PonJun03A Conf

7/28/2019 PonJun03A Conf

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Abstract Mining operations are facing the challenge of

competitiveness because of low metal prices and operating

costs. The only way to solve the problem is applying new de-

sign concepts in management, process operation, R&D and

technologies, where industrial electronics and information

technologies are taking a decisive role concerning process con-

trol, automation and personnel safety. The size reduction,

crushing and grinding, is one of the main stages in mineral

processing, because of energy, material and maintenance

costs. Looking at economy of scale, new projects consider theuse of big mill grinding circuits, where large power gearless

motor drives with cycloconverters in the range around 20 MW

are being applied for semiautogenous (SAG) mills and re-

cently, ball mills are also being considered.

This work addresses some current issues involving design

and application of high-power drives employed in SAG grind-

ing mills, highlighting industrial electronics aspects:

Complex nature of the process with large units

Reliability and availability

Operation within weak networks

Power quality and harmonics control

Process and knowledge-based instrumentation

Mechanical and mechatronics aspects

Based on actual cases of application, the main constructive

and operation aspects are described and the open problems

for improving reliability and availability are discussed.

Index Terms Drives, power converters, harmonics, mills,

mining

I.INTRODUCTION

Cycloconverter-fed Motor Drives have inherent advan-tages for handling high-power low-speed applications withgearless motor drives intended for grinding mills, employedin cement plants and ore concentrators [1], [2]. Other mod-

ern applications in the MW range are flywheel energy andpumped hydraulic storage systems [3]. General considera-tions and criteria for evaluation of mill drives options havebeen discussed in [4], [5], [6]. For power greater than10.000 Hp, mechanical pinion coupled drives have beenconsidered as a limit for single drives because of the risk oflong-time outages due to mechanical maintenance. In addi-tion, for variable-speed operation including zero-speed andstarting with full torque within limited short-circuit level,high-power gearless motor drives have shown advantages.Installations with units in the range of 20 MW are beingrecently designed and applied, and higher powers areconsidered to be feasible. High-dynamic performance has

been achieved using field orientation techniques as well asobservation methods for sensorless control [7] - [12]. How-

ever, the larger grinding units, especially the semiautoge-nous (SAG) mills have the drawback of being a bottleneckwhen one unit goes out of service, that is why, reliabilityand availability are the main concerns. The most commonlyused converter is the cycloconverter, because of its inherentadvantage to handle high power with low output frequencyand 4-Quadrant operation. Under network disturbances, thephase-controlled cycloconverter must be blocked in orderto avoid commutation failures, with the same nature of

HVDC stations [13]. After a network disturbance, a re-closing may be very dangerous, like the situation whicharose by re-closing large induction motors, as reported by[14] and [15]. Operation within a weak network should becarefully taken into account from the initial conceptual pro-ject stage, with a design based on specific modeling andsimulation for meeting power quality requirements underthe variable operating conditions [16] - [19]. There are alsoissues of mechanical and mechatronics engineering to befaced at the design stage in order to avoid mechanical reso-nances excited by the electromagnetic torque and forces[20], [21], [22].

II.BACKGROUND:SEMIAUTOGENOUS GRINDING (SAG)MILL WITH A GEARLESS DRIVE

Fig. 1 shows the general scheme of a mill with gearlessmotor drive. The rotor poles of the synchronous machineare bolted to the mill. There is no gear, because the elec-tromagnetic torque is generated directly to the mill. The sta-tor has a ring form, giving the alternative name ring-motor to this wrapped-around configuration. Typical value

J. PONTT1, Member IEEE, J. RODRIGUEZ2, Senior Member IEEE, W.VALDERRAMA3,G. SEPLVEDA4, P. CHAVEZ5, B. CUITIO6, P. GONZLEZ7, G. ALZAMORA8

1, 2, 3, 4, 5, 6, 7

Universidad Tcnica Federico Santa Mara, Valparaso, Chile, e-mail:1

[email protected] Minera Los Pelambres, Salamanca, Chile,e.mail: [email protected]

Current Issues on High-Power Cycloconverter - fedGearless Motor Drives for Grinding Mills

Fig. 1, General scheme of a Gearless Mill Motor drive [24].


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of the airgap is in the range of 15-17 mm. A tolerance of

+/- 3 mm is considered for setting alarms and protections.The motor is fed with variable frequency by a cyclocon-verter in order to regulate the torque and operating speed ofthe mill. The excitation is controlled by a 6-pulse controlledbridge.

