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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