Apparatus and method of mechanically commutating a brushless motor

A commutation assembly mechanically commutates an electric motor. It includes a plurality of distinct electrical contacts forming a circular surface around a motor shaft. At least some of the electrical contacts are electrically coupled to distinct coils within the motor. The assembly further includes a conductive flexible ring and an arm with two rollers. The conductive flexible ring has a circumference that is less than the circumference of the circular surface. In use, the arm is rigidly attached to the motor shaft and has a roller disposed at a distal end for forcing an electrical connection between the flexible ring and specific electrical contacts during rotation of the arm. The electrical connection causes at least one of the distinct coils to be selectively energized.

FIELD OF THE INVENTION
 The present invention relates to electric motors and, more particularly, to
 an apparatus and method for mechanically commutating electric motors.
 BACKGROUND OF THE INVENTION
 Electric motors are nearly ubiquitous today and range from very small in
 size, such as those found in compact disk players, to very large such as
 those found in industrial applications. The type of motor, its size, power
 and control requirements all depend on the particular implementation.
 In general, electric motors include a shaft, a rotor and a stator. Driving
 a workpiece may be as simple as connecting the shaft to a platen for
 spinning an object such as a compact disk in a compact disk player.
 Alternatively, the shaft may drive the workpiece through one or more gears
 or transmissions for imparting rotational force under desired torque
 conditions or for imparting translational force.
 There are several different types of DC electric motors, including brush
 and brushless motors. In brush motors, the rotor includes coils called the
 armature which must be connected to a power source to create torque on the
 rotor. The connection of the rotor coils to the power source is made
 through brushes, typically carbon, which slide over a metal cylindrical
 surface that is part of the rotor. The stator includes either a fixed
 permanent magnet or fixed coils which exert torque on the rotor via the
 armature.
 There are several problems associated with brush motors, most of which
 relate to application of power to the rotor through the brush itself.
 These problems include wear of the brush and rotor contact during use,
 arcing, resistance and heating at the brush-contact interface, and burning
 of the brush during temperature extremes.
 In brushless motors, permanent magnets are implemented in the rotor instead
 of coils. The stator includes fixed coils that may be selectively
 energized to create torque on the rotor. Because the permanent magnets do
 not require connection to a power source, no brush is required. Thus, the
 problems associated with the brush-rotor contact interface are avoided.
 Brushless motors tend to be more reliable over time than brush motors and
 are ideal for aerospace applications.
 All brushless electric motors must be commutated in order to create torque
 and rotation on the shaft. During commutation, one or more coils of the
 stator are momentarily energized in a rotating fashion around the axis of
 rotation of the rotor. Each energized coil creates a magnetic field which
 imparts electromotive force ("EMF") between the energized coil and a
 magnetic pole of the rotor. It is the selective energizing of the coils
 which imparts torque on the motor shaft.
 Traditionally, commutation has been done electronically using electronic
 components. Electronic commutation is accomplished by using position
 sensors on the motor which determine the position of the rotor relative to
 the stator and a series of switches which energize the stator coils based
 on the rotor position. Electronic commutation is reliable but expensive
 and is difficult to implement when stator coil currents are high.
 There is a need for a new commutation technique for brushless electric
 motors which does not require expensive electronics and which can handle
 high coil excitation currents. The technique needs to be inexpensive and
 reliable and should avoid problems associated with a brush-rotor contact
 interface.
 SUMMARY OF THE INVENTION
 According to the present invention, problems associated with sliding brush
 contacts, expensive electronics and current limited switches for exciting
 stator coils are avoided by mechanically commutating a brushless motor.
 To accomplish mechanical commutation of the motor, one end of the stator
 coils is connected to a series of distinct contact elements arranged to
 have an inner cylindrical surface which makes electrical contact to
 achieve commutation. The electrical contact is made with a flexible
 cylindrical ring. Its outer diameter is slightly smaller than the diameter
 of the inner cylindrical surface of the stator contact elements. The
 electrical contact is made at two discreet points (180 degrees apart) by
 deforming the ring outward so that it contacts the stator elements. This
 is shown and described with reference to FIGS. 3A & 3B. Connected to the
 motor shaft is an arm with rollers at each end which deforms the flexible
 ring outward. The arm with the rollers is aligned with the permanent
 magnetic polls of the rotor.
