electrical-engineering-portal.com-selection of crane duty motors part 3

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Sele ction Of Crane Duty Motors (O n photo: Single gird er o verhead traveling crane - max. 12.5t, 4-30m of Demag Cranes &

Comp onents via Direc tIndustry.com)

Ashok Parikh

Selection Of Crane Duty Motors (Part 3)

Continued from second part: Selection Of Crane Duty Motors (Part 2)

Derivation Of Cyclic Duration Factor From Different Duty Cycles

S3 Intermittent Periodic Duty
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S3 - Intermittent Period ic Duty

S4 - Intermitte nt Perio di c Duty with starting

N= Duration of motor o peration under rated

conditions

R= Duration of motor at rest and de-energised

Max= Maximum temperature attained during Duty

cycle

Unless o therwise specif ied, the duration of the duty

cycle is 10 minutes. The recommended values for

CDF are 25, 40 and 60 percent.

S4 Intermittent Periodic Duty with starting

D= Duration of starting

N= Duration of motor o peration under rated

conditions

R= Duration of motor at rest and de-energised


cycle

S5 Intermitt ent Periodic Duty with starting and

breaking

D= Duration of starting

N= Duration o f motor operation under

ratedconditions

R= Duration of motor at rest and de-energisedF= Duration f or electric braking


Cycle

Starting Of Crane Duty Motors

The squirrel cage motorsare started with direct-

on- line startersin mos t o f the cranes. Few cranes,operated more or less on continuous basis, are

f ound operated through a variable voltage variable

f requency (VVVF) drive.

For intermittent duty cranes, provision o f VVVF

would be cost ly af f air.
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S5 Intermittent Perio dic Duty with starting and b reaking

.

K= Constant depends on line voltage drop;

varies f rom 0.6 to 1.0. Generally a value 0.8 can

be considered.

VR= Rotor voltage (Volts)

IR= Rotor Current

TFL = Full load torque (kg-m)

TLR= Locked Rotor (Starting) TorqueRrt = Rotor res istance in Ohms per phase

Rext = External roto r resistance per phase to

be added (Ohms) t o get to rque TLRat stand

still.

Rrtis generally small compared to Rextand may

be neglected. However, if required, approximate

Rrtcan be calculated by following relation:

The starting torquecan be increased up to thevalue of maximum torque available for that

particular design. Usually, it is possible to

obtain the starting to rque as high as 2.5 times

the normal torque.

Selection Of Motors

Choice of cage and wound motors may be based on the following criteria.

Squirrel Cage Motors may be used for various applications as f ollows:

The driven equipment is to be accelerated rapidly with a f ixed sequence of operation and unif orm

load conditions, e.g. mechanical workshop crane.

If the load conditions are almost identical f or both directions of rotat ion, e.g. long travel or cross

travel of gantry crane.

In the cranes, which are running at single speed without speed control.

If site conditions are dusty, corro sive, these moto rs with totally enclosed fan cooled construction

would be robust and would provide services with least maintenance.

Where the cost f actor is to be considered, as cost of cage motor is less t han that o f slip-ring motor;

The squirrel cage motors f or crane duty are available normally up to 250M f rame size.

Slip-ring motors may be used for various applications as follows:

Where very precise speed contro l is required f or the crane, e.g. inching, slow and f ast handling of

load during hoist ing and lowering, alignment o f crane over a f urnace opening, etc.

In case of non-unifo rm loading conditions and operation is to be carried out in nos. of sequences.

The cranes are required to perform large number of starting and reversals during operation.

The cranes are required starting torque of more t han 2.5 times t he rated to rque in general.
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Selection

The crane duty moto rs are always supposed to o perate under varying load conditionsand sequential

switchingdue to requirement o f handling materials of varied weights (i.e. loads).

Many continuous duty moto rs even operate under varying load conditions due to chemical process

requirements.

As a common practice, where such variable loads are to be operated, a motor rating is selected based

on the highest anticipated load.However, more ef f icient and cost ef f ective approach is to select the

motor with optimum rating on the basis o f the load duration curve f or t he particular application.

Selection made on this basis also provides equally ef f ective and satisf actory operation.

Thus as an alternative, it is better to select the motor having rating slightly lower than the peak anticipated

load and let it be operated at overload f or a short t ime duration, rather than selecting the moto r of high

rating that would operate at f ull capacity for only a short period providing opt imum ef f iciency only for that

much duration. Only concern f or motors operating at higher than its rat ing is t he thermal capacity ofmotor, which determines the speed of degradation of the winding insulation .

Applications of various f actors discussed in f orego ing paragraphs combined with this suggestion would

provide better result. However, accurate ratingis very dif f icult to determine f or crane duty applications.

CONCLUSION

Most vital and primary technical consideration f or selection o f the moto r f or any particular application is the

torque requiredby the load during starting and acceleration periods, especially the relationship between

the maximum to rque or breakdown to rque generated by the moto r and the start -up torque or locked roto r

to rque fo r both periods.

The thermal capability of motor is determined based on the duty / load cycle.Additionally, where t he

totally enclosed fan cooled (TEFC) motoris o perated with speed variations, t he cooling may be

insuf f icient when the motor is operated at speeds below its rated value.

If the ambient temperature is more than 40C, derating factor would affect the selection of motor.

Good estimate o f the switching f requency can help in selecting the appropriate motor f or the proposed

duty cycle.

Reliabilityis of prime importance in all the services, however, it is general practice to grossly oversize the

motor unnecessarily f or better reliability, which results into sub-opt imal energy performance. Bett er

understanding of the power system and knowledge of operating parameters can certainly aid in reducing

over s izing with no adverse ef f ect on reliability.

References:

1. Ef f icient Electric Motor Systems Handbook, by Todd Litmann

2. IS:12824-1989; Types of Duty and Classes of Rating Assigned to Rotating Electrical Machines

3. The Technical Literature of Indian Moto r Manufacturers

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