Availability and Reliability of Siemens' Gearless Drives

www.siemens.com/mining

Mining Technologies

Availability and Reliability of Siemens’ Gearless DrivesKurt Tischler and Todd Kennedy

111111_Reprint_Mining_A4_02RZ.indd 2 02.01.12 16:02

SAG 2011;Fifth International Conference about Autogenous and

Semiautogenous Grinding

September 25 to 28, 2011; Vancouver, British Columbia, Canada Page 1 of 11

Availability and Reliability of Siemens’ Gearless Drives

Kurt Tischler1 and Todd Kennedy2

1Siemens AG Schuhstr. 60

Erlangen, Germany 91052

2Siemens Industry Inc. 100 Technology Drive Alpharetta, GA 30005

USA

.




AVAILABILITY AND RELIABILITY OF SIEMENS’ GEARLESS DRIVES

ABSTRACT

This paper presents the field experience of Siemens with regard to Gearless Drives in the mining industry … We consider the elapsed production hours of Siemens Gearless Drives to be extensive enough to provide a reliable estimation of availability calculated on the basis of data representing repair time, downtime, and operating time. The users of Siemens Gearless Drives confirm an availability of more than 98% after exclusion of some major issues. The paper addresses all of these major issues of Siemens Gearless Drives, describes successfully applied corrective measures, explains the influences on the design of the drives, and lists the downtimes associated with each of these issues.

KEYWORDS

Gearless Drive, downtime, availability,

INTRODUCTION

The first Siemens Gearless Drive in mining began operation in 1980 and was installed on an iron ore grinding mill. As of September 2011, 40 Siemens Gearless Drives will have accumulated 136,000 machine days of operating time. According to pure rules of statistical analysis, this accumulated operating time may be too low to be used for a sample of field data to provide precise statistical values of availability, however we consider it large enough to provide indicative values of worthwhile consideration. This paper deduces the availability statistics of Siemens Gearless Drives in the mining industry from information received from end users and factors in all major issues that Siemens has experienced with our Gearless Mill drives. This paper does not consider Gearless Drives in other industries such as the cement industry. The cement plants, especially in the northern hemisphere, interrupt production for several months in the winter. This lengthy downtime is used to maintain the complete plant including the mills, their motors and their drives. The mining industry has more stringent and higher production requirements. In the mining industry, production must run 24 hours per day, 365 days a year.

Availability

We may define availability of a machine in simple terms. Essentially it is the ratio of the time a machine is operating over the time a machine is expected to be available and ready for operation. With regard to grinding mills many users factor in all unavailable hours of the mill including planned hours for maintenance. Under that scenario the resulting equation, uses a denominator of 365 days for calculating the availability ratio for one year. Other users reduce the denominator by the number of planned maintenance hours for the mill, lowering the base of 365 days, and use the result as the denominator in the availability equation.

When the mill is out of operation for maintenance, the Gearless Drive is off also. Accordingly we consider planned downtime of a mill as time the Gearless Drive is not expected to be available and ready to operate. Scheduled maintenance tasks for the Gearless Drive can be accommodated within the planned downtime of the mill. Therefore additional downtime for the purpose of drive maintenance is not required and users plan maintenance of the Gearless Drive into intervals of planned downtime for other mill maintenance activities. Experience indicates that the denominator for the availability equation then varies between 345 and 355 days per year.




Field data

When we ask the users of our Gearless Drives about availability relevant to their installation, we normally hear figures between 92% and 96%. When we ask about the circumstances that lead to down time we find it is low for electrical equipment and that significant contributors are apparatus such as feed chutes, liners, grates and screens. However, these mechanical components are not components of the Gearless Drive.

Many operators are not immediately aware of the failures and corresponding downtime of their Gearless Drive. After checking the data, they indicate an electrical availability with figures around 99%, and in all cases better than 98%. These values represent downtimes of between three and seven days per year.

Often when we ask our clients for exact statistics the data is not made available to us either because they consider such data as company confidential, the information is not available, or there are other reasons for withholding these statistics. As a result we can base our analysis only on the unofficial information made available to us and test our results against the client reported 99% availability mentioned earlier.

