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EVOLUTION OF DIRECT COUPLED PINION DRIVE TECHNOLOGY FOR GRINDING

MILLS

F. Tozlu1, K. Lim

1, L. Galarza Castillo

2, J. Sobil

3

1Metso Minerals

2715 Pleasant Valley Rd

York, PA 17402, USA

2Siemens Industry Inc.

100 Technology Drive

Alpharetta, GA 30005, USA

3Siemens Canada Ltd

150-4011 Viking Way

Richmond, BC V6V 2K9, Canada

ABSTRACT

For mining operations globally, the scale and criticality of the grinding mill drive has meant that selection

of the correct mill drive technology is paramount. With so much depending on the successful

implementation of the electrical drive system— safe operation, timely installation, availability, energy

efficiency, minimized maintenance and extended use as tool to assist during mill liner change—customers

are justified in their scrutiny and slow adoption of new drive technology. From the first installation of

low, fixed speed synchronous motors to the introduction of the current source and further voltage source

variable frequency drive, innovations require long periods of time and mining customers willing to push

the industry forward. In recent years, the variable frequency drive has been the stepping stone for a new

generation of mill drive technology. In use for decades in other heavy industrial applications, the drives

allow for system optimization in a myriad of ways. The most dramatic of changes is in the design of the

motor. Where in the past motor design was bound by line frequency, today the designer can optimize

machine size and, more importantly, eliminate the need entirely for the synchronous motor. Induction

motor design has now progressed to the point where low speed operation is possible at efficiency levels

nearing those of the synchronous design. Moreover, the simplicity of the squirrel cage design means that

availability, reliability and spare part requirements are greatly improved. As the next step in the evolution

of large pinion driven grinding mills, the low speed induction drive offers a new reliable and robust

system to the market.

KEYWORDS

Low Speed Induction Motor, Low Speed Synchronous motor (High and Low number of Poles), Wound

Rotor Induction Motor, Dual Pinion Mill Drives, SAG Mill, AG Mill, Ball Mill

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INTRODUCTION

As deposit grades continue to decline, new ways of processing ever greater concentrator throughputs have

been required to maintain throughput efficient and profitable operations. Furthermore, the high risk nature

of mining developments requires a constant search for safer, higher availability and more efficient

equipment design. Limitation of downtime is a key factor in ensuring mining operations can be sustained

through volatile market conditions. Often processing the entire mine throughput through just 1 or 2 lines,

the crushing and grinding circuits are of particular criticality to the uptime and, thus, profitability of any

mining operation.

For many grinding mill installations, the use of pinion driven mills is appropriate. Advances in girth gear

manufacturing have pushed the limits of pinion drive applicability. In recent years, pinion drive

technology has pushed into higher power ranges with installations above 17 MW and moving higher.

With a relatively wide range of capital costs to consider in terms of the type of drive configuration,

careful scrutiny is required to assess the best choice of technology. Failure of the system or added

maintenance resulting in increased downtime could make a significant the difference to the success or

failure of a given project.

The most drastic changes in technology have been witnessed in higher power ranges where direct coupled

motors to the pinion shaft (without additional mechanical speed reducer gearbox in-between). These

motors provide high torque at low speed and thus eliminate the need for a gear reducer. At higher power

ranges (+5MW per pinion), this technology is suitable due to its higher efficiency, footprint, maintenance

and functionality considerations. This paper will evaluate several factors in choosing the correct mill

drive technology. Furthermore, it will discuss the adoption of the next step in pinion drive technology, the

low speed induction motor.

MILL DRIVE DESIGN: CURRENT STATE OF THE ART

In order to illustrate the current state of the technology, let us first examine the wide range of

configurations currently available in the market place. Many different options exist for pinion mill drives,

and each can be put in one of two categories: high speed or low speed referring to motor rpm.

High Speed

For high speed applications, three main solutions exist; (i) high speed wound rotor induction motor

(WRIM) with liquid rheostat starter (LRS), (ii) WRIM with LRS and slip energy recovery (SER), and (iii)

high speed induction motor (IM) with variable frequency drive (VFD). Each have distinct characteristics

from an electrical standpoint, however all require the application of a gearbox reducer. Figure 1

illustrates the general high speed configuration for mill drives.