 The flexible ring is restrained by, for example, keys on the stator. These
 keys and the two rollers are to be electrically insulated to completely
 isolate the flexible ring. Because the flexible ring does not rotate, it
 can be connected to the external power source. Consequently, the stator
 coils are sequentially activated at the desired time to achieve
 commutation. Commutation is thus accomplished without electronically
 controlled switches and without reliability problems associated with a
 sliding electrical interface like a brush-contact interface.
 In one embodiment, a commutation assembly for mechanically commutating a
 brushless electric motor includes a plurality of distinct electrical
 contacts forming a circular surface around a motor shaft. At least some of
 the electrical contacts are electrically coupled to distinct coils within
 the motor. The assembly further includes a conductive flexible ring and an
 arm. The conductive flexible ring has a circumference that is less than
 the circumference of the circular surface. The arm is for rigid attachment
 to the motor shaft and has a roller disposed at a distal end for forcing
 an electrical connection between the flexible ring and some of the
 electrical contacts during rotation of the arm. The electrical connection
 causes at least one of the distinct coils to be selectively energized.
 The roller may make substantially non-sliding contact with the flexible
 ring. In some embodiments it may be a bearing assembly while in others it
 may be a pin and wheel assembly such as a bushing. The conductive flexible
 ring may be made of beryllium copper and may be coupled to a source of
 electrical power.
 A method of commutating a brushless electric motor according to one
 embodiment of the invention includes the steps of providing, coupling and
 attaching. In a providing step, a plurality of distinct electrical
 contacts are provided to form a circular surface around a motor shaft. In
 the coupling step at least some of the electrical contacts are coupled to
 distinct coils within the motor. In another providing step, a conductive
 flexible ring is provided having a circumference that is less than the
 circumference of the circular surface. In the attaching step an arm is
 attached to the motor shaft. The arm has a roller disposed at a distal end
 for forcing an electrical connection between the flexible ring and some of
 the electrical contacts during rotation of the arm. This causes at least
 one of the distinct coils to be selectively energized during motor
 operation and therefore commutation of the motor.

DETAILED DESCRIPTION
 Electronic Commutation of the Prior Art
 FIG. 1 depicts an illustrative DC brushless electric motor configuration
 according to the prior art. Referring to FIG. 1, a motor 10 includes three
 stator coils 20 surrounding a rotor 30 which is rigidly attached to a
 shaft 40. The rotor 30 includes magnetic poles 180 degrees apart which may
 be created using permanent magnets or using current supplied to coils
 within the rotor. The rotor 30 is rigidly attached to the shaft 40 and is
 configured to rotate around the axis of the shaft 40 thus rotating the
 shaft 40.
 The motor 10 may be configured to include any convenient number of stator
 coils 20, but is shown for convenience to include three. The stator coils
 20 in general remain stationary relative to the rotor. The stator coils 20
 are distinct electrically conductive coils that are selectively energized
 in order to impart electro-motive force (EMF) between the magnetic poles
 of the rotor and the energized coils of the stator. During a process of
 "commutation," one or more of the stator coils 20 are momentarily
 energized in a rotating fashion around the axis of rotation of the rotor.
 Each sequentially energized coil exerts torque on the magnetic pole(s) of
 the rotor and rotates the rotor and the shaft in the desired direction.
 The sequence of energizing the coils 20 is structured to rotate the shaft
 and impart torque on it. Under low-torque operating conditions, the rate
 of shaft rotation is equal to the rate of rotation in energizing the
 stator coils.
 All brushless electric motors must be commutated as described above in
 order to create torque and rotation on the shaft. Commutation is typically
 done using electronic components such as position sensors on the motor,
 which determine the position of the rotor relative to the stator, and a
 series of switches which energize the stator coils based on the rotor
 position. Electronic commutation is reliable but expensive and is
 difficult to implement when stator coil currents are high.
 A typical electronic commutation configuration according to the prior art
 is depicted in FIG. 2. Referring to FIG. 2, the motor 10 is illustrated to
 include three stator coils 20 used to control the rotation of the rotor
 30. One end of each of the stator coils is connected together. The other
 three ends of the three stator coils 20 are connected through switches
 104-114 to a source of electrical power 120, 122. For example, 120 could
 be applied to a positive terminal of a power source and 122 could be
 applied to a negative terminal of the power source.
