Selection of Induction Motors Part 3

7/28/2019 Selection of Induction Motors Part 3

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

http://electr ical-engineering-portal.com/selection-of- induction-motors-for- industr ial-applications-part-3 May 13, 2013

Selection of Induction Motors for Industrial Applications(part 3)

Ashok Parikh

Continued from first part: Selection of Induction Motors for Industrial Applications (part 2)

Design Considerations (cont.)

Different Rotor Classes

The different rotor class, i.e. KL7, KL10, KL13, KL16, KLp, etc. are available in case of themot or 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 t he 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 rot or could take maximum starting torque up to 180 % of the full load

torque, and for KL16 class, it could go up to 200 %, but for very minimum time exert ing morestress to the rotor.

Such practice reduces the motor life in no uncertain terms.

It is therefore essential to obtain the technical data f or the torque requirement of driven
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equipment during starting and incorporate in the specif ication so that the motor of correctrot or class will be installed.

Constructional features of motor

Based on the application requirements, the constructional features of the mot or are to be

selected as follows:

Mounting Arrangement

Different types of mounting arrangement for the mot ors 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.

International standards specify mounting arrangementwith some different notation.

It is important t o specify correct mount ing arrangement f or satisfactory installat ion of motorand its coupling with the driven equipment. Details of driven equipment may also be furnished ifdesired by the motor vendor.

Enclosures

The enclosures are classif ied under two categories as follows. It is selected based on thespecif ic application and location of the motor.

Open Type Totally Enclosed Type

1. Screen prot ected drip proof(SPDP)

1. Tot ally enclosed f an coo led (TEFC)

2. Splash wat erproof 2. Tot ally enclosed surf ace 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. Tot ally enclosed for marine applications

Frame size


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General Electric triclad induction motor f rame

Motor directly coupling

The frame size ofthe motoris to beselected consideringambient conditionsand environment insurroundings,where it is to be

installed.

If the ambienttemperature isexpected to beabnormally high, themot or with onehigher frame size f orthe same ratingprovides bet terservices due to

availability of morecooling surface areadue to higher framesize. This factor isthus related to location of the motor.

Mechanical design features

Coupling Arrangement with Load

It is necessary to mention whether the mot or isto be directly coupled with the drivenequipment, orcoupled through a belt/chaindrive, or gearbox.

For belt-drives, the motor shaft diameterand length depend on t he 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 Equipment

For better selection o f 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 starting time of the mot or and anticipatedtemperature rise during starting.
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Motor terminal box

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

Terminal Box

The size of cable with number of cores shouldbe specif ied so as to enable the mot or

manufacturer 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 modified arrangement may bespecified to facilitate the cable termination.

If required, the cable splitter box should also bespecif ied along with t he main terminal box forfacilitating splitting of cable cores beforetermination.

Where the space heater is provided in themotor, it is desirable that t he leads of spaceheater are brought out in a separate terminalbox so as to avoid unnecessary congestion ofcables in the main terminal box.

Moreover, it is essential to specify whether the terminal box should be located on side or onthe top of the mot or considering anticipated encroachments at site due to structures,pipelines, etc. around locat ion of motor. Depending on the site location, it may be necessary tospecify Degree of Protection required fo r the terminal box.

It is general practice to maintain same Degree of Prot ection for the motor and terminal box.

Earthing Terminals

In accordance with t he Indian standard specif ication, two separate and distinct earthingterminals should be essentially provided on the body of the motor and one earthing terminalshould be provided internal in the cable termination 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
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Hazardous Area Motors - Explosion

Proof & Purged and Pressurized

It is essential to include all the possible details oflocation and surroundings in the t echnical 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 attacks not only the mot orwinding insulation, but also the housing, statorlaminations, rotor, shaft and bearings. For the motorsworking in the corrosive areas, the winding should beapplied a specif ic varnished impregnat ion t reatment, anti-acidic treatment and overhangs should be applied withepoxy based varnish.

Corrosion to metal body is prevented by applyingepoxy based resin paint.

Service factor or overload capacity

The 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 insulating 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 eff icient motors of fer opt imum eff iciency when operatedbetween 65% to 95% of their rating.

Although the motors operat ing between their full load rating 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 imeduration, even if permitted by the service factor. A high service factor can be used as anindication of a high quality, more reliable mot or.

Conclusion

Effort made in technical article covers as many factors for selection of motor as possible,through the subject is vast. The complete technical specif ication would greatly facilitate themotor vendor to propose a type of motor required for a specif ic application.

Effective selection and applicat ion of modern day electric mot ors require a thorough technicaland practical knowledge of basics of rotat ing machines as well as an in-depth awareness of thelatest technical developments via continuous contact.

Operating conditions such as duty cycle, number of starts, ambient conditions, 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 conditions to support theselection procedure.

It can also be seen from the above
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High-speed operation with the new synchronous reluctance

motors can eliminate use o f mechanical power transmissionelements such as gearboxes in some applications.

discussion that most of thetechnical requirementsare interwoven and closely relatedwith each other. Consideration ofone factor may affect t heot her factor and hence it isimportant to adopt an integratedapproach to the total specification.

Finally, the number of specificrequirements is going to raisethe cost of the motor.

For example, the initial cost of higheff iciency motor will be more,depending on design and specificmaterial aspects, but theconsistent lower operating costdue to less drawl of power will

prove economically benef icial in along run. Besides consideringvarious general design features asmentioned above, thespecif ic requirements in view o f abnormal site condit ions and application 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 t hat t he repayment would be in a shortest duration withreduced outage of the mot ors.

Last but not least, the selection 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 application of the electric motor.

EEP recommends

Ashok Parikh - Working as Electrical Engineering Consultant located atVadodara, India providing System Design & Engineering services to variousindustries, possessing 40 years of experience in diversif ied industries andconsultancy.
http://jptechnosoft.com/

Documents

Selection of Induction Motors Part 3