CA1093135A - Noise reduction in a permanent magnet synchronous motor - Google Patents

Abstract

NOISE REDUCTION IN A PERMANENT
MAGNET SYNCHRONOUS MOTOR

Abstract of Disclosure One set of rotor poles of a rotor of a synchronous permanent magnet motor is shorter than another set and a member of magnetizable material is disposed from and cooperates with the poles to axially bias the rotor in a predetermined direction as the rotor rotates.
Preferrably the one set of poles is longer in one axial direction only and the member of magnetizable material is disposed in line and opposite the extended length. The biasing of the rotor prevents axial oscillations of the rotor to reduce motor noise.

Description

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SUMMARY OF TtlE INVENTION
Generally speaking, the present invention relates -to permanent magnet synchronous motors and more particularly to a method of reducing noise in a permanent magnet synchronous motor and to the structure resulting therefrom which comprises providing interleaved first and second sets of rotor poles, the length of each pole in one of the ~irst or second sets of rotor poles being longer in relation to the length of the poles in the other set, the longer lengths taken in one direc~ion only and in a direction substantially parallel to the rotatlonal axis of the perrnanent rotor ~or the rotor poles and further providlng a member of magnetizab1e material spaced from the first and second rotor poles in the one direction whereby an unbalance of magnet flux ls generated to reduce axial oscillation of the rotor thus reduce noise caused by oscillation.
BACKGROUND OF THE INVENTION
.
There are a multitude of applications for small synchronous motors of the type having a permanent magnet rotor. Many of these applications ; are in timing devices such as clocks and appliances such as clotheswashers and dryers which utilize timed seqùence swltches. One problem associated with these types of synchronous motors is noise. Objectionable levels of noise are difficult to prevent, especially in appl;ances where motors have been attached in such a manner that the housing of the appliance amplifies the sound produced by the motors. Because the noise is caused by a number of sources, each source must be considered in reducing the noise. One of the more significant sources of noise is caused by axial oscillat10n of the rotor along its shaft as tt rotates resulting in a pers~stent bang~n0 of the rotor agalnst the housln~. To overcome this problem, means have been provlded for magnetlcally dampenlng the ax~al osc111at~ons of the rotor. For example, tilere 15 descrlbed ln U. S. Patent 4,056,744 "Nolse ~ampenln~J Means For A Permanent Magent 5ynchronous Motor" issued November 1, 1977, a magnetlc dampening means ~ 1 -~, 7 ~ ~

3L(~ 3~L~3 5 wherein dampening of the axial oscillations is provi~ by shortenin~ one set of stator poles with respect to another set. Wh~le this method has proven, for the most part, to be successful, it has been found that substantial dampening of the axial oscillations has been di~ficult if not impossible under certain operating conditions of the motor.
OBJECTS OR FEATURES OF THE IN~ENTION
It is, therefore, a feature of the invention to provide a means to reduce noise generated in a synchronous motor by axial oscillations of the rotor of the rnotor. It is another feature of the invention to provide such a means wherein one set of rotor poles carried by -the rotor is shorter than the other set and wherein there is a member of magnetizable material disposed from the rotor. Still another feature of the invention is the provision of such a means wherein the member of magnetizable material is disposed from the rotor but in working relation thereto and in a direction to which axial bias;ng of the rotor ;s desired. Yet another featu~e o~ the invent;on ;s the prov;sion of such a means wherein the member of magnetizable material is a ring carried by a hub of the rotor. And still another feature of the invent~on is the provision of such a means wherein the longer set of poles extends beyond the shorter set in one direction only and wherein the meniber of magne-tizable material is disposed opposite the extended length.
According to the above features, from a broad aspect, the present invention provides a synchronous motor wherein a permanent magnet rotor rotates about and i~ axi~11y dis-placeable along an axi~ Th~ irnprov~r~rlt: ln the mot:or i~
characterized by a ~irst ~et of pole8 ~pacod abo~t: arld carried by the p~rman~nt rnaynct rotor. A ~econd ~et o~
poles is carried by the permanent m~c~net rotor and inter~
meshed between the ~irst ~et of pole~. ~rhe uecond ~t of poles is of shorter length than the ~ir~t set, A mernber of magnetizable material i3 diupo~ed erom thc permanent mac3net

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rotOr but in working relation thereto, :in a direction -to which axial biasing of the rotor is desired.
According to a further broad aspect of the present in~
vention, there is provided a method of reduciny noise in a synchronous motor due to axial dispLacement of the rotor of the motor. me method cornpri~eg providing flrst and second sets of intermesh~d pole~ akout the periphery of the rotor, the second set being of shorter length than the first set, and providing a member of magnetizable material in working relation to the fir~t and second sets of poles in a direction to which biasing of the rotor is desired.
These and other features of the invention will become apparent from the following description taken in conjunction with the accompanying drawings.
DESCRIPTION OF_THE DRAWINGS
In the drawing~:
Figure 1 is a cro~s section of a motor employing the features of the invention.
Figure 2 is an exploded view of the motor.
DETAILED DESCRIPTION
Re~erring to the drawings, the component parts of the present - 2a -~B