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Figure 1: High Speed solution mill drive

While this solution can provide an overall lower initial cost, increases in maintenance, spare parts &

footprint, paired with decreased efficiency must be compared to alternative, direct drive technologies. As

mill power trends higher, the market is increasingly looking to direct drive for future installations. With

the inherent limitation to high speed technology in this regard, our focus from this point forward will be

on low speed mill drive systems.

A Brief History of the Low Speed Drive System

Like the High Speed solution, the low speed drive solutions have been in use for many decades as well,

beginning with DC drives prior to the 1960s, as illustrated in Figure 3, leading to today’s modern

installations of high torque AC drive systems. Though innovation occurs at a slow pace, there are

intermittent step changes in technological development. Synchronous AC motors first appeared in

response to the need for increased mill power. DC drives and the corresponding capital cost and

maintenance issues were no longer economical at the high current demands. Introducing synchronous AC

technology drastically increased the limits of power draw for milling. In a relatively short period of time,

large air-clutch start solutions, current source drive applications and gearless mill drives were being

installed.

Figure 2: 3000hp, 150rpm, 6900V Synchronous Motor for Ball Mill designed in 1968

• 16 Bearings

• 4 couplings

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Figure 3: Evolution of Low Speed Grinding Mill Drive System

Low Speed Drive System Configurations

At higher power ranges, in particular, the operating cost advantages and increased uptime of the direct

coupled, low speed pinion drive become apparent. Once again, the options outlined in Figure 4 for drives

can be broken down into three prominent solutions; (i) Low speed synchronous motor (LSSM), (ii) LSSM

with VFD, and (iii) Low speed induction motor (LSIM) with VFD. In all cases, note the reduction in the

number of bearings and couplings, the primary sources of failure increases the system safety, energy

efficiency and reliability, as compared to high speed solutions.

Figure 4: Low Speed (Direct Coupled) Mill Drive

Low Speed Synchronous Motor

With a long history in the mining industry, the low speed synchronous motor has been the traditional

drive system for grinding mills. It provides several advantages; including high efficiency as no drive

losses are present, inherent VAR compensation and system simplicity as these installations have fewer

components. If the LSSM is a fixed speed motor (no VSD), then the speed is predetermined in the design

• 8 bearings

• 2 couplings

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phase and constrained by line frequency and motor pole count. As illustrated in Figure 5, the designer

must choose from a limited number of speeds (mill design points) and thus could limit the level of

optimization that goes into the design of the mill. Once in operation there is no way in which to

compensate for inconsistencies or errors made in the initial design and so the mill may not operate at its

optimal level, it requires an external inching device with the associated extra risk and collateral downtime

/ production loss during liner exchange or mill inspection.

Figure 5: Possible Motor Speeds for LSSM

To start a mill with a fixed speed LSSM, an air clutch must be used as it is incapable of developing the

necessary starting torque through a direct on line start. The motors are brought to full speed un-clutched.

Upon reaching nominal speed, the clutch is engaged, bringing the mill to speed in 5-10 seconds. Over

time, clutches must be maintained as a normal wear item. This further detracts from the availability of the

mill drive.

Introduction of VFDs for SAG and Ball Mills

In recent years, some notable changes in the functionality of grinding mill drives have been introduced to

the market place due to the application of VFDs. Traditionally, VFDs were applied only to SAG mill

installations, however new projects are increasingly seeing the benefits of adding variable speed to

secondary grinding as well. Furthermore, the addition of variable speed to the Ball mill gives added

confidence to the mill designer that he or she can adjust the process to local conditions and inputs once

the mill is installed to optimize the comminution circuit. As ore variability increases and marginal

improvements to processing are increasingly important, the added flexibility and process control that

variable speed enables is proving valuable for Greenfield and Brownfield concentrator design.

Furthermore, mechanical loading, when starting pinion drives, is a critical factor in ensuring longevity of

the equipment. As noted in the previous section, mills were traditionally started using an air clutch. VFDs

allow for softer and safer starting, more consecutive starts and start tries per hour, speed control for

process optimization, controlled stopping and detection of a frozen charge. Whereas the need for

increased mill power propagated the introduction of AC synchronous motors, so the need for process

flexibility, softer starting (electrically and mechanically) and speed control options has brought about the

broad application of VFDs to modern mills. The first step is to evaluate which type of motor best suits a

grinding mill application.