 Position sensors 130 are used to measure the position of the rotor 30 of
 the motor 10 relative to the stator coils 20. The motor control logic 140
 receives the output of the position sensors 130 and issues control signals
 to the commutator 100. The motor control logic may calculate, for example,
 the position, velocity or acceleration of the motor shaft 30 based on the
 output of the position sensors 130. The motor control logic may then
 output motor control signals, including a rotor position signal, to the
 commutator 100 for controlling the commutation of the stator coils 20.
 The commutator 100 receives input from the motor control circuit and issues
 control signals 102 to the switches 104-114 based on the motor control
 signals. In order to commutate the coils 20, the commutator 100 generally
 closes two out of the six switches to energize a pair of coils at any
 given time. For example, a typical commutation sequence would be to
 energize the following pairs of coils 20 in sequence: BA, CA, CB, AB, AC,
 BC. To achieve this, the commutator must turn on the following switch
 pairs in sequence: 108, 112; 104, 112; 104,114; 106, 114; 106, 110; and
 108, 110.
 It is apparent from FIG. 2 that each pair of coils 20 is energized through
 two switches. One switch carries current from a first terminal into a
 first coil. The first coil is then connected to a second coil and the
 second switch carries current from the second coil to the second terminal.
 The switches themselves, depending on how they are implemented, may not be
 capable of carrying high coil excitation currents. For example, if the
 switches are implemented on an integrated circuit chip, the current may be
 severely limited. Moreover, if larger switches are used in order to carry
 more current to the coils the size of the switches may be undesirable
 and/or expensive depending on the application.
 Mechanical Commutation Embodiments of the Invention
 According to the present invention, a commutation assembly for mechanically
 commutating an electric motor is provided. The commutation assembly
 according to one embodiment of the invention is depicted in FIG. 3. At a
 high level, the commutation assembly mechanically attaches to the shaft 40
 of the motor 10 and electrically attaches to the stator coils. During
 attachment, the commutation assembly registers the position of the rotor
 relative to the stator based on its geometry and mechanically connects the
 appropriate stator coils 20 to the appropriate electrical contacts for
 energizing them.
 Referring to FIG. 3, the commutation assembly 200 includes an arm 210, a
 flexible conductive ring 220 and an outer ring 230. The outer ring
 includes twelve electrical contacts 250, spaced 30 degrees apart, and
 isolators 260 between each pair of adjacent contacts. The contacts 250 and
 isolators 260 form a circular inner surface having as its center the axis
 of the shaft 40. Each contact 250 is connected to one end of a unique
 stator coil 20. Each stator coil 20 is connected at its other end to a
 terminal of a power supply. The conductive flexible ring 220 is smaller in
 diameter than the diameter of the inner surface of the outer ring. It is
 held stationary relative to the housing of the motor 10 and is connected
 to the other terminal of the power supply.
 The arm 210 is rigidly coupled to the shaft at its center point and has two
 distal ends with opposing rollers 240. The rollers 240 are electrically
 non-conductive and make non-sliding contact with the conductive flexible
 ring 220. Moreover, the rollers 240 deform the conductive flexible ring
 220 and outwardly bias it so that the conductive flexible ring 220 makes
 mechanical and electrical contact with only two of the electrical contacts
 250 at a time. This electrical contact between the conductive flexible
 ring 220 and each particular pair of contacts 250 causes power from the
 conductive flexible ring 220 to energize the corresponding pair of stator
 coils 20 and exert torque on the shaft 40 of the motor in the direction of
 rotation of the shaft.
 FIGS. 4A-4C illustrate the mechanical and electrical connection between the
 commutation assembly 200 and the motor 10. Referring to FIG. 4A, each of
 the twelve electrical contacts 250 is shown connected to a respective one
 of stator coils 20 labeled A-L. FIG. 4B illustrates the motor 10 looking
 into the shaft 10. The motor 10 includes the 12 distinct stator coils 20,
 labeled A-L, positioned around a rotor 30. The rotor 30 includes four
 magnetic poles--two north poles on opposing sides of the shaft 40 and two
 south poles on opposing sides of the shaft 40 and spaced 90 degrees apart
 from the north poles. The stator coils 20, when energized, exert EMF on
 the magnetic poles of the rotor 30.
 The arm 210 is shown with the rollers aligned with the magnetic poles of
 the rotor 30. In general, the arm 210 should be positioned relative to the
 magnetic poles of the rotor such that the stator coils energized by the
 arm outwardly biasing the conductive flexible ring create maximum torque
 on the magnetic poles of the rotor.
