LARGE CAGE INDUCTION MOTORS FOR OFFSHORE MACHINERY DRIVE APPLICATIONSCopyright Material IEEEPaper No PCIC-2006-35

T A GallantLaurence, Scott & Electromotors Ltd, Norwich, UK.

Abstract - Offshore electrical power systems are ofnecessity very compact with low fault levels. Despite thislimitation, modern production increasingly requires highpower electric motors as a clean and cost effectivealternative to other forms of prime mover. In order to startsuch motors on a small power system necessitatesequipment that is reliable, compact, cost-effective anddesigned with the system in mind.This paper sets out the main design criteria that need to besatisfied to meet these difficult requirements, as an aid to thespecifiers and operators of such equipment. In particular thefollowing topics will be covered:* Offshore specification requirements, including NEMA,

EEMUA, API and IEC.* Power system considerations, including configuration

and modelling.* Practical solutions, including transformer assistance,

star-delta starting, soft-starting, special cage design &the comparative costs of different solutions.

* The design of cage induction motors for low startingcurrent applications <2.8 times Full Load Current,including the prediction of performance, comparison withtest and experience in service.

* The hazardous environment implications of low startingcurrent cage induction motor applications, includingcage temperatures, purging & pressurising and motorauxiliaries.

* Experience in major offshore applications of the lowstarting current induction motors enables potentialpitfalls to be identified and avoided.

1. INTRODUCTION

The ever-increasing world demand for oil and gas supplieshas led to the exploration of the sea for such resources. Thisrequires offshore drilling or exploration rigs that can be fixedor mobile, for example in the form of Floating Production,Storage and Offloading vessels (FPSO). When oil isdiscovered or produced there is normally a significantquantity of gas available. This can be utilised as fuel forturbines on board the rig or vessel. In the early stages ofsuch exploration any surplus gas was often wasted by flaringoff. This was the safest solution in many cases, but it iswidely recognised that such a valuable commodity shouldpreferably be processed in the most efficient manner. Indeedmany countries now have regulations that control thewasteful use of such products in their coastal waters andrestrict emissions of certain gases.

K M AndrewsFlakt Woods Ltd Colchester, UK.

Gas turbines are used to drive on board electrical generatorsor other machinery. There is no question that local electricalgeneration by this means is likely to be the only practicalsolution, since land based power supplies are not normallyfeasible even with under sea cable links for example. Theuse of direct turbine drives as the prime mover for suchmachinery as gas compressors or pumps is therefore alsovery common, but increasingly there is an incentive toreplace these turbines with electric motors. The reasons forthis are mainly the relatively low purchase cost, lowmaintenance and high reliability associated with an electricmotor. For the largest drives (above about 20MW) asynchronous motor would be a practical solution. Forultimate simplicity, reliability and low maintenance costshowever, the cage induction motor has many advantages,even if a gearbox is required to attain the high output speedsassociated with some gas compressors. This paper thereforeconcentrates mainly on the design aspects of large HV. orMV cage induction motors, with particular reference to theissues connected with starting such machines."Large" in the context of this paper covers machine outputsof between 4 and 20MW, although many of the issues thatare raised will have relevance outside this range.

"HV" refers to voltages up to 15kV, however experience inthis field at higher outputs will predominately be in the range1 1 to 13.8kV at 50 or 60Hz respectively as these are typicalsystem voltages both for onshore and offshore systems. Itshould be recognised that some useful work is currentlybeing carried out at even higher voltages, using speciallywound machines of around 40MW [1], although for the rangeup to 20MW under discussion it seems likely that theconventional approach will continue to predominate.

Since the production rigs or vessels are processing potentiallyflammable or explosive material in both liquid and gaseousform, the equipment must be operated in accordance with thehazardous area classification and all motors will normally becertified for use in such areas by an independent authority.There will be restrictions on the surface or internaltemperatures that can be attained during normal operation orduring starting and these will be an important consideration bythe motor manufacturer at the design stage.

II. STARTING

An induction motor at start has a low impedance, as themagnetic flux developed by the stator winding currents cannotpenetrate the stationary cage, because the passing speed ofthe flux induces large counteracting currents in the cage itself.

1-4244-0559-9/06/$20.00 02006 IEEE 1

Impedances

Fig. 1,Equivalent Circuit of a Squirrel Cage Induction Motor,

showing how Impedance is Small at Start.

