CA1281066C - Ironless, brushless d.c. motor with wave winding - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A simplified fan and brushless DC motor has rotor with an annular permanent magnet that is magnetized in segments about its circumference and is associated with à
wave-wound stator coil. Alternate circumferential segments of the magnet are oppositely polarized. Fan blades are located within the annular magnet. The wave-winding varies in position to present alternate magnetic fields opposite in direction to the rotor magnet outer surface to cause rotation thereof. The stator is ironless. The coil can have two bifilar windings alternately energized. A Hall device is, typically, employed for commutation along with suitable switching circuitry.

Description

~8~0~6 ACRGROtlND OF T~ INVENT_ON

This inven~ion relates to brushless DC motors that are electronically commutated. More particularly, it relates to a wave-wound, ironless, brushless DC fan motor that is of simple construction, inexpensive to manufacture, and reliable.
A typical goal for manufacturing a fan motor is to make one that is very simple and, consequently, ha~ a low pro- -duction cost. Side armature AC motors come close to a~hieving - these ~oals. Recently, however, DC ~an motors have become increasingly attractive, particularly for fans used to cool electronic circuits where DC power is available~
Brushless C motors using Hall effect devices to sense the commutation p~ints as the rotor rotates are well known in the art. One or more stator coils are repeatedly energized or have their ener~ization reversed ~o e~fect a relocation of the ma~netic field produced by the stator coil or coils~ A permanent magnet rotor is contihually at:tracted ~z~ _' 6~

by the new magnetic field. For commutation, one or more Hall effect devices sense the location of the poles of the permanent magnet ro~or to rontrol the energization of the stator coil or coils, or a Ball effect device detects the S position of one or more co~mutation magnets mounted to rotate with the rotor and provided especially to indicate, by chang-ing the state of the Hall effect device, the commutation points as the rotor turns.
Many brushlecs DC motors have been complex in both ~heir s~ructure and ~heir commutation circuitry, with a con-comitant production cost. In situations where simple, low-cost, reliable fan motors have been needed, these brushless DC motors--which might, more appro~riately, have been used for precision disc or tape drives, for instance--have been too expensive for the simple purpose of f~n rotation.

Commonly owned, u.S. patent number 4,563,622 of C. Deavers and J. Reffelt, dis-closes a simple brushless ~C fan motor. ~he mvtor has an annular permanent-maqnet rotor that is radially magnetized ~polarized) in circumferential segments. Alternate circum-ferential segments have opposite polarities. The motor also has a ~tator with an electro~agnetic structure including a coil, wound on a bobbin, a core, and a pair of arms terminat-ing in pole pieces. The electromagnetic structure is locatedin a compartment at one location at the bottom of a generally circular housing. A coil is energized to produce a masneto-motive force that exerts a ~orque for turning the rotor. A

commutation circuit, which includes a positio~ detector, ~81C~

preerably a ~all effect device, selectively energizes the coil. The position detector detects the position of the rotor with respect to the stator and supplies a siqnal for controlling the commutation circuit.
Commonly owned V.S. Patent 4,553,075 of Fred Brown and Alan Grouse discloses a further brushless DC motor with an annular permanent magnet rotor magnetized in oppositely polarized segments to present alter-nate ~pposite poles to an external electromagnet structure loca~ed at one location along the circum~erence of the rotor.
In this motor two coils are wound in bifila.r fashion and are alternately oppositely en@rgized to rever~e the field pre-sented by pole pieces in magnetic conducting relation to the coils.
lS The DC fan motors disclosed in U . S . Patents Nos . 4, 563, 622 and 4, 553, 575 are simply constructed, inexpensively manufactured, and reliable. The electromagnetic structure and coil are arranged so that there is a resultant radial magnetic force on the rotor bv virtue of ~he electro-mag~etic structure being located a~ one location along the periphery of the permanent magnet~ For example, in the arrangement illustrated i~ Figures 1 and 2 of U.S~ Patent No. 4,563,622, when the stator coil is ener-gized, the magnetic field created produces a radial forces directed towards the stator magnetic pole pieces at one side of the rotor. When the stator coil is deenergized, these Ladial forc~i are removed. ~he resultan~ of these radial forces is a radial force in one radial direction each ti.~e the stator coil i~ energized. For very quiet operation, ~ree of vibration, and having less demanding bearing requirements, it would be desirable to provide the counterbalancing or more nearly counterbalancin~ radial forces that multiple coils located about the entire periphery of a rotor can pro-vide while still providing many of substantial benefits ofthe simple, inexpensive and reliable motor of the aforesaid application. In that case, in the desi~n and manufacture of the motor, attention to vibration due to repeated radial forces on the rotor could be substantially reduced.