Design Enhancements for Siemens Gearless Drives

The above figures do not include downtimes, resulting from major issues encountered with Siemens Gearless Drives. We address all of the lessons learned and corrective actions taken in the following cases and calculate their impact on the production availability of this technology in each of the cases.

Resonance vibration on a 40ft Gearless Drive Motor

This issue is the most widely known in the mining industry. There are several publications about these events and the impact they have had on Gearless Drive Motor design. Chris Meimaris, Bill Lai and Leigh Cox published in SAG 2001(1) the root cause analysis and the development of the solution to eliminating the motor vibrations on Cadia’s 40ft Gearless Drive Mill. Horst Kummlee and Peter Meinke(2) published the impacts on design tools and ultimately the design of Siemens Gearless Drive Motors.

How long was the downtime, caused by this vibration issue? R. Dunne, S.Morell, G. Lane, W.Valery and S. Hart(3) state in SAG 2011 “Vibration and deflection in the SAG mill motor was a problem at start-up and for some months thereafter”. Cadia stated in a letter to Siemens that the availability of the Gearless Drive was 98.5% during that time. At the end of 1999 Cadia and Siemens installed the final solution to the problem, a “strong back” on the stator housing designed to stiffen of the motor frame. With the strong back installed, the Gearless Drive no longer exhibits any vibration concerns. The installation of the strong back required a downtime of seven days. During the installation of the strong back, Cadia’s contractor changed the mill liners. The required and planned change out of the mill liners required 5 days. The installation of the strong back then resulted in two additional days of downtime.

Please refer to picture 1.




Picture 1: Cadia’s stator with the strong back installed.

Cracked welds on rotor pole segment supports

Cadia’s rotor segments

In 2003 Cadia’s maintenance team detected cracks at support rib welds on a pole segment of their SAG-Mill. See picture 2 and picture 3.

Picture 2: Rotor pole segment support with cracked weld




Picture 3: Rotor pole segment with support ribs

The issue was investigated and temporarily repaired during a planned liner change. The engineering analysis and repair concept was developed by Siemens together with Cadia and EAnD. Cadia published a paper in the SAG-conference 2006 (4). Please find all details there.

The final repair was performed during a scheduled downtime of 15 days in June 2005 in conjunction with a planned liner change, which normally would have needed four days. These repairs resulted in additional downtime of one day for the investigation and temporary repair and additional downtime of eleven days for the final repair.

Rotor pole segment on a 38ft SAG-Mill in Chile

In 2005 Siemens the same fault was discovered on a 38ft SAG-Mill in Chile. This mill started operation in 2003 and its Gearless Drive is of the original design with regard to the rotor pole segments and their support structures. This Gearless Drive did not include the improvements Siemens began applying to all ring motors fabricated after the issue in Cadia 2003.

During the weekend of August 31, 2008 the 38ft SAG-Mill stopped several times and was successfully restarted each time. Finally, on September 1, at 4:48 a.m. the air gap supervision tripped the motor and did not permit a restart. The owner’s maintenance team inspected the air gap and rotor and discovered there was weld crack on a rotor pole segment support (Picture 4). Specialists from Siemens Germany arrived at site on September 3 and inspected the motor. They found another crack on the same segment as well as another segment with similarly conditions. They began the repair on September 4 and the mill was put back into production on September 8, 2008 at 9:10 a.m.

Cracked weld




Picture 4: Cracked weld at rotor pole support

The downtime for this unforeseen issue was 9 days for investigation and preliminary repair.

Failure analysis of the cracks at this weld

Upon discovery of the weld cracks on Cadia’s Gearless Drive motor, Siemens did not investigate the failure because the root cause of these fatigue cracks was previously established and known. In the first 18 months of its operation this Ring motor endured additional metal fatigue loads attributed to cyclic magnetic forces that were resulting from deflections of the stator. These stator deflections were more pronounced than those considered in the design calculations. As a consequence, in 2003 Siemens introduced a new design of the rotor segment designed to avoid any possibility of the experienced weld fatigue recurring in new motors

After discovering the cracks in the Chilean Ring motor, Siemens investigated the fatigue loads imposed on the concerned welds because there were no deflections of the stator that exceeded the design of that motor. During its lifetime the ring motor never had an air gap alarm or trip. The investigation used a Finite Element Model of the rotor, analyzing cyclic loads on the weld. With the same Finite Element Model, Siemens engineers verified rotor pole segment design integrity for the improved design.

