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The present invention relates to a d.c. motor.
D.C. motors require commutation i.e. the process of reversing the current
in each armature coil. This is carried out while the commutator segments
to which the coil is connected are short-circuited by a brush. Very high
rates of change of current are involved, which change is opposed in
conventional d.c. motors by the inductive e.m.f. induced in the coil. If
the reversal is not completed by the time that the short-circuit is
removed, then sparking will occur with consequential damage to the brushes
and the commutator itself.
It is known to assist commutation by the provision of extra poles (or
compoles) located between the main poles of the stator. These compoles are
arranged so as to induce an e.m.f. in the short-circuited coils which
opposes, and therefore acts in a direction to neutralise, the induced
e.m.f. Even with the aid of compoles, sparkless commutation cannot be
achieved in conventional d.c. motors unless the inductive e.m.f. is
limited to about 10 volts per coil and the mean voltage between adjacent
commutator segments does not exceed about 20 volts. These factors impose
considerable limitations on the design of conventional d.c. motors.
Shunt connected d.c. motors produce a relatively low torque and series
connected d.c. motors are used for high torque. However, series connected
motors suffer from very poor speed regulation and reduced efficiency owing
to the increased I.sup.2 R losses in the series field coils.
An object of the present invention is to provide a d.c. motor in which the
above problems associated with commutation are overcome and which will
provide high torque with good speed regulation.
In accordance with this invention as seen from one aspect there is provided
a d.c. motor comprising a cylindrical stator, a rotor journalled on a
shaft for rotation within the stator with a uniform gap between the rotor
and stator as the rotor rotates, armature windings inserted in slots in a
cylindrical surface on one of the stator and rotor and being divided into
similar coils mutually spaced around the shaft axis, field generating
means located on the other of said stator and rotor for forming poles
mutually spaced around the shaft axis, and switching means adapted to
connect said armature coils to a d.c. source in timed synchronism with
rotation of the rotor such that switching of the d.c. to each armature
coil occurs when a pole generated by that coil is substantially in
alignment with a rotor (or stator) pole of opposite polarity and such that
immediately before switching the e.m.f. induced in that armature coil is
approaching maximum value and opposes the applied e.m.f. and immediately
after switching those poles are of like polarity and the induced e.m.f.
assists the applied e.m.f.
In accordance with this invention as seen from another aspect there is
provided a d.c. motor comprising a cylindrical stator, a rotor journalled
on a shaft for rotation within the stator with a uniform gap between the
rotor and stator as the rotor rotates, armature windings inserted in slots
in a cylindrical surface of the stator and being divided into similar
coils mutually spaced around the shaft axis, field generating means
located on the rotor for forming poles mutually spaced around the shaft
axis, and switching means adapted to connect said armature coils to a d.c.
source in timed synchronism with rotation of the rotor so as to generate a
rotating field which is in arrear of and oposes the rotor field to exert a
repelling force on the rotor poles.
The switched windings may be provided on the stator or on the rotor, i.e.
either the stator or the rotor forms the armature. The field generating
means may comprise windings divided into similar coils forming the field
poles mutually spaced around the rotor (if the stator is the armature), or
around the stator (if the rotor is the armature). Instead, the field
generating means may comprise a permanent magnet.
In the motor in accordance with this invention, the induced e.m.f. due to
relative movement of the field is used to assist commutation so that
immediately before commutation the inducd e.m.f. reduces current to a
minimum value whereby sparkless commutation can be obtained even with
applied e.m.f.'s of high value (e.g. several hundred volts) and a voltage
of high value (e.g. several hundred volts) between commutator segments.
Further, the induced e.m.f. also serves to increase the torque produced by
the motor because immediately after commutation the induced e.m.f. assists
the applied e.m.f. to increase the rate of increase of current and
therefore magnetic flux which in turn produces a greater torque.
The stator and rotor fields are maintained in opposition to one another
such that the e.m.f. induced in the switched winding due to a change in
current is neutralised by the e.m.f. induced in the windings due to an
equal and opposite change in the field flux linking the winding.
Accordingly the normal inductive properties of the switched windings are
obviated and these display essentially resistive characteristics.
