Selection of Induction Motors f or Industrial Applications (photo by http://www.iecee.org)

http://electrical-engineering-portal.com/selection-of- induction-motors- for- industrial-applications-part-1 May 9, 2013

Selection of Induction Motors for Industrial Applications(part 1)

IntroductionAll types of industries are invariably required to install dif ferent types of electric motors as primemover for driving process equipment part icipat ing in their respect ive product ion line up. Thecont inuous process of technical development has resulted into availability of highly diversif iedtypes of electric motors.Hence, an utmost care should be exercised in select ion of most appropriate type of motorconsidering number of technical factors for each application, so that the motor wouldprovide desired and optimum performance.The characterist ics of motors vary widely with the nature of their applicat ion and the type ofduty they are expected to perform. For example, the applicat ions like constant speed, constanttorque, variable speed, cont inuous/intermit tent duty, steep/sudden starts, f requent start /stops,etc. should be taken into considerat ion carefully when deciding for the type of a motor for thatspecif ic applicat ion.Moreover, the motors are required to perform quite often under abnormal conditions duringtheir total service life.

Stator and Rotor Damages

In view of above, an incorrect selection of motor always lands the industrial buyer into all sortsof problems, including premature failure of the motor, causing severe product ion curtailments.Like one ment ioned above, a number of other factors and design features like weatherconditions, stringent system conditions, abnormal surroundings, hazardous area, dutycycle, motor efficiency, etc. should be considered while deciding the rat ing and subsequent lydrawing out the technical specif icat ions of the motor.

Abnormalconditions andeffectsThe usual abnormalconditionsencountered by themotors are givenbelow.

1. AbnormalSystemConditions

1. Voltage1. Undervoltage2. Overvoltage3. Unbalance in 3-phase4. Single phasing5. Voltage surges

2. Frequency1. Low frequency2. High frequency

2. Abnormal Operating conditions

1. Locked rotor or stalled rotor2. Reswitching/Frequent start-stops3. Momentary interrupt ion/Bus transfer4. Overloading5. Improper cable sizing

3. Environmental conditions

1. High/low ambienttemperature2. High alt itude

3. High humidity4. Corrosive atmosphere5. Hazardous atmosphere/surroundings6. Exposure to steam/salt -laden air/oil vapour

4. Mechanical problems

1. Seized bearings2. Incorrect alignment/foundat ion levelling3. Incorrect f ixing of coupling4. High vibrat ion mount ing5. External shock due to load

5. Condition at location

1. Poor vent ilat ion2. Dirt accumulat ion3. Exposure to direct sunlight

Though, above ment ioned abnormalit ies may prevail for short or long duration or maybe transient in nature, major impact of the listed abnormal condit ions is overheating of themotor along with one or several of the other ef fects as follows.Change in the motor performance characterist ics like drawl of more power andconsequent deteriorat ion in motor ef f iciency, etc.Increase in mechanical stresses leading to:

1. Shearing of shafts2. Damage to winding overhang3. Bearing failures4. Insulat ion failures

Increase in stator and rotor winding temperature leading to:1. Premature failure of stator or rotor insulat ion (For wound rotor motor)2. Increased f ire hazard3. Breakage of rotor bar and/or end ring (For squirrel cage motors)

All the motors encounter few or several of these abnormalities during the course of theirservice lives. Considerat ion of listed abnormal condit ions at design stage great ly helps tominimise the effects of abnormal conditions to maintain a consistent performance.

Design Considerations

Following are the most important design factors required to be considered when select ing amotor for any of the diversif ied industrial applicat ions.

Output in kW/HPThere are two principle limitations for selecting the motor output:

1. Mechanical limitation

The breakdown torque, which is the maximum torque that the motor can produce whenoperat ing without stalling. This is a crit ical design factor in motor applicat ions, particularly forthe motors subjected to occasional extreme load conditions.Another crit ical factor is the locked-rotor torque, which is the maximum torque that the motorcan produce during startup from steady-state condit ion, a crit ical design feature for conveyordrives.

