This invention relates to permanent magnet rotary dynamo electric
machines of the type in which relative rotation occurs between a plurality of permanent
magnetic poles and a plurality of wound poles. The wound poles being wound in the
sense that they have associated therewith electric current carrying conductors.
"Permanent magnetic poles" are generally provided by high strength permanent magnets
formed of ceramic ferrites or rare earth magnets but they can also be provided
by single or multi-turn closed loop(s) superconductors where the magnetic poles
are permanent so long as the conductor is energised and remains in a superconducting
state. The term "permanent magnet rotary dynamo electric machines" includes both
motors and generators of direct or alternating current, and thus includes direct
current generators as well as alternators.
Electric motors and generators/alternators have traditionally been
constructed in coaxial cylindrical formation with a central rotor having a plurality
of wound poles formed by windings on steel laminations or on a soft iron core.
The stator is a cylindrical casing surrounding the rotor, and requires accurate
construction as there is only a narrow cylindrical gap between the rotor and stator
of typically less than 0.25 mm for small machines, less than 5 kw. The stator also
has a plurality of wound poles formed by windings in laminations inside a steel
casing. Such silicon steel laminations generally have a high magnetic permeability
of the order of 2000 (relative to air). Magnetic permeability of a material is conventionally
expressed as a numerical value showing how many times it is greater than the magnetic
permeability of air.
In the case of synchronous and universal motors the connections to
the rotor are by way of a commutator and brushes or slip rings and brushes which
are prone to wear. These traditional electric motors, and to a lesser extent alternators,
suffer from problems arising from the use of iron in the wound poles in either
the rotor, stator, or more typically both. The iron imposes a frequency limitation,
typically a DC electric motor is limited to an internal operating frequency of
no more than 500 hertz, because of the induced eddy current losses arising arising
from the iron present.
For example the small high speed electric motors used in vacuum cleaners
may run up to 30,000 rpm (ie 500 hertz) but are limited to operation of about one
hour at a time because of eddy current heating problems associated with the iron
laminations associated with the wound poles of both stator and rotor. In order
to achieve these high speeds, within the frequency limitations imposed by the iron
laminations they use only 2 brushes. If the number of brushes were increased there
would be much greater losses, as well as greater complexity in the circuit used
to control the motor.
Attempts have been made to build permanent magnet rotary dynamo electric
machines of similar coaxial cylindrical configuration but using generally conventional
constructional techniques so that there is a narrow air gap between permanent magnets
on an outer stator and the wound poles are wound in the iron laminations of an
internal rotor. The following US patents show examples of permanent magnet machines:
All of these patents show motors/alternators utilising permanent magnets of different
configurations. The oldest patent shows a magneto, the Rawlings patent shows a
bicycle generator, and the others show various motors/alternators, mostly with
permanent magnets aligned with their axes parallel to the rotor shaft. The West
patent shows a permanent magnet starter motor having a conventional laminated (iron)
wound rotor and only 4 arcuate permanent magnets. This patent is concerned with
shields for the permanent magnets positioned on the inside of the surrounding stator
so as to prevent demagnetization of the corners of the magnet. The other U.S. patents
listed above show motors having between 4 and 12 permanent magnets all having conventional
laminated iron rotors with wound poles wound in slots in the laminations. These
constructions all suffer from the same eddy current and hysteresis losses because
of the need for iron laminations associated with the wound poles.
- 1,958,043 Heintz, 1934
- 2,104,707 Rawlings, 1938
- 3,296,471 Cochardt, 1967
- 3,531,670 Loudon, 1970
- 3,564,306 Ott, 1971
- 3,818,586 Harkness et al, 1974
- 4,303,843 Arnoux et al, 1981
- 4,417,167 Ishii et al, 1983
- 4,471,252 West, 1984
- 4,636,671 Terada, 1987
- 4,638,201 Feigel, 1987
- 4,651,066 Gritter et al, 1987
Another approach involves the use of permanent magnets and an ironless
stator with a low effective coupling between them, to produce a motor exhibiting
low torque. This approach is suitable for low power high speed DC motors.
