Thought Leadership: Magnetic propulsion — A Game Changer for eVTOL Efficiency and Reliability

Effi­cien­cy and reli­a­bil­i­ty remain two of the biggest chal­lenges for the devel­op­ment of viable eVTOL air­craft.

Gary Rodgers, CEO of Mag­no­mat­ics, explains how scal­able mag­net­ic motor sys­tems address these chal­lenges while pro­vid­ing man­u­fac­tur­ers with ver­sa­til­i­ty and cost-effec­tive­ness.

A nov­el approach to propul­sion

With urban road­ways unable to cope with grow­ing traf­fic vol­umes, eVTOL air­craft promise an effec­tive way to com­bat con­ges­tion while pro­vid­ing trans­port that is quick, qui­et, clean and green.

For that promise to be realised, the devel­op­ment of these air­craft requires detailed plan­ning and metic­u­lous research. Dur­ing this process, count­less pro­to­types are cre­at­ed and analysed to ensure that an eVTOL’s design is safe, reli­able and ener­gy-effi­cient and that it com­plies with reg­u­la­tions and meets the needs of poten­tial buy­ers.

With regards to propul­sion, focus­ing on ener­gy-effi­cient alter­na­tives is vital for deliv­er­ing air­craft that offer man­u­fac­tur­ing, oper­a­tional and main­te­nance cost sav­ings. Com­pared to stan­dard sys­tems, mag­net­i­cal­ly geared motors are sub­stan­tial­ly more com­pact, light­weight and ener­gy effi­cient.

These attrib­ut­es not only offer ener­gy-sav­ing propul­sion dur­ing flight, but they also enable man­u­fac­tur­ers to build high­ly effi­cient eVTOL air­craft. More­over, being scal­able, they offer excep­tion­al ver­sa­til­i­ty for eVTOL devel­op­ment as they can be used in var­i­ous appli­ca­tions, includ­ing small-scale eVTOL projects, like drone deliv­er­ies, per­son­al air taxis and emer­gency ser­vices.

For devel­op­ing com­mer­cial­ly viable air­craft, mag­net­ic motors offer impor­tant ben­e­fits for man­u­fac­tur­ers. They reduce man­u­fac­tur­ing costs due to their easy instal­la­tion, com­pat­i­bil­i­ty with stan­dard elec­tron­ics and the need for few­er com­po­nents; main­te­nance costs are low­ered because of built-in safe­ty and reli­a­bil­i­ty fea­tures; and oper­at­ing costs are min­imised by the motor’s excel­lent ener­gy effi­cien­cy.

This effi­cien­cy, togeth­er with their qui­et oper­a­tion and the need for few­er raw mate­ri­als dur­ing man­u­fac­ture, means mag­net­i­cal­ly geared motors are intrin­si­cal­ly more sus­tain­able than tra­di­tion­al sys­tems.

Issues with per­ma­nent mag­net motors

Per­ma­nent mag­net (PM) motors are the con­ven­tion­al way to gen­er­ate the required pow­er den­si­ty in elec­tric propul­sion. How­ev­er, when utilised in eVTOL appli­ca­tions, the PM motor out­put speed, which can exceed 10,000rpm, has to be reduced to 1/7 of the orig­i­nal speed.

While mechan­i­cal gear­box­es are the tra­di­tion­al way to reduce speed, there is the poten­tial that an unde­tect­ed sin­gle-point fail­ure can cause rotor jam­ming and result in a dis­as­trous loss of propul­sion pow­er. This means mechan­i­cal gear­box­es are com­plete­ly unsuit­able for eVTOL appli­ca­tions where safe­ty is crit­i­cal, espe­cial­ly for pas­sen­ger flights and trans­port over urban areas. As a result, both pas­sen­ger and non-pas­sen­ger-car­ry­ing eVTOLs require safer, more reli­able propul­sion sys­tems.

Main­te­nance is anoth­er con­cern for mechan­i­cal gears. Prone to phys­i­cal wear and fail­ure, reg­u­lar phys­i­cal inspec­tions will be a neces­si­ty, and worn parts will need replac­ing more fre­quent­ly. This will result in high­er main­te­nance costs and increased down­time, mak­ing eVTOL air­craft that use mechan­i­cal gears less mar­ketable to poten­tial buy­ers.

A final issue with mechan­i­cal gears is that they are noisy. This noise not only affects pas­sen­ger com­fort but can also result in noise pol­lu­tion, espe­cial­ly when the air­craft are tak­ing off, land­ing and fly­ing at rel­a­tive­ly low alti­tudes over urban areas. There is the
poten­tial that noise lev­els could result in air­craft fail­ing to meet cur­rent or future reg­u­la­tions, result­ing in noise cer­ti­fi­ca­tion or licences being refused.