SUMMAR~_OF THE INVENTION

- In accordance with this invention, a simple DC
brushless motor has a rotor with an annular permanent magnet and a wave-wound stator. The annular magnet is a relatively large ring ma~netized in segments or sectors about its cir- -cumference. Each succeeding segment is oppositely polarized.
~he wave winding can encircle the rin~, presenting alternat-ing magnetic fields to the outer surface of the magnet acro~s an air ~ap.
One or more wave-windings are distributed around the rotational path of the annular magnet in a wave li~e pattern that varies the location of the winding axially as one proceeds around the rotor magnet. That is to say, the wave of the winding proceeds from a location closer the front o~ the motor at one point on the stator to a location closer to the back ~f ~he motor at another ooint ~nd ç~ntinueC ~or-ward and back until the rotor has been encircled. When ener-gized, the winding's plurality of field locations acting on o~;

the annular magnet at a plurality of locations around the magnet produce offset~ing maqnetic forces to diminish or eliminate the resultant radial ~orce on the rotor. Vibration from the repetitive radial fo~ce resultant can thus be totally S or substantially eliminated. ~his manne!r of winding takes up very little ~paoe in the radial direc~ion so that this motor can be quite compact. Bearing demands are not as great and the likelihood of noise is greatly diminished.
In the fan of this invention, as with those o the ab~ve-mentio~ed applications, fan blades are located within and affixed to ~he annular magne~. The ma~net and the fan blades are mounte~ for rotation on a central hub. The stator includes a housing and support structure extending from pro~-imate the magnet to the hub; this structure supports the rotor, including the hub, the fan blades and the magnet, for rotation. ~ ~all device can be ~sed to control commutation~
A small compartment in the housing may be provided to hold commutation circuitry or that may be remotely located.
The wave-winding is wound on cylindrical stator structure surrounding the permanent magnet. It is su~ported closely proximate and just radially outward of the annular rotor magnet. Two coils can be wave wound in bifilar fash-ion and oppositely energized to present alt rnating fiel~s attracting succeeding oppositely magnetized segments of the annular permanent magnet of the rotor during operation.
The stator of the subiect motor is ironless, which is to say that there is no high permeability or ma~netic materials directi~g flux to stator poles. The motor can thus be liqht wei~ht, and because there are no salient poles of magnetic material, there is no attraction of the rotor magnet to a cog position as ~he rotor comes to rest.
The above and further eatures of the invention will be better understood with reference ~o the several fig-ures of ~he attached drawings and the following det~iled description of the preferred embodiments.

_RIEF ~ESCRIPTION OF T~E DRAWINGS

In the drawings:
Figure 1 is a top plan view of a fan and ~otor according to this invention with a rotor having alternately ~ radially magne~ized segments and a stator having wave-wound stator coils, Fiyure 2 is a side elevational view of the 4an and motor shown in Figure 1 and better il-lustrates the wave wound coils; and Figure 3 is a diagramatic illustration of a detector and commutation circuit suitable to operate the rotor o~
Figure 1 and ~.