One example of such a motor is described in GB 1500955 by Teldix.
In the case of the Teldix motor the magnets are spaced apart from one another with
resin in between them, so that by using a large number of small width bar magnets,
the magnets in the external rotor correspond fairly closely to the radius of curvature
of the inside of the rotor. Teldix suggests the use of 24 magnets grouped in such
a way that there are only eight poles. These spaced apart magnetic poles create
a low effective coupling between the flux lines and the windings as the Teldix
windings intersect the flux lines at a shallow angle.
Another example of a motor having a low effective coupling is described
in US 4,618,806 by Grouse. The Grouse patent shows a motor having a ring magnet
and a zig zag or meandering winding on the stator. This meandering winding reduces
the available torque to about 40% to that available from an axial winding.
It is an object of this invention to provide an improved permanent
magnet rotary dynamo electric machine, or one which will at least provide the
public with a useful choice.
STATEMENT OF INVENTION
In one aspect the invention provides a brushless permanent magnet
rotary dynamo electric machine including: at least one generally cylindrical stator
and at least one generally cylindrical rotor rotatable about an axis and having
a generally cylindrical surface facing said generally cylindrical stator and spaced
apart therefrom by a generally cylindrical gap, a plurality of permanent magnetic
poles in the form of bands parallel to the rotational axis on said surface of
the rotor and positioned adjacent said generally cylindrical gap, said stator having
a plurality of wound poles on or in a substrate of the stator, the majority of
said wound poles being positioned on or in that surface of the stator which faces
said rotor so that the wound poles are adjacent said cylindrical gap and at least
said majority of the wound poles being positioned substantially parallel to the
rotational axis of the rotor, whereby at least the outer portion of the stator
does not contain any ferromagnetic material, and the flux paths owing to said
rotor magnetic poles extend in a region intersected by said majority of said wound
poles, said region and the substrate of the stator having both a low relative magnetic
permeability and being substantially non-conducting, so that the flux paths do
not pass through any ferromagnetic material in the stator, characterised in
that the permanent magnetic poles are closely spaced around the generally cylindrical
circumference of the rotor surface such that the magnetic flux paths are relatively
short substantially semi-circular magnetic flux paths between circumferentially
adjacent permanent magnetic poles, whereby said majority of said wound poles intersect
substantially all of the magnetic flux paths substantially at right angles thereto.
Preferably there are a relatively high number of poles on both the
rotor and the stator of a rotor/stator pair so as to create a correspondingly
short magnetic flux path between adjacent permanent magnetic poles. In most cases
a single rotor/stator pair will suffice although this invention can be applied
to multiple rotor/stator pairs. It is also preferred that the wound poles are of
shallow depth and are wound on or close to the surface of the substrate. The substrate
can be made of any material having a low magnetic permeability e.g. wood, fibreglass,
plastics, plastics resins, or in some cases ferrites. Preferably the magnetic permeability
of the substance is below 20 (relative to air). The wound poles could be wound on
a removable mould and encapsulated within a plastics resin so that the resin forms
Preferably the wound poles are provided on or close to the outer cylindrical
surface of a stator so that the permanent magnetic poles are provided on the inner
surface of a surrounding cup shaped rotor.
Alternatively, the rotor can be positioned within the stator, ie the
positioning is reversed, in which case the permanent magnetic poles will be positioned
on the outside of the rotor, and the wound poles will be on the inner cylindrical
face of the stator.
Where the permanent magnetic poles are to be positioned on the inner
face of a rotating outer cylinder, it would be generally convenient to use a plurality
of high strength bar magnets such as ceramic or rare earth magnets, mounted adjacent
one another with their axes parallel to the axis of the rotor. By mounting them
on the inside of the rotor, it is possible to withstand greater rotational speeds
than would be possible with the magnets on the outside surface of the rotor.