Grinding or size reduction of mineral ore is made withlarge-diameter rotating mills where the mineral itself isused as a grinding media. This process is called autoge-nous grinding. A semiautogenous grinding mill (SAG)uses complementary steel balls of typical 5-inch diameter tosupport the grinding media. Fig. 2 shows the possible tra-jectories inside the mill. The tube mill has internal steel lin-ers with lifters for lifting the load. The load builds a kidney

form with a cataracting effect with different balls androcks trajectories. The cataracting with trajectories impact-ing at the toe of the load is the most effective grinding ef-fect. For higher throughput, large diameter mills are beingemployed to use this impacting effect. The movement ofthe load depends strongly on several operating variableslike geometry, viscosity, ore size-distribution and rotationspeed. In addition, the wearing of the lifters also plays alsoa role because its geometry changes with time. There are

also harmful impacts because of the ball trajectories im-pacting in the lifters, with energy loss, accelerated steel ball

wear and jeopardizing the life of lifters and the availabilityof the mill. That is why variable speed is needed for high-power mills. The useful range of operation speed is around75% and 80% of the critical speed. The critical speed is de-fined as the speed at which a steel ball remains at the shellof the mill without falling when the centrifugal force equalsits weight and is given by:

)(

2

dD

gw

= (1)

Where w is the critical speed, g=9.81 [m/s2], D is the in-ternal mill diameter and d is the ball diameter. If d


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nominal speed of 77% the critical speed is 9.8 [rpm]. Thebigger the diameter, the lower the nominal speed of rota-tion.

Fig. 3 depicts the scheme using a synchronous motorwith two separated stator windings. There are also applica-tions with only one stator winding. There is a current dis-cussion concerning the advantages and drawbacks of bothalternatives regarding issues of: efficiency, cooling, size,short-circuit behavior and reliability, among others.

Fig. 4 shows the connection of a twin 6-Pulse cyclo-converter system to a 2-stator winding machine. The trans-formers have secondary windings with 30 degrees dis-placement in order to build a 12-Pulse equivalent cyclo-converter at the network side. A symmetry current controlis needed to ensure the proper balanced operation.

Fig. 5 depicts the use of a single stator winding ma-chine. The 12-pulse cycloconverter is built by the seriesconnection of two 6-pulse cycloconverters.

In both cases, the cycloconverter has a three-phase cur-rent control [17] with feedforward scheme using internalvoltages of the machine for better tracking of the three-phase current references.

Fig. 6 shows the zero-crossing of the controlled currentin one phase of the motor. A deadtime is set for ensuring asafe transition from a positive and negative converter withno-circulating current. The longer the deadtime, the largerthe motor current distortion and vice-a-versa. Current dis-tortion produces undesirable torque oscillations. It is veryimportant to meet a trade-off between a desired small dead-time for a good quality of controlled current versus a longer

deadtime with a higher safety margin at transition. Offsets

in the analog current measurement may jeopardize this keybehavior.Fig.7 depicts the simplified space-vector diagram of the

synchronous machine. Stator resistance is neglected as afirst approximation. The - axis is fixed to the phase aof the stator. The d-q axis is fixed the rotor of the machine,where the excitation winding is along the d-axis, with rela-tive position m. The x-y axis is chosen with the resultingflux along the x-axis with relative position s. The load-angle r is defined as the relative position between the fluxand the magnetomotive force given by the excitation cur-rent ie. A vector control method is employed in order tohave a good dynamic performance with decoupled control

of torque and flux of the machine. Torque is commandedwith the product of resulting flux and active currentcomponent iy. The flux is controlled by commanding thereactive component ix and excitation current ie. The operat-ing power factor of the machine is cos, where is the an-gle between the current i and voltage es at the machineterminals.

Fig.8 shows a scheme for the vector control of the ma-chine. There are three main control loops: speed controller,Flux-controller and a second flux controller for the excita-tion control. For sensorless operation, a voltage- and a cur-rent model are employed in order to construct the internal

variables of the machine. There are other internal controlloops (not shown here) for improving the performance ofthe three-phase stator current control and for reducing the

Fig. 7, Space-vector diagram of the vector-controlled synchronous ma-chine.

Fig. 8, Sensorless Vector Control for a Gearless Synchronous machineDrive (CT: Coordinates Transformation).

