This invention relates to a rotor assembly for an electrical
machine such as an electrical motor or generator.
Electrical motors are widely used for many different applications
and are commonly used in domestic appliances. For example, in a vacuum cleaner a
motor is used to drive a fan that causes dirty air to be sucked through a dirty
air inlet. The dirty air passes through some form of separation device such as a
cyclonic or bag separator that separates dirt and dust from the airflow, and finally
the air is exhausted from an air outlet.
Switched reluctance machines have become increasingly popular
in recent years. In a switched reluctance motor, a stator has sets of poles that
are sequentially energised to rotate a rotor into line with the energised pair of
poles, under the influence of the magnetic fields associated with each set of poles.
By rapidly switching between different pairs of poles, it is possible to cause the
rotor to rotate at a very high speed.
Switched reluctance machines have an advantage in that
they do not use carbon brushes, which need to be replaced periodically and which
emit particles of carbon into the atmosphere as they wear down. Furthermore, the
motor has a relatively long life and its speed is not limited by the need to maintain
a reasonable brush life.
In order to energise sequentially the stator poles, the
position of the rotor with respect to the stator poles has to be determined with
great accuracy. It has been proposed to use an encoder disk mounted on the shaft.
The encoder is arranged, in use, to rotate with the shaft and interrupt intermittently
light travelling between a transmitter and a detector. Thus, during manufacture,
the encoder has to be mounted carefully on the shaft in a predetermined position
and orientation with respect to the rotor so that the rotational position of the
rotor can be determined. Alternatively, the encoder can be mounted on the shaft
in an arbitrary orientation, which then must be determined with respect to the rotor
discloses an encoder and a balancing device, radially spaced on a rotor
The invention as defined in the independent claim 1 provides
a rotor assembly comprising a shaft carrying a rotor and a balancing member located
in a predetermined position with respect to the rotor, the balancing member comprising
a main body from which material is selectively removable, characterised in that
the balancing member further comprises a sleeve which carries an encoder member,
axially spaced from the main body.
A balancing member according to the invention is defined
in the independent claim 13.
A method of manufacturing a rotor assembly according to
the invention is defined in the independent claim 15.
The provision of a balancing member having an integral
encoder member greatly simplifies the manufacture and set-up of the rotor assembly.
The balancing member is located in a predetermined position with respect to the
rotor; hence the integral encoder member is also located in a known position and
orientation with respect to the rotor. Furthermore, a predetermined distance between
the main body and the encoder is maintained.
Preferably, locating means is provided in the form of a
lug located between adjacent poles of the rotor. This facilitates the locating of
the balancing member in the predetermined position.
Advantageously, the integral encoder member comprises a
disk having at least one aperture, otherwise known as a chopper.
The invention is applicable to switched reluctance machines,
and is particularly useful in such machines that operate at high speeds of, say,
100,000 revolutions per minute.
While the following embodiments describe the invention
as applied to motors which are used to drive a fan in a vacuum cleaner, it will
be appreciated that the invention can be applied to both motors and generators,
for any type of application, and is not limited to vacuum cleaners or the field
of domestic appliances.
The invention will now be described, by way of example,
with reference to the accompanying drawings, in which:-
- Figure 1 shows a rotor assembly constructed in accordance with the invention;
- Figure 2 is an exploded view of the rotor assembly of Figure 1;
- Figure 3 is a sectional view of the rotor assembly of Figures 1 and 2;
- Figure 4 is a sectional view of a motor incorporating the rotor assembly of
Figures 1 to 3;
- Figure 5 is a side view of a vacuum cleaner incorporating the motor of Figure
Like reference numerals refer to like parts throughout
Figures 1 to 3 show a rotor assembly constructed according
to the invention and indicated generally by the reference numeral 1. The rotor assembly
1 comprises a rotor shaft 2 having a rotor member 3. The rotor member 3 comprises
an axially laminated stack of steel plates, arranged to form a pair of poles 3a,
3b. The shaft 2 also carries a coaxial impeller 4 having a plurality of blades 5
arranged to direct fluid flow from the shaft 2 to the periphery of the impeller
in tangential directions.
Bearing assemblies 7, 8 are provided on the shaft 2. Each
bearing assembly 7, 8 comprises a bearing 9, 10 supported on the shaft 2 by a housing
11, 12. The bearings 9, 10 are arranged to press-fit onto the shaft and into their
respective housings 11, 12. Each bearing 9, 10 comprises an inner race 9a, 9b, an
outer race 10, 10b and a plurality of ball bearings (not shown) held between the
races. The bearings 9,10 permit the rotor 3 to be rotatably supported in a stator
13, such as is shown in Figure 4.