DETAILED DESCRIPTIO~ OF TBE PREF~RRED E~BODIM~NTS
In Figures 1 and 2, a fan and motor combination 10 accarding to the invention includes a rotor 11 and a stator 12. ~he rotor 11 has an annular permanent magnet 13 magne-tized in eight segments 1~ about its circum~erence; each alternate seqmen~ 14 is oppositelv polari~ad in th~ radia1.
direction, as shown by the north (N) and south (S) magnetic pole designations. The annular magnet 13 is secured on a ~.~a~066 ring 15. Fan blades 16 extend ~ro~ the ring 15 to a central No. 4,563,622 provides de~ails for one possible mountin~ and ~uppor~ ~rrangement for the rotor 11 (including the hub 17); the mounting and support arrangement employed i5 not an essential feature of this invention.
The stator 12 includes a base 18, which, as shown, has a substantially rectangular shape, bu~ may have other shapes, such as circular. The base 18 ~upports a substan-tially cylindrical member ~r wall 19 having lugs 20 and 20'~
farther from and closer to the base 18, respectively, and projecting radially outward. A coil 21 is wound onto the lugs 20 and 20'. As illustrated, the winding wire is aiter-nately wound over one lug and under the next, with this pat-1~ tern being repeated to form the wave shape of the coil. The crests and valley of the wave vary ~xially in p~sition, with respect to the axis of the rotor's rotation. Figures 1 and 2 show the coil waveshape as a triangle wave. However, other wave shapes may be used, e.g., a sinusoidal or square wave.
In adaition to the lugs shown, the wave-shape of the coil 21 may be formed by winding the turns of wire in a channel, groove or other support structure in or on the wall 19.
Other manners of producing the coil may occur to those skilled in the artt such as the use of printed circuit, techniques, for e~ample. The coil 21 comprises two bifilar-wound wind-ings Ll and 12 (Figure 3), each separately energi~eable in opposite directions by a commutation circuit 22. When current flows through one of the windings ~1 or L2, alternate north and south magnetic poles, eight poles altogether, are produced -7~