Fig. 9, Industrial Power System with 4 Gearless Motor Drives (GMD).

Fig. 6, Zero crossing of motor current @ 7.1 Hz (simulation).


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errors of the observed internal variables. Dynamical per-formance depends on the effective construction of flux andangular positions for the decoupled control. An importantissue is operation at a very low speed (frequency) becausethe inching and creeping functions concerning the ac-curate positioning of the mill for maintenance and mill-lining changes. Chapter V (the next chapter) show how

vector control variables may help process control

III.POWERQUALITY ISSUES

Reliability, protection coordination, power factor andharmonics are the main issues to be faced at the very be-ginning of a project concerning new or retrofitting installa-tions. Cycloconverters inject a distorted current ID into thenetwork with superposition of harmonic and interharmonicscurrents components. For a 12-pulse configuration, a su-perposition of the harmonic components is given by:

{ }{ } { } { } )3(432

1

0010101 }613{}611{}6{

fff

kffkffkffIk

D

+++

++==

{fh +/- 6kf0 } is a term comprising the characteristic fre-

quency component fh and its lateral sidebands. f1 is the fun-damental current component of the network side (50 Hz), f0is the output frequency of the cycloconverter, and k is aninteger value k= 0,1. In addition, non-characteristic har-monics components f2, f3, f4 should also be considered,especially at low-damped networks with high non-linearloads where resonances may be excited. A more complex

problem arises with multiple cycloconverter-fed drives op-erating at different output frequencies. Fig. 9 depicts the

simplified electrical system of a modern concentrator plant

with 4 gearless motor drives. A careful study for the powerfactor and harmonic control had to be carried out at the de-sign stage.

A great number of operating conditions of the electricalsystem were taken into account because of the complexityof the productive system. Special procedures and softwaretools were needed [19].

General specifications included a minimum power fac-tor of 95% (typical) at the PCC and the compliance ofIEEE-Std.519-92 for different productive conditions.

Therefore, 4 units of harmonic filter modules were de-signed (see fig. 10). Each module has 4 steps in order tomeet the stringent specifications. A control system gives

the ON/OFF command for the different modules for coarseand fine voltage control of 23 kV buses.Fig. 11 shows the spectral impedance at the bus 3 with

one filter module. Branch filters tuned to 2nd, 3rd and 4thharmonic order with C-Type [18] were needed in order tomitigate the resonances at the medium voltage 23 kV buses.Branch filters of the high-pass type tuned to the 5th, 7th and11th harmonic orders were considered to control the charac-

Fig. 10, Harmonics Filter Module.

23 kV Main Busbar5

4

3

2

1

0

Frequency [Hz]

Impedance[Ohms]

50 150 250 350 450 550 650

23 kV Main Busbar5

4

3

2

1

0

Frequency [Hz]

Impedance[Ohms]

50 150 250 350 450 550 650

Fig. 11, Harmonic Impedance Z (f) at bus 3 with One Filter Module.

0.00%

0.50%

1.00%

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2.00%

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3.00%

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3 Phase Average Limite

Voltage Total Harmonic Distortion

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 .2 % 0 .4 % 0 .6 % 0 .8 % 0 .9 % 1 .1 % 1 .3 % 1 .5 % 1 .7 % 3 .1 %

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uen cy

Percentil 95


Minera Escondida Domeyko

Substation 220 kV Busbar

Phase IV Startup

23/10/2002 to 28/10/2002

Average = 1.21%

Std. Dev.= 0.28%

Limit = 1.50%

Perc. 95 = 1.71%

Fulfill = NO

140 MW Industrial Plant

220 kV Busbar

Total Harmonic Voltage Distortion


0.00%

0.50%

1.00%

1.50%

2.00%

2.50%

3.00%

3.50%

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0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 .2 % 0 .4 % 0 .6 % 0 .8 % 0 .9 % 1 .1 % 1 .3 % 1 .5 % 1 .7 % 3 .1 %

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uen cy

Percentil 95




Phase IV Startup

23/10/2002 to 28/10/2002

Average = 1.21%

Std. Dev.= 0.28%

Limit = 1.50%

Perc. 95 = 1.71%

Fulfill = NO


220 kV Busbar

0.00%

0.50%

1.00%

1.50%

2.00%

2.50%

3.00%

3.50%

22-10-02

23:59

23-10-02

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0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 .2 % 0 .4 % 0 .6 % 0 .8 % 0 .9 % 1 .1 % 1 .3 % 1 .5 % 1 .7 % 3 .1 %

