Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to D.C. motors of the type
having a permanent magnet rotor, and more particularly to
brushless motors of this type.
Conventional D.C. motors employ commutators and
brushes for accomplishing the switching required to energize
and deenergize the motor coils. However, mechanical
commutators pose problems in applications requiring high
reliability and long life. The sliding contact between
brushes and commutator causes wear on the parts. Furthermore,
when such a motor is used in certain types of environments, - -
such as highly corrosive atmospheres, the electrical contact
between brush and commutator may be interfered with due to
contamination of the parts. There are other well known
deficiencies of commutator-brush devices.
In order to avoid these defiaiencies, brushless D.C.
motors have been suggested in the past. These motors eliminate
the mechanical switching provided by a commutator and brushes.
An example of brushless D.C. motors is a motor employing a
plurality of light sources and associated light sensors. A
light-interrupting means rotates with the motor shaft to
sequentially block and transmit light from the light sources
to the light sensor~, and thereby effect switching as a
function of motor shaft position.
Another example of brushless D.C. motors is one
employing Hall effect resistors fixed to the stator and arranged
in the field of the permanent magnet rotor. As the rotor
rotates, each resistor develops an output voltage which is a
function of the rotor position, and these voltages are used
to effect switching of the stator coils.
An important problem presented by brushless D.C.
motors used in the past involves their sensitivity to temp~r-
ature variations. The outputs of light emitting diodes and of
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Hall effect resistors vary considerably with temperature.
Hence, where extremes of temperature are encountered by the
motor, operation becomes unreliable. Furthermore, atmospheric
contaminants can interfere with performance of motors using
light sources for switching. In addition, in this type of
motor if a lamp is used in place of a light emitting diode,
lamp life becomes a possible problem.
It is an object of the present invention to overcome
these problems by providing a brushless D.C. motor employing
a commutator which is not subject to wear and is completely
insensitive to temperature variations.
It is a more specific object of the invention to
provide a brushless D.C. motor incorporating an electro-
magnetic commutator wherein switching is accomplished by
coupling and decoupling the primary and secondary windings
of a transformer.
It is a further object of the invention to provide
such a brushless D.C. motor wherein a high radio frequency
signal is employed as the energy coupling the primary and
secondary windings of the transformer.
It is another object of the present invention to
provide a brushless D.C. motor in which all electronic com-
ponents can, if desired, be separated from the motor and
commutator so that these components are not subjected to the
immediate environment surrounding the motor and commutator.
An additional object of the invention is to provide
a brushless D.C. motor having high unit-to-unit uniformity
without the need for trimming during manufacturing. Trimming
i8 often required when optical and Hall effect com~ltators are
used due to the wide manufacturing tolerances of semiconductor
devices.
It is a further object of the invention to provide a
brushless D.C. motor employing a commutator which is both
highly reliable and can readily be manufactured economically
on a mass production basis.
Additional objects and features of the invention
will be apparent from the following description, in which
reference is made to the accompanying drawings.
In the drawings:
Fig. 1 is a partial longitudinal cross-sectional
view of a brushless D.C. motor according to the present invention,
the lower portion of the motor being shown in elevation;
Fig. 2 is a cross-sectional view taken along lines
2-2 of Fig. 1 showing the face of a printed circuit board
carrying the transformer primary winding;
Fig. 3 is a cross-sectional view taken along line
3-3 of Fig. 1, showing the face of a printed circuit board
carrying a plurality of secondary windings;
Fig. 4, on the first sheet of drawings, is a
cross-sectional view taken along line 4-4 of Fig. 1, showing
the face of a shield rotatable with the rotor;
Fig. 5 is a schematic view of the electromagnetic
commutator circuitry;
Fig. 6 is a diagram illustrating the variations in ; -- -
voltage in the three secondary windings of the transformer;
Fig. 7, on the first sheet of drawings, is a
cross-sectional view taken along line 7-7 of Fig. l; and
Fig. 8 is a schematic diagram illustrating an alter-
native arrangement of transformer secondary windings to permit
reversing the direction of rotation of the motor.
The brushless D.C, motor chosen to illustrate the
present invention, and shown in Fig. 1, includes a housing 10
within which are a plurality of stationary stator windings 11,
only one being visible in Fig. 1. In the present illustration,
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three stator windings are employed, these being identified by
reference numerals lla, llb, and llc in Fig. 5. However, it
is understood that two or more stationary stator windings
can be used, and preferably at least three are employed.
Within the stator windings 11 is a cylindrical permanent
magnet rotor 12 fixed to a motor shaft 13 which extends beyond
both ends of the rotor 12.
Shaft 13 extends freely through openings 14 in the
opposite ends of housing 10. Hollow bosses 15 surround open-
ings 14 and project into the housing from the inner surfaces
of the housing ends. Ball bearings 16 are fixed within bosses
15 and rotatably support shaft 13 with respect to the housing.