2. Electrical limitation due to insulation provided on the motor windings

The electrical load on the motor can be imposed t ill the winding insulat ion is able to withstandthe prescribed temperature rise over an ambient for that part icular class of insulat ion. Life ofthe motor great ly depends on the temperature rise of the windings.Ant icipated life-span of the motor can be achieved provided it is operated at its rated outputwithout overloading and the prescribed prevent ive maintenance pract ices are religiouslyfollowed.

Speed of the MotorMost of the motors are directly coupled with the driven equipment where in the speed of themotor and the driven equipment will be same.In order to meet the speed of the driven equipment, the devices like gearbox, chains or beltsare introduced between motor and driven equipment. In this case, it may be necessary toprovide the rotor shaft suitable for its at tachment with the speed decreasing or increasingdevice and hence the specif icat ion should include such specif ic requirement.In case a variable speed drive is to be used for the speed variat ion, the motor shouldbe compat ible for this specif ic applicat ion. The standard motor may not provide desiredperformance when operated via variable speed drive.Cant see this video? Click here to watch it on Youtube.

Power Supply Voltage and Frequency VariationsVariat ions in the power supply parameters, i.e. voltage and frequency signif icant ly af fect overallperformance of the motor. As provided in IS:325-1996, the permissible voltage variation is 5to 10%, permissible frequency is 50Hz 3%, and permissible combined variation is 6 %.The effect of undervoltage is more serious than that of overvoltage.The higher torque, result ing f rom overvoltage, can handle a lit t le overload without undue

Large burned out induction motor

heat ing of the winding, but only for a short durat ion. Cont inuous operat ion with undervoltagecondit ion increases the current at the rate of about 20% for every 5% reduct ion in the supplyvoltage, increasing the rated copper loss.This results into heating and prolonged temperature rise, and f inally the burning of winding.During a motor start-up, the torque reduces by 10% for each 5% reduct ion in the supplyvoltage, causing more start ing current and consequent ly more rapid heat ing of thewinding.

Themotoroffers

reduced efficiency at either overvoltage or undervoltage. Power factor drops sharply withhigher voltage and improves with lower voltage. Even when motor is light ly loaded, over-voltagecause rise in current and temperature thus reducing the life of motor. The variat ion in f requencyby +5 % decreases the torque by about 10% and vice-versa at 5% frequency, the torqueincreases by about 10%.It is, therefore, of utmost importance to consider the combined ef fect of variat ion in voltageand frequency both when purchasing the motor.Unbalance in the supply voltage results into a current unbalance of 6 to 10 times thepercentage voltage unbalance. This in turn results into generat ion of negat ive sequencecurrents in the rotor causing its overheat ing and premature failure.It is therefore vital to specify the permissible limits of variat ions in the power supply parametersfor the motor in accordance with the requirement of the driven equipment. However, thepermissible limits should never be more than provided in the applicable Indian Standard IS:325-1996 (Reaff irmed in 2002).

Selection of Induction Motors f or Industrial Applications (part 2) - photo by TCD Systems

http://electrical-engineering-portal.com/selection-of- induction-motors- for- industrial-applications-part-2 May 9, 2013

Selection of Induction Motors for Industrial Applications(part 2)

Motor Ef f iciency

The new IEC 60034-30 motoref f iciency standard could have majorenergy-saving impact f or industrial

motors worldwide.

Though standard motors are now available with abetter ef f iciency, this factor (motor ef f icency) requiresdue at tent ion when making the select ion of the motorfor a specif ic applicat ion in view of substant ial quantumof power consumed by the motors in the industries.The motors running cont inuously should be as ef f icientas possible to reduce the power consumption.Improvement of even 1% in efficiency results intosaving of enormous quantum of power over a lifespan of the motor as well as the cost of power.For the drives to be in service round the clock, dueconsiderat ion should be given install the energyefficient motors having EFF 1 or EFF 2 class even atthe higher cost, as the premium paid in the form ofcapital investment will be paid backhand somely in theform of cost saving due to signif icant energy savingwhen the drive will be kept in cont inuous service.