U.K. patent specification No. 2028598A discloses a d.c. motor having a
stator with two pairs of salient poles (in contrast to a cylindrical
stator which forms a uniform gap with the rotor as the latter rotates):
these salient stator poles are connected to a d.c. supply and switched so
as to maintain a constant polarity sequence of N,N,S,S which advances
around the poles in synchromism with the rotation of the rotor shaft. This
motor is dependent for its operation upon the provision of the two pairs
of salient poles and a substantial air gap between adjacent said poles and
the maintaining of the N,N,S,S sequence in synchronism with the rotor
rotation. The motor of U.K. patent specification No. 2028598A does not
provide the above-described characteristics of the motor in accordance
with the present invention and its speed is proportional to the sum of the
unidirectional flux generated by the rotor and the alternating flux
generated by the stator, whereas in the motor in accordance with the
present invention the speed is proportional to the product of the rotor
flux and the stator flux.
In known motors of the brushless d.c. type, typically 3-phase winding$ are
provided on the stator and fed from the d.c. supply by a controller to
generate a field which rotates ahead of the rotor and which therefore
"pulls" the rotor around with it: e.g. the North pole of this field
rotates ahead of the South pole of the rotor to exert a force of
attraction on the rotor South pole. The rotating field generated by the
stator and the rotor field therefore assist each other. In contrast, in
the motor in accordance with the present invention, the field generated in
the stator (when this is the armature) rotates in arrears of the rotor and
therefore "pushes" the rotor around: e.g. the North pole of the stator
field rotates in arrear of the North pole of the rotor to exert a force of
repulsion on the rotor North pole. The rotating field generated by the
stator and the rotor field therefore oppose each other. It can be shown
that the energy required to energise the windings of the brushless d.c.
motor (the fields assisting each other) to produce a given force on the
rotor, is very much greater than the energy required to energise the
windings of the motor in accordance with the present invention (the fields
opposing each other). Also, in a brushless d.c. motor with 3-phase stator
windings, at any instant only two windings are energised whilst the third
must be disconnected from the supply and instead connected to a
dissipating circuit for the purposes of commutation: in the motor in
accordance with the present invention, it is not necessary for the winding
being commutated to be de-energised and all windings can be energised all
of the time, further increasing the available torque from the motor in
comparison to a brushless d.c. motor of similar size. A further advantage
of the motor in accordance with the invention is that if the load is
increased, tending to slow the rotor, the rotating stator field will in
effect move closer to the rotor field and increase its "pushing" force:
whilst in the case of a brushless d.c. motor if the load is increased
tending to slow the motor, the fields will move further apart and the
"pulling" force consequently diminished.
In the motor in accordance with the present invention the means for
connecting the armature coils to the d.c. source can be electronic or
mechanical. If the stator forms the armature, the motor can conveniently
include a mechanical commutator and slip rings connected in series between
the stator coils and the d.c. source. If instead an electronic means is
employed, the commutator, brushes and stator slip rings are replaced by
rotor position sensing devices such as Hall effect or photodiodes. When a
mechanical commutator is used, the commutator and slip ring assembly may
be housed outside the machine and driven by gears or a timing belt or
alternatively may be fitted to the shaft with provision for easy removal.
In both cases the comutator and slip rings may be connected in one of two
ways: the stator coils may be connected via brushes to the slip rings and
the commutator to the d.c. source via brushes, or alternatively, the
stator coils may be connected to the commutator via brushes and the slip
rings connected to the d.c. source via brushes. In the latter case, the
commutator is divided into as many segments as there are rotor poles and
alternate segments are connected to two slip rings which connect to the
d.c. source via brushes.
The stator windings may be connected in parallel, in series or in a
series-parallel combination and the terminals connected to brushes evenly
and sequentially distributed around the commutator. By this means the
required magnetic polarity sequence is established and maintained
sequentially in synchronism with rotation of the rotor.
A motor in accordance with the present invention has characteristics not
found in known designs. For example, both speed and torque are directly
proportional to the field current whilst in known designs the speed is
inversely proportional to the field current.
The motor presents an essentially resistive load to the supply and this
enables the use of an armature having a few large windings such as those
used in conventional alternating current (a.c.) machines in contrast to
conventional d.c. armatures which have a large number of small windings.
Thus the d.c. motor can be designed using conventional and less expensive
a.c. components, in particular a.c. type armatures. Doubly-excited a.c.
machines such as three phase alternators, synchronous motors and brushless
d.c. motors can easily be converted to d.c. motors in accordance with the
present invention.
