Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
_
BACKGROUND OF THE INVENTION
This invention relates to brushless DC motors
electronically commutated and more particularly to brushless
DC fan motors that are of simple construction, inexpensive
to manufacture, and reliable.
A typical goal in the manufacture of fans is a
motor that is very simple and consequently has a low manu-
facturing cost. In AC motors for fans, the side armature AC
motor comes closest to achieving these goals. However,
recently, DC motors for fans have become more and more
attractive, particularly for fans used to cool electronics
where DC power is available.
Brushless DC motors using ~all effect devices to
sense the commutation points as the rotor rotates are well
known in the art. One or more stator coils are repeatedly
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energized or have their energization reversed to effect
relocation of the electromagnetic field produced by poles of
the stator core. A permanent magnet rotor is continuously
att~acted to the new electromagnetic pole locations. For
commutation, one or more Hall effect devices senses the
location of the poles of the rotor permanent magnet to
control the eneryization of the stator winding or windinys,
or a EIall device detects the~position of oIle or more com-
mutation magnets mounted to rotate with the rotor and
provided especially to indicate, by changing the state of
the Hall device, the commutation points as the rotor turns.
Many brushless DC motors have been complex in both
their structure and their commutation circuitry. Where
simple, low cost and reliable fan motors have been needed
these brushless DC motors, whlch might more appropriately
have been used for, say, precise disc or tape drives, have
been too expensive for the simple purpose of fan rotation.
BRIEF SU~ARY OF THE INVENTION
In accordance with this invention, a simple DC
brushless motor has a rotor with an annular permanent magnet
and a stator coil and electromagnet structure outside the
annular magnet. The annular magnet of the rotor is mag-
netized in segments about its circumference, each succeeding
segment being oppositely magnetized in the radial direction.
The electromagnet structure includes pole pieces magnetized
by the coil and closely proximate the outer cylindrical
surface of the annular magnet. A Hall effect device senses
,
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the passage of the rotor magnet segments to turn the coil on
and off. The location of the electromagnet pole pieces and
of the Hall effect device are such that, each time the coil
is energized, the corr~ct magnetic polarities are established
at the pole pieces to attract the next approaching s~gments
or poles of the annular ma~net.
A further permanent magnet supported on the stator
structure close to the periphery of the annular rotor magnet
magnetically detents the annular magnet so that the rotor is
correctly positioned for start up.
In the fan of the invention, fan blades are located
within and affixed to the annular magnet. The magnet and
the fan blades are mounted for rotation at a central hub.
The stator includes a housing and support structure extending
from proximate the magnet to the hub and supporting the rotor
hub for rotation. The housing encircles the annular magnet
and the fan blades. The support structure includes mounting
means formed in the housing about the periphery of the motor.
~ small compartment in the housing houses the stator coil,
the electromagnet structure and all circuit elements of the
relatively simple switching circuitry, including the commuta-
tion-effecting ~all device~
Whereas, with its large annular rotor and external
stator structure, the motor of this invention misht not be
adva~tageous for certain other applications, it is particu-
larly suitable for fans. The tips of fan blades ordinarily
define a circular path and the area within the circle is
necessarily used. Driving the fan blades directly from a
magnet secured to the blade tips thus re~uires little
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additional space and permits a narrow fan since there is no
need to couple a motor to the hub or shaft supporting the
blades centrally. The drive is ef~icient and is economically
accomplished in that the rotary force applied to the magnet
is imparted directly to the blades. Less force is required
to move the blades against a load than when force is applied
where the blades are centrally supported. Furthermore,
using the rotor of the current invention a large proportion
of the circular area from the blade tips inward can be
devoted to air 10w, inasmuch as no space therein is taken
up by motor. The advantages of the combination fan and rotor,
then, surpass the apparent sum of the advantages of each.
The above and further features of the invention
will be better understood with reference to the several
figures of the attached drawings and the following detailed
description of a preferred embodiment.
DESCRIPTION OF THE DRAWINGS_
. _ . .. . .
In the drawings:
Figure 1 is a top plan view of the fan and motor
according to this invention and shows the rotor's annular
permanent magnet magnetized in segments about its periphery
and an external coil and electromagnet stator structure.
Figure 2 is a side elevational view of the fan and
motor of Figure 1, partially in section, along the line 2-2
of Fig. 1, and illustrates the mounting of the rotor and fan
blades on a hub for rotation about a central support section
of the stator.
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Figure 3 is a schematic diagram of an energizing
circuit for the stator coil.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In Figure 1 a fan and motor combination 10 accord-
ing to the invention includes a rotor 11 and a stator 12.