 FIG. 4C depicts the electrical connection between each coil 20, a
 corresponding one of the electrical contacts 250, the conductive flexible
 ring 220 and a controlled power source 350. Referring to FIG. 4C, each of
 the coils 20 is connected at one end to a terminal of the controlled power
 source 350. At the other end each of the coils 20 is connected to a
 corresponding one of the twelve electrical contacts 250. The conductive
 flexible ring 220 is shown connected to the other terminal of the
 controlled power source 350. The controlled power source 350 may be
 switched such that the polarity of the terminals may be changed. Moreover,
 the voltage or current supplied by the controlled power source 350 may be
 variable to adjust the position, speed, acceleration or torque of the
 shaft.
 During operation of the motor, the arm 210 forces the conductive flexible
 ring 220 to come into contact with two electrical contacts 250. This
 creates the switched connection between the power terminal 120 and the
 coil 20 illustrated in FIG. 4C and causes the coil 20 to be energized.
 This in turn causes torque to be exerted between the energized coil 20,
 and the magnetic poles of the rotor 30, which causes rotation of the shaft
 40 and the arm 210. When the arm moves into registration with the next
 pair of electrical contacts 250, the next pair of stator coils is
 energized. The sequence of energizing the stator coils is A, B, C, D, E,
 F, G, H, I, J, K, L and commutates the motor.
 It will be appreciated that innumerable variations of the embodiments
 described with reference to FIGS. 3-4C are possible. For example, the
 number of contacts 250/stator coils 20 and the number of magnetic poles of
 the rotor may be any convenient number depending on the implementation.
 With respect to connecting the stator coils 20 to the controlled power
 source 350, a resistor may be placed in series with the power source in
 order to limit the maximum current drawn by the stator coils. In some
 embodiments, more than one commutation assembly may be operatively
 connected to a given motor 10. Moreover, there may be several electrically
 conductive rings and arms and contacts in each commutation assembly. The
 rotor was described to include four magnetic poles. However, one of
 ordinary skill in the art will appreciate that any reasonable number of
 poles may be implemented in the rotor.
 Having described the embodiments shown in FIGS. 3-4C at a high level, the
 commutation assembly itself will be further described in more detail
 below. Referring to FIG. 3, the outer ring 230 includes a plurality of
 contacts 250 and isolators 260 disposed adjacent to each other around the
 outer ring 230. The contacts 250 include at least an electrically
 conductive surface 255 and a path for carrying electrical current between
 the electrically conductive surface 255 and a conductor connected to a
 stator coil 20 and/or a power source. The isolators 260 electrically
 isolate adjacent contacts 250 from each other.
 The isolators 260 and the electrically conductive surfaces 255 of the
 contacts 250 should be arranged so that the outer ring 230 includes a
 circular inner surface, having the axis of the shaft as its center, with
 electrically conductive surfaces 255 that are electrically isolated from
 one another. The dimensions of the conductive regions and isolation
 between the conductive regions should be chosen to optimize transferring
 current to the appropriate stator coil 20 to create rotor rotation.
 The conductive flexible ring is disposed within the outer ring 230. It must
 have a diameter that is less than the diameter of the inner surface of the
 outer ring 230. It is capable of conducting electrical current and may,
 for example, convey electrical current from one electrical contact 250 to
 one or more other electrical contacts 250. The conductive flexible ring
 may also convey current between a power source and one or more electrical
 contacts 250. For this purpose, the ring should have a sufficient
 cross-sectional area and sufficient conductivity to convey the desired
 amount of current without an excessive amount of resistive loss.
 The conductive flexible ring 220 may be made of any conductive material
 that is resilient enough not to work harden or permanently deform over
 operating time requirements and which has a natural frequency higher than
 the maximum rotational frequency of the motor shaft. The ring may be made
 of, for example a metal or alloy. It may be coated with gold to ensure
 good electrical contact with the electrically conductive surfaces 255,
 which may be similarly coated. In one embodiment of the invention, the
 conductive flexible ring is made of beryllium copper (BeCu). The
 conductive flexible ring should be keyed to the stator, the motor housing
 or another member so that it does not rotate relative to the outer ring
 230.