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around the circumference of the rotor. Current through t~e other of the coils in the opposite direction produces fields of the opposite polarity. As will be d iscussed below, in relation to the cir~uit of ~igure 3, the current is controlled by a ~all switch ~1 of the commutation circuit 22 to produce poles correctly timed t~ exert torque on the annular magnet 13 and, con equently, turn the rotor 11. In Figu~es 1 and 2, the ~all switch ~1 is ~hown ~ounted upon the exterior of the cylindrical wall 19.
The stator 12 further includes struts 2~ radiating outwardly from a ~entral stator projection 23 to the ~ase.
18. Mounting bQ6ses 25 define holes 26, which enable the motor and fan combination 10 to be moun~ed by, Çor instance, bolts or screws (not shown) passed through the ho~es 26.
1~ An exemplary embodiment o a circuit 22, suitable to ~ontrol the motor, i~ shown in U.S. Patent - No. 4,563,622 and is reproduced in Figure 3. Other suitable circuits are known and disclosed in the patent literature.
At start-up~ the coil 21 is energized to apply starting torque to the an~ular magnet, The ~all effect device ~1 controls e~nergization of the coil 21 such that the coil windings ~1 and L2 are alternately energized to attract approaching poles of the annular rotor magnet. When the ~5 Hall effect device ~1 detects changing of the magnetic field produced by the rotor 11 due to the arrival of a new, oppo-sitely magneti~ally polarized segment proximate the device, the magnetic field at the annular permanent magnet is reversed by means of the circuit of Figure 3 to attract the poles of the annular rotor magnet next approaching. ~his sequence repeats itself until the rotor comes tc~ an equilibriuJn speed where the aerodynamic load balances the motor power.
. In Figure 2, the poles produced by the wave-winding are produced in the areas 30, 31, 32 and 33 below and above the windings alternately and continuing around the rotor.
Consideration OL the path and direction of ~he lines of flux produced around a conductor will indicate that each succeea-ing area's field is opposite in direction from the preceding area and reversal of the current flow will reverse the field in each area. Moreover, it will be seen that each area in which a field is produced exerting a radial attraction on the magnet 13 has an area exactly 180 away exerting a like radial force in the opposite direction such that these counter-balance to cancel repeated rad;al forces tending to produce vibration and noise. The stator windings may be wound to provide fewer or more pole areas and the rotor magnet may likewise have fewer or more oppositely polarized segments. In each case, the best location for the Hall device ~ easily empirically determined such that switch-ing occurs just as new segments move into position to be attracted to a new stator field area.
In Figure 3, the Hall device l~l, which can be a Hall ~witch, for example, is seen controlling transistors Ql and Q2, each of whose collector-emitter circuits is in series with a different one of coil windings Ll and L2. One such Hall switch WhiCh may ~e use~ is the ~all effect digital switch UG~-3013T of Sprague Electric Co., Worcester, Massachusetts. Exposure of the ~all effect device Hl to _g_ ~ 66 a magnetic field of one direction or polarity open~ (breaks) a conduction path from a line 51 (pin 3) to ground (pin 2), while exposure of the device Bl to an opposite polarity closes (makes) a conduction path from line ~1 to ground.
When the path is open, there is very li~tle volt-age drop across resistors R2, R3 and R4, and the Ql base-emitter voltage is sufficient ~o ~urn on Ql, which is an NP~T
Darlington pair, commonly housed and sold as a single compo-nent. The Ql Darlington pair has a high gain so that the small base current through ~2, R3 and R4 is adeguate to turn on ~i and energize the winding L~. When the path through Hl is open, the Q2 base-emitter voltage is insufficient to tur~
on Q2, a PNP power transistor that requires a greater base drive to conduct. So with Q2 of, no current flows thr~ugh winding Ll. When the ~all effect device Hl experiences a magnetic fie~d of opposite direction or polarity~ connecting line 51 to ground, a voltage drop nearly e~ual to the full input vcltage, for example about 12 volts, appears across resistors R2 and R3, creating a Q2 base-emitter voltase suf-ficient to turn on Q2. Conduction by Q2 causes current to flow through winding Ll. However, the Ql base-emitter vol~-age is insufficient to turn Ql on, so that no current flows through winding L2. Wi~dings Ll and L2 are thus alternatively energized and deenergized in response to detection of differ-ent magnetic fields by Ball 2evice Hl. Instead of providing two separate coil windings Ll and L2, only one winding may be provided along with circuit means to al~ernately reverse the current in the winding.

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The remainder of the Figure 3 circuit comprises capacitor Cl~ which damps transients that arise fr~m the abrupt switching of the coil 21, a current limiting resistor Rl, appropriate protective fusing such as fuse ~Pl, and ~iode CRl, which ~reverlts current reversals back through the input leads to the supply. The circuit elements may be mounted on a printed c;rcuit board of essentially the same shape as a Gavity that forms ~ compartment (not hown) in the base 1 or the circuit 22 may be remotely located.
From the foregoing i~ will be seen that a simplified brushIess DC motor is provide~ in which the field producing provlsions of the stator, such as the windings 21, add very little to the radial dimension. In a fan9 a large proporW
tion of the motor's r~dial dimension can be dedioated to the air moving structure. ~he ~tator's radi~l dimension, e.g., the radial thickness of the cylifldrical wall 19, and coil 2l, and the lugs 20 and 20 is no greater than the thickness in the radial direction of the ring 15 and magnet 13. So in this case, where an ob jective is to provide a large central opening surrounded by relatively narrow structure, this strator arrangement is particularly satisfactory even though the coil surrounds the narrow annular magnet. It wil~ be readily apparent to those skilled in the art that many varia-tions in the foregoing exemplary, preferred embodiments can be made without departing from the spirit and scope of the invention, as defined in the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A DC motor comprising a rotor with an annular per-manent magnet defining a large central opening and magnetic segments radially polarized, wherein segments of like polariza-tion are spaced apart circumferentially about the annular magnet; an ironless stator with coil means wherein the stator includes a housing defining a wall on the periphery of the rotational path of the annular magnet and lugs spaced apart circumferentially about said wall; and said coil means is in the form of an axially oscillating wave along said wall with the windings of the coil means supported on said lugs.