- Self-exciting dynamos have been known for a long time, and the principle here is essentially the same, although it is not necessary to make them self-excite if external power is available to provide excitation under electronic control.
- the resistance of the windings When it is in the generator mode the resistance of the windings generates a voltage which is subtracted from the generator emf, and the terminal voltage is lower than the generator e.m.f., and again, as must be, the resistance appears as a loss. So the voltage applied to the rotor windings has to be negative, to start 'extracting' current from the motor, sufficient to start the mutual-coupling generation. Since the circuit complexity of introducing a negative voltage to start the rotor current adds complexity, it is practically easier to start the system by using a reverse connection on one of the switched windings.
- Fig. 13H where the conductive path is formed by radial windings, as illustrated in Fig. 13H, it is convenient to use the interstices between individual conductors for cooling.
- Motors of this sort may have either gas (air) or liquid cooling.
- gas air
- liquid cooling is the natural choice in higher power motors.
- Water is a common cooling fluid with a high specific heat and a low viscosity, but it is corrosive, conductive when contaminated and in use may be contaminated with particulate matter.
- high-stability non- conductive liquids such as transformer oils, which will be better in most cases.
- liquid cooling may have other requirements, such as ensuring adequate resistance to corrosion, and insulation on conducting parts. If liquid cooling is used, there needs to be an inlet and outlet on the shaft. This can be achieved by use of conventional rotary seals such as oil seals, and connection ways to a matrix of cooling channels which connect via the hub.
- FIG 22 A preferred method of building the motor is as shown in Fig 22. This shows a motor with a driven shaft 40. The torque is high and so the output shaft 40 has a minimum diameter. As has been shown, high torque to weight ratio motors tend to be short
- the inner end of the shaft 40 can comprise a boss 45 onto which rotors 20a can be mounted.
- Rotors 20a and stators 20b can then be placed onto the motor in turn, with the dimensions of the rotor inner axial length determining the axial spacing of the rotors 20a, and the outer axial dimension determining the axial position of the stators 20b.
- the set of hub machine screws 21 can then be tightened to secure the rotors 20a.
- a "back" end plate 15b can then be put in position and tie rods or studs 22 in the outer skin of the motor can be tightened to secure the stators 20b in place.
- the motor is at that time mechanically complete.
- the shaft end and inner face of the "back" end plate 15b can contain the inductive coupler 55.
- Switching electronics 50 can then be secured in place and connected up; it is convenient to do this on the "back" end plate 15b, with an electronics housing cover 15c placed over the top.
- first discs are coupled to a shaft, and the seconds discs to the frame of a machine
- hub motors, fans and underwater thrusters can commonly be made 'inside out 1 , where it is the shaft that is static, and the rotating part (wheel fan, propeller) is built onto or coupled to, the outside of the rotating frame.
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Synchronous Machinery (AREA)
- Permanent Magnet Type Synchronous Machine (AREA)
- Dynamo-Electric Clutches, Dynamo-Electric Brakes (AREA)
Abstract
Description
Claims
Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
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US13/133,325 US8922093B2 (en) | 2008-12-18 | 2009-12-16 | Axial flux motor and generator assemblies |
CA2745533A CA2745533C (en) | 2008-12-18 | 2009-12-16 | Axial flux motor and generator assemblies |
EP09775250.5A EP2377230B1 (en) | 2008-12-18 | 2009-12-16 | Axial flux motor and generator assemblies |
CN200980151828.4A CN102265483B (en) | 2008-12-18 | 2009-12-16 | Axial flux motor and generator assemblies |
JP2011541580A JP5619022B2 (en) | 2008-12-18 | 2009-12-16 | Axial flux motor and generator assembly |
Applications Claiming Priority (2)
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GB0822992A GB2466436A (en) | 2008-12-18 | 2008-12-18 | Axial flux motor and generator assemblies |
GB0822992.4 | 2008-12-18 |
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WO2010070285A2 true WO2010070285A2 (en) | 2010-06-24 |
WO2010070285A3 WO2010070285A3 (en) | 2011-01-20 |
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PCT/GB2009/002898 WO2010070285A2 (en) | 2008-12-18 | 2009-12-16 | Axial flux motor and generator assemblies |
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US (1) | US8922093B2 (en) |
EP (1) | EP2377230B1 (en) |
JP (1) | JP5619022B2 (en) |
CN (1) | CN102265483B (en) |
CA (1) | CA2745533C (en) |
GB (1) | GB2466436A (en) |
WO (1) | WO2010070285A2 (en) |
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CN102412664A (en) * | 2011-11-21 | 2012-04-11 | 李明山 | Conductor plate-type electrical generator |
Also Published As
Publication number | Publication date |
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GB2466436A (en) | 2010-06-23 |
CN102265483B (en) | 2014-09-10 |
JP5619022B2 (en) | 2014-11-05 |
JP2012513184A (en) | 2012-06-07 |
WO2010070285A3 (en) | 2011-01-20 |
US8922093B2 (en) | 2014-12-30 |
CA2745533A1 (en) | 2010-06-24 |
US20110291511A1 (en) | 2011-12-01 |
CN102265483A (en) | 2011-11-30 |
CA2745533C (en) | 2017-10-03 |
EP2377230A2 (en) | 2011-10-19 |
GB0822992D0 (en) | 2009-01-28 |
EP2377230B1 (en) | 2013-08-28 |
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