ELEC­TRIC MO­TOR BA­SICS

Wheels (Australia) - - Fortune Special -

Elec­tric mo­tors are ev­ery­where around us, whirring away ac­cord­ing to prin­ci­ples dis­cov­ered al­most two cen­turies ago. Yet for most of us their in­ner work­ings are a mys­tery. Wheels read­ers able to dis­cuss the finer points of the in­ter­nal com­bus­tion en­gine in all its com­plex va­ri­ety will likely have only the faintest un­der­stand­ing, if any, of how an elec­tric mo­tor con­verts cur­rent into kilo­watts.

This is how it’s done. Cre­ate a mag­netic field, put a con­duc­tor like a cop­per wire in that field, then con­nect the con­duc­tor to a source of elec­tric­ity. Re­sult? The mag­netism pushes on the wire. This core con­cept ap­plies to all elec­tric mo­tors, and grasp­ing it is the equiv­a­lent of un­der­stand­ing that burn­ing fuel mixed with air in a closed space cre­ates pres­sure.

Com­pared to the typ­i­cal com­bus­tion en­gine’s cylin­ders, pis­tons, con­nect­ing rods, cranks, valves and cams, an elec­tric mo­tor is ele­gant sim­plic­ity. Ar­range mag­nets around the pe­riph­ery of a cylin­dri­cal cas­ing, set a bunch of con­duc­tors length­ways into a cylin­der free to ro­tate in­side that cas­ing, and or­gan­ise some way to sup­ply them with cur­rent, and you’ve got an elec­tric mo­tor. The sta­tion­ary sec­tion is called the ar­ma­ture or sta­tor, the ro­tat­ing part is called the ro­tor. So far, so easy...

There are many types of elec­tric mo­tors, named ac­cord­ing to the way their mag­netic fields are cre­ated, how the ro­tor con­duc­tors are sup­plied with cur­rent and, fi­nally, what kind of elec­tric­ity they’re made to con­sume. These de­tails are the source of the baf­fling terms you’ll see in al­most any de­scrip­tion of an elec­tric mo­tor.

One big and ba­sic di­vi­sion is whether the mo­tor uses DC (di­rect cur­rent) or AC (al­ter­nat­ing cur­rent). Mo­tor mag­nets can be per­ma­nent (typ­i­cally us­ing the rare-earth el­e­ment Neodymium), elec­tro­mag­nets (cop­per wire wound around an iron core and need­ing elec­tric­ity to pro­duce mag­netism), or hy­brid com­bi­na­tions of both. Cur­rent can be sup­plied to the ro­tor by wear-prone brushes or with­out a phys­i­cal con­nec­tion via elec­tric in­duc­tion, the same ef­fect that en­ables in­duc­tive charg­ing of bat­ter­ies in ev­ery­thing from tooth­brushes to EVS.

The three-phase AC in­duc­tion mo­tor has be­come the dom­i­nant type in EVS. It’s the kind of mo­tor that gets elec­tri­cal en­gi­neers very ex­cited. “Judged in terms of fit­ness for pur­pose cou­pled with sim­plic­ity, the in­duc­tion mo­tor must rank along­side the screw thread as one of mankind’s best in­ven­tions,” is the way one ex­pert de­scribes it in print.

It’s easy to un­der­stand the en­thu­si­asm. These mo­tors con­vert elec­tric­ity into driv­ing force with great ef­fi­ciency. From 85 to be­yond 95 per­cent is the typ­i­cal range, which makes the 40 per­cent ef­fi­ciency of the best automotive com­bus­tion en­gines look down­right waste­ful.

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