Ambient TemperatureAs per normal standards, the motor output is given by the vendors based on 40C ambienttemperature.If ambient temperature is expected to be high for a longer durat ion, then the motor is requiredto be checked for its suitability to maintain the specif ied output at higher temperature, orotherwise, the deration factor is to be applied to know the actual ant icipated output at highertemperature.In order to maintain the motor output at higher temperature as per the power requirement ofdriven equipment, it may be necessary that the motor with a higher f rame size for the samerat ing is selected to avoid adverse effect of derating.

AltitudeThe standard motor outputs are specified by the manufacturers for site altitude up to 1000m.For alt itudes of more than 1000 m, the motor rat ingis required to be checked for its suitabilityto maintain the specif ied output, or otherwise the durat ion factor is to be applied to knowactual ant icipated output at higher alt itude. Criteria for the select ion of motor remains thesame as provided for higher ambient temperature.

Method of Starting and Number of StartsThe start ing performance of the motor depends

DOL starter, with enclosure, less overload,contains TeSys Model D contactor (Ratings:

4kW, 9A, AC3, 240V, less O/L)

The start ing performance of the motor dependson the method of start ing deployed, i.e.directon-line, star-delta, high resistance, autotransformer, variable frequency drive, etc.The direct-on-line start ing (DOL motor starter) isthe most common method adopted in which thestarting current is 6-7 times the rated full loadcurrent of the motor. For high start ing torque,the direct-on-line start ing is essent ial. If themotor driving a load requiring high start ing torqueis started using star-delta start ing, either thespeed may not pick-up af fect ing the motoraccelerat ion, or may take a very long t ime tocome up to its rated speed under loadedcondit ion inducing severe electrical andmechanical stresses respect ively in the windingand core.Where the starting torque requirement is notso critical, the star-delta starting or any otherreduced voltage method of start ing is used.Where the start ing with very heavy load, such as with hoist or crane drives, and speed controlover a wide range is required, it is advantageous to consider the slip-ring type (or wound rotortype) motor with a drum controller starter or resistance starter.As per modern day technology, a soft starter can also be considered for such applicat ions.It is essent ial to specify ant icipated number of starts per hour or per shif t of 8-hrs durat ion aswell as number of consecut ive starts required when the motor is started from cold or hotcondit ion for facilitat ing the design of motor windings and select ion of correct class ofinsulat ion to encounter ant icipated temperature rise due to number of starts.Large rated motors are often started via soft starters. It is desirable to explicit ly specify thisrequirement so that the motor, compat ible for such applicat ion, is designed and manufactured.

Duty CycleSelect ing the proper electric motor also depends

Laf ert Electric motor that combines brushlesspermanent magnet (PM) and AC induction

motor technologies.

Select ing the proper electric motor also dependson whether the load is steady, variable over af ixed t ime durat ion, following a repet it ive cycle ofvariat ion, or load with pulsat ing torque or shocks.The motors to be kept min service round theclock, such as driving pumps, fans, etc., may beselected on the basis of continuous load andother factors discussed in this art icle.This is the Duty Cycle required to be performedby the motor.The motors driving the equipment likeautomatically controlled compressors, cranes,hoists start and stop a number of t imes per hourand those in some machine tools start and stopmany t imes per minute.The Duty Cycle is a f ixed repet it ive load patternover a given period of t ime which is expressed asthe rat io of on-t ime to cycle period. Whenoperat ing cycle is such that electric motorsoperate at idle or a reduced load for more than25% of the time, Duty Cycle becomes a factor in sizing electric motors. Also, energy required tostart electric motors (that is, accelerat ing the inert ia of the electric motor as well as the drivenload) is much higher than for steady-state operat ion, so f requent start ing could overheat theelectric motor.When the motor is supposed to operated at idle or reduced load for more than 25% of the t imein accordance with its operat ing cycle, the Duty Cycle becomes a vital factor in sizing themotor.Also, the energy required to start the motors, i.e. during accelerat ing along with driven load, ismuch higher than that required for steady-state operat ion, so f requent start ing, in mostprobability, is likely to overheat the motor.

