[PDF] Force Characteristics of Solenoid Electromagnet with Ferromagnetic

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MK0500041

Forc e Characteristic s o f Solenoi d

Electromagne

t wit h Ferromagneti c Dis c i n th e Coi l

Vultcha

n GUEORGIEV , Iva n YATCHEV , an d Alexande r ALEXANDRO V

Technica

l Universit y o f Sofia , Dept . o f Electrica l Apparatus , 115
6 Sofia , Bulgari aTel. +359 2 965 3873, Fax: +359 2 962 4196,

E-mail

: vulchy@vmei.acad.bg , yatchev@vmei.acad.b g

Abstract

. Th e pape r present s th e constructio n an d characteristic s o f a solenoi d electromagne t wit h ferromagneti c dis c place d i n th e coil . Th e presenc e i f th e dis c lead s t o chang e o f th e forc e characteristi ccompared with conventional solenoid electromagnets - increasing the forc e a t larg e ai r gap s an d decreasin g th e forc e a t smal l ai r gaps . Thi s coul d b e ver y usefu l fo r som e actuators . I t ha s bee n studie d ho w th eforce characteristic depend on disc size, position and material. Finite elemen t metho d ha s bee n use d fo r field an d forc e calculation s o f th e electromagnet . l . INTRODUCTIO NSolenoid electromagnets are used in a wide variety of industrial applications, mainl y a s actuators . Th e co-ordinatio n betwee n th e electromagneti c an d th e loa d forc e an d obtainin g desire d forc e characteristi c i s o f majo r importanc e i n th e desig n o f thes eactuators. Usually the change of the characteristics is obtained either by varying geometry , e.g . th e shap e o f th e pol e pieces , o r b y externa l control . Th e pape r present s a solenoi d electromagne t tha t give s th e opportunit y o fchanging the force characteristic by placing a ferromagnetic disc inside the coil. 2 . SOLENOI D ELECTROMAGNE T WIT H FERROMAGNETI C DIS C Th e electromagne t unde r consideratio n feature s a ferromagneti c dis c place d i n th e coi l i n suc h a wa y tha t th e coi l i s divide d i n tw o part s - Fig . 1 . I t i s subjec t o fBulgarian patent [1,2]. The location of the disc is somewhere in the zone of the air ga p o f th e electromagnet . 9 4

Ferromagnetic

dis c Figur e 1 : Principl e constructio n o f th e electromagne t Th e ferromagneti c dis c i s cu t i n radia l directio n i n orde r t o avoi d it s behavio r a s a secondar y windin g a t dynami c conditions . Onl y th e magneti c field redistributio n du e t o th e presenc e o f th e dis c i s thu s obtaine d an d it s influenc e o n th e forc e ha s bee n studied . 3 . FORC E CHARACTERISTIC S A T DIFFEREN T CONDITION S

Insertin

g a ferromagneti c dis c i n th e coi l lead s t o a chang e o f th e forc e characteristi c (electromagneti c forc e versu s ai r gap) . Thi s ca n b e see n fro m Fig . 2 wher e forc e characteristic s withou t an d wit h ferromagnetic : dis c ar e show n fo r a n electromagne t o f th e sam e magneti c circuit . -Withou t Dis c-With Disc 4 6 8 1 0 Ai r Ga p ? , m m1214 : Figur e 2 : Stati c forc e characteristic s o f solenoi d electromagne t withou t an d wit h ferromagneti c dis c 9 5 All the results in Fig. 2 are experimentally obtained. Whe n th e dis c i s insid e th e ai r gap , i.e . a t larg e gaps , th e dis c strengthen s th e workin g magneti c flu x throug h th e armature . Thes e result s i n increasin g th e electromagneti c forc e compare d t o th e cas e withou t disc . Th e flu x densit y i n th e dis c i s relativel y low , th e dis c i s no t saturate d an d it s influenc e o n th e forc e i s significant . Whe n th e dis c i s outsid e th e ai r ga p zone , i.e . a t smalle r gaps , th e dis c weaken s th e workin g magneti c flux , whic h lead s t o lowe r electromagneti c forc e i n th e cas e withou t disc . Th e ferromagneti c dis c i s saturate d an d it s influenc e o n th e forc e i s relativel y low . B y varyin g th e positio n o f th e dis c differen t forc e characteristic s ca n b e obtained . I n Fig . 3 compute d characteristic s fo r thre e differen t dis c position s ar e shown . Th e dis c thicknes s i n thes e thre e case s i s A d = 0. 5 mm . Th e result s sho w tha t th e characteristic s ar e raise d i n th e zon e o f th e disc . Th e forc e increase s i n th e are a right afte r th e disc . Othe r factor s tha t coul d influenc e ar e th e dis c materia l an d dimensions , i n particula r it s thickness . I t i s als o possibl e t o inser t mor e tha n on e disc . A n exampl e o f characteristic s wit h dis c o f differen t materia l i s show n i n Fig . 4 . Th e first curv e (dis c 1 ) i s obtaine d wit h simpl e silico n iro n an d secon d curv e i s fo r qualit y iro n wit h highe r inductio n o f saturation . Therefore , i n secon d cas e th e dis c i s les s saturate d an d ha s stronge r influenc e o n th e forc e characteristic . -Dd=8.5m m -HB-Dd=4m m Dd=10m m 1 2 1 4 Ai r Gtp , m m Figur e 3 : Stati c Forc e Characteristic s fo r Thre e Differen t Dis c Position s 9 6 - Deltad=0.5mm -»-Deltad=1mm Deltad=2mm 16 0 14 0 12 0 10 0 8 0 6 0n \»''/; *' •* • - ,' -V '.- r * r < * *4" • •"^• - 6 8 Ai r Gap , m m10 1214 Figur e 4 : Stati c Forc e Characteristic s wit h Differen t Dis c Material s Forc e characteristic s fo r th e sam e dis c positio n an d differen t dis c thicknes s ar e give n i n Fig . 5 . 16 0 14 0 12 0 10 0 8 0 6 0 - l \ -vfc-Disc 1 • _Disc 2 !..J ••** * . - 1'-"" "j 4 6 8 1 0 Ai r Cap,m m12 Figur e 5 : Stati c Forc e Characteristic s fo r differen t Dis c Dimension s 9 7 Force characteristics for the same disc position and different disc thickness - Aj are give n i n Fig . 5 Th e thinnes t dis c (A d = 0.5mm ) i s saturate d earlie r an d thi s determin e it s relativel y wea k influenc e o n th e characteristi c variation . A t th e opposite , th e larges t dis c (A d = 2mm ) stay s unsaturate d an d work s lik e magneti c shunt . I t lead s awa y par t o f th e magneti c flux , thu s decreasin g th e force . 4 . FIEL D ANALYSI S B Y TH E FINIT E ELEMEN T METHO D Fo r theoretica l stud y th e finit e elemen t metho d ha s bee n employed .