Consequences for other operating installations

In September 2008 Siemens investigated the possibility of similar faults in all other operating Ring motors and found that all our motors, except for three, were not susceptible to this type of failure, . Siemens informed the users of those three installations and asked them to inspect their rotors. The users of two of the three installations found cracks in their rotor segments, one of those installations is located in Australia, and the other is in Indonesia.




Rotor pole segments of a SAG-Mill in Australia

The gearless driven SAG-Mill in Australia has operated since 1998. Siemens and the owner’s maintenance team inspected the rotor during planned plant maintenance in November 2008 and February 2009. They found cracks and implemented temporary repairs.. The preliminary repair consumed two days of additional downtime. Due to a failure on a mine winder, this plant suffered a downtime of several months from the end of 2009 to mid 2010. At the owner’s request, Siemens performed the permanent repairs on the rotor and a complete upgrade of the ring motor in early 2010. Due to the downtime of the mine winder, the repair of the ring motor did not cause additional downtime.

Rotor pole segments on a SAG-Mill motor in Indonesia

The gearless driven SAG-Mill in Indonesia has been in operation since 1997. Siemens and the owner’s maintenance team inspected the rotor during planned mill maintenance in April 2009. They found cracks and applied a temporary repair. This preliminary repair needed half a day of additional downtime.

This plant has scheduled the final repair under a different approach compared to the other plants. The concept is to distribute the repair over scheduled mill maintenance activities with the objective to reduce additional mill downtime by utilizing the planned mill downtime over several years. The schedule is:

June 2010: 3 days of downtime in parallel to a liner change: additional downtime : 0 days January 2011: 4 days of downtime in parallel to a liner change: additional downtime : 1 day January 2012: 5 days of downtime in parallel to a liner change: additional downtime : 2 days September 2012: 5 days of downtime in parallel to a liner change: additional downtime : 2 days January 2013: 4 days of downtime in parallel to a liner change: additional downtime : 1 day

In this way the additional downtime will sum up to only six days.

Final repair of the rotor pole segments in the Chilean plant

The owner decided to realize this repair during a scheduled downtime of the SAG-Mill in July 2009 coinciding with a scheduled liner change. The mill and Gearless Drive were stopped on June 30, 2009. After three days, the team had replaced two pole segments and had dismounted two additional pole segments. While subsequently rotating the Ring motor again, its protection system tripped the operation and switched the drive off due to earth faults in the stator windings and rotor pole coils. The team investigated the 15 mm air gap between stator and rotor and found debris, which damaged the winding insulation during rotation of the motor. (Picture 5)




Picture 5: Debris found in the ring motor

The debris consists of foreign material (not used in the motor construction) as well as motor parts that had been recently removed to enable the rotor repairs. To enable investigation of the damage inside the air gap, it was necessary to shift the stator. The simultaneous liner change could not be continued because the Ring motor was no longer available to inch the mill. Bolts that had been left in the motor instead of being removed from the machine during repairs to the rotor had become entangled in the rotating machine and subsequently cut into and damaged the coils of two rotor poles. Debris had damaged the surface of several stator windings thereby destroying their insulation system. (Picture 6)

Picture 6 Damage caused by debris in the air gap

The repair team finished the stator winding and field coil repair on July 29 and shifted the stator back to operating position and assembled it. After additional tests, the Gearless Drive was deemed ready for operation on August 1. This allowed the continuation of the rotor repair work and change out of the liners. After change out of a rotor segment on August 3, the start procedure was interrupted because the feeding switchgear failed suffering from an earth fault within the switchgear. The repair of the switchgear required five days and the Gearless Drive continued operation on August 8. On August 12, 2009, the liner change was finished and the rotor improvement work was then interrupted to start production Due to the lengthy downtime of the SAG-Mill, the owner decided to prioritize production over the continuation of the rotor repairs.

The downtime of this event sums up to 43 days, which includes the time for liner change of the SAG-mill. Considering four days for liner change, the additional downtime due to this event is 39 days. The pending repair of the other rotor segments is planned to be realized in a later date. Siemens estimates the downtime for the pending repair to 24 days minus four days for mill maintenance.