Electrical conductors (not shown) for supplying electric
current to the motor pass through a hole in housing 10 lined
by a grommet 17.
Secured to one end of housing 10 is a cup-shaped cap
20, Four long threaded studs 21 extend from that end of the
housing and through four holes in the cap 20. A nut 22
threaded on each stud 21 holds cap 20 against housing 10.
Supported on studs 21, within cap 20, are a support
plate 23, and two printed circuit boards 24 and 25. Each of
the suppo~t plate 23 and printed circuit boards 24 and 25 is
provided with an enlarged central opening through which shaft
13 passes freely, and with four holes through which studs 21
pass. A tubular spacer 26 surrounds each stud 21 between the
end of housing 10 and support plate 23, a spacer ring 27
surrounds each stud 21 between support plate 23 and printed
circuit board 24, a spacer 28 and washer 29 surround each
stud 21 between the two printed circuit boards 24 and 25, and
a nut 30, threaded on to each stud 21, is tightened against the
outer face of printed circuit board 25.
Support plate 23 carries an electronic package 34
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comprising the components illustrated schematically in Fig. 5
Where necessary or desirable, support plate 23 could be elimin-
ated, and electronic package 34 arranged at a location remote
from the motor, the electronics being connected to the motor
by suitable conductors. This may be desirable when the motor
is used in an environment, such as a high temperature
environment, which would adversely affect the semiconductor
or other electronic components.
Printed circuit boards 24 and 25 constitute a trans-
former. As may be seen most clearly in Fig. 2, board 24
carries a printed circuit primary winding 35. As shown in
Fig. 3, board 25 carries three printed circuit secondary
windings 36, 37, and 38. Three secondary windings are
employed because the stator of the motor includes three
stationary windings lla, llb, and llc. The number of second-
ary windings carried by board 25 equals the number of stator
windings of the motor since, as will be apparent from the
description which follows, one secondary winding of the trans-
former is allocated to each winding of the stator.
Arranged between printed circuit boards 24 and 25 is
an electrically-conductive plate-like shield 39. Shield 39
is of a smaller diameter than boards 24 and 25, so that it
fi~s within the space defined by the four studs 21 without
interfering with the studs. Furthermore, shield 39 is fixed
to, and rotatable with, shaft 13 by means of a collar 40. ;
As shown most clearly in Fig. 4, shield 39 has a generally
circular shape, but with a sector removed. The size of the
removed sector is such that at any one time shield 39 can ~ ~ -
cover all but one of the secondary windings 36-38. In the
present example, since there are three secondary windings,
the removed sector of shield 39 subtends an angle of 120.
If board 25 were carrying four secondary windings, the removed
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section of shield 39 would subtend an angle of 90.
Shield 39 is formed of a material capable of interrupt-
ing the transfer of energy between primary winding 35 and
secondary windings 36-38. In the present example, a radio
frequency signal is applied to primary winding 35. Therefore,
shield 39 is formed of a material, such as aluminum, capable
of blocking transmission of the radio frequency signal from
primary winding 35 to secondary windings 36-38. However, it
will be obvious that in the region of the removed section of
shield 39, transfer of energy between primary winding 35 and
secondary windings 36-38 is permitted.
The electromagnetic commutator of the present invention
will now be further described with reference to Fig. 5. All
of the components illustrated in Fig. 5, except for the trans-
former coils and the stator coils, may be included in the
; electronic package 34. An oscillator 44, of any conventional
suitable type, is arranged to provide a radio frequency signal
to primary coil 35. The frequency of the signal is preferably in
the ~egaHertz range, a typical frequency being 2.5 MHz. Higher
frequencies may be used, and in fact one of the advantages of
the present invention accrues from the use of a high frequency,
since as a result of the high frequency used to couple the
transformer primary and secondary windings, the commutator is
capable of extremely fast switching. Furthermore, it is easier
to shield higher frequencies than lower frequencies, and
smaller components can be used to filter higher frequencies.
one end of each of the secondary coils 36-38 is
connected to ground, and the other end is connected to a diode
45. If desired, a capacitOr 46 may be shunted across each
secondary coil to tune the coil so as to produce a somehwat
higher signal level. The other side of each diode is connected
to the input of an amplifier 47. If desired, a capacitor 48
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may be connnected between the input of the amplifier and
ground to filter the D.C. signal leaving diode 45.
The outputs of amplifiers 47 are connected to the
bases of transistors 52, 53, and 54, respectively. The
emitters of all three transistors are connected together by
conductor 55, and through common emitter resistor 56 to
ground. The collectors of transistors 52-54 are connected
to the bases of transistors 57, 58, and 59, respectively.