The reduced number of armature windings, three in the case of a three phase
armature, allows the design of a rotating field d.c. motor. The reduced
number of armature windings also allows the use of solid state switching
devices instead of the conventional brushes and multisegment commutator.
A d.c. motor in accordance with the present invention, and having a
permanent magnet field, would have similar constructional features to a
conventional brushless d.c. motor. However, if the armature windings are
wye-connected, current is applied to each phase for a complete cycle
whilst in the case of conventional brushless d.c. motors current is
applied for only two thirds of a cycle. Since current is applied for a
complete cycle, the output power of a given machine arranged in accordance
with the present invention is similar to that of a comparable conventional
d.c. motor and some one and a half times greater than a brushless d.c.
motor operating in the conventional manner.
Embodiments of the present invention will now be described by way of
examples only and with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic longitudinal section through an embodiment of a
d.c. motor in accordance with the present invention;
FIG. 2 is a schematic diagram of the electrical connections of the motor of
FIG. 1;
FIGS. 3, 6, and 9 are diagrammatic sections on the line A--A in FIG. 1,
showing the rotor in three different positions:
FIGS. 4, 7, and 10 are diagrammatic sections on the line B--B in FIG. 1,
corresponding to the rotor positions of FIGS. 3, 5, and 7 respectively;
FIGS. 5, 8 and 11 are schematic diagrams of the electrical connections of
the armature windings corresponding to the rotor positions of FIGS. 3, 5
and 7 respectively;
FIG. 12 is a schematic diagram of a transistor inverter to replace the
commutator and brushes assembly of the motor shown in FIGS. 1 to 11;
FIG. 13a is an end view of a disk component of a rotor position sensor used
with the inverter of FIG. 12:
FIG. 13b is a section through the disk and showing also l.e.d. and
photodiode pairs;
FIG. 14 is a graph of the waveforms at output terminals of the inverter;
FIG. 15 is a table of the switch sequence of the power transistors of the
inverter; and
FIG. 16 is a graph of the output of the inverter when variable pulse width
is used to control the motor.
CONSTRUCTION
Referring to FIGS. 1 and 3 of the drawings, there is shown a d.c. motor
comprising a laminated cylindrical stator core 1, with semi-enclosed slots
to accomodate windings, and formed of steel sheets riveted or seam-welded
together. A salient pole rotor 3 is journalled on a shaft 5 to rotate
within the cylindrical stator, rotor 3 having acurate ends 3a, 3b of
diameter slightly less than that of the stator (exaggerated in FIG. 3).
The rotor also has parallel flat side surfaces 3c, 3d around which a rotor
winding is wound whereby the rotor can be magnetically polarised along its
longer transverse axis.
The rotor illustrated in FIG. 3 has two poles but the motor could be
constructed with any desired number of poles, in which case the rotor
could also be of cylindrical construction with slots to accommodate the
rotor windings. Further variations in rotor construction include the
"claw" type and the rotor could be a permanent magnet.
The stator forms the armature and is provided with windings 2a, 2b, 2c
inserted into slots in the stator core so as to form similar magnetic
poles or phases evenly distributed around the stator. In the example
illustrated in FIG. 3, a 12-slot stator is wound with three single layer
windings consisting of two full pitch coils in two slots per pole per
phase.
It will be appreciated that the construction described thus far is
identical in principle to that of a conventional a.c. alternator.
The terminals of the armature windings are connected to brushes 7a, 7b, and
7c with the start of winding 2a connected to brush 7a, the start of
winding 2b connected to brush 7b and the start of winding 2c connected to
brush 7c. The finish of the windings are connected together in a
wye-configuration, however the windings may instead be connected in a
delta configuration.
The stator windings shown in the example produce three phases, however, in
other embodiments any number of phases may be used. Alternatively, the
three phases could be produced from six windings with the same result.
There are several known types of star-connected three phase windings of
either the distributed (sinusodial) or concentrated (trapezodial) form
which can be used in the motor.
The rotor winding 4 is connected to a pair of slip rings 12 and 13 (FIGS. 1
and 2) which connect with brushes 14 and 15 connected, in use, to a d.c.
source to provide the field.