Low Speed Synchronous with VFD

There are two different solutions when using low speed synchronous motors for grinding mill drives. The

first is to use a traditional synchronous motor with +30 poles and a nominal frequency of 50/60 Hz. The

argument in favour of this solution is that, in the event of a drive failure, the motor can be run directly on

line through a bypass switching system. As the availability of VFDs has steadily risen, however, the need

for this solution has decreased. With features such as cell bypass and the fast change out of power cells,

VFD failure is not a limiting factor to plant availability. With proper maintenance and inspection, the

drive uptime can be maintained at a high level; in some cases effectively eliminating the drive as a source

of operational failure (Eaton, Rama, Hammond, 2003).

0

1

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140 150 160 170 180 190 200 210 220 230 240 250

50 Hz

60 Hz

[Motor RPM]

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More recently, manufacturers have implemented drive systems with low pole count motors. Since the

manufacturer is no longer constrained by a set frequency, motors can be built with 6, 8, or 10 poles and

run at a nominal output frequency of approximately 10 Hz. These have the benefit of being lower cost and

more compact than their predecessors. By foregoing a bypass option, the system is greatly simplified,

both electrically and mechanically. No longer is a complex switching arrangement required, nor is there a

need for an air clutch for emergency starting.

Low Speed Induction with VFD

To further simplify, and thus increase availability, induction motors have been introduced to the market

place. Similar to the 6, 8, or 10 pole synchronous solution, the induction motor is run at low frequency to

optimize motor design and compactness in the mill. While these machines provide equal performance to

the synchronous options above, the induction motor requires fewer components and further reduces the

physical volume occupied at the mill.

Comparing Synchronous and Induction

In general, most applications for industry globally require induction motors, also known as squirrel cage

motors. Torque is developed by inducing current in the rotor via the applied stator waveform.

Synchronous motors require an applied field current to specially mounted rotor poles which then interacts

with the stator waveform to produce torque. The rotor is the fundamental difference between the two

technologies. Figure 6 illustrates the added complexity of the synchronous rotor, highlighting additional

manufacturing steps or potential failure points.

Figure 6: Synchronous Rotor Construction

The need for synchronous motors for grinding mill applications was originally driven by the need for low

speed. Synchronous motors can be built with high pole counts and yet produce unity or leading power

factor, whereas induction motors built with similar pole counts would achieve only a power factor of 0.5

or less. In the past, mills running on pinion drive trains were smaller and designed to run at fixed speed;

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thus synchronous motors were the correct and economical choice. As noted previously, the advantages of

the VFD in for both SAG and Ball Mill applications are clear so we must now ask: why is a high pole

synchronous motor necessary?

PARTING WITH TRADITION

Application of a VFD to the system provides separation between the main line and the motor. Motors can

operate in a wide range of output frequencies which allows for innovative steps in motor design.

Applying design principles taken from the metal rolling industry, motors are run with a lower pole count

at lower frequency to achieve the desired pinion speed. This is done both for synchronous and induction

installations since the lower pole count motor is more compact, lower cost, equal in efficiency, and (in the

case of induction) capable of a power factor greater than 0.85. In order to run these motor, however, they

must be fed from the VFD continuously. The motor, therefore, is permanently separated from the line

removing the key obstacle from the past preventing the use of low speed induction motors. Therefore, the

use of high pole synchronous motors for grinding mill applications was historically driven, and induction

motors provide equal performance with a greater degree of simplicity, as 8 or 10 pole synchronous motors

also offer simplicity when compared to the high pole synchronous motors.

System Simplification

One prominent disadvantage to the use of synchronous motors is the requirement for an external

excitation control and power supply. Separate transformer, control panel and a means of transferring the

current to the rotor either through a brushless exciter or brushed mechanism are all additional equipment

requirements in the system. As shown in Figure 7, the induction motor eliminates the need for these

added components. This further simplifies the system as a whole by reducing the complexity of the motor

construction.

