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Choosing the Right Motors for your Mills

Alex Doll and Derek Barratt

DJB Consultants, Inc., Canada

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Overview of Mill Selection

• rinding testwor! to characterise the

"ro#ect$s orety"es

•Characterise hardness varia%ility or loadgrinding results into %loc! &odel

• Select &ill si'es to achieve a desired

through"ut (or select a through"ut %ased

on desired &ill si'es)

• Select &otor(s) to fit the &ills

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Ste" *+ Mill overload "ower 

• Deter&ine a set of o"erating conditions

that reflects the &ai&u& -overloaded

condition that the &ill should %e a%le to

handle+

 / S0 &ills+ 123 v4v %all charge, 523 v4v filling

and 673 critical s"eed with worn liners

 / Ball &ills+ 583 v4v filling and 673 criticals"eed with worn liners

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Ste" *+ Mill overload "ower 

• S0 &ill, use a

9ent Diagra& to

identify the design

re:uire&ents

• ;a&"le, S<=.2

57$> ? *@$ ;A

S0• Re:uires 1*. M

at &ill shell

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Bench&ar!ing ower 

2 B2 *22 *B2 122 1B2 5222

B

*2

*B

12

1B

52

Aarge Mill Dower 

5=$ 58$ 57$ =2$ =1E

 

   M   C 

20% balls

Upgraded Freeport mill

Upgraded Pelambres mill

First 42' mill

D1.B×;A

*222

First two 40' millswere underpowered

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Ste" 1+ ower &easured

whereF• 9he "rocess conditions reflect "ower

delivered to the &ill shell

• 9he &otor "ower reflects the &otor out"ut

• Motors with "inions and reducers &ust

allow &echanical losses

 / &a!es the &otor %igger to "roduce the sa&e

shell "ower 

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Ste" 5+ hich 9y"eF

• 9hree co&&only used &ill &otor ty"es+

 / earless drives

• largest si'e, u" to 17 M

• highest efficiency

 / Synchronous with "inion

• one or two "inion configuration, u" to *.8 M

 / Induction &otor with s"eed reducer • one or two "inion configuration

• chea"est ca"ital, %ut "oorest electric efficiency

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Ste" 5+ hich 9y"eF

• ;lectrical efficiency

 / earless is highest

 / Induction is lowest

• Directly affects

o"erating costs

 / inefficient &otors

consu&e &ore

"ower  Mill Shell

Motor Output

0.97 0.96 2.@8

1.0 0.985 2.@6

CCV/Transformer Input

Motor Input

0.98 0.99 2.@@

earless

Synchronous

Induction

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Ste" 5+ hich 9y"eF

• earless is always varia%leGs"eed

• Synchronous and induction are nor&ally

fiedGs"eed

 / Can %e &ade varia%le s"eed %y adding &ore

e:ui"&ent and sacrificing electrical efficiency

 / In al&ost all cases, varia%le s"eed is costG

effective for S0 &ills

 / Haria%le s"eed for %all &ills de"ends on the

"articular "ro#ect and ore

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Ste" =+ Select S"eed

• 9he rated s"eed of a &otor is no&inated

%y the engineer 

• 9he tor:ue out"ut of a &otor is constant

u" to the rated s"eed

• 9he "ower out"ut of a &otor is constant

a%ove the rates s"eed

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Case Study

• South 0&erican ironGore "ro#ect

• Haria%ility in ore density

• 82,222 tonnes4day on 63 of sa&"les• Single line

 / * S0, 57$ > ? *@$ ;A

 / 1 %all &ills, 11$ > ? 57$ ;A

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Case Study+ o&inal o"eration

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Case Study, 1*. M S0

• Density =.2 t4& • Density 5.*6 t4&

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Case Study, Design S"eed

• 673 of critical • 63 of critical

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Conclusion

• Si'e grinding &ills for no&inal o"erating

conditions (through"ut targets)

• Si'e &otors for -overload conditions that

&ay reasona%ly %e encountered

 / Kigh %all charge, volu&etric filling

 / Kigh density ore

 / orn liners