Nevertheless, it is possible to construct an electric motor or generator/alternator
in accordance with this invention, with the magnets on the outer face of a rotor
which is placed within a cylindrical stator. In such case, it would be generally
convenient to use a single ceramic ring magnet, which is magnetized in such a way
that the ring has regions of alternate polarity.
Preferably, the cylindrical gap between the rotor and stator is greater
than that used with conventional electric motors or conventional generators/alternators
which require the presence of iron within the stator in order to provide a magnetic
flux path in the stator.
Other aspects of this invention, which should be considered in all
its novel aspects, will become apparent from the following description, which is
given by way of example only, with reference to the accompanying drawings in which:
- Figure 1a: is a drive end view (with mounting plate removed) of a permanent
magnet rotary dynamo electric machine showing the external rotor construction using
- Figure 1b: is a section on line AA of Figure 1, (with mounting plate
in position), showing the placement of the bar magnets in the outer rotor, and
the relative position of the stator within the rotor.
- Figure 2a: shows the drive end view of a permanent magnet rotary dynamo
electric machine having an external rotor formed from a one piece ring magnet.
(The stator construction has been omitted from this drawing.)
- Figure 2b: is a section on line BB of Figure 2a, showing the construction
of the rotor only.
- Figure 3a: shows a flux diagram for a typical permanent magnet rotary
dynamo electric machine of this invention.
- Figure 3b: shows a voltage graph for a typical permanent magnet rotary
dynamo electric machine of this invention.
- Figure 4: shows a three phase winding construction for a typical permanent
magnet rotary dynamo electric machine of this invention.
- Figure 5a, 5b, 5c: show three segments of a circuit diagram for the operation
of a three phase motor constructed in accordance with Figures 1a/1b using direct
- Figure 6: is a block diagram for the motor operation of Figures 1a/1b
using direct current supply.
- Figure 7a: shows the drive end view (end cover removed) of a permanent
magnet rotary dynamo electric machine showing an internal rotor construction using
a cylinder magnet.
- Figure 7b: shows a section on line CC of Figure 7a, in exploded view,
of the end cover, internal rotor, and external stator.
- Figure 8: shows the relationship of magnetic flux length to magnet size.
The motor/alternator of this invention is preferably constructed using
a series of adjacent bar magnets inside a steel annulus to form the rotor as shown
in figure 1a. The steel annulus provides two important functions:
- 1. A mechanical support for the magnets against the high centrifugal forces
encountered when the rotor is running at high rpm. It will be apparent that ceramic
and rare earth magnets have a low tensile strength when compared to the steel cup
shaped rotor of figure 1b.
- 2. A return path for the magnetic flux between adjacent magnets. The bar magnets
could equally well be replaced by a cylindrical "ring" magnet, as shown in figure
2a, with alternate north and south poles around its circumference.
The motor alternator of this invention can also be constructed with
the rotor as the inner element and the stator as the outside element, see figures
7a and 7b, but is not as advantageous as the external rotor construction because
of the lower maximum rpm that the rotor of figure 7b can sustain due to the low
tensile strength of ceramic and rare earth magnets when compared to the steel cup
shaped rotor of figure 1b.
The internal rotor construction does have applications in the low
to middle speed area as it lends itself to current construction techniques as used
to manufacture small induction motors and alternators.
In the following examples, it is most convenient to provide the permanent
magnets on the rotor so that electrical connections can be readily made to the
wound poles on the stator. Such a construction can be referred to as an iron-less
stator motor/alternator. Other configurations are possible. For example, if the
co-axial iron-less direct current motor construction is controlled by carbon brushes
and a commutator, it would then have the magnets stationary (stator) and the windings
and commutator turning (rotor) and thus would have an ironless rotor rather than
an ironless stator.
If the co-axial iron-less stator direct current motor is controlled
by electronic means, the construction would follow, see figure 1 or figure 2, and
therefore the magnets would turn (rotor) and windings would be stationary (stator).