Fig. 1 demonstrates this on the equivalent circuit of aninduction motor. At start the large magnetising reactance Xm iseffectively short-circuited by the reflected low rotor resistanceR2/s and the starting current is limited only by the statorresistance, R1 and leakage reactance Xi.A consequence of the large rotor starting current is that thecage temperature rises rapidly and a large starting torque isdeveloped on the rotor.With a small power system, the system impedance is high, tokeep the short circuit current low. When an induction motorstarts on such a system, its large starting current can drawdown the terminal voltage to such a reduced level that startingis delayed. In addition, other machinery connected to thepower system is affected. It is not possible to devise generatorAutomatic Voltage Regulators (A.V.R.) to counteract all theseeffects, so large induction motors need to have a system ofassisted-start, or they must be designed with a low intrinsicstarting current.

III. STANDARDS & APPLICATIONS

A typical cage induction motor having standard performancewill achieve a starting current in the order of 6 times FullLoad Current. Where lower levels are required, theperformance of the machine might be worse in terms ofefficiency and/or power factor. This would give an unrealisticcontractual limit where the times Full Load Current seemslow but in terms of actual current in amperes is not as low aswould at first appear. It should be stressed that it is the linecurrent at start, which is important in this case.This is recognised in US standards such as NEMA MG1, byhaving a table of code letters that can be applied, each ofwhich gives a range of starting kVA per HP. By this means, itis not possible to produce an apparently attractive times FullLoad Current at start by offering a poor running performance.API specifications refer to motor starting and runningconditions and invoke NEMA MG1.European standards such as EN 60034-12 (formerly IEC34-12) also tabulate starting current limits in a similar way,except that these are in terms of kVA per kW, although thisstandard applies to LV machines of limited output. In eitherevent by multiplying the tabulated value by the per unit

efficiency and power factor of the machine in question, acomparable starting current in terms of times Full LoadCurrent is arrived at.For the example of a 14210 HP machine cited later in thispaper the kVA/HP gave a letter designation, based on NEMAMG1, of'A'.Some user-related standards also influence starting criteria.Typical of these is EEMUA (formerly OCMA) Elec 1. Suchstandards often require a torque margin, usually 10%, to bemaintained during the complete starting cycle when thesupply voltage is between 100% and 80% of normal. This isan onerous requirement when considering a means ofreducing the starting current to the lowest possible level. Theassumed driven torque characteristic is usually square law,which is less onerous than some compressors can producein normal operation. A more practical approach might thenbe to take into account the voltage recovery that will usuallyoccur due to operation of the generator A.V.R.. This restoresthe supply voltage to normal within a few seconds and canenable a large motor to continue accelerating, even againstan onerous torque characteristic, with consequentiallyextended starting times. There still will be a need to limit theinitial voltage dip, usually to around 15 - 20%, and a lowcurrent motor design can achieve this.Close co-operation between the motor vendor and user isimportant in such cases. There are usually economicreasons for producing optimum starting conditions, such asthe need to avoid or minimise flare-off of waste gas or toreduce the amount of starting equipment, which adds weight,space and cost in a restricted offshore environment.Another influence on the starting condition will be the need toadhere to the requirements for operation in hazardous areas.International Standards prescribe temperature limitations forelectrical equipment in such an environment and this mayhave to be applied to any part of the equipment including insome cases the rotor cage. The current relevant USstandard is NEC505 (European IEC 60079.) It is thereforeimportant for the motor manufacturer to have designprograms that can accurately predict temperatures in thisand other areas under all operating conditions.

IV. ASSOCIATED STARTING EQUIPMENT

Table 1 shows the typical equipment required to start aninduction motor of >5000 hp (4 MW), giving an approximateindication of the relative costs of the equipment including themotor. Although the popular option of star-delta starting hasbeen included for comparison purposes, it is not reallyapplicable to the large machines under discussion. Theseare predominately MV and HV, generally with star connectedstators. An autotransformer, (with closed circuit transition), isthe more practical alternative. It can give a reduced linecurrent without such significant detriment to the motoraccelerating torque. Whilst a reactor will reduce inrushcurrent, it has a much more adverse effect upon motortorque than the autotransformer. Torque can also be aproblem with soft start equipment that only reduces thevoltage. The variable voltage and frequency method is muchbetter in this respect but attracts a cost penalty, especially asa full variable speed drive.