Also, a commutator 6 (FIGS. 1 and 2) is fixed to the shaft 5 and a second
pair of slip rings 8 and 9 are provided which connect with brushes 10 and
11 connected to the d.c. source. The commutator in this embodiment is
divided into two segments 6a and 6b, segment 6a being connected to slip
ring 8 and segment 6b being connected to slip ring 9. Stator brushes 7a,
7b, and 7c are fixed to an insulated support plate 16 (FIG. 1) arranged so
that the brushes are distributed evenly round the commutator in
approximate alignment with the stator poles, the brush support plate being
angularly adjustable relative to the stator so as to allow limited
movement around the commutator for final adjustment of the commutation
position.
In the present embodiment, the commutator is divided into as many segments
as there are rotor poles. Alternatively, since the commutator and slip
rings are in series between the d.c. source and the stator windings, the
latter windings can be connected via brushes to three slip rings, in which
case the commutator is divided into the number of phases, three in this
example.
The commutator and slip rings effectively convert the d.c. input from the
supply to a.c. in synchronism with and in phase with rotation of the rotor
poles. It will be appreciated that this task could equally well be
performed by electronic means.
OPERATION
The basic operation of the illustrated motor is as follows. Consider the
rotor winding 4 (FIG. 3) connected to a d.c. source so that the right hand
pole 3a of the rotor, as illustrated in FIG. 3, is a North pole and the
left hand pole 3b is a South pole, the commutator segments 6a and 6b (FIG.
4) being connected to the d.c. source such that segment 6a is permanently
connected to the positive terminal and segment 6b to the negative
terminal.
The brushes 7a, 7b and 7c are located at positions substantially central of
the stator windings 2a, 2b and 2c respectively. The latter windings are
connected in a wye-configuration with the start of winding 2a connected to
brush 7a, the start of winding 2b connected to brush 7b, the start of
winding 2c connected to brush 7c and the finish of all three windings
connected together.
At the instant illustrated in FIGS. 3 and 4, the start of winding 2a is
connected to the positive segment 6a via brush 7a, and the start of
winding 2c is connected via brush 7c to the negative segment 6b. Windings
2a and 2c are therfore connected in series across the d.c. supply as
illustrated in FIG. 5. Similarly, the start of winding 2b is connected to
the positive segment 6a via brush 7b and therefore winding 2b is also
connected in series with winding 2c across the d.c. supply.
The resulting current distribution around the stator at this instant is
illustrated in FIG. 3. The conductors of winding 2a in slots 1 and 2 carry
current into the plane of the paper and the return conductors are in slots
7 and 8. The conductors of winding 2b in slots 9 and 10 carry current into
the plane of the paper and the return conductors are in slots 3 and 4. The
start of winding 2c is connected to the negative terminal of the supply so
that the direction of current in winding 2c is reversed compared with the
other two windings so that the conductors of winding 2c carrying current
into the plane of the paper are in slots 11 and 12 and the return
conductors are in slots 5 and 6.
At this instant therefore the conductors on one half of the stator carry
current in one direction and conductors on the other half carry current in
the opposite direction which results in the formation of two stator
magnetic poles, a North magnetic pole in the region between slots 2 and 3
and a South magnetic pole in the region between slots 8 and 9: these are
indicated by reference N between slots 2 and 3 and reference S between
slots 8 and 9.
At the instant illustrated in FIGS. 3 and 4 the rotor poles are about 120
degrees out of alignment with the stator poles and there will therefore
exist a rotational torque on the rotor in a direction which would result
in the alignment of the fields i.e. the stator North pole in alignment
with the South pole of the rotor and the stator South pole in alignment
with the North pole of the rotor. At this instant, therefore, the stator
North pole exerts a force of repulsion on the North pole of the rotor
producing a clockwise torque on the rotor and similarly the stator South
pole will exert a force of repulsion on the South pole of the rotor which
again produces a torque in the clockwise direction on the rotor.
After a clockwise rotation brush 7b breaks contact with the positive
segment 6a and makes contact with the negative segment 6b as illustrated
in FIG. 7. At the instant depicted in FIGS. 6 and 7 therefore, the start
of winding 2b is connected to the negative terminal of the supply via
brush 7b as illustrated in FIG. 8 and the direction of current in the
slots housing winding 2b is reversed so that the conductors in slots 3 and
4 carry current into the plane of the paper and slots 9 and 10 carry the
return conductors. The direction of current in the remaining slots is
unchanged.