Slotted optical switches have been used in the following examples
to accurately sense position of the rotor and control transistors to switch direct
current into the three phase stator windings, see figures 3, 4, 5, and 6. The rotor
has a series of protrusions, one per magnetic pole pair and 120 degrees electrical
to allow sequential current injection into the stator windings. Three optocouplers
are used with logic gates to provide drive signals to the transistors, as shown
in figure 5a, 5b, 5c. Also non-overlap logic is used, so that only one winding
at a time has current flowing in it. Two or more phases could be used if desired,
but generally three phases gives optimum efficiency with current conduction of
120 electrical degrees per phase.
EXAMPLE 1 - FIGURES 1a & 1b
In this example, a co-axial motor or alternator is illustrated, having
an external rotor construction utilizing bar magnets. Whether the unit is used
as a motor or an alternator will depend upon the application required, and whether
or not current is extracted from these from the stator windings, or whether current
is supplied to the stator windings to operate the unit as a motor.
Preferably the motor/alternator 10 has a cylindrical sleeve 11 which
is conveniently in the form of a cup having an end face 12, which is attached to
a central shaft 13. This shaft is preferably mounted within bearings 14, 15 mounted
within a stator 16. Conveniently, the shaft has a tapped end 17 for connection to
Preferably, the inner face 20 of the sleeve 11 is provided with a
plurality of side by side bar magnets 27, aligned with their axes parallel to the
axis of the rotor. It will be appreciated that there will be an even number of
such closely spaced magnets, so that the polarity of the permanent magnetic poles
alternates as one travels around the inner circumference presented by these magnets.
[This differs from traditional motor designs where the magnetic poles are widely
spaced and there are much longer flux paths through the iron of the stator/core].
The magnets are preferably rare earth or ceramic bar magnets, and
20 such magnets are shown in Figure 1A, for the purpose of illustration. Any even
number of such magnets can be used depending upon design criteria such as size,
weight, price, availability and frequency. For medium speed machines 12 to 30 poles
are preferred, with 20 permanent magnetic poles providing optimum performance for
a motor of the type illustrated in figures 1a and 1b.
By closely spacing the magnets around the motor circumference, the
optimum ratio of magnetic circuit pole length (I) to magnet thickness (t) can be
chosen, as shown in figure 8. The free air flux lines are shown as semi-circular
whilst the metal rotor R provides a metallic return path for the flux
The maximum length flux lines (f) approximate to semi-circular paths
for higher pole numbers, and thus figure 8 approximates to a rotor of an infinitely
large radius. The maximum length flux line (f) can be expressed as:
2t = pi/2 x 1
t = 0.785 1
This relationship applies to strontium ferrite magnets, rare earth magnets such
as samarium cobalt and neodymium iron, and also for air cored single or multiple
loop superconductor electromagnets.
Preferably the bar magnets are formed from either rare earth or ceramic
magnets, and have a high field strength enabling them to provide a higher magnetic
flux across a much wider air gap than is possible with conventional magnets, but
at the same time it is preferred that the adjacent permanent magnetic poles are
close together to provide a short magnetic flux path between adjacent magnetic
Preferably, the rotor sleeve and end face, are formed of steel although
other materials could be used.
The stator 16 is preferably connected to a mounting plate 22, which
may also support slotted optical switches 23 (only one of which is shown) in order
to detect the position of the magnets. The slotted optical switches 23 conveniently
detect the position of protrusions 26 on the end face of the sleeve, which protrusions
26 may be associated with magnetic poles of a particular polarity.
It is preferred that the rotor and stator are spaced apart by a relatively
large cylindrical air gap 28 of the order of 0.25mm to 1.5mm and preferably 0.75mm
for the 20 pole motor/alternator of this example. This enables the wound poles
to intersect the magnetic flux path as shown in figure 3a. The air gap is preferably
less than the depth of the magnets and should be of such a size as to allow for
normal engineering clearances and tolerances.