Axisymmetrica

l formulatio n fo r th e modifie d magneti c vecto r potential rA i s use d fo r th e solutio n o f th e magneti c fiel d proble m governe d b y pseudo-Poissonia n equation . Thi s i s a specia l (an d easie r t o model ) cas e o f th e mode l presente d i n [3] .

Homogeneou

s Dirichle t boundar y condition s ar e impose d o n th e boundar y o f a buffe r zon e aroun d th e electromagnet . Th e proble m ha s bee n solve d a s a non-linea r one , takin g int o accoun t th e materia l characteristic s o f th e cor e an d th e disc . Th e non - linearit y i s treate d b y th e Newton-Raphso n metho d wit h relaxatio n factor . Th e electromagneti c forc e i s compute d b y th e Maxwel l stres s tenso r approach. Fo r maintainin g simila r accurac y a t differen t position s o f th e armature , th e mes h i s generate d i n a wa y ensurin g th e sam e mes h fo r th e armatur e an d fo r th e ai r surroundin g i t wher e th e Maxwel l integratio n surface s ar e located . Cod e develope d b y th e author s i s use d fo r th e finit e elemen t analysis . I t i s writte n i n FORTRA N an d offer s som e pre - an d post-processo V features . Th e stati c forc e characteristi c o f th e electromagne t wit h on e dis c i s compute d an d compare d wit h th e experimenta l one , bot h show n i n Fig . 6 . -Experimenta l Dat a-Computed Results 20 0 15 0 10 0 g . 5 0

0•I: -r • -:

1 - - • -"^l.- , .f i 6 8 Ai r Ga p , m m101214 Figur e 6 : Stati c forc e characteristic s o f solenoi d electromagne t wit h ferromagneti c dis c 9 8 The computed and measured characteristics are in a relatively good agreement. Th e difference s ar e mainl y du e t o th e differenc e betwee n th e rea l materia l propertie s fro m thos e use d fo r th e modeling . 5 . CONCLUSIO N

Insertio

n o f a ferromagneti c dis c i n th e coi l o f solenoi d actuato r coul d chang e significantl y it s forc e characteristic . Th e chang e o f th e force i s i n a desire d wa y - increas e a t larg e gap s an d decreas e a t smal l gaps . B y varyin g th e positio n o f th e disc , it s dimension s an d materia l differen t forc e characteristic s coul d b e obtained . Th e finit e elemen t metho d ha s bee n use d fo r theoretica l analysis . Possibl e furthe r studie s coul d includ e mor e detaile d analysi s o f th e influenc e o f othe r factor s o n th e forc e characteristic , a s wel l a s synthesi s o f electromagne t o f desire d characteristic .

References

[I] A. Alexandrov, A. Pisarev, and S. Pavlov: "DC solenoid electromagnet" Bulgarian patent Mo. 18140,
1972.
[2] A. Pisarev, A. Alexandrov and S. Pavlov: "Solenoid electromagnet with possible change of the characteristic " in Proc.of Int. Symp. SIELA '74,vol 1, pp.234-238, 1976.
[3] I. Yatchev, R. Miltchev, K. Hinov, A. Alexandrov. "Coupled problem for a solenoid actuator", in Proc.of Xl-th Int. Symposium SIELA'99, vol. II, pp. 216-223,
1999.
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