The distribution of current around the stator is again such that the
conductors on one half carry current in one direction and the conductors
on the other half carry current in the opposite direction so that two
magnetic poles are formed but at this instant the stator North pole is
located in the region between slots 4 and 5 and the stator South pole in
the region between slots 10 and 11 as illustrated in FIG. 6. The stator
field has therefore advanced in a clockwise direction so that the rotor
poles are again about 120 degrees out of alignment with the stator poles
and again the stator North pole exerts a force of repulsion on the North
pole of the rotor producing a torque in the clockwise direction and
similarly the stator South pole exerts a force of repulsion on the South
pole of the rotor again producing a torque in the clockwise direction.
With further clockwise rotation brush 7c breaks contact with the negative
segment 6b and makes contact with the positive segment 6a as illustrated
in FIG. 10. At the instant depicted in FIG. 10 therefore, the start of
winding 2b is connected to the positive terminal of the supply via brush
7c as illustrated in FIG. 11 and the direction of current in the slots
housing winding 2c is reversed so that conductors in slots 5 and 6 (FIG.
9) carry current into the plane of the paper and slots 11 and 12 contain
the return conductors.
The distribution of current around the stator is again such that the
conductors on one half carry current in one direction and those on the
other half carry current in the opposite direction so that two magnetic
poles are formed but, at the instant depicted in FIGS. 9 and 10, the
stator North pole is located in the region between slots 6 and 7 and the
stator South pole is located in the region between slots 1 and 12. The
stator field has therefore advanced further in a clockwise direction so
that the rotor poles are again some 120 degrees out of alignment with the
stator poles and again the stator North and South poles exert forces of
repulsion on the North and South poles, respectively of the rotor
producing a torque in the clockwise direction.
With continued rotation in the clockwise direction brush 7a breaks contact
with the positive segment 6a and makes contact with the negative segment
6b and the above cycle is repeated but with opposite polarities.
It will accordingly be seen that torque is produced from the force of
repulsion between two magnetic fields, a rotating magnetic field being
produced in the stator which is used to "push" the rotor around. This
effect is in direct contrast to brushless d.c. motors in which torque is
produced from the force of attraction between two magnetic fields and the
rotor is "pulled" around by the rotating stator field.
It will be noted in particular that the two fields are always maintained in
opposition to one another, i.e. the phase difference is maintained between
90.degree. and 180.degree. and this provides the unique performance
characteristics of the motor. Since the two fields are always in
opposition a change in one is accompanied by an equal and opposite change
in the other: if one field expands the other must contract and conversely
if one field contracts the other must expand.
When the field produced by a winding expands the resulting induced e.m.f.
opposes the applied e.m.f. and conversely when the field contracts, the
induced e.m.f. assists the applied e.m.f. In the d.c. motor being
described, an increase in the stator current which would cause an increase
in the field produced by the stator winding would also be accompanied by
an equal reduction in the rotor flux linking the stator winding and
similarly a reduction in stator current would cause a decrease in stator
flux and an increase in the rotor flux linking the stator winding.
The e.m.f. induced in the stator winding due to a change in stator flux is
therefore equal and opposite to the e.m.f. induced in the winding due to a
change in the rotor flux linking that winding so that the algebraic sum of
the induced e.m.f.s due to a change in stator current is zero:
consequently the motor presents an essentially resistive load to the
supply. This characteristic of the motor allows current to the stator
windings to be switched at any point on the cycle without producing the
high inductive e.m.f.s which are normally produced when large inductors
are switched.
The inductive e.m.f. induced in the stator windings due to movement of the
rotor are not similarly neutralised. The instant immediately after
commutation of winding 2a is illustrated in FIG. 3: the magnetic field
which would be produced by stator winding 2a alone would have a North pole
between slots 4 and 5 and a South pole between slots 10 and 11, i.e. 180
degrees out of phase with the rotor poles at that instant so that the
induced e.m.f. due to a change in rotor position is a maximum in the
direction which assists the applied e.m.f. The start of winding 2a remains
connected to the positive terminal of the supply until the rotor has
completed one half revolution at which instant the rotor poles are again
in alignment with the stator North and South poles that would be produced
by winding 2a alone: the e.m.f. induced in stator winding 2a is now a
maximum in the direction which opposes the applied e.m.f.