~3~3 invention may be visualized. A permanent magnet synchronous motor 1~
includes a housing 12~ a rotor assembly 14, a stator winding 16, a first set of stator poles 18, and a second set of stator poles 20.
Housing 12 comprises a field plate 24 and a stat~r cup 26 held ~ogether ~o ~orm a substantially cyl;ndr;cal structure ~or housing 12.
The material for stator cup 26 and f;eld plate 24 may be ordinary cold-rolled steel, preferably annealed, but may be of other magnetically attractable materials. The first set of stator poles 18 is formed by lancing radial strips out of the flat portion of stator cup 26 and forming them parallel to the rotational axis of the motor. In the same manner, the second set of stator poles 20 are lanced from field pla~e 24. The assembled combinat;on of stator cup 26 and field plate 24 provides both sets of stator poles 18 and 20 interleaved in a c;rcular pattern. The stator pole arrangement is asymnetrlcal and prov;des alternate flux at individual poles ;n a typical manner well known in the art of permanent magnet synchronous motors.
Rotor assembly 14 includes a permanent magnet 32 sandwiched between ~wo spiders 34 and 36, the rotor and the spiders be-ing carried by a hub 38. Hub 38 includes a platform 40 having a plural~ty of studs 42 projecting therefrom for providing spacers between rotor poles 28. The hub is carried on a shaft 44 to be rotatable thereon. The hub includes a motor output pinion 46. As is clearly shown in Figure 2, spiders 34 and 36 have integral legs formed parallel to the center axis of the spiders so as to intermesh alternately w1th the legs o~ the opposite sp1der. The spiders can be formed o~ cold-rolt~d stee1, preff~rably dnne~led, The permanent magnet 32 is magnetized so that one face ls north and the opposite south. Thus the splder 1e!Js are ma~netlzed correspond1ngly so that the periphery v~ khe rotor contalns a pr0deterInlned number of salient poles 28' and 28" magnetlzed a1ternately north and south.

~o~ a..3s Stator w1nding 16 includes a coil Or wire 50 wound on ~ bobbin 52 Lead wires 56 carry electric current to the co11 ~/1re 50 In operation, when an alternat;ng current is applied to sta-tor wtnding 16, rotor assembly 14 will begin to rotate due ~o the maynetlc flux paths generated bekween rotor poles 28, and stator poles 1~ an~ 20 The start;n~
of rotor 32 ;s ;nitiated by an asymmetrical arrangement o~ stator poles 18 and 20. In t~le event the rotor starts to run in a wrong-way direction, a directional pawl 58 engages a stop ~not shown) carried on hub 38 and causes the rotor to rebound and start ln the rlght d1rection.
During the running o~ the motor as descr;bed hereln, rotor 32 often tends to oscillate in an axlal direction along shaft 44. I~ the motor is positioned in a manner that the osc~lla~lng rotor can bang aga~nst a ~ace of the field plates or against a pllot for shaft 44 ~not shown), an annoyiny ; noise can result. In accordance with the lnvention, a means 60 dampens ; such oscillat~ons and thus the noise generated by such oscillatlons.
According to the present invention, dampenlng means 60 includes a member of magnetizable material 62 ~n combinatlon w1th shortened rotor poles, which in the illustrative embodiment are poles 28". The member of magnetizable material 62 lncludes ring 64 carried about and having an - 20 interference fit with hub 38. Ring 64 may be made of cold-rolled steel, for example. The ring is posikioned in a direction toward which the rotor is to be biased and in the preferred embodiment shown, such direction is also determined by the d1rection toward ~hich the longer rotor poles extend beyond the shorter poles. Thus in the embodiment shown, the longer poles 28' extend beyond the shorter poles 28" 1n one d~rectlon only and ring 64 ls pos1ttoned oppos1te s~ch extended len~th. Wlth thls arrangement, the rotor wlll be blased toward plate 24.

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Claims (6)

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

1. In a synchronous motor wherein a permanent magnet rotor rotates about and is axially displaceable along an axis, an improvement charac-terized by:
a) a first set of poles spaced about and carried by said permanent magnet rotor.
b) a second set of poles carried by said permanent magnet rotor and intermeshed between said first set of poles, said second set of poles being of shorter length than said first set, and c) a member of magnetizable material disposed from said permanent magnet rotor but in working relation thereto, in a direction to which axial biasing of said rotor is desired.

2. In a synchronous motor according to claim 1 wherein said member of magnetizable material is a ring carried by a hub of said rotor.

3. In a synchronous motor according to claim 1 wherein the length of said first set of poles provides an extended length beyond said second set in one direction only and said member of magnetizable material is disposed opposite said extended length.

4. In a synchronous motor according to claim 3 wherein said first and second sets of poles are salient poles disposed about and spaced from said permanent magnet rotor.

5. A method of reducing noise in a synchronous rotor due to axial displacement of the rotor of the rotor comprising:
a) providing first and second sets of intermeshed poles about the periphery of the rotor, said second set being of shorter length than said first set and b) providing a member of magnetizable material in working relation to said first and second sets of poles in a direction to which biasing of the rotor is desired.

6. A method according to claim 5 wherein said first set is longer than said second set in one direction only and said member of magnetizable material is disposed in said direction.