The stator 13 comprises a stack of steel laminations arranged
to have four inwardly projecting salient poles. Two of the poles 13a, 13b, diametrically
opposite each other, are shown in Figure 4. Each pole supports a winding 14a, 14b
which together form a first phase. The other diametrically opposite poles (not shown)
similarly accommodate respective windings, which represent a second phase. Each
winding 14 comprises a large number of turns (e.g. 50+ turns) of an insulated electrical
conductor around the respective stator pole.
Each of the housings 11, 12 carries a pair of o-rings 15a,
15b and 16a, 16b. The o-rings of each pair are located at positions corresponding
approximately to the end portions of the bearing within the respective housing.
This soft mounting of the rotor assembly 1 against the stator assembly permits the
rotor assembly 1 to find its own centre of rotation in use. Thus, the rotor assembly
1 rotates about its centre of mass, with little excursion, and so the clearance
between the rotor poles and the stator can be made relatively small. The smaller
the gap, the smaller the magnetic reluctance between the stator and the rotor and
hence the more power that can be generated by the motor with a given electrical
input. Thus, the efficiency of the machine is improved.
The bearing assemblies 7, 8 are located at the extreme
end portions of the rotor shaft 2. This feature aids the balancing of the shaft
2, particularly at the high speeds experienced by the rotor assembly 1.
The housings 7, 8, for the bearings 9,10 are thermally
conductive. Heat generated by the bearings 9, 10 is dissipated by the housings 7,
8. Thus, the rotor assembly can be rotated at very high speeds for prolonged periods
without the bearings overheating.
The housings 7, 8 also contain respective reservoirs 17,
18 of fluid, such as grease, which are arranged to provide lubrication to the bearings
9, 10 in use. Typically, the ball bearings are coated with grease that, over time,
gets pushed out of the races. The reservoirs 17, 18 of grease supply the ball bearings
with lubrication throughout their lifetime.
If the mass of the rotor assembly 1 is unevenly distributed,
the rotor assembly may wobble as it rotates, which can put strain on some of the
components and subject them to uneven wear. Thus, the shaft 2 of the rotor assembly
1 also carries a balancing member in the form of a disk-shaped main body 40 on a
sleeve 41. The disk 40 is formed of plastics material. The rotor assembly 1 may
be placed in a balancing apparatus (not shown), in which the shaft 2 is arranged
to spin. The balancing apparatus is arranged to detect imbalances of the rotor assembly
1. Tools are then employed, preferably under automatic control, to remove material
from the balancing disk 40 by shaving, drilling or cutting. Material is removed
from the disk 40 in order to balance the rotor assembly 1.
In accordance with the invention, the balancing member
further comprises an integral encoder member. In this embodiment, the encoder member
comprises a disk 42 having a plurality of apertures 48, 49. The encoder disk 42
is carried by the sleeve 41, and is axially displaced from the balancing disk 40.
The encoder disk 42 is located on the sleeve 41 in a predetermined
position, which position includes a predetermined orientation. Thus, when the balancing
member is introduced onto the shaft 2 and arranged to abut the rotor member 3, the
encoder 42 automatically occupies a predetermined position and orientation with
respect to the rotor member. In particular, the positions of the edges of the apertures
48, 49 with respect to the rotor poles are known. Hence, the position of the rotor
member 3 can be determined accurately. Previously, the encoder member was introduced
onto the shaft separately, and thus the relative position and orientation of the
encoder and rotor poles had to be determined carefully before the rotor assembly
could be used. Thus, the manufacture and set-up of the rotor assembly of the invention
is more straightforward than hitherto.
To ensure that the balancing disk 40, and hence the encoder
member 42, occupies a predetermined position, locating means is provided in the
form of axially-projecting lugs 43, 44, which are arranged to engage the rotor member
3 in the region between the poles 3a, 3b of the rotor member. Hence the balancing
member may simply be slotted into the desired position.
The balancing member, including the encoder disk 42, has
a diameter smaller than that of the rotor member 3, which facilitates manufacture
of the rotor assembly. During manufacture, the components of the rotor assembly
are assembled on the shaft, and the entire rotor assembly is simply slotted into
the aperture 19 provided for the rotor member 3, with the housing 11 abutting the
end cap 21. Previously, the individual components of the rotor assembly were balanced
separately before being incorporated into the motor or generator, which produced
a less than ideal balance condition of the completed rotor assembly. However, the
rotor assembly of the present invention may be completed before final assembly of
the motor. Thus, the complete rotor assembly may be balanced in one operation by
means of the balancing member, as described above.