The e.m.f. induced in the stator windings due to movement of the rotor is
therefore a maximum in a direction which assists current at the beginning
of a cycle and a maximum in a direction which opposes current at the end
of a cycle. Immediately after commutation therefore, the growth of current
and consequently of stator flux, is assisted by the induced e.m.f. and at
the end of a cycle the induced e.m.f. opposes the applied e.m.f., to
reduce current and therefore assist commutation. Accordingly sparkless
commutation can be achieved with inductive e.m.f.s of several hundred
volts and several hundred volts between segments.
Producing torque from the reaction between two opposing magnetic fields
provides another effect. Consider an increase of the field current and
consequently of the rotor flux: there will be a reduction in stator
current owing to the contraction of the stator field but the force exerted
on the fields will be greater because of the additional rotor flux. There
will therefore be a decrease in stator current and the additional force
will either cause an increase in speed if the torque is maintained
constant or an increase in torque if the speed is maintained constant. The
motor therefore has the characteristic that both speed and torque are
proportional to field current. The stator input current, however, is
inversely proportional to field current and it therefore follows that the
efficiency is also proportional to the field current.
At the end of a cycle the e.m.f. induced in a stator winding due to a
change in rotor position opposes the applied e.m.f. and an increase in
field current will also increase the e.m.f. at this part of the cycle.
After a further increase in field current a point is reached when the
e.m.f. induced in the stator winding exceeds the applied e.m.f. over a
portion of the cycle and the machine behaves as a generator and this
effect limits the speed of the motor. However, the e.m.f. induced as a
result of a change in rotor position can be reduced by increasing the load
and thereby decreasing the speed and consequently the e.m.f. induced in
the winding to movement of the rotor.
ELECTRONIC COMMUTATION
The commutator and brushes of the motor so far described convert the d.c.
supply to an alternating supply in phase and in synchronism with rotation
of the rotor and the waveforms appearing at the commutator brushes 7a, 7b
and 7c are illustrated by Va, Vb and Vc respectively in FIG. 14. The same
waveforms can equally well be produced by an electronic means, an example
of which will now be described.
Referring to FIGS. 12 and 13, the commutator and brushes assembly is
replaced by a rotor position sensing transducer 22 comprising a disc 23
fixed to the shaft 5. The disc has three slots 23a, 23b and 23c
corresponding to the magnetic poles produced by the stator windings. In
this example there are three rows of slots corresponding to three phases
and one slot per row corresponding to the number of pairs of rotor poles.
Each slot has a span of 180 degrees which again is determined by the
number of pairs of stator poles. A four pole motor, for example, would
have two slots per row with each slot having a span of 90 degrees.
The transducer 22 further comprises a light emitting diode (l.e.d.) and
photo diode pairs 24a and 25a, 24b and 25b, and 24c and 25c (FIG. 13)
positioned such that the light emitted from the l.e.d.s 24a, 24b and 24c
pass through the slots in the disc 23 and illuminate the photodiodes 25a,
25b and 25c respectively. An alternative arrangement would be to
distribute the emitting and photo diode pairs around the disc in positions
corresponding to the positions of brushes 7a, 7b and 7c around the
commutator, in which case only one row of slots would be required on the
disc.
The signals from the photodiodes are processed by an electronic circuit 26
in FIG. 12 and used to switch six solid state power switching devices 20a,
20b, 20c, 20d, 20e and 20f which connect the motor armature windings to
the terminals of the d.c. source. The start of armature winding 2a is
connected to terminal 30a which connects with the transistor pair 20a and
20b, armature winding 2b is connected to terminal 30b which connects with
the transistor pair 20c and 20d and armature winding 2c is connected to
terminal 30c which connects with the transistor pair 20e and 20f.
In this example, when light falls on any given photodiode, the associated
power transistor connected to the positive terminal of the supply is
switched on and the transistor connected to the negative terminal is
switched off: otherwise the transistor connected to the negative terminal
of the supply is switched on and the transistor connected to the positive
terminal is switched off.
At the instant depicted in FIG. 13, light from l.e.d. 24a passes through
slot 23a to illuminate photodiode 25a and the resulting signal from the
photodiode is used to turn on transistor 20a which connects the start of
stator winding 2a to the positive terminal of the supply. Similarly, the
light from l.e.d. 24b passes through slot 23b to illuminate photodiode 25b
and switch on transistor 20c to connect the start of stator winding 2c to
the positive terminal of the supply. The light from l.e.d. 24c is blocked
by the disc because slot 23c is not in alignment and therefore transistor
20f is switched to connect the start of winding 2c to the negative
terminal of the supply. The instant depicted in FIG. 13 is therefore
equivalent to that depicted in FIGS. 3, 4 and 5 and a clockwise rotation
of the rotor will produce the same switching sequence as the commutator
and brushes as illustrated in the Table of FIG. 15.