Insulation ClassThe permissible temperature rise for six insulat ion classes is based on the ambienttemperature of 40C as shown in following table for dif ferent class of insulat ing materials.

ThermalClass ofInsulat ion

Normalpermissibletemp. rise over40C

Normaltotaltemp.C

Maximumpermissibletemp. rise C

Insulat ing materials

A 60 100 105 Cotton, Silk, Impregnated/Coatedpaper

E 75 115 120 Synthet ic enamel based on polyvinylacetate, Polyurethane, Epoxy onpolyamide resin

B 80 120 130 Mica, Glass f ibre, Asbestos withsuitable

bond, viz. Synthet ic resin varnishes,Epoxy resin shellac, Asphalt orbituminous compounds

F 100 140 155 Mica, Glass f ibre, Asbestos withsuitable bond, viz. Alkyd epoxy resin,add silicon alkyd resin

H 125 165 180 Silicon elastomers, Mica, Glass f ibre,etc. with bonding substances likesilicon resins

G 170 210 225 Mica porcelain, Other mica classquartz, with bonding materials ofsilicon resin

It may be remembered that for every 10C rise in operating temperature, the insulation lifereduces by 50% of its usual life.Thus the temperature rise in motor is usually the dominat ing ageing factor of inf luence on thewinding insulat ing materials and insulat ion systems. Hence it is essent ial to specify proper classof insulat ion for the motor based on design ambient temperature, if it is more than standarddesign temperature of 40C.The endurance of the insulat ion is adversely af fected by many other ageing factors, such assurroundings, electrical and mechanical stresses, vibrat ion, deleterious atmospheres andchemicals, moisture, dirt and radiat ion.

Selection of Induction Motors f or Industrial Applications (photo by ABB)

http://electrical-engineering-portal.com/selection-of- induction-motors- for- industrial-applications-part-3 May 9, 2013

Dif ferent Rotor ClassesThe dif ferent rotor class, i.e. KL7, KL10, KL13, KL16, KLp, etc. are available in case of themotor to fulf il the functional torque requirements of the driven equipment. The rotor classesindicate against what quantum of the load torque the motor would be able to start easily.The motor with KL10 class of rotor, when started direct-on-line, would accelerate safely to itsrated speed against the load torque of 100 % of its rated torque. Similarly, the motor with KL16class of rotor would be capable of start ing against the load torque of 160 % of its ratedtorque.Though, KL10 class rotor could take maximum starting torque up to 180 % of the full loadtorque, and for KL16 class, it could go up to 200 %, but for very minimum t ime exert ing morestress to the rotor.Such pract ice reduces the motor life in no uncertain terms.It is therefore essent ial to obtain the technical data for the torque requirement of drivenequipment during start ing and incorporate in the specif icat ion so that the motor of correctrotor class will be installed.

Constructional features of motorBased on the applicat ion requirements, the construct ional features of the motor are to beselected as follows:

Mounting ArrangementDif ferent types of mount ing arrangement for the motors are Horizontal foot mounted (B3),Horizontal flange mounted (B5), Flange-cum-foot mounted (B3/B5), Vertical flangemounted with shaft downwards (V3), etc and so on.Internat ional standards specify mounting arrangement with some dif ferent notat ion.It is important to specify correct mount ing arrangement for sat isfactory installat ion of motorand its coupling with the driven equipment. Details of driven equipment may also be furnished ifdesired by the motor vendor.