The rotor assembly 1 further comprises a second balancing
member 45, located adjacent the other end portion of the rotor member 3. The second
balancing member 45 has a set 46 of lugs arranged to project into the region between
the poles 3a, 3b of the rotor member 3. A further set 47 of lugs is provided and
arranged to engage in apertures in the impeller 4. The provision of balancing members
40, 45 at both ends of the rotor member 3 helps to support the laminations of the
The encoder disk 42, or chopper, is associated with position
detecting means, indicated generally in Figure 4 by the reference numeral 50. The
position detecting means 50 comprises a source of optical radiation and an optical
sensor. The encoding disk 42 is positioned between the source and detector, the
plane of the disk being substantially perpendicular to the direction of optical
radiation. The apertures 48, 49 allow light from the source to be transmitted to
the sensor. As the encoder disk 42 rotates with the shaft 2 of the rotor assembly
1, light from the source is interrupted intermittently. Thus, the optical sensor
receives a pulsed light signal. Signals from the optical sensor are transmitted
to a controller (not shown).
The controller is electrically connected to the drive circuit,
to which the windings 14a, 14b on each of the stator pole portions 13a, 13b are
connected. Torque is produced by switching current on in each phase winding in a
sequence, so that a magnetic force of attraction results between the rotor and stator
poles that are approaching each other. The current is switched off in each phase
before the rotor poles nearest the stator poles of that phase rotate past the aligned
position. Thus, knowledge of the rotational position of the rotor poles is essential.
The impeller 4 rotates with the rotor shaft 2 and thus
draws air into the motor. The bearing assembly 8 forms a nose cone located at the
end of the shaft 2, upstream of the impeller 4. Hence, air being drawn in by the
impeller 4 will firstly flow over the bearing assembly 8. Heat generated by the
bearing 10 is dissipated by the thermally conductive bearing housing 12. The airflow
over the bearing assembly 8 serves to cool the bearing housing 12.
There is also provided an inlet 22 for a second airflow
for the bearing assembly 7 at the other end of the shaft. Heat generated by the
bearing 9 is dissipated by the thermally conductive housing 11, which is cooled
by the flow of air from the inlet 22.
The stator 13 and windings 14 are encapsulated by plastics
material 20 by means of an injection-moulding process, by which plastic granules
are melted, then injected into a mould cavity under pressure to create the required
shape. During this process, the aperture 19 for the rotor assembly 1 and an end
cap 21 for receiving one of the bearing housings 11 are formed simultaneously.
Figure 5 shows one example of a vacuum cleaner 30 in which
the motor may be used. The motor-driven impeller 4 draws dirty air into the cleaner
30 via a nozzle 31 and a hose and wand assembly 32. The dirty air enters a separator
33, which serves to separate dirt and dust from the dirty air. The separator 33
can be a cyclonic separator, as shown here, or some other separator, such as a dust
bag. Cleaned air leaves the separator 33 before entering the motor housing located
within the main body 34 of the cleaner. A pre-motor filter is typically placed in
the airflow path before the impeller to filter any fine dust particles that were
not separated by separator 33.
In use, the motor rotates the impeller 4 at a very high
speed (of around 100,000rpm). The pumping action of the impeller 4 draws air through
the cleaner. The air then flows over the bearing housings and is redirected by the
impeller blades 5 through diffusion outlets 23 into the scroll 24.
A post-motor filter may be placed in the airflow path after
the scroll 24. However, the provision of a brushless motor reduces the requirement
for such a filter. The cleaned air is then exhausted from the cleaner to the atmosphere
via a suitable outlet.
Variations to the described embodiments will be apparent
to a skilled person and are intended to fall within the scope of the invention as
defined in the appended claims. For example, while a four-pole stator, two-pole
rotor machine has been described, the invention can be equally applied to machines
having other numbers of poles on its stator and rotor and with motors having other
Although the balancing member and integral disk have been
described as being of plastics material, it will be appreciated that any non-magnetic
material may be used.
The lugs employed to fix the balancing member in a predetermined
location on the shaft may be replaced with other locating means. For example, grooves
may be provided to co-operate with lugs on the rotor and/or the shaft.
The rotor assembly of the invention is equally applicable
to motors and generators, not necessarily of the switched reluctance type, and may
be employed in appliances other than domestic vacuum cleaners, such as lawn mowers,
air conditioners, hand dryers and water pumps.