2

V. OFFSHORE POWER SYSTEMS

2:$ t-o

M it

i Io10L s

o>

\ rii t Ne

Fig. 2Example of a Typical Small Offshore Power System, taken

from [11].

Offshore power systems typically consist of three or moregenerating sets feeding a segregated busbar system at 13.8or 11 kV, 60 Hz, as shown in Fig. 2. HV/LV transformers stepdown power to the voltage required for the majority ofauxiliaries and the hotel load, however, large pumps andcompressors will be fed directly from the HV busbar throughcircuit breakers and control equipment in that switchboard.Therefore any additional control equipment, such as thatdescribed above, will need to be incorporated into thatswitchboard, incurring an additional weight, space and costburden.

VI. MOTOR DESIGN & PERFORMANCE

Thirty years ago, induction motors with particular startingrequirements were designed with double cages, see Schwarz[2]. These transformed into cages with bars with a narrowthroat design, such as are described by Schwarz et al in [3]and Parry et al [9]. In recent years designers, such as Pestleet al [4], have discovered that by using deep narrow, parallel-sided bars, with the appropriate resistivity, with modificationsto existing induction motor design programs, it is possible toachieve most of the needs for low starting current. This allowsthe designer to avoid the use of shaped bars, whichexperience reduced fatigue performance during starting,because of high frequency torque pulsation experiencedduring that period, see Lloyd et al [5] and Fig. 3.

VIl. ELECTROMAGNETIC ANALYSIS

The effect on motor performance of different shapes andmaterials of rotor bars has been investigated numerically,particularly using finite elements. This has been done in muchmore detail than is possible with classical induction motordesign programs. See Williamson et al, [6], Siyambalapitiya etal, [7] and Williamson et al [8]. Ref 6 shows how the startingcurrent for a motor with a complex shaped bar can bepredicted and Refs 7 & 8 show how the temperature rise at thetop of the bars can be predicted under starting conditions.

Fig. 3Example of Prediction of Rotor Bar and End Ring

Temperature Rises during Start on an Induction Motor for aHazardous Area. Taken from [7]

The latter will be particularly important for motors destinedfor a hazardous environment, such as offshore, where rotorcage temperatures must be restrained during starting. Anexample of the prediction of rotor cage bar and end ringtemperature rise, taken from [7], is shown in Fig. 3, wheredesign programs give a very close approximation tomeasurement.Parry et al, [9], gave a description of the steps necessary tosatisfy such conditions for a motor with a shaped cage bar,using existing induction motor design programs.These references show that whilst the performance ofparticular rotor designs can be predicted by designprograms, more detailed analysis can be undertaken if theapplication or the duty requires.

Vil. POWER SYSTEM MODELLING

Having designed a motor with an adequate starting torque andlow enough starting current for the application, it then remainsto confirm that its starting can be accommodated by the powersystem. Again analysis has shown how that can be achievedand Figures 4 and 5 are the results of analysis on the

U--

I -

LW-

'2! X

32 4 6 6.4

Fig. 4Typical Starting Torque Simulation Showing Dynamic Torque

Pulsation at Start.

3

rI

is

II

sl.ki.L-d.

Ri1,(117

jLt

.SG)

I

-L11 "Iim1-i,l-,L-a

1- --.

;TI , I

SG

A rjw"to

0 a

power system shown in Fig. 2. Fig. 4 shows clearly the largedynamic pulsation in torque during starting as the rotoraccelerates. Figure 5 shows the

Fig. 5Typical Starting Current Simulation For Low Starting Current

cage Induction Motor

consequential pulsation in current. However, more importantlyfor this paper, it shows the predicted starting current ofapproximately 3.8 times Full Load Current. The simulateddynamic current is shown to be the same as predicted by thedesign program but it experiences a substantial drop as themotor start current draws down the generator supply voltage.This is corrected by the system A.V.R.