Noise reg­u­la­tions for eVTOLs are still evolv­ing and the Euro­pean cer­ti­fi­ca­tion reg­u­la­tor, EASA, has drawn up the first
pro­pos­als for assess­ing noise gen­er­at­ed by eVTOL air­craft.

An inno­v­a­tive approach for eVTOL design

Giv­en the issues with PM motors, mag­net­ic motor sys­tems offer a fea­si­ble alter­na­tive that erad­i­cates the need for mechan­i­cal gears and pro­vides a range of oth­er advan­tages for eVTOL air­craft.

An inno­v­a­tive design fea­tures a con­cen­tric com­bi­na­tion of a mag­net­ic gear and a per­ma­nent mag­net­ic motor. With the mag­net­ic gear mount­ed inside the sta­tor and the gear’s out­er mag­net­ics attached to the stator’s inner bore, this enables the stator’s cop­per wind­ings to dri­ve the mag­net­ic gear’s inner rotor.

This design is ide­al for eVTOL appli­ca­tions that demand high torque and low speed, as the system’s topol­o­gy max­imis­es the propul­sion system’s torque capa­bil­i­ty. Sur­pass­ing the lim­i­ta­tions of tra­di­tion­al direct-dri­ve elec­tri­cal machines, the mag­net­ic gear deliv­ers the same uplift in out­put torque as a sin­gle-stage mechan­i­cal gear.

In addi­tion, with a ful­ly inte­grat­ed cen­tral gear ele­ment, the dual use of the inner rotor, com­bined with the slight increase in
mass from the out­er mag­nets and pole-piece rotor array, results in a light­weight, com­pact design.

Cru­cial­ly, the design also improves safe­ty and reli­a­bil­i­ty. The lack of mag­net­ic gears means it is less sus­cep­ti­ble to fail­ure than an equiv­a­lent mechan­i­cal gear and full load torque can be main­tained fol­low­ing a phase-bank fail­ure.

Addi­tion­al­ly, the ther­mal, elec­tri­cal, mechan­i­cal and mag­net­ic iso­la­tion of phas­es and banks deliv­ers full duplex redun­dan­cy, avert­ing fire and lim­it­ing the impact of elec­tri­cal shocks, while the fault-tol­er­ant motor acts as a pas­sive­ly reset­table torque fuse, remov­ing the need for a clutch or shear pins if jam­ming occurs.

When devel­op­ing a reli­able, ener­gy-effi­cient and light­weight elec­tric motor, effec­tive ther­mal man­age­ment is vital. Heat trans­fer paths can vary con­sid­er­ably, depend­ing on the motor’s engi­neer­ing para­me­ters, ther­mal inter­faces and com­po­nent mate­ri­als. Rather than an elec­tric-pow­ered fan, a more appro­pri­ate alter­na­tive for eVTOL air­craft is Total­ly Enclosed Forced Ven­ti­la­tion (TEFV).

With an eVTOL’s rotor load being approx­i­mate­ly pro­por­tion­al to the square of the rotor speed, a lighter and more com­pact cen­trifu­gal fan can be mount­ed to the motor shaft, util­is­ing its high speed. This approach enables rel­a­tive­ly high heat trans­fer with­out recourse
to inter­nal air­flow, mit­i­gat­ing con­cerns linked to exter­nal flu­id cool­ing sys­tems, such as com­plex­i­ty, addi­tion­al mass and reli­a­bil­i­ty.

Ther­mal man­age­ment is also enhanced through advanced wind­ing tech­nol­o­gy. Pre­ci­sion wire place­ment of round con­duc­tors, com­bined with mod­u­lar sta­tor build meth­ods, results in a high­er pack­ing fac­tor that increas­es wind­ing ther­mal con­duc­tiv­i­ty and min­imis­es eddy cur­rent loss­es. As a con­se­quence, heat rejec­tion require­ments are min­imised.

More­over, there is a sub­stan­tial decrease in cop­per vol­ume as the mag­net­ic gear­ing reduces the Min­i­mum Mar­ketable Fea­ture (MMF) require­ments. Wind­ing tem­per­a­tures are thus low­ered due to the low­er loss­es, high ther­mal con­duc­tiv­i­ty and small­er wind­ing cross-sec­tion.

For eVTOL man­u­fac­tur­ers seek­ing a scal­able and ver­sa­tile solu­tion suit­able for a wide range of appli­ca­tions, mag­net­i­cal­ly geared motors offer the reli­a­bil­i­ty, cost-effec­tive­ness and ener­gy effi­cien­cy crit­i­cal for devel­op­ing com­mer­cial­ly viable air­craft.