Flashover at slip ring connections

This issue occurred at a 12MW Siemens Gearless Drive on a 36 ft SAG-Mill in Chile that has been in operation since 1993. The owner normally does not contract Siemens for maintenance. On March 15, 2010 the Gearless Drive shut down due to over current protection after disturbances on the electrical power supply network, SIC, on March 14, 2010. The user tried to restart operation on March 16, at 2:45 am. The Gearless Drive interlocks prevented start up. On March 16, at 3:38 pm Siemens received notice of failure. Siemens motor and drive specialists arrived at site on March 17. The specialists inspected the motor, the cycloconverter, the excitation system and the controls of cycloconverter and excitation systems. Picture 7 records the observations made during the inspection.

Picture 7: Flash over damage

A flashover occurred between the bus bars that connect the field windings to the slip rings. Neither the stator nor its windings suffered any damage. No other electrical systems including the cycloconverter, excitation system etc. suffered any failures from the flashover or supply disturbance. The analysis shows the following failure sequence: - Supply network disturbances caused a short circuit to occur in the Gearless Drive power section. - The protection system reacted correctly and shut the drive off without damages. - The short-circuit current in the stator circuit induced an overvoltage in the rotor winding circuit. - The overvoltage protection of the excitation rectifier did not react, because the overvoltage was too low. - The low overvoltage caused a flash over at the rotor, due to the large accumulation of carbon dust on the bus support insulators.

The specialists finished the repair on March 20, 2010, five days after the detection of the failure which was four days after Siemens was informed.




Evaluation of availability for Siemens Gearless Drives

The following table 1 sums up the downtime resulting from equipment failure events.

Table 1

Event Down-time

Downtime for planned mill maintenance activities

Resulting downtime attributable to the gearless drive

Cadia vibrations installation of the strong back

7 days 5 days 2 days

Cadia, weld failures on the rotor pole segments Inspection and temporary repair

5 days 4 days 1 day

Cadia, weld failures on the rotor pole segments Final repair


USA failure of converter cooler

5 days 5 days

38ft Gearless Drive in Chile, weld failures on the rotor pole segments Inspection and temporary repair

9 days 9 days

38ft Gearless Drive in Chile, weld failures on the rotor pole segments First part of repair, including repair of failed welds


38ft Gearless Drive in Chile, weld failures on the rotor pole segments Final repair, yet to be scheduled

24 daysestimated

4 days 20 days

Australian SAG-Mill, weld failures on the rotor pole segments Inspection and temporary repair; Nov. 2008 & Feb. 2009

2 x 5 days

2 x 4 days 2 days

Indonesian SAG-Mill, weld failures on the rotor pole segments Inspection and temporary repair; April 2009

.5 days

Indonesian SAG-Mill, weld failures on the rotor pole segments Final repair, as scheduled above

6 days

12MW Gearless Drive in Chile, March 2010 Flashover at slipring connections

5 days 5 days

Sum of all major downtimes 100.5 days




All major events on Siemens’ Gearless Drives sum up to a total unplanned downtime of 100.5 days.

DISCUSSION

Impact on the availability of Siemens’ Gearless Drives

The statistics require a comparison of the sum of unplanned downtimes to the total expected or available operating time of all Siemens Gearless Drives in operation. As of September 2011, 40 Siemens Gearless Drives will have accumulated 136,000 machine days of operating time. The ratio of the 100.5 days of the discussed downtimes to the 136,000 machine days results in less than .1 %.

Our estimations then fully agree with the general statements from the users of Siemens Gearless Drives that indicate that they experience about 99% availability. The resulting .1%unplanned and lost availability is a relatively minor impact when all things are considered.

CONCLUSIONS

The high and substantiated availability of around 99% makes Siemens Gearless Drive Technology the most reliable mill drive technology on the market.