Figure 7: Synchronous and Induction System Layout

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Motor Construction and Installation

Whereas high pole count motors are built with pedestal bearing and based, low speed induction or

synchronous motors are built as a compact, bracket bearing design. The enclosure is typically rated for

IP55 (either TEWAC or TEAAC); however ODP ratings are also available if required. Figure 8 shows the

compact design of the low pole count induction motor with a TEWAC enclosure. An important attribute

is the lower diameter of the machine. This opens space at the mill for other components such as the feed

chute as well as for mechanical maintenance on the mill itself. High pole count synchronous motors are

traditionally built as updraft machines which greatly simplifies the cooling of the motor, however requires

regular internal inspection for dust build up.

The simplicity of the low pole count design provides for ease of shipping and fast installation at site. Only

3 crates are required; (i) motor, (ii) cooling assembly and (iii) terminal box and anchor bolts. With this

simplicity of the construction and delivery, these motors typically take only one week to install and

commission. High pole count machines are typically shipped disassembled and reassembled at site.

Typically, this takes on the order of 2 weeks for installation, inclusive of the air clutch.

Figure 8: Low speed induction motor

Rotor Design

The rotor is a simple squirrel cage construction as shown in Figure 9. Whereas the synchronous motor

requires added components for the applied excitation field (either brushless other DC-Rectifier for a brush

type), the induction rotor requires only rotor bars and end-rings to complete the circuit. This is the most

robust and reliable AC-Machine design that exists with almost no maintenance requirements. In addition,

servicing induction motors can be done with standard techniques and know-how.

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Figure 9: Induction Rotor Construction

As noted above, induction motors differ fundamentally from synchronous motors in the construction of

the rotor. Due to need of an externally applied rotor field, synchronous motors require rotor mounted

poles, rotor circuitry and a means of transmitting the energy. This means that an excitation circuit is built

into the rotor, poles wound separately and bolted on, and an excitation generator mounted to both rotor

and stator. Each step of a synchronous motor adds complexity, increased maintenance, requires more

spare parts and has more potential failure points.

A further advantage to the induction motor is that without the need for excitation components, the motor

is in fact smaller in footprint than the equivalent synchronous machine. Figure 10 illustrates the difference

between the two designs, evident in the length and diameter of the machine.

Figure 10: Motor Footprint Comparison (>5MW)

In terms of footprint, the induction option has a clear advantage over the alternatives. The reduction in

size realized by removing the exciter components has a significant effect on space inside the mill. For a

typical 6.5MW motor, there is a 17% improvement in volume as compared to the low pole count

synchronous option (Matthews, 2015).

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Regarding efficiencies, modern induction motors are approaching the levels of the synchronous motor.

The current gap in efficiency between the two designs is 0.5% in favour of the synchronous motor

without excitation circuit. While efficiency is always important, the capital and operating cost reduction

of the induction motor make it a suitable alternative in many cases. In fact, the efficiency difference is

more pronounced in the losses realized by the VFD. Table 1 shows a summarized comparison of the key

elements to consider in evaluating mill motors.

Table 1: Comparison Summary (Low Speed Solutions)

Feature Fixed or Variable Speed Variable Speed Variable Solution 30-40 Pole Synch 8-12 pole Synch 8-12 pole Induction Diameter/Width Large Smaller Smaller Efficiency 97% 97% 96.5% System PF 1.00 0.96/1.00 0.96/1.00 Availability Lower Lower High* Installation +2 weeks 1 week 1 week Performance Equal for all Maintenance Additional for Exciter Additional for Exciter Cost High (Pole count) Lower Lower Inching Drive Required Optional Optional

*Availability of the induction motor is higher compared to the synchronous motor due to fact that the

synchronous machine with its excitation generator and control has a higher number of essential parts.

This, in turn, reduces the mean time between failure and increases maintenance requirements.

Choosing a Variable Frequency Drive

For low frequency, high torque applications, two preferred options exist for the VFD: (i) the Voltage

Source Converter (VSI) and (ii) the Cycloconverter (CCV). For grinding mill applications each must be

evaluated based on their merits as applied to the specific site requirements.