The rotor 11 has an annular permanent magnet 14, magnetized
in segments about its circumference. As illustrated, each
succeeding segment about the circumference is oppositely
magneti~ed in the radial direction. The magnet 14 is
secured on a ring 16. Fan blades 17 extend from the ring 16
to a central hub 19.
In Figure 2, the hub 19 carries a shat 21 sup-
ported in the sleeve bearing 22 located in a bore 23 formed
ln a central projecting portion 25 of the stator 12. C-rings
28 and 29 retain the bearing and secure the rotor and stator
together. Spacers or shims 31 take up any excess end play
in the assembly. The particular mounting employed to locate
the rotor assembly and fan blades rotatably ~ithin the stator
structure i5 exemplary only and not to be construed as essen-
tial to the invention. For example, roller bearings or otherbearing structure can be substituted for the sleeve bearing
22 and other arran~ements than the shaft 21 and bore 23 will
readily be envisioned.
Three struts 33 radiate outwardly from the central
stator projection 25. These connect to a generally circular
housing 34. Mounting bGsses 35 define holes 36 ab~ut t~
periphery of the housing, enabling the entire motor and fan
to be mounted by, e.g., bolts passed through the holes 36.
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At one location, as seen at the bottom of Figure 1,
the housing 34 forms a compartment 40 housing a stator coil
41, electromagnet structure 4~, a Hall effect device Xl, and
the remaining circuit elements of the commutation circuit,
not shown in Figure 1. The coil 41 is wound on a bobbin 43.
A core 44 o magnetic material extends through the bobbin
from end to end and forms a part of the electromagnet
structure 42. Alternatively, the bobbin can be a part of
the electromagnet structure. That structure also includes a
pair of arms 46 and 47 secured in flux conducting relation
to the magnetic core 44 and terminating in pole pieces 48
and 49 closely proximate the outer surface of the permanent
magnet 14. As can be seen in Figure 1, the spacing of the
pole pieces 48 and 49 is such that, with the magnet located
as shownr energization of the coil 41 to make pole piece 48
north and pole piece 49 south will affect clockwise rotation
of the rotor structure and the fan blades.
A further permanent magnet 50 magnetically detents
the annular rotor magnet in a position such that starting of
the motor and fan is assured. At start-up, the coil is ener-
gized to applyr via the pole pieces 48 and 49, starting
torque to the annular magnet. The Hall effect device X1
controls eneryization of the coil 41 such that the coil is
energized to attract approaching poles of the annular rotor
magnet and then deenergized when the magnetic field of the
annular permanent magnet is reversed at the ~all device as a
result of movement of a new, oppositely magnetically polar-
ized segment into proximity with the deviceO Deenergization
of the coil allows passage o= the next poles past the pole
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pieces 48 and 49 until the Hall device Xl senses the rever-
sal of the magnetic field, signaling the approach of the
next segrnents. The coil is reenergized and the pole pieces
attract the next two segments. This sequence repeats itself
until the rotor comes to an e~uilibrium speed where the
aerodynamic load balances the motor power.
This operation allows use of a simplified com-
mutation circuit. In Figure 3, the Hall device Xl, which
can be a Hall switch, for example, is seen controlling a
transistor Ql whose collector-emitter circuit is in series
with the stator coil 41. One such Hall switch is the
sprague UGN-3013T of Sprague Electric Companyt Worcester,
Mass. Exposure of the Hall effect device X1 to a magnetic
field o the correct direction or polarity opens (breaks) a
conduction path from a line 51 to ground. With this path
thus open, a resistor Rl supplies base drive to Ql, biasing
Ql into conduction and energizing the coil 41. When the
Hall effect device Xl experiences a magnetic field of
reverse direction or polarity, or no field at all, the
current path from ground to Rl and the base of Ql closes.
This path of conduction, now closed, starves Ql of base
drive and Ql no longer conducts, the coil 41 is deenergized
until Xl experiences a field of the appropriate direction
again. Capacitor Cl damps transients that arise from the
abrupt switching of the coil 41. The diode CRl prevents
current reversals back through the input leads to, for
example, the electronics being cooled. Typically, the input
power to this simple circ~it arrangement is taken from the
DC bias voltage available nearby in the cooled electronics.
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In a preferred embodiment, the circuit elements are mounted
on a printed circuitboard of essentially the same shape as
the cavity that forms ~he compartment 40 in E`igure 1. The
circuitboard is supported above the cavity with the circuit
elements projecting downward and housed within the cavity so
that the fan and motor has the compact shape seen in Figure 1
and the narrow profile shown in Figure 2.
From the foregoing it will be seen that a simple
and quite original combination of motor and fan has been
provided. It will be readily apparent to those skilled in
the art that many variations in the foregoing exemplary
embodiment can be made without departing from the spirit and
scope of the invent.ion, as defined in the depen~ant claims.
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