REFERENCES

(1) Meimaris, Chris & Lai, Bill & Cox, Leigh (2001). REMEDIAL DESIGN OF THE WORLD'S LARGEST SAG MILL GEARLESS DRIVE. SAG 2001

(2) Dr.-Ing. Horst Kuemmlee & Prof. Dr.-Ing. Peter Meinke. (2001). A MECHATRONIC SOLUTION DESIGN AND EXPERIENCE WITH LARGE GEARLESS MILL DRIVES, SAG 2001

(3) R. Dunne & S. Morrell & G. Lane & W. Valery & S. Hart, M. (2001). DESIGN OF THE 40 FOOT DIAMETER SAG MILL INSTALLED AT THE CADIA GOLD COPPER MINE. SAG 2001

(4) Gunn, Phil (2001). PROBLEM DEFINITION AND REPAIR OF THE ROTOR POLE STRUCTURE ON ONE OF THE WORLD’S LARGEST GEARLESS DRIVE SAG MILLS. SAG 2006

For further information contact:Siemens AG Industry SectorDrive Technologies DivisionSchuhstr. 60 91052 Erlangen, Deutschland

www.siemens.com/mining

Order No. E20001-A170-T195-X-7600

Printed in Germany |

GB 111111 WS 12110.5 | © 12.2011, Siemens AG

The information provided in this brochure contains merely general descriptions or characteristics of performance which in actual case of use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of contract.

All rights reserved. Subject to change without prior notice.

Authors:

Kurt Tischler Siemens AG Schuhstr. 60 91052 Erlangen, Germany

Todd Kennedy Siemens Industry Inc. 100 Technology Drive Alpharetta, GA 30005, USA

111111_Reprint_Mining_A4_02RZ.indd 1 02.01.12 16:02

Prinect PDF Report 3.1.096 - 1 - 02.01.2012 16:17:29

Dokument ÜbersichtDateiname: Siemens_Reprint_Mining_4c.pdfTitel: -Erstellt mit: Adobe InDesign CS4 (6.0.6)Anwendung: Adobe PDF Library 9.0Verfasser: -Erstellt am: 02.01.2012 16:02:06Geändert am: 02.01.2012 16:16:26Dateigröße: 623.2 KByte / 638163 ByteTrapped: NeinOutput Intent: -PDF/X Version: -PDF-Version: 1.4Anzahl Seiten: 16Medien-Rahmen: 232.82 x 319.82 mm / 226.58 x 313.58 mmEndformat-Rahmen: 210.00 x 297.00 mm

Zusammenfassung Fehler Warnung Repariert InfoDokument - - - -PDF/X - - - -Seiten - - - -Farben - 11 - -Schriften - - - -Bilder - 19 - -Inhalt - 1 - -

FarbenFarbraum: Kalibriertes RGB (3-13)

BilderAuflösung von Farbbildern 52 dpi ist unter 250 dpi (8)Auflösung von Farbbildern 147 dpi ist unter 250 dpi (11)Auflösung von Farbbildern 150 dpi ist unter 250 dpi (3-13)Auflösung von Farbbildern 159 dpi ist unter 250 dpi (6)Auflösung von Farbbildern 162 dpi ist unter 250 dpi (10)Auflösung von Farbbildern 186 dpi ist unter 250 dpi (10)Auflösung von Farbbildern 199 dpi ist unter 250 dpi (10)Auflösung von Farbbildern 200 dpi ist unter 250 dpi (6,10)

InhaltTransparenz ist vorhanden (1)

Sonstige InformationenFarbseparationen: 4

CMYK

Prinect PDF Report 3.1.096 - 2 - 02.01.2012 16:17:29

FarbräumeDeviceCMYK / DeviceGray / SeparationICCBased (CMM erforderlich)

Schriften: 7Helvetica TrueType (CID) / Identity-H / eingebettete UntergruppeSiemensSans-Bold Type1 / Custom / eingebettete UntergruppeSiemensSans-Roman Type1 / Custom / eingebettete UntergruppeSiemensSerif-Semibold Type1 / Custom / eingebettete UntergruppeTimesNewRomanPS-BoldMT TrueType / WinAnsi / eingebettete UntergruppeTimesNewRomanPS-ItalicMT TrueType / WinAnsi / eingebettete UntergruppeTimesNewRomanPSMT TrueType / WinAnsi / eingebettete Untergruppe

Availability and Reliability of Siemens' Gearless Drives

Documents

Transcript of Availability and Reliability of Siemens' Gearless Drives