Efficiency

Cycloconverters have been operated in mill applications for over 40 years. The drive can provide

significant savings as comminution in a typical gold or copper mine consumes an average of 36% of the

site’s electrical load (Ballantyne, Powell, 2015). The CCV provides an efficiency advantage of +1% over

the VSI due to a single conversion taking, rather than converting first to DC then back to AC through

pulse width modulation. Table 2 compares the difference in drive efficiency between the CCV and VSI

for a typical grinding mill application. At a typical industrial kilowatt-hr price of $0.08, Table 3 shows the

total savings on a yearly basis when utilizing a CCV for a large pinion driven mill.

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Table 2: Efficiency Comparison of VSI and CCV (9 MW)

Voltage Source Converter 12p DFE (diode front end)

Cycloconverter Star Connection

Topology

Power Range (single/parallel connection)

12 / 24 MVA 12 / 24 MVA

Transformer Power (9 MW motor)

10 MVA 13 MVA

Reactive Power (9 MW motor at rated point)

3 MVA 8.6 MVA

Line Harmonics (at 34.5 kV)

THDi = 9 A (12p) THDi = 3 A (24p)

THDi = 40 A (6p) THDi = 23 A (12p)

Losses (at 9 MVA) ~90 kW ~55 kW Cooling Water (water cooling unit/water supply

required) Air (Air conditioning recommended)

Table 3: Efficiency Comparison of CCV and VSI (18MW mill power rating)

Harmonics

All variable frequency drives require extensive consideration of the effects of harmonic distortion. VSIs

provide relatively good performance with regards to the harmonics. When using multi-level drives with

low harmonic outputs, the filtering requirement is generally minimal. It should be noted, however, that

filtering is typically still required and the use of an active front end drive (to provide leading reactive

power compensation) provides added uncertainty of the harmonic effects to the system as well as to the

final efficiency value of the drive.

When using the CCV, careful consideration must be made regarding filtering and power factor

compensation. It is recommended to approach this issue in a holistic manner by recognizing that the site

as a whole will inevitably require capacitor banks and filters. By managing this issue at the site level,

filtering and power factor compensation can be centralized in a safe area away from plant and pit

VSI CCV

Transformer 99.0% 99.0%

Drive 98.5% 99.5%

Motor 96.5% 96.5%

System 94.1% 95.1%

Mill Power 18,000 18,000

Consumed 19,128 18,936

Power Differential - -192

Operation (hours per yr) 8760

Total Cost ($ per yr) 13,405,032$ 13,270,308$

Savings ($ per mill-yr) - 134,724$

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electrical. Regardless of which drive is chosen, there will never be complete certainty as to the extent of

filtering until a final design and harmonic study is complete.

Construction and Availability

Significant differences exist in the construction of the respective drives. Though it is an older technology,

the CCV is a much simpler topology than the VSI. As illustrated in Figure 11, the VSI consists of an

isolation transformer, diode or active front end (diode shown), DC link with capacitors, and IGCT output

converter. By contrast, the CCV has an isolation transformer and thyristor bank, limiting the amount of

critical parts. Not only are the number of different components less for the CCV, the absolute number of

total components is also lessened. As the power needs increase above approximately 7MW, the VSI

requires two separate bridges to meet the power and overload requirements. This further complicates

drive construction and drastically increases the number of parts within the drive. In high torque, low

speed mining applications, the CCV has been shown to have an availability of 99.5% (Combes, Dirscherl,

Roesch, 2014). Table 4 outlines the different part counts for each power range of the two drives based on

a typical 150% overload requirement.

Figure 11: Variable Frequency Topology Comparison

Table 4: Part Count Comparison of Drive

Drive Type VSI (<7MW) VSI (>7MW) CCV (<7MW) CCV (>7MW) Diode 30 48 IGCT 12 24 DC Link Capacitor 6 12 Thyristor 36 36 Total 49 76 36 36 MTBF Improvement (yrs) +2 +2

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The drives also differ significantly in footprint and cooling. This is an important consideration when

designing electrical rooms as space in a mine is always at a premium. The VSI, with front end, DC link

and output, is inherently larger than the CCV. For higher power ratings (+5MW), the VSI must be water

cooled which adds a water cooling unit with two circuits (de-ionized and external raw water). Figure 12

illustrates the difference in building space requirement for the respective drive types.