The stator has an annular generally cylindrical substrate 24 of low
magnetic permeability material with a plurality of wound poles 25 on its outer
cylindrical surface. A preferred substance is glass reinforced plastics as this
can be formed into a sufficiently rigid cylindrical surface which on a prototype
machine without a fan has not distorted in use. The number of wound poles correspond
to the number of permanent magnetic poles inside the rotor. The wound poles are
relatively shallow in that they are formed on or close to the surface of the substrate
(unlike conventional wound poles which are wound within slots formed in steel laminations).
The depth of the wound poles on or close to the surface of the stator will depend
upon the size of the stator and required rating of the motor. In the example shown,
the depth would be of the order of 1mm to 10mm, and preferably about 3mm.
It will be generally convenient to provide the wound poles as wave
windings on that surface of the stator facing the permanent magnetic poles. For
example figure 4 shows the wave windings W1, W2, W3 each providing a plurality
of wound poles 201 - 211, 201A - 211A, and 2018 - 211B on the surface of a substrate
for a three phase stator winding as used in the motor/alternator of figures 1a/1b.
As will be discussed below a three phase winding is preferred for most applications
but other phases have their uses for particular applications. They may be exposed
to the air on encapsulated in a plastic resin of low magnetic permeability.
The wound poles may be provided in a variety of forms and may provide
for one or more phases. As the substrate is of low magnetic permeability there
is consequently no iron (at least in the outer portion of the substrate) to provide
a magnet flux path in the stator. The wound poles on the surface of the stator are
so positioned as to intersect the magnetic flux lines connecting adjacent ceramic
magnets as the flux lines essentially form a series of loops from one magnet to
the next as one travels around the inner circumference of the rotor. This is shown
in figure 3a which illustrates the relative position of two wound poles say 201
and 202 of wave winding on the surface of the stator and the relatively short magnetic
flux paths between adjacent permanent magnetic poles on the rotor which intersect
an outer annulus on the stator containing the wound poles.
It is preferred that the average length of the magnetic flux path
depicted as a series of semi-circles in Figure 3a (eg from point 'X' to point 'Y')
is of the order of 16mm when the air gap between stator and rotor is 0.75mm and
the depth of the wound poles is about 3mm. The average magnetic flux path length
between adjacent permanent magnetic poles will be shorter than the average flux
path length in a conventional synchronous motor where the path length is determined
primarily by the size and geometry of the steel laminations surrounding the wound
ELECTRONIC COMPONENT PARTS LIST - Figure 5a, 5b, 5c
Figure 3b shows the stator voltage for different rotor positions for one phase only
of the three phase stator windings of the motor/alternator of figures 1a &
The circuit diagram of Figures 5a, 5b, 5c is shown on three separate
sheets for convenience. The letters shown in circles, eg a-k show how the components
of Figure 5a connect to the components both of Figure 5b and Figure 5c. For convenience
the drawing of Figure 5a should be laid out to the left of the drawing of Figure
5b, and the drawing of Figure 5c should be placed in a position below that of Figures
5a and 5b. These three drawings 5a-5c have the following components.
OPERATION: 12 VOLT DC ELECTRONIC CIRCUIT DIAGRAM - Figures 5a, 5b, 5c
- 1. Semiconductors
LED1-LED3 Light emitting diode, 3mm, green high efficiency
BYV42 - 50
Tr1 - Tr4
Tr5 + Tr6
Tr7 - Tr26
Tr27 + Tr28
Slotted Optical Switch
OPB 865 T51
Quad Exclusive Or
Quad Op - amp
- 2. Capacitors
C1 + C2
C5 - C8
C9 - C15
C17 - C19
C21 - C23
- 3. Resistors
R2 - R4
R5 - R7
R10 - R15
R20 - R22
R23 - R38
R40 + R41
R42 + R43
R46 - R50
R51 - R53
R56 + R57
- 4. Transformer
1 - 2
3 - 4
5 - 6
- 5. Inductor
- 6. Switches
The operation of a three phase motor as shown in figure 1a/1b will
now be described with reference to Figures 5a, 5b, 5c and 6. The block diagram
of Figure 6 shows the relationship of the following subsystems relative to the
- 100 -
- the rotor position sensing means (provided by the slotted optical switches).