The motor presents an essentially resistive load to the supply and in the
electronically commutated example this feature can be utilised to provide
an improved motor control system. To vary the speed of a d.c. motor over a
wide range it is necessary to vary the supply voltage and the most
efficient method of voltage control is to use a pulsed technique whereby
the controlling element is either in the on state or off so that, unlike a
resistor for example, the power lost in the switching device is a minimum
and the average output voltage is then proportional to the mark-space
ratio. In conventional brushless d.c. motor and synchronous motor control
systems the mark-space ratio is modulated over one cycle to produce a
quasisine waveform (pulse-width modulation--p.w.m.). However, in the motor
described herein such a waveform would be inefficient and seriously
degrade the performance mainly because of the low voltage value at the
beginning of the cycle which would reduce the rate of increase of current,
the peak value of the current and the average value of the torque produced
by the motor.
One method of controlling the d.c. supply to the motor described herein is
to use a switched-mode system whereby the signal to the power switching
devices consist of a series of pulses with a variable mark space ratio as
illustrated in FIG. 16, showing four pulses either side of the instant of
commutation. It will be appreciated that the greater the number of pulses
per cycle the finer the degree of control. In conventional p.w.m. systems
providing a high degree of control, the modulating frequency is in the
audible range which is undesirable and, because of the necessity to vary
the modulation over a cycle, the use of higher frequencies is complicated
and expensive. In the motor control system shown in FIG. 12, for a given
adjustment of a mark-space ratio control 27, the mark-space ratio is
constant over the whole cycle and this allows the use of much higher
frequencies, preferably in the ultrasonic range, thereby overcoming the
objectionable noise generated in conventional p.w.m. systems.
PERFORMANCE
Conventional three phase doubly excited machines such as alternators,
brushless d.c. motors and synchronous motors are readily converted to d.c.
motors in accordance with the present invention and this feature is useful
in comparing the performance of one machine with another. In a
conventional d.c. motor the power converted to magnetic energy is given
by:
PELEC=E I Watts (1)
where E is the open circuit e.m.f. at a given speed and I is the armature
current. In the case of a three-phase machine this becomes:
PELEC=E I 31/2 Watts (2)
In the case of a permanent magnet field or a constant field current the
open circuit e.m.f. is proportional to speed and if the speed is expressed
in radians per second (w) a useful constant (K.sub.E) for a given machine
can be derived:
K.sub.E =E/w Volts/rad.sup.-1 sec.sup.-1 (3)
Substituting for E in equation (2):
PELEC=(Ke w) I 31/2 Watts (4)
The mechanical output power is given by:
PMECH=w T Watts (5)
where T is the torque and, neglecting losses, the mechanical power is equal
to the electrical power so that:
w T=K.sub.E w I 31/2
Dividing by w gives:
Torque T=K.sub.E 31/2 I Nm (6)
From equation (6) it can be seen that the electrical constant K.sub.E can
also have mechanical units and is often referred to as the torque constant
or torque sensitivity and in three phase machines the constants are
related thus:
K.sub.T =K.sub.E 31/2 (7)
and in d.c. machines K.sub.E =K.sub.T so that in all cases:
Torque T=K.sub.T I Nm (8)
The performance characteristics of the motor in accordance with the present
invention, when the armature windings are connected in wye-configuration,
therefore compare with conventional d.c. motors but in the case of
inverter driven synchronous motors and brushless d.c. motors current is
applied for only two-thirds of a cycle and consequently the effective
electrical constant (K.sub.E) is reduced by about 31/2. Further the
performance characteristics of the motor in accordance with the present
invention are the same as those of conventional d.c. motors with the
exception that both speed and torque are directly proportional to the
field current whilst in conventional d.c. motors speed is inversely
proportional to the field current.
The motor of the present invention has utility in vehicle steering systems
and is believed to be of advantage in such usage because of the
possibility of achieving a high torque output for a small sized motor
unit. An example of a vehicle steering gear illustrating the manner in
which the d.c. motor may be incorporated in a steering unit is disclosed
in EP 0 101 579.
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