Life cy­cle of mag­nets

DEMM Engineering & Manufacturing - - CONTENTS -

Per­ma­nent mag­net ma­te­ri­als are now be­ing used in elec­tri­cal ma­chine ap­pli­ca­tions, and a lot of this equip­ment will be ex­pected to have long life­cy­cles.

If per­for­mance and life­cy­cle tar­gets are to be met, de­tailed char­ac­ter­i­sa­tion at the de­sign stage us­ing ad­vanced elec­tro­mag­netic and mul­ti­physics sim­u­la­tion tech­nol­ogy will be crit­i­cal, ac­cord­ing to Cob­ham Tech­ni­cal Ser­vices in the UK.

High field strength, rare earth per­ma­nent mag­nets, are key com­po­nents in many of the new elec­tri­cal ma­chine de­signs now ap­pear­ing on the mar­ket. Typ­i­cal long-life ap­pli­ca­tions for such tech­nol­ogy in­clude wave and wind power gen­er­a­tors, and the trac­tion mo­tors for hy­brid and elec­tric ve­hi­cles.

Sim­u­la­tion soft­ware al­ready plays a vi­tal role in the de­vel­op­ment of such ma­chines by short­en­ing de­sign-to-man­u­fac­ture times and re­duc­ing the num­ber of test pro­to­types that need to be pro­duced.

How­ever, many of the mod­el­ling ap­proaches and de­sign soft­ware tools cur­rently used are un­suit­able for cap­tur­ing and characterising grad­ual degra­da­tion of mag­netic per­for­mance over time – es­pe­cially if this is due to a com­bi­na­tion of fac­tors.

Cob­ham has built up con­sid­er­able ex­per­tise in help­ing de­sign­ers op­ti­mise the per­for­mance of elec­tri­cal ma­chines, and its Opera soft­ware is used by com­pa­nies world­wide to max­imise per­for­mance of equip­ment, in­clud­ing per­ma­nent mag­net (PM) gen­er­a­tors.

The soft­ware is also be­ing used by a ma­jor re­search project in the UK, which is in­ves­ti­gat­ing the tech­nol­ogy that will be needed for low carbon ve­hi­cles.

A key part of this re­search will in­volve ex­am­in­ing the fac­tors that cause the per­for­mance of PM trac­tion mo­tors to de­te­ri­o­rate with age, with a view to de­vel­op­ing bet­ter ma­te­ri­als, mo­tors or con­trol tech­niques to over­come the prob­lem.

Al­though par­tial de­mag­ne­ti­za­tion of a mo­tor’s per­ma­nent mag­nets is be­lieved to be re­spon­si­ble, at present it is not clear whether this is due to nat­u­ral degra­da­tion, over­heat­ing caused by ex­ces­sive power de­mands, tem­po­rary fault con­di­tions that are rec­ti­fied dur­ing the life of the ve­hi­cle, or in­her­ent short­com­ings in the de­sign of the mo­tor it­self.

Ac­cord­ing to Chris Ri­ley of Cob­ham: “The need to be able to ac­cu­rately char­ac­terise the de­mag­ne­ti­za­tion of high field strength per­ma­nent mag­nets at any stage in their life cy­cle, with a view to im­prov­ing long term per­for­mance, has never been more acute.

“PM trac­tion mo­tors are a prime ex­am­ple; their en­vi­ron­men­tal op­er­at­ing con­di­tions and duty cy­cles are far more se­vere than those of most in­dus­trial or con­sumer ap­pli­ca­tions, yet users ex­pect them to last for the life­time of the ve­hi­cle – up to 14 years – with lit­tle or no change in per­for­mance.

“And al­though wind power gen­er­a­tors have a more con­sis­tent duty re­quire­ment, off-shore plat­forms will need to en­dure very hos­tile cli­matic con­di­tions. They will also be re­quired to have long de­sign lives – of 20 to 25 years – which again raises con­cerns about mag­net longevity.”

At present, most PM-based trac­tion mo­tors for hy­brid and all-elec­tric ve­hi­cles use neodymium-iron-boron mag­nets, but their mag­netic field strength re­duces with in­creas­ing tem­per­a­ture, and above about 220-de­grees C the de­mag­ne­ti­za­tion ef­fects can be ir­re­versible. The amount of de­mag­ne­ti­za­tion de­pends on a num­ber of fac­tors, in­clud­ing the phys­i­cal shape of the mag­net and its mag­netic cir­cuit, as well as the grade of ma­te­rial and the shape of its mag­netic char­ac­ter­is­tic.

Some de­sign­ers are us­ing neodymium-iron­boron mag­nets in which a small per­cent­age of the neodymium is re­placed with dys­pro­sium to raise the co­er­civ­ity and the tem­per­a­ture at which per­ma­nent de­mag­ne­ti­za­tion oc­curs.

Cob­ham’s Dr Dan Ilea says: “At present, nearly all dys­pro­sium comes from China, and is mainly used for nu­clear, laser and mag­netic data stor­age ap­pli­ca­tions.

“There is likely to be a se­vere short­fall of this el­e­ment within a cou­ple of years, es­pe­cially if it is taken up by high vol­ume in­dus­tries like au­to­mo­bile man­u­fac­tur­ing. It is there­fore crit­i­cal to max­imise the life of neodymi­u­m­iron-boron mag­nets by de­sign­ing elec­tri­cal ma­chines that are not sus­cep­ti­ble to self-de­mag­ne­ti­za­tion.”

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