- 110 -
- pulse width modulation means for starting and speed control.
- 120 -
- power transistors and resistive braking means
- 130 -
- general control means for
- on/off starting control
- low voltage shut-down
- high temperature shut-down
- motor turning indication.
The rotor position is sensed by the slotted optical switches OPTO1-OPTO3
and this information is converted by logic into three 120 degree non-overlapping
signals to control the power transistors. The power transistors Tr17-21, Tr12-16
and Tr7-11 are connected to each of the three phase windings on the stator of the
When the power transistors Tr17-21 are switched 'on' by the control
logic, current flows through winding W1 which magnetically attracts the rotor,
which if free to move, turns 120 electrical degrees. After the rotor has turned
through 120 electrical degrees, transistors Tr17-21 are switched off and transistors
Tr12-16 are switched on. After the rotor has turned a further 120 electrical degrees
transistors Tr12-16 are switched off and transistors Tr7-11 are switched on. After
the rotor has turned a further 120 electrical degrees transistors Tr7-11 are switched
off and transistors Tr17-21 are switched on again completing the cycle.
At starting the input current is only limited by the resistance of
the circuit because there is no back EMF voltage produced, as the rotor is stationary.
The starting current can be reduced to an acceptable level by using pulse-width
modulation techniques (PWM). IC6 is a pulse-width modulation device which can govern
the rpm and maximum input current to the motor by varying the pulse width of a series
choke DC-DC converter formed by inductor L and Transistors Tr22 - Tr26.
Motor input current limitation is achieved by sensing the voltage
drop across resistor R60, which IC6 senses via IC4/b and OPTO4 and limits the pulse-width
to provide current limiting.
Motor rpm governing is achieved by rectification of the AC voltage
generated in winding W3 and sensing of this voltage by IC6 via IC4/a and OPTO 4
to provide a pulse-width of suitable duty cycle.
Rotor position is sensed by slotted optical switches, OPTO1 - OPTO3
which detect 120° electrical degree protrusions 26 on the rotor. IC1 and IC2 provide
non-overlap logic so that only one winding at a time has current flowing in it.
Also a time delay in switch-on resistors R11, R12 and R13 and capacitors C6; C7
and C8 provides additional 'dead-time' to allow current flowing in the previous
motor winding to reduce to zero before the next winding has current flowing in
IC3 provides the logic necessary for th electronic brake. While the
motor is switched on the main power transistors Tr7 - Tr21 are turned on and off
sequentially under the control of the optical slotted switches OPTO1 - OPTO3. The
motor can be switched off by three different means:
- 1. Switch SW1 turned off under control of the operator.
- 2. Low battery voltage causes IC4/d to 'latch-up' and turn off the motor.
- 3. High temperature on the motor windings or main power transistors causes IC4/c
to 'latch-up' and turn off the motor.
When the motor is turned off IC3 turns on all the power transistors,
on the three motor windings. This applies a short-circuit between all three windings,
which stops the motor turning in 1 - 2 seconds, providing a useful braking action
for the motor.