EnclosuresThe enclosures are classif ied under two categories as follows. It is selected based on thespecif ic applicat ion and locat ion of the motor.

Open Type Totally Enclosed Type 1. Screen protected drip proof(SPDP)

1. Totally enclosed fan cooled (TEFC)

2. Splash waterproof 2. Totally enclosed surface cooled 3. Pipe vent ilated 3. Totally enclosed pipe vent ilated 4. Weather protected 4. Hose and splash proof

5. Increased safety motor with enclosure e forhazardous areas 6. Totally enclosed for marine applicat ions

Frame size

General Electric triclad induction motor f rame

Motor directly coupling

The frame size ofthe motor is to beselected consideringambient conditionsand environment insurroundings,where it is to beinstalled.If the ambienttemperature isexpected to beabnormally high, themotor with onehigher f rame size forthe same rat ingprovides betterservices due toavailability of morecooling surface areadue to higher f ramesize. This factor isthus related to locat ion of the motor.

Mechanical design featuresCoupling Arrangement with Load

It is necessary to ment ion whether the motor isto be directly coupled with the drivenequipment, or coupled through a belt/chaindrive, or gearbox.For belt -drives, the motor shaft diameterand length depend on the type of belt and pulley,as the standard shaft length suitable for f latbelt may not be suitable for V-belt drive andwidth of pulley. It is necessary to specify theshaft extension requirement.

Type of Driven EquipmentFor better select ion of motor, it is necessary to specify the type of driven equipment,viz., centrifugal pump, compressor, blower, bowl mill, conveyor drive, etc.The moment of inertia and accelerating torque of the driven equipment along with its dutycycle should also be specif ied to consider start ing t ime of the motor and ant icipatedtemperature rise during start ing.

Motor terminal box

The design of windings would greatly depend on this technical data.

Terminal BoxThe size of cable with number of cores shouldbe specif ied so as to enable the motormanufacturer to provide correct type and sizeof cable terminal box. When a number ofparallel cables are to be terminated, as in caseof star-delta started motor, a special terminalbox with a modif ied arrangement may bespecif ied to facilitate the cable terminat ion.If required, the cable split ter box should also bespecif ied along with the main terminal box forfacilitat ing split t ing of cable cores beforeterminat ion.Where the space heater is provided in themotor, it is desirable that the leads of spaceheater are brought out in a separate terminalbox so as to avoid unnecessary congest ion ofcables in the main terminal box.Moreover, it is essent ial to specify whether the terminal box should be located on side or onthe top of the motor considering ant icipated encroachments at site due to structures,pipelines, etc. around locat ion of motor. Depending on the site locat ion, it may be necessary tospecify Degree of Protection required for the terminal box.It is general pract ice to maintain same Degree of Protect ion for the motor and terminal box.

Earthing TerminalsIn accordance with the Indian standard specif icat ion, two separate and distinct earthingterminals should be essent ially provided on the body of the motor and one earthing terminalshould be provided internal in the cable terminat ion box is to be provided.Addit ionally, one earthing terminal may also be provided on either one side of the terminalbox housing or internal in the terminal box housing for enhancing overall safety of theequipment.

Location data

Hazardous Area Motors - ExplosionProof & Purged and Pressurized

It is essent ial to include all the possible details oflocation and surroundings in the technical specif icat ion.The motor with a suitable enclosure should be selectedbased on its location, whether it is indoor, outdoor,semi-outdoor, prone to dust, corrosive vapours,chemically hazardous explosive area, salt laden air, etc.Corrosive chemical vapours at tacks not only the motorwinding insulat ion, but also the housing, statorlaminat ions, rotor, shaft and bearings. For the motorsworking in the corrosive areas, the winding should beapplied a specif ic varnished impregnat ion treatment, ant i-acidic t reatment and overhangs should be applied withepoxy based varnish.Corrosion to metal body is prevented by applyingepoxy based resin paint.