IX. TYPICAL EXAMPLE OF A LOW STARTINGCURRENT PROJECT

An example of the practical application of these techniqueswas the machine shown in Figure 6, a large 14210 hp, 11kV, 60 Hz motor, installed on a FPSO vessel driving anHP/Export Gas Compressor. The torque-speed, current-timecharacteristics in Figs. 7 & 8 show how the designperformance achieved, using the methods and designprograms described above, resulted in a starting current forthis machine of 2.75 times Full Load Current. The cage ofthis machine was manufactured with high performancecopper alloys using modern brazing techniques to give long-term fatigue life. This gives the motor the potential for manystarts and service experience with this machine shows that ithas exceeded the operator's expectations.

X. POTENTIAL PITFALLS

It should also be recognised that the overload capacity (pullout or breakdown torque) of a machine having a very lowstarting current will be lower than that of a motor with anunrestricted value in this respect. This does not matter if theforegoing recommendation is applied but the peak torquelevel might fall below that invoked in some national or userspecific standards. This could preclude future opportunitiesto increase the rating of a motor at a later date [13], or toinstall it in a different application where more torque isrequired. Having said this, many instances of subsequent re-rating of low starting current machines have in practice beeninvoked with no reported problems.

A machine with a very restricted starting current maynecessitate a slightly larger frame size with its associatedweight and cost, but this is, by experience, more than offsetby the potential benefits and the weight/cost penalty of mostother solutions.

The design might be limited by thermal considerations, noton the stator winding as would normally be the case, but bythe rotor with its need for special cage designs. Although thisis not a problem for a reputable motor manufacturer, someconsideration will have to be made in order to assess thetemperature for example by means of change in slip. All thispresupposes that the machine will operate in a hazardousenvironment being typically Exe Exp or even Eexe.

There could be a small penalty in terms of motor efficiency insome cases, but this reduction would be in the order of 0.5%or less. Bearing in mind that the actual efficiency of a largeinduction motor would be in the region of 97 to 98%, wherethe gas turbine alternative might be in the order of 35% thisis a small price to pay. The variable speed induction motordrive option will be affected by the losses in inverter andtransformers. This could be between 2 and 5%, negating anyefficiency gains with respect to the motor alone. A poorlydesigned inverter can induce additional losses in the motorthus detracting from overall drive efficiency.

Care should be taken with manufacturer's claims that appearto give a relatively low times FLC when in reality they arequoting a poor power factor. Looking at kVA/kW ratios forcomparison as indicated earlier is sound advice or checkingthe actual current in Amperes because that is what reallymatters.

Clearly the importance of close collaboration between themotor vendor and the driven equipment supplier cannot beoveremphasised, particularly when approaching the lowestpractical starting currents.

4

Xi. CONCLUSIONS

This paper concludes that:* Offshore power systems have low fault levels and yet need

to incorporate high power induction motors.* US & European standards provide for low starting current

induction motors, but this requirement can produce a poortorque and power factor.

* Starting current can be reduced by using a conventionalmotor and starting equipment. In general a standardmotor with starting equipment will cost more and take upmore space than a motor designed with a low startingcurrent.

* Powerful squirrel cage induction motors, > 20000HP (15MW) up to 13.8 kV, can be designed with very lowstarting currents to satisfy the requirements of large driveson low fault-level power systems in offshore installations,an example has been shown. Starting currents of lessthan 2.8 times Full Load Current are possible for suchmachines without prejudicing reliability or performance.

* Such motors need to be designed with close collaborationbetween the user and motor manufacturer to achieveoptimum performance.

* The performance of such machines can be modelledagainst the designed load in a modern power systemprogram, to demonstrate to prospective clients theviability and effectiveness of a proposed design.

* The cages of these machines can be designed by modernfinite element methods and manufactured with modernmethods to give long-term fatigue life.

* The machine cages can also be designed to meet thetemperature rise requirements of hazardous environmentcodes.

STANDARDS1. NEMA MG1-1998, Motors and Generators, published

by the National Electrical Manufacturers Association,USA.

2. EN 60034-12:1995, Rotating Electrical Machines. Part12 Starting Performance of Single-Speed Three PhaseCage Induction Motors for Voltages up to and Including690 V, 50 Hz. Formerly IEC 34-12.

3. EEMUA Publication No 132, 1988, Specification forThree Phase Induction Motors, published by theEngineering Equipment and Material Users Association,UK. Formerly OCMA Elec 1.

4. API Standard 541, Third Edition, April 1995, Form-Wound Squirrel Cage Induction Motors-250 Horsepower

5. and Larger, published by the American PetroleumInstitute, USA.