As an added safety device IC7 and switch SW2 provide a three second
enabling period during which switch SW1 must be operated to allow the motor to
ROTOR POSITION CONTROL
The three phase motor of Figure 1a/1b utilises slotted optical switches
to provide rotor position information to control the point of switching of current
into the motor windings. Due to the 120 electrical degree conduction angle technique
used for this design, there exists other alternatives for accurately sensing rotor
position. This is due to the 240 electrical degree period in which the voltage across
the motor windings is purely the back EMF voltage generated by the action of the
By differentiating the waveform of winding W2 during the 240 electrical
degree non-conducting angle period, it is possible to produce an accurate signal
to indicate the point at which current should start to flow in say wound pole 201
of winding W1 and stop flowing in wound pole 201B of winding W3. Also wound pole
201B of winding W3 can indicate the point at which current should start to flow
in wound pole 201A of winding W2 and stop flowing in wound pole 201 of winding
W1. This applies to all the wound poles of the windings so that in general, winding
W1 can indicate the point at which current should start to flow in winding W3 and
stop flowing in winding W2. This can be used for accurate stepper motor control
by switching the different windings to provide incremental movement.
Another method of determining rotor position from the back EMF of
the motor windings is by sensing the crossing points of the three voltage waveforms
and using this information to determine when to switch "on" and "off" the appropriate
Magnetic sensors such as Hall Effect devices can be used in place
of the optical slotted switches especially in dusty environments where problems
with fully enclosing the motor exist.
Also provided with the electronic design are the following features:
EXAMPLE 2 - FIGURES 2a & 2b
- (1) Low voltage shut-down, if battery powered then to avoid harmful over-discharge
of the battery, the electronics switches off the motor current, if the battery
voltage drops below 10.5 volts.
- (2) Thermal shut-down, if the windings, or power transistors overheat then the
electronics switches off the motor current.
- (3) Current limiting to limit torque and starting current.
This arrangement is similar to that of example 1 except that a single
cylindrical ring magnet 30 is provided on the inside of the cup shaped rotor 31.
Similar protrusions 32 are provided around the edge of the rotor to identify the
positions of the permanent magnetic poles of the ring magnet to a slotted optical
switch as shown in figure 1b. The stator (not shown) can be the same as that of
EXAMPLE 3 - FIGURES 7a & 7b
This arrangement has a single cylindrical ring magnet 41 on an internal
rotor 40, i.e. the permanent magnetic poles are on the outer face of the rotor
40, and are surrounded by the wound poles 42 on the inner cylindrical face of the
stator 43. The wound poles 42 are provided on a low-magnetic substrate 44 forming
part of the stator. The rotor is mounted on shaft 45 which is mounted in bearings
46, 47 in end plates 48, 49. End plate 48 has an internal slotted optical switch
50 which detects the position of protrusions 51 on the edge of the rotor.
ADVANTAGES OF THE PREFERRED MACHINES OF THIS INVENTION
- 1. As there are no steel laminations used in the stator, there are no 'eddy-current'
and hysteresis frictional losses. This allows for higher efficiency at part or
low loading of the machine.
- 2. As there are no steel laminations used in the stator, there is no saliency
"break-out" torque normally associated with permanent magnet machines. This is
important in areas where low-speed torque is low and might not be sufficient to
allow the machine to start.
- 3. Very low self inductance of the stator windings due to the large air-gap
between alternate magnet poles allows good output regulation of voltage if rpm
held constant. This also allows very simple control by transistors in the motor
- 4. Very high power to weight ratio when compared to equivalent induction motors
and "universal" brush motor and gives a definite weight saving (as there is no
iron in the stator).
- 5. Electronic control allows a reduction of starting current to any desired
level; existing electric motors generally draw five to ten times their normal rated
current during starting.
In the above examples a three phase motor/alternator is described
but other phase windings may be used, and in particular 1,2 and 4 phase configurations
will now be described.
SINGLE PHASE CO-AXIAL MOTORS/ALTERNATORS
TWO PHASE CO-AXIAL MOTORS/ALTERNATORS
- 1. Single phase co-axial alternators are the most simple configuration
of stator for this machine and can find uses where simplicity of construction outweighs
absolute output power. The three phase equivalent produces twice the output power
for the same resistive losses, and 50 per cent additional copper in the stator
- 2. Single phase co-axial motors are the most simple configuration of
stator for this machine, but required some means of starting (similar to single
phase induction motors) and therefore require an additional winding or mechanical
means to ensure starting. In areas of low starting torque requirements i.e. direct
coupled fans there could be a use for single phase motors.