 The arm 210 is rigidly coupled to the shaft 40. In general, it is important
 to situate the arm on the shaft so that it has a desired orientation
 relative to the rotor 30. The arm may include, for example a bore 212 for
 receiving the shaft 40. The bore may be any desired shape and may be
 affixed to the shaft in any convenient manner including, for example, by
 adhering it, welding it or fastening it with a bolt, nail, clip, key or
 other technique. The arm may also be integrally formed with the shaft. Any
 convenient attachment technique is contemplated for attaching the arm 210
 to the shaft 40.
 The arm includes two distal ends 214 relative to the shaft 40. The arm
 further include a roller 240 situated at each distal end. The roller 240
 may be a pin mounted bearing or bushing. The surface of the roller which
 contacts the conductive flexible ring 220 should be electrically
 non-conductive to electrically isolate the rotating shaft.
 The dimensions of the arm, and in particular the distance of the distal
 surface of the roller from the central axis of the shaft, is important. In
 general, this distance should be set approximately equal to the radius of
 the inner surface of the outer ring 230 minus the thickness of the
 conductive flexible ring 220. When there are two distal ends 214 of the
 arm 210, as illustrated in FIG. 3, this permits the rollers of the arm to
 force the conductive flexible ring 220 into mechanical contact with the
 inner surface of the outer ring 230 in the region between the roller and
 the outer ring 230. When the inner surface is an electrically conductive
 surface 255, the conductive flexible ring makes mechanical and electrical
 contact with it. Moreover, because the diameter of the conductive flexible
 ring 220 is less than the diameter of the outer ring 230, the conductive
 flexible ring only makes mechanical and electrical contact with the inner
 surface of the outer ring where it is forced between a roller 240 and the
 inner surface of the outer ring 230. This arrangement permits application
 of electrical current to select ones of the contacts 250, and therefore to
 select stator coils 20, based on the position of the arm, and therefore
 the rotor, relative to the stator.
 FIG. 3b illustrates a cross-sectional view of an embodiment of the
 commutation assembly through the arm and along the axis of the shaft. FIG.
 3b illustrates recess 270 in the distal ends 214 for the rollers. Also
 shown are an attachment region 280 of the arm 210. The attachment region
 280 surrounds the shaft 40 and may include one or more threaded bores 290
 for receiving a bolt to fasten the arm 210 to the shaft 40. Also
 illustrated in FIG. 3b is the interface between the rollers 240, the
 conductive flexible ring 220 and the electrically conductive surface 255.
 The contact 250 further includes a connection between the electrically
 conductive surface 255 and the conductor 295 which are connected to a
 distinct motor coil.
 Ring Confinement Embodiments
 It should be further noted that many different embodiments of the
 relationship between the conductive flexible ring 220, the arm 210 and the
 inner surface of the outer ring 230 are contemplated. The conductive
 flexible ring 220 must be positionally constrained to prevent it from
 rotating about the axis of the shaft and prevent it from moving along the
 axis of the shaft. This may be done in many different ways including
 constraining the ring from the motor case, non-rotating portions of the
 mechanical commutation assembly and/or the rollers.
 For example, the inner surface of the conductive flexible ring 220 may have
 inwardly protruding edges to define a track for the rollers 240. This
 constrains movement of the conductive flexible ring along the shaft. In
 this scenario, the edges of the track would be electrically
 non-conductive. Alternatively, the conductive flexible ring 220 may
 include a circumferential, central slot in which the rollers ride to keep
 the flexible ring from moving along the axis of the shaft. The conductive
 flexible ring and the roller may include mating teeth. Moreover, although
 the arm has been illustrated with two distal ends bringing the conductive
 flexible ring 220 into contact with the inner surface of the outer ring
 230, embodiments, may be desired where only one or three or more distal
 ends contact the inner surface. When only one distal end forces contact
 between the conductive flexible ring and the inner surface, the arm may
 include at least one other roller at an opposing end for outwardly biasing
 the conductive flexible ring, even through the additional rollers do not
 force contact between the conductive flexible ring 220 and the inner
 surface of the outer ring.
 Methods of attachment of the Mechanical Commutation Assembly to the Motor
 The commutation assembly, or multiple commutation assemblies, may be
 operatively connected to a motor 10 in any convenient manner. In general,
 the outer ring 230 is fixed relative to the stator coils 20. This may be
 accomplished by attaching it to the motor housing internal or external to
 the motor housing. Attachment may also be accomplished by securing the
 outer ring to a platform to which the motor 10 is also attached. During
 attachment, the outer ring should be situated relative to the motor 10
 such that the center of the inner circular surface of the outer ring has
 as its center the central axis of the shaft.