Figure 12: E-Room Layouts Space Comparison

As outlined in previous sections, there are a large number of considerations in choosing the appropriate

mill drive technology for each project. For high power pinion drives, robust design, reliable operation and

energy efficiency are key factors in evaluating the best drive system for the project. This evaluation is

best shown by the example of a mill drive system installed in a new concentrator built in Arizona.

PROVEN IN INDUSTRY: A CASE STUDY

In June 2014, Metso and Siemens completed commissioning of two (2) 24’ x 40’ ball mills, each with a

power rating of 2 x 6.5 MW fitted with twin pinion, low speed induction motor drive systems. The end

customer challenged the equipment manufacturers to provide a system that would run continuously with

100% availability. At the same time, energy efficiency was a paramount consideration based on the

design philosophy of the customer.

After careful consideration, low speed induction motors driven by cycloconverter technology were

provided. Siemens scope consisted of low speed induction motors, cycloconverters, isolation

transformers, modular e-rooms, 13.8kV switchgear and the complete system harmonic study. Combining

the advantages of both the cycloconverter and the low speed induction motor, this was the solution

selected based on the constraints set forth by the end customer.

The motors were supplied as water-cooled, compact design, with totally enclosed IP55 rated enclosures.

Each unit was fully factory-tested and shipped as a single piece. Supplied with bracket type sleeve

bearings, each with dedicated jacking unit, the motors produce 345kNm of torque at 180 RPM. By

developing full torque at 0 RPM, the design did not require installation of an air clutch or the

corresponding control and pressurized air supply. Instead, the machines are direct coupled with a torque

limiting coupling.

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The air-cooled cycloconverter, rated at 8 MVA, is a SINAMICS SL150, equipped with load sharing

controller for smooth operation under all working conditions for the dual pinion configuration. The

control, part of Siemens’ standard Mill Technology Control package, includes added mill functionality

features including: smooth start-stop operation modes, creeping & inching mode for maintenance/mill

positioning for liner change, and detection of frozen charge. This drive system has the added advantage

that it can run at 115% overload continuously, while still meeting the 150% overload starting requirement

(1.15 service factor). Siemens installed and pre-commissioned the drives, switchgear and control in

containerized e-rooms. This facilitated fast installation and commissioning at site.

In the end, the mill drives were successfully installed and commissioned on a condensed schedule and

have been in operation without issue for over 16 months. The drive system has demonstrated excellent

reliability and performance through this time period. Throughout operation, maintenance costs have been

at a minimum (Galarza et al, 2015).

Figure 13: 10,000hp, 180rpm, TEWAC Ball Mill Induction Motors commissioned in June 2014

CONCLUSIONS

Many factors influence the choice of drive technology for grinding mills. As the industry looks for new

technologies to push the limits of functionality, availability and efficiency, it is important that each

solution be analyzed by its own merits. As one of the most critical pieces of equipment at any mine site,

the mill drive must be evaluated in a holistic approach whereby the effects on efficiency, reliability,

space, and harmonics are well understood. How these drives integrate into the site can have dramatic

effects to the overall electrical design and eventual operation.

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This holistic approach of simplicity was demonstrated recently at a mine in Arizona. This low speed

induction and cycloconverter were chosen for robustness of design, reliability with the least number of

parts, dedicated mill functions, and energy efficiency. This combination meets or exceeds modern mill

drive requirements in the mining industry.

REFERENCES

Eaton, D., Rama, J., Hammond, P. (2003): Five Years of Continuous Operation with Adjustable

Frequency Drives, IEEE. Catlettsburg, USA; New Kensington, USA.

Matthews, V. (2015): Low Speed Asynchronous Motor for Mill Applications, SME. Alpharetta, USA.

Ballantyne, G. R., Powell, M. S. (2015): Development of the Comminution ‘Energy Curve’. Brisbane,

Australia.

Combes, M., Dirscherl, C., Roesch, T. (2014): Increasing Availability Through Advanced Gearless Drive

Technology, CMP. Santiago, Chile; Littleton, USA; Erlangen, Germany.

Galarza, L., Kitz, W. (2015): Dual Pinion Drive System Running Smoothly. Alpharetta, USA; Erlangen,

Germany.

Mular, A., Halbe, D., & Barratt, D. (2002): Mineral Processing Plant Design, Practice and Control, SME.

Littleton, USA.