Service factor or overload capacityThe service factor of a motor indicates how much it could be overloaded withoutimmediately failing. Generally, the motors are designed with 1.15 service factor with thedevelopment of high quality insulat ing materials that can withstand higher temperatures.Most of the motors do not operate at maximum efficiency when overloaded. Theopt imum eff iciency is obtained from the standard motors when operated at around 80 to 95%of their rat ing and the energy ef f icient motors of fer opt imum eff iciency when operatedbetween 65% to 95% of their rat ing.Although the motors operat ing between their full load rat ing and their service factor rat ing donot fail immediately, their service life certainly becomes shorter.Hence, it is best to avoid designing of motors to operate with overload except for short t imedurat ion, even if permit ted by the service factor. A high service factor can be used as anindicat ion of a high quality, more reliable motor.

ConclusionEffort made in technical art icle covers as many factors for selection of motor as possible,through the subject is vast . The complete technical specif icat ion would great ly facilitate themotor vendor to propose a type of motor required for a specif ic applicat ion.Effect ive select ion and applicat ion of modern day electric motors require a thorough technicaland pract ical knowledge of basics of rotat ing machines as well as an in-depth awareness of thelatest technical developments via cont inuous contact .Operat ing condit ions such as duty cycle, number of starts, ambient condit ions, and datafor locat ion, environment, driven equipment, etc. are important considerat ions for themotor ef f iciency and reliability. It is also imperat ive to seek advice of the motor manufacturermany a t imes if motor will be operated under any unusual service condit ions to support theselect ion procedure.It can also be seen from the above

High-speed operation with the new synchronous reluctancemotors can eliminate use of mechanical power transmission

elements such as gearboxes in some applications.

It can also be seen from the abovediscussion that most of thetechnical requirementsare interwoven and closely relatedwith each other. Considerat ion ofone factor may af fect theother factor and hence it isimportant to adopt an integratedapproach to the total specif icat ion.Finally, the number of specificrequirements is going to raisethe cost of the motor.For example, the init ial cost of higheff iciency motor will be more,depending on design and specif icmaterial aspects, but theconsistent lower operat ing costdue to less drawl of power willprove economically benef icial in along run. Besides consideringvarious general design features asment ioned above, thespecif ic requirements in view of abnormal site condit ions and applicat ion requirements mustbe considered and brought out in the purchase specif icat ion.The extra cost, required to be incurred for some specif ic features, would simplify themaintenance to such an extent that the repayment would be in a shortest durat ion withreduced outage of the motors.Last but not least , the select ion and applicat ion of electric motors has become morecomplex than ever before because of the emergence of high efficiency and premiumefficiency (PE) motors as a part of cont inuous process of technical developments. As a result ,one must extremely careful look to any applicat ion of the electric motor.

electrical-engineering-portal.com-Selection_of_Induction_Motors_for_Industrial_Applications_part_1 (1)Selection of Induction Motors for Industrial Applications (part 1)IntroductionAbnormal conditions and effects1. Abnormal System Conditions2. Abnormal Operating conditions3. Environmental conditions4. Mechanical problems5. Condition at location

Design ConsiderationsOutput in kW/HP1. Mechanical limitation2. Electrical limitation due to insulation provided on the motor windings

Speed of the MotorPower Supply Voltage and Frequency Variations

electrical-engineering-portal.com-Selection_of_Induction_Motors_for_Industrial_Applications_part_2Selection of Induction Motors for Industrial Applications (part 2)Motor EfficiencyAmbient TemperatureAltitudeMethod of Starting and Number of StartsDuty CycleInsulation Class

electrical-engineering-portal.com-Selection_of_Induction_Motors_for_Industrial_Applications_part_3Different Rotor ClassesConstructional features of motorMounting ArrangementEnclosuresFrame size

Mechanical design featuresCoupling Arrangement with LoadType of Driven EquipmentTerminal BoxEarthing Terminals

Location dataService factor or overload capacityConclusion