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uen cy

Percentil 95




Phase IV Startup

23/10/2002 to 28/10/2002

Average = 1.21%

Std. Dev.= 0.28%

Limit = 1.50%

Perc. 95 = 1.71%

Fulfill = NO


220 kV Busbar

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 .2 % 0 .4 % 0 .6 % 0 .8 % 0 .9 % 1 .1 % 1 .3 % 1 .5 % 1 .7 % 3 .1 %

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uen cy

Percentil 95




Phase IV Startup

23/10/2002 to 28/10/2002

Average = 1.21%

Std. Dev.= 0.28%

Limit = 1.50%

Perc. 95 = 1.71%

Fulfill = NO


220 kV Busbar



Fig. 12, Power Quality assessment of an industrial power system plantwith 4 GMDs. Measurement of the Total Harmonic Distorsion at PCC220 kV.

0.00%

0.50%

1.00%

1.50%

2.00%

2.50%

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3 Phase Average Limit

PST Flicker

0

50

100

150

200

250

300

350

400

450

0 .1% 0.3% 0. 4% 0.5 % 0.6 % 0 .8% 0.9% 1.0% 1. 1% 2. 1%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uency

Pe rcentil 95

PST Flicker

Average = 0.6%

Std. Dev.= 0.1%

Limit = 1.0%

Perc. 95 = 0.9%

Fulfill = YES


220 kV Busbar

Pst Flicker

0.00%

0.50%

1.00%

1.50%

2.00%

2.50%

22-10-02

23:59

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PST Flicker

0

50

100

150

200

250

300

350

400

450

0 .1% 0.3% 0. 4% 0.5 % 0.6 % 0 .8% 0.9% 1.0% 1. 1% 2. 1%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uency

Pe rcentil 95

PST Flicker

Average = 0.6%

Std. Dev.= 0.1%

Limit = 1.0%

Perc. 95 = 0.9%

Fulfill = YES


220 kV Busbar

0.00%

0.50%

1.00%

1.50%

2.00%

2.50%

22-10-02

23:59

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PST Flicker

0

50

100

150

200

250

300

350

400

450

0 .1% 0.3% 0. 4% 0.5 % 0.6 % 0 .8% 0.9% 1.0% 1. 1% 2. 1%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uency

Pe rcentil 95

PST Flicker

Average = 0.6%

Std. Dev.= 0.1%

Limit = 1.0%

Perc. 95 = 0.9%

Fulfill = YES


220 kV Busbar

0

50

100

150

200

250

300

350

400

450

0 .1% 0.3% 0. 4% 0.5 % 0.6 % 0 .8% 0.9% 1.0% 1. 1% 2. 1%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

A cum . Freq uency

Pe rcentil 95

PST Flicker

Average = 0.6%

Std. Dev.= 0.1%

Limit = 1.0%

Perc. 95 = 0.9%

Fulfill = YES


220 kV Busbar

Pst Flicker

Fig. 13, Power Quality assessment of an industrial power system plantwith 4 GMDs. Measurement of the Flicker PST at PCC 220 k.


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teristic injected harmonic spectrum of cycloconverters in-cluding the interharmonics.

Fig.12 depicts the measurements of harmonic distortionat PCC 220 kV and fig. 13 shows the measured flicker dur-ing the power quality assessment of the installation duringsetting-up and commissioning. Recommendations for im-

proving the performance were made.

IV.MECHANICAL AND MECHATRONICS ISSUES

The trend in mineral processing plants is to employ lar-ger mills: SAG mills and recently ball mills, in order tohave economy of scale. Nowadays, mills with diameters 34feet or greater with powers 10 MW or higher, have beendesigned with gearless drives. Some major problems affect-ing availability have been observed due to different causesconcerning new problems imposed by size scaling or sys-tem compatibility:

a) Stator deformations and foundation issues due to

huge short circuit forces. This may happen when the cyclo-converter is regenerating and a network disturbance resultsin a commutation failure or by unstable reclosing as de-scribed in [13], [14], [15]. Right coordination of electricalprotections and development of new intelligent devicesmay be considered.

b) Isolation failure of stator windings due to wet mois-ture. By wet grinding, water splashing may contaminate thewindings if seals are imperfect. Development of intelligentisolation surveillance may be considered.

c) Vibrations and deflections due to asymmetrical airgap [20], [21]. Bigger machines are distributed systems

with several degrees of freedom. Electromagnetic linearand non-linear excitation forces may be produced underfailure conditions. Deformation modes and forces distribu-tion along the poles of the rotor should be carefully ana-lyzed during design stage, in order to ensure global systemcompatibility without dangerous airgap fluctuations and

other non-desirable behavior.

