Selection and Design Considerationsfor AC Crane Duty Motors -

Technically Speaking

Based on a previously published Article in IEEMA Journal in May 1996; - tribute to Late P. Mazumder

1. IntroductionOver the years of successful field performance associated witheasier maintainability have made three phase induction craneduty motors as the workhorse for material handling industry.

AC Slipring crane duty motors associated with improvisedcontrol system became obvious choice to perform for most ofthe critical applications so far reserved for DC crane controlsystems. With the introduction of Variable Speed Drives,squirrel cage motors have started to replace the slip ringmotors to a large extent in recent times.

Due to increasing emphasis on higher productivity associatedwith the need of better reliability in service and considerationof overall cost of the system, the selection and design of motorand its control system have become more critical.

With the increasing trend of using large AC Slipring motorsand Squirrel Cage motors with Variable Speed Drives for craneduty applications eg hot metal cranes, proper selection, properdesign consideration and proper use of sophisticated controlfor such critical drives have gained importance.

Motor design and manufacturing technology is also becomingcritical for long term sustainability of the product under thesearduous conditions.

We will look into the various methods of selection of the craneduty motors, understand the emphasis of the typical dutyratings against design parameters.

2. Motor Selection

2.1 Standard MethodMost of the crane manufactures in India follow therecommended procedures as per IS 4137 and IPSS SPEC.No. IPSS : 2.02.004. The European manufacturers adopt theprocedures recommended in Federation Europeans DE LaManutention (FEM) Part 1.

Over the years of successful field performance for crane dutymotors selected on the basis of recommendations as per aboverecommendations endorsed considerable level of confidencein the method. Yet, a careful study of the methods detailed inIS 4137 / IPSS : 2.02.004 reveals that it does not represent allconditions of drive requirements leading to selection of motor.

A comparative analysis of the considerations adopted by IS4137/IPSS specification and FEM for selection of crane dutymotors is tabulated in Annexure A. The primary differencebetween FEM and Indian Standards lie in evaluating motorrating.

FEM recommends motor rating as means equivalentpower/torque based on RMS duty cycle ignoring rest timeMean eque./power/torque rating should be corrected forambient temp. altitude and electronic control. Duty cycle isspecified as per actual duty cycle of drive. FEM recommendsminimum POT as 190% of the maximum rated torque for thedrive.

Indian standards specify motor output calculated based onmaximum power loading associated with service factor. Servicefactor is selected based on service class of crane establishedfrom application criteria of drive. Detail recommendation forservice class is given in the specification. Maximum powerloading, such calculated is corrected for ambient, altitude, slipfactor and thyristor loading to arrive a required motor outputat 40% CDF.

Minimum POT is recommended as 275% FLT correspondingto 40% CDF obviously output at 40% CDF is much higher thanactual mechanical loading of drive. This has been done toaccommodate duty cycle requirement of drive. It is obviousfrom the above that motor size selected based on Indiastandards may be oversized as compared to European practice.

Indian standards also do not directly considered equivalent no.of starts of % CDF of actual duty cycle in calculation for hoistduty calculation all considerations are lumped in service factor.However for travel/traverse drive - factor of inertia isrecommended in formula recommended by IPSS. Moreover,Indian standards do not clearly specify duty classification.

This has resulted various options for crane manufacturers ininterpreting the duty as follows :

S3-40% CDF

S4/S5-40% CDF

Some manufacturers as also specifying S4 / SS duty with CDFas per actual duty cycle.

The implication of such varying duty specification in selectingmotor frame size is shown in Table 1.

It is important to note that over sizing a motor for duty cycleloading does not necessarily help since energy losses duringstart is directly proportional to total incrtia. Where motorinertia becomes considerable compared to load inertia - Asubstantial amount of motor rating caters for its own inertia.

Keeping in view of the above, it is recommended that S3 dutyneeds to be clearly mentioned in IS 4137 and requirement ofPOT should be related to maximum mechanical KW.

Ananda Majumdar GM Business Development &

Technical ServicesPart I

to be continued ...

M A R A T H O N K O L K A T A N E W S

POWER SRI LANKA 2012 INT’L EXPO” HELD ATSRI LANKA EXHIBITION & CONVENTION CENTRE(SLECC), COLOMBO – SRI LANKA FROM JUNE14TH - 16TH, 2012Marathon Electric together with M/S Macs Marketing Private Limited (ourdistributor in Srilankan) participated in 8TH POWER SRI LANKA 2012 INT’LEXPO” held at Colombo from 14th June to 16th June 2012.

The stall was inaugurated by Minister of Industry and Commerce (Govt ofSrilanka) Mr.Rishad Bathiudeen.

Deputy Chairman of Macksons Group Mr. M. Milfer was also present.

The stall displayed the capacity of Marathon.

The footfall at the stall was good.

Dr. N. Rajashekar – GM Export and Alternator represented Marathon duringthe event.

8TH