 The arm 210 is attached to the shaft and should be either fixed at a
 desired angular offset relative to the rotor or should be fixed and the
 desired angular offset measured. In the latter scenario, the outer ring
 should be positioned relative to the stator prior to affixing it to the
 motor so that the angular position of the electrically conductive surfaces
 255 relative to the stator coils accounts for the angular offset between
 the arm and the rotor. The goal is to align the electrically conductive
 surfaces 255 relative to the stator coils 20 and the arm 210 relative to
 the rotor so that as the arm and rotor rotate at the same angular
 velocity, the arm sequentially forces electrical contact with each of the
 electrically conductive surfaces 255 to sequentially energize the stator
 coils 20 and commutate the motor 10. To accomplish commutation, the
 electrical contacts 250 of the outer ring should be connected to the
 appropriate stator coils 20 over conductors 295 so that the stator coils
 20 are energized in the proper sequence to ensure commutation. Electrical
 connection over, for example, a conductor 297 may also be accomplished to
 deliver power to the conductive flexible ring 255 or to deliver current
 from the conductive electrical ring to control logic or switches.
 When the commutation assembly is attached to the motor 10 as described
 above, commutation and operation of the motor 10 is as follows. In the off
 state, the arm of the commutation assembly (and the rotor) is stopped
 relative to the outer ring (and the stator). In the stopped position, the
 roller at the distal end of the arm 210 forces mechanical and electrical
 contact between the conductive flexible ring 220 and at least one of the
 electrically conductive surfaces 255. When power is applied to the motor
 10, and it may be applied in controlled amounts, the stator coils 20 are
 energized based on which electrically conductive surface 255 is in contact
 with the conductive flexible ring 220. Energizing the stator coils 20
 creates EMF between the energized coils and the magnetic poles of the
 rotor 30. This causes the rotor 30 and therefore the arm 210 to rotate.
 As the arm 210 rotates about the axis of the shaft 40 and in unison with
 the rotor 30, the conductive flexible ring 220 sequentially moves out of
 contact with one electrically conductive surface 255 and into contact with
 the next electrically conductive surface 255. This transition causes a new
 pair of stator coils 20 to energize which exerts additional torque on the
 rotor 30 and causes further rotation of the rotor 30 in the desired
 direction. This is repeated as the arm 210 and the rotor 30 trace a 360
 degree rotation about the axis of the shaft to accomplish commutation. The
 acceleration, velocity and position of the shaft may be controlled by
 adjusting the level of power applied to the rotor. At higher power levels,
 the acceleration of the shaft is correspondingly higher during motor
 startup and the peak velocity is also correspondingly higher. Reversing
 the power connection will reverse the direction of the motor.
 Multiple Commutation Assembly Embodiments
 FIG. 5 depicts an embodiment with two commutation assemblies 400 and 402
 positioned on the same shaft. The first commutation assembly 400 has its
 conductive flexible ring 220 coupled to one terminal of a power source,
 terminal 122, and the second commutation assembly 410 has its conductive
 flexible ring 220 connected to the other terminal of the power source,
 terminal 120. The motor 10 includes 3 stator coils 20. Each commutation
 assembly is configured as shown in FIG. 4 to include an outer ring 230
 having 12 contacts 250 and an arm 210 with two distal ends for forcing
 contact between the conductive flexible ring 220 of each commutation
 assembly and the corresponding inner surface of each outer ring 230.
 Referring to FIG. 5, each contact 250 is shown connected to either stator
 coil A, B, or C of the motor 10 which is also depicted. The arms 210 of
 each commutation assembly 400 and 402 are aligned and rotate around the
 axis of the same shaft 40. As the shaft 40 rotates the arm through a full
 rotation, it will be noted that pairs of coils are energized in the
 following sequence: BA, CA, CB, AB, AC, BC. This is the same commutation
 sequence as the elaborate electronic commutation arrangement of FIG. 2
 except it is accomplished mechanically and without switches in the coil
 energizing path. Thus, high coil excitation currents are possible and the
 complexity of the motor electronics are reduced.
 While particular embodiments of the present invention have been described,
 it will be appreciated by one of ordinary skill in the art that changes
 may be made to those embodiments without departing from the spirit and
 scope of the invention.