V. PROCESS AND KNOWLEDGE BASED INSTRUMENTATION

Availability and safety together with process stabilityand optimization are the main issues for ensuring produc-tivity and costs in mineral processing facilities. Non-scheduled maintenance and process instabilities must bemanaged for reducing risks and losses. Main causes fornon-scheduled maintenance and shutdowns are broken lin-ers inside a mill. Fig.14 depicts a new instrument calledImpactmeter for assessing the impacts behavior inside theSAG mill (patent pending). The goal is the early detection

of harmful impacts on the mill liners based in the measure-ment of acoustical behavior around the mill in order to con-trol the operating variables to reduce the risk of broken lin-ers. Fig. 15 presents the measurement of a typical acousti-cal signal from one sensor during 1.2 [s]. A DSP-based pat-tern recognition scheme is employed for identifying harm-ful impacts.

The load filling of the SAG mill is a critical variable.Commonly, the bearing pressure signals (weight measure-ments) are used for forecasting the filling of the mill, butglobal density is very dependant on size distribution ofminerals and their viscosity. In addition, mechanical valvesand piping devices are prone to failures and losses ofcalibration.

Impactmeter

Plant Control

SensorsRight side

SensorsLeft side

Controlvariable

Impacts Level

Fig. 14, General scheme of the Impactmeter.

Some possible Impacts

1Volt

-1Volt

Time

Fig. 15, Acoustical signal from one sensor, durationof 1.2 seconds.

Millgeometric

centerMass center ofthe load

Weight of the load

MonSAG

Plant Control

Ring Motor

Electricalvariables

ControlvariablesLoad filling

Fig. 16, General scheme of the MONSAG system.

Fig. 17, Improvement in the process stabilization employing MONSAGinstead the bearing pressure.


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bration.The Fig.16 shows a new monitoring system

(MONSAG) intended for indirect measurement of the millfilling based on electrical variables of the motor vector-control and process variables, without dependence on me-chanical variables, complementing the typical bearing pres-sure measurement (patent pending). Fig.17 depicts a betterprocess quality gained at the measurement of the mill fill-ing of a SAG mill. Production average was improved byabout 2.5% and specific energy consumption (kWh/Ton)was improved by 2.4%.

VI.COMMENTS AND CONCLUSIONS

Main current issues of cycloconverter-fed gearlessdrives for grinding applications were presented, highlight-ing: a) Complex nature of the process with large units, b)Reliability and availability, c) Operation within weak net-works, d) Power quality and harmonics control, e) Processand knowledge-based instrumentation, f) Mechanical andmechatronics aspects.

The challenge of improving safety, reliability and pro-ductivity, regarding the application of complex and largergrinding mills, requires the integration of multidisciplinarywork, with aspects of industrial electronics, informationtechnologies, process-, mechanical- and mechatronics engi-neering.

Further work must be done for identification and con-trol of critical variables in order to improve robustnessagainst perturbations to the whole grinding process control.Network disturbances, power quality, cycloconverter con-trol, machine control, airgap surveillance, grinding operat-ing variables and excitation of non-desired modes of oscil-lations must be managed, where new methods and instru-

mentation systems based on process knowledge play a sig-nificant role.

VII.REFERENCES

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[13] C.V. Thio, J.B. Davies, K.L. Kent, Commutation failures in HVDCTransmission Systems, IEEE Trans. on Power Delivery, Vol.11,No.2, April, 1996, pp.946-957.

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[15] High Speed reclosing system, GE Web site:http://pm.geindustrial.com/FAQ/Documents/750_760/GER-3224.pdf

[16] K. S. Smith, L. Ran, Input current Harmonic Analysis of Pseudo 12-Pulse 3-Phase to 6-Phase Cycloconverters, IEEE Trans. on PowerElectronics, Vol. 11, No.4, July, 1996, pp. 629-640.

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