THREE PHASE CO-AXIAL MOTORS/ALTERNATORS
- 1. Two Phase Co-axial Alternators. This machine provides a 40% increase
in output power for the same resistive losses of a single phase machine.
- 2. Two Phase Co-Axial Motors. Due to two phase operation there are no
starting problems, but if the machine is to be electronically controlled from a
direct current supply then efficiency could be low if 180 degree current injection
is used. The two phase motor could be electronically controlled to provide the
180 degree current injection just for starting and reduce to 120 degrees or less
for normal running operation.
GREATER THAN THREE PHASE CO-AXIAL MOTORS/ALTERNATORS
- 1. Three Phase Co-axial Alternators. This machine provides an increase
in output power for the same resistive losses as a single or double phase machine.
If the three phase machine uses the same amount of copper as a two phase machine
and the stator windings of both machines occupy the full 360° available, then the
three phase machine can produce a further 7% output power over the two phase
machine for the same resistive losses (ie 47% increase over the single phase machine).
- 2. Three Phase Co-Axial Motors. Due to three phase operation there are
no starting problems, and the machine can easily be electronically controlled from
a direct current supply as described with reference to figure 6. Due to the 120
degree split with three phase operation sufficient back EMF voltage exists over
the 120 degrees to provide efficient current injection from a direct current supply,
and this design allows for accurate control of the rotor position, e.g. for a stepper
- 1. Co-axial Motors. Slightly higher efficiency possible due to possible
smaller current injection angle. The four phase motor has possible applications,
due to simplicity of electronically controlled reversing of rotor direction while
still only requiring two slotted optical switches to sense rotor position.
- 2. Co-axial Alternators. No additional benefits other than a slightly
higher efficiency and lower ripple voltage if the output of the alternator is being
rectified to provide a direct current power supply.
In all of these examples the wound poles are formed on one or the
other of the rotor or the stator and the permanent magnetic poles are provided
on the remaining one of the etator or the rotor. However it in possible to provide
other configurations, e.g. the rotor may be provided with a band of permanent magnetic
poles then a band of wound poles (which may be repeated along its length) and the
stator is provided with the opposite configuration so that a band of wound poles
on the stator face the band of permanent magnetic poles on the rotor, and a band
of permanent magnetic poles on the stator face the band of wound poles on the rotor.
The absence of iron in the substrate adjacent the gap and the plurality
of permanent magnetic poles allows the flux path from pole to adjacent pole to
be relatively shallow so that a composite motor or composite generator/alternator
could be built up with a cylindrical stator within an annular cylindrical rotor
which in turn is within an annular cylindrical stator (etc.).
The closely spaced permanent magnetic poles can be provided by a single
ring magnet (in which case the poles are contiguous) or by separate magnets which
touch one another as in Figure 1a, or are separated from one another by a small
gap to allow for engineering tolerances (or thermal expansion). Such a gap would
generally be less than 2% of the width 1 of the magnet as shown in Figure 8. For
very large motors in which the magnets may be 150mm in width the gap would be of
the order of 3mm.
The number of permanent magnetic poles will be chosen to suit the
particular application of the motor and in particular the required maximum speed
of the motor. Between 4 and 30 poles are preferred, the lower pole numbers being
suited to small high speed machines, with the higher pole numbers being suited
to medium speed and larger machines. In general, the estimated maximum motor speed
(s) in revolutions per minute (rpm) can be determined from the formula
s = 3000 x 60 / (n)
where n = number of pole pairs
Thus for a 4 pole machine ( ie 2 pole pairs) the maximum speed
s = 90,000 rpm
but for 30 poles ( ie 15 pole pairs) the speed drops to:
s = 12,000 rpm.