6. IEC 60079-0, 2004, Electrical Apparatus for ExplosiveAtmospheres. General Requirements.

REFERENCES[1] Lamell J 0, Trumbo T, Nestli T F "Offshore Platform

Powered with New Electrical Motor Drive System" IEEEPCIC2005-29.

[3] Bone JCH, Schwarz KK, Large ac motors, Proc IEE,V120, 1973, pp1111-1132.

[4] Pestle JP, Gallant TA, The modern design of reliablesquirrel cage induction motor rotors, IEE Conf, Pubn 282,1987, pp140-144.

[5] Lloyd MR, Smith JR, Buckley GW, The prediction oftorque and current requirements for large induction motordrives, IEE Conf, Pubn 170, 1978, pp 81-90.

[6] Williamson S, Robinson MJ, Lim LH, Finite elementmodels for induction motor analysis, Trans IEEE, IAS,1989, V256.

[7] Siyambalapitiya DJT, McLaren PG, Tavner PJ, Transientthermal characteristics of induction motor rotor cages,Trans IEEE, PES, 1988, pp223-228.

[8] Williamson S, Lloyd M R, Cage rotor heating at standstill,Proc IEE, V134, PtB, No 6, 1987, pp 325-332.

[9] Parry GE, Middlemiss JJ, Design and construction toachieve low starting currents on hazardous area motors.IEE Colloquium, Feb 1999, Digest 1997/057, pp 5/1-5/4.

[10]Smith JR, Response Analysis of AC Electrical Machines,Research Studies Press, Taunton, UK, 1990.

[11]Smith JR, Chen MJ, Three Phase Electrical MachineSystems, Research Studies Press, Taunton, UK, 1993.

[12] Gallant TA, Tavner PJ Low Starting Current CageInduction Motors for the Offshore Petrochemical IndustryIEE PEMD 2002.

[13] Tavner PJ Gallant TA Improving The Ratingof Large Electrical Machines IEE PEMD 2004.

14210 HP (10.6MW) 4P 11kV 60Hz Cage Induction MotorFor Offshore Gas Compressor Application.

[2] Schwarz KK, Bone JCH, Design of reliable squirrel cages,LSE Engineering Bulletin, Vol 9, No 3,1967, p1.

5

TORQUE0SPEED CHIARACTEMSTP

ly HPEXOPORT COMPUMMO MOWTOR

ICURRENTMM]ME CUANRACTERFST1CS

'10WMMv IIKV 3PH 4*ZIMM 2KNM.!y HP EKP RT r-QqWfWR OTIOR

Fig 7.

Predicted Torque - Speed Characteristics of a14210HP(10.6MW) Cage Induction Motor Driving a Gas

Compressor.

Fig 8.

Predicted Current -Time Characteristic of a 1421 0HP(10.6MW) Cage Induction motor.

6

Type of Start Motor Switchgear Control Gear Typical Costs

Direct on Line Normal design One contactor None 100%Star-Delta (LV) Two contactors Timing only 150%

Reactor Normal design Two contactors Reactor & timing 160%Auto Transformer Normal design Two or three Auto 175%

contactors Transformer & timingSoft starter Normal design One contactor Soft starter 200%Variable Speed Starter Special design One contactor Complex 250%Low start current motor, Special design One contactor None 125%Direct on Line

Table 1 Typical Comparative Cost For Starting Large Cage Induction Motors

XII. VITA

Terry Gallant is a Chartered Engineer and aMember of the UK Institution of Electrical Engineers(IEE) He has written a number of technical paperson various aspects of machine design and iscurrently employed by Laurence, Scott &Electromotors Ltd Norwich UK as Chief ElectricalEngineer, having also held a number of other seniormanagement positions in all aspects of machinedesign and operations. He has been responsible forthe design and development of both variable speedac motors and large high voltage cage inductionmotors, particularly those intended for operation inhazardous areas.

Ken Andrews is also a Chartered Engineer andFellow of the UK Institution of Mechanical Engineers(I Mech.E). He is currently Engineering Director,Flakt Woods Ltd., Colchester UK and waspreviously Head of Engineering, LSE, Norwich overthe past five years. He has considerable experiencein the mechanical design and development ofmachines including dynamometers, hydraulic liftingequipment and more recently, electric motors andgenerators.

7