The emitters of transistors 57-59 are connected to ground by
a common conductor 60, and the collector of each of these
transistors is connected to one end of one of the stator ~,
coils lla-c, respectively. The opposite ends of coils lla-c
are connected together to a source of D.C. power.
As shield 39 rotates with motor shaft 13, a high
frequency voltage i8 induced in each of the secondary coils
36, 37, and 38, in sequence. The voltages induced in the
secondary coils are illustrated in Fig. 6, At the start of
one complete revolution of shield 39, the voltage 36' in coil
36 is rising. The voltage reaches a maximum value and then
begins to fall, After shield 39 has rotated through 120, the
voltage 37' in coil 37, which has been rising, crosses and
exceeds vsltage 36' which shortly thereafter drops to zero
when coil 36 is completely shielded from primary coil 35.
After 240 of rotation of shield 39, voltage 38' in coil 38
crosses and exceeds the falling voltage 37', and at the com-
pletion of the revolution, voltage 36' crosses and exceedc
voltage 38', to begin the cycle again,
The signal induced in each of the coils 36-38 is
rectified by diode 45, and the resulting D.C. signal is applied
to amplifier 47. The outputs from the amplifiers are applied
to the bases of transistors 52-54, respectively. Resistor 56
serves to back-bias the two transistors which at any instant
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receive the lower driving voltages. In other words, only the
transistor connected to the amplifier 47 which is providing the
highest output voltage is turned on, while the other two trans-
istors are turned off. It will be appreciated from an inspect-
ion of Fig. 6 that, as a result, transistors 52, 53, and 54
are turned on and then off in sequence. More specifically,
as shield 39 passes the 120 degree point in its rotation,
transistor 53 turns on, transistor 52 turns off, and trans-
istor 53 remains off. As shield 39 passes the 240point,
transistor 54 turns on and transistors 52 and 53 are off.
When shield 39 passes the 360 point, transistor 52 turns
on and transistors 53 and 54 are off.
In response to transistors 52, 53, and 54 being turned
on and off in sequence, transistors 57, 58, and 59 are also
turned on and then off in sequence. when transistor 57 is
turned on, a circuit is completed from the D~aC~ source through
coil lla, transistor 57, and conductor 60 to ground, thereby
completing a circuit for energizing the coil lla. In similar
fashion, the turning on of transistors 58 and 59 causes
energization of coils llb and llc, respectively. It will be
seen, therefore, that coils lla-c are energized in se~uence.
As is usual in such motors, energization of the stator coils
in sequence causes rotation of the permanent magnet rotor 12.
It will be appreciated that proper functioning of the
motor depends upon an appropriate relative angular po~itioning
between the stator coils 11 and the secondary coils 36-38. If
coils 36-38 "lead" the stator coils, rotor 12 will rotate in
one direction, and if the secondary coils "lag" the stator
coils, the rotor will rotate in the opposite direction. Con-
sequently, it is a relatively simple matter to provide for
reversability of the motor according to the present invention,
As indicated in Figs. 3 and 7, the holes in printed
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circuit board 25, through studs 21 pass, are actually elong-
ated arcuate slots 64. when rotation in one direction is
desired, board 25 is adjusted so that each stud 21 is located
at one end of its respective slot 64, and nut 30 is tightened to
retain this position of adjustment. Should it be desired to
reverse the direction of rotation of rotor 12, nuts 30 are
loosened, board 25 is rotated so that each stud 21 is arranged
at the opposite end of its respective slot 64, and nuts 30 are
retightened.
An alternative way of providing for reversability of
the present motor involves providing an auxiliary printed
circuit board 65 identical to board 25, as indicated in Figs.
1 and 8. In this alternative arrangement, the holes in boards
25 and 65 need not be slots, as indicated at 64 in Fig. 3,
since no adjustment of either board is necessary, However,
board 65 is arranged so that the secondary coils 66, 67, and
68 which it carries are angularly offset from the coils carried
by board 25. Arranged between the coils of each circuit
board and the remainder of the commutator circuit is a three
pole, double throw switch 69. In Fig. 8, the three movable
contacts of switch 69 are shown in solid lines connecting the
coils of printed circuit board 25 to the remainder of the commu-
tator circuit, and the coils of printed circuit board 65 are
unconnected to the circuit. when it is desired to reverse the
direction of rotoation of rotor 12, switch 69 is shifted 80
that its movable contacts are in their dotted line positions
wherein they connect the coils of printed circuit board 65 to
the remainder of the circuit and simultaneously disconnect the
coils of printed circuit board 25 from the circuit.
The invention has been shown and described in pre-
ferred form only, and by way of example, and many variations
may be made in the invention which will still be comprised
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within its spirit. It is understood, therefore, that the
invention is not limited to any specific form or embodiment
except insofar as such limitation are included in the
appended claims.
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