The fu­ture of VSE mo­tors, Amin Al­masi

DEMM Engineering & Manufacturing - - CONTENTS - By Amin Al­masi

The cost of the ro­tat­ing ma­chin­ery trains could amount to be­tween 30 and 60 per­cent of the to­tal cost of many plants.

It is the gen­eral trend of ma­jor oper­a­tion com­pa­nies to re­place the gas tur­bine and steam tur­bine driv­ers (and other driv­ers) with the vari­able-speed elec­tric mo­tors due to bet­ter re­li­a­bil­ity and ef­fi­ciency.

The vari­able-speed elec­tric mo­tors, us­ing vari­able-speed­drives (VSD), are be­com­ing more pop­u­lar. To avoid any prob­lem with the vari­a­ble­speed elec­tric mo­tors, the VSD should pro­vide an out­put with the fol­low­ing qual­i­ties: Volt­age and cur­rent with good phase bal­ance and min­i­mal DC com­po­nent. Limited har­monic con­tent. These two fac­tors can min­i­mize cir­cu­lat­ing cur­rent losses and min­i­mize torque pul­sa­tions. The char­ac­ter­is­tics and ad­van­tages of the elec­tric VSDs should be con­sid­ered as an im­por­tant el­e­ment of con­trol that pro­vide flex­i­ble and safe oper­a­tion, in­stead of the more com­mon con­cept found in lit­er­a­ture that see the VSD only as an en­ergy sav­ing de­vice.

In­duc­tion elec­tric mo­tors

An in­duc­tion elec­tric mo­tor is a type of AC ma­chine where power is sup­plied to the ro­tor by means of elec­tro­mag­netic in­duc­tion. These elec­tric mo­tors are widely used in in­dus­trial drives, be­cause they are rugged, sim­ple and re­li­able.

Their speed is de­ter­mined by the fre­quency of the sup­ply cur­rent to elec­tric mo­tors. The most com­mon type is the squir­rel cage elec­tric mo­tor, this term is some­times used for in­duc­tion mo­tors gen­er­ally.

The sta­tor of an in­duc­tion mo­tor con­sists of poles car­ry­ing sup­ply cur­rent to in­duce a mag­netic field that pen­e­trates the ro­tor. To op­ti­mise the dis­tri­bu­tion of the mag­netic field, the wind­ings are dis­trib­uted in the slots around the sta­tor, with the mag­netic field hav­ing the same num­ber of north and south poles.

The main com­po­nents of a con­ven­tional in­duc­tion mo­tor ro­tor are usu­ally the shaft, the core (could be con­structed of lam­i­nated steel punch­ing), the con­duc­tor bars, and the con­duc­tive end-rings.

While this type of con­struc­tion is rel­a­tively sim­ple and ro­bust, for a high­speed and high-power den­sity ap­pli­ca­tion, the cen­trifu­gal and ther­mal stresses re­quire rig­or­ous re­li­a­bil­ity stud­ies of each of mo­tor com­po­nents. A fab­ri­cated ro­tor cage is usu­ally the only op­tion in high-speed, high-power ap­pli­ca­tions given the strength lev­els re­quired from the end-ring and the bar ma­te­ri­als. A syn­chro­nous elec­tric mo­tor is an AC mo­tor in which the shaft ro­tat­ing speed is syn­chro­nized with the fre­quency of the AC cur­rent. The syn­chro­nous elec­tric mo­tors con­tain elec­tro­mag­nets on the sta­tor of the mo­tor that cre­ate a mag­netic field, which ro­tates in time with the os­cil­la­tions of the sup­ply cur­rent.

The ro­tor turns in step with this field, at the same rate.

Con­tin­ued on page 28

In other words, the elec­tric mo­tor does not rely on ‘slip' un­der usual op­er­at­ing con­di­tions, and as a re­sult pro­duces torque at the syn­chro­nous speed.

Syn­chro­nous mo­tors can be con­trasted with the in­duc­tion mo­tors, which should slip in or­der to pro­duce torque. The speed of the syn­chro­nous mo­tor is de­ter­mined by the num­ber of mag­netic poles, as well as the fre­quency of the power sup­ply to the mo­tor.

The mo­tors are avail­able in high­horse­power di­rect-cur­rent ex­cited in­dus­trial sizes. In large sizes, the syn­chro­nous mo­tor pro­vides two im­por­tant func­tions; first, it of­fers a rel­a­tively high ef­fi­ciency; sec­ond, it can op­er­ate at leading or unity power fac­tor and thereby pro­vide the power-fac­tor cor­rec­tion.

Pre­vi­ously, there were only two ma­jor types of syn­chro­nous mo­tors; ‘nonex­cited' and ‘di­rect-cur­rent ex­cited', which had no self-start­ing ca­pa­bil­ity to reach syn­chro­nism. Ad­vances in in­de­pen­dent brush­less ex­ci­ta­tion con­trol of the ro­tor wind­ing set have of­fered a new type of syn­chro­nous mo­tor.

The ‘brush­less wound-ro­tor elec­tric mo­tor' is the most suit­able type of syn­chro­nous mo­tor with all the the­o­ret­i­cal qual­i­ties of the syn­chro­nous mo­tor, and the wound-ro­tor mo­tor com­bined.

Fea­tures in­clude the power fac­tor cor­rec­tion, the high­est power den­sity, the high­est po­ten­tial torque den­sity, rel­a­tively low cost elec­tronic con­troller, the high­est ef­fi­ciency, and oth­ers. Brush­less syn­chro­nous type elec­tric mo­tors are com­monly used for the high power level ap­pli­ca­tions.

Out­put rat­ings of two-pole syn­chro­nous mo­tors are the­o­ret­i­cally un­lim­ited in terms of power, since gen­er­a­tors of this type have been built for above 100MW with many suc­cess­ful ref­er­ences. Many of these gen­er­a­tors are above 200MW.

How­ever, the shaft speed op­er­at­ing ranges of such elec­tri­cal ma­chines are limited for rea­sons, such as: The cen­trifu­gal forces act­ing on dif­fer­ent ro­tat­ing com­po­nents and their sup­ports. In other words, the me­chan­i­cal strength of the ro­tor or the sup­port­ing com­po­nent ma­te­ri­als, in

• gen­eral, can be­come a lim­it­ing fac­tor. The length of the ro­tor (the bear­ing span) is limited to cope with the lat­eral vi­bra­tions and ro­tor­dy­nam­ics is­sues. The brush­less ex­citer, which should sup­ply full field power from stand­still to rated speed, is usu­ally solidly flanged to the non-drive end of the elec­tric mo­tor and some­times has its own bear­ing.

This ro­tor as­sem­bly is known as the “three-bear­ing ro­tor”, of­ten used for high­speed elec­tric mo­tors. This can re­sult in a well-de­fined (in terms of lat­eral vi­bra­tion be­hav­iour) ro­tor sys­tem with three bear­ings and a con­trolled ro­tor­dy­nam­ics re­sponse.

Three bear­ings elec­tric mo­tors of­fer rel­a­tively high stiff­ness and ex­cel­lent dy­namic be­hav­iour.

For rel­a­tively low speeds, sleeve bear­ings are usu­ally em­ployed. Some­times, there is a re­quire­ment for bet­ter sta­bil­ity and additional damp­ing of tilt­ing-pad bear­ings for rel­a­tively high­speeds.

Usu­ally, ad­vanced tilt­ing-pad bear­ings should be used; par­tic­u­larly for speeds above 2000 rpm, tilt­ing-pad bear­ings are al­ways pre­ferred.

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