Selection Of Crane Duty Motors
Selection Of Crane Duty Motors (On photo: Single girder overhead traveling crane - max. 12.5t, 4-30m of Demag Cranes & Components via

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

Derivation Of Cyclic Duration Factor From Different Duty Cycles

S3 – Intermittent Periodic Duty

Intermittent Periodic Duty
S3 - Intermittent Periodic Duty

N = Duration of motor operation under rated conditions
R = Duration of motor at rest and de-energised
ØMax = Maximum temperature attained during Duty cycle

CDF formula for S3

Unless otherwise specified, 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

S4 - Intermittent Periodic Duty with starting
S4 - Intermittent Periodic Duty with starting

D = Duration of starting
N = Duration of motor operation under rated conditions
R = Duration of motor at rest and de-energised
ØMax = Maximum temperature attained during Duty cycle

CDF formula for S4

S5 – Intermittent Periodic Duty with starting and breaking

S5 – Intermittent Periodic Duty with starting and breaking
S5 – Intermittent Periodic Duty with starting and breaking

D = Duration of starting
N = Duration of motor operation under ratedconditions
R = Duration of motor at rest and de-energised
F = Duration for electric braking
ØMax = Maximum temperature attained during Duty Cycle

Starting Of Crane Duty Motors

The squirrel cage motors are started with direct-on-line starters in most of the cranes. Few cranes, operated more or less on continuous basis, are found operated through a variable voltage variable frequency (VVVF) drive.

For intermittent duty cranes, provision of VVVF would be costly affair.

The slip-ring or wound rotor motors are usually started by means of variable resistance in the rotor circuit to get required starting torque at reduced starting current. The value of external starting resistance can be calculated as under.

External starting resistance formulae

K = Constant depends on line voltage drop; varies from 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) Torque
Rrt = Rotor resistance in Ohms per phase
Rext = External rotor resistance per phase to be added (Ohms) to get torque TLR at stand still.

Rrt is generally small compared to Rext and may be neglected. However, if required, approximate Rrt can be calculated by following relation:

Rotor resistance in Ohms per phase

The starting torque can be increased up to the value of maximum torque available for that particular design. Usually, it is possible to obtain the starting torque 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 follows:

  • The driven equipment is to be accelerated rapidly with a fixed sequence of operation and uniform load conditions, e.g. mechanical workshop crane.
  • If the load conditions are almost identical for both directions of rotation, 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, corrosive, these motors with totally enclosed fan cooled construction would be robust and would provide services with least maintenance.
  • Where the cost factor is to be considered, as cost of cage motor is less than that of slip-ring motor;
  • The squirrel cage motors for crane duty are available normally up to 250M frame size.

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

  • Where very precise speed control is required for the crane, e.g. inching, slow and fast handling of load during hoisting and lowering, alignment of crane over a furnace opening, etc.
  • In case of non-uniform 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 than 2.5 times the rated torque in general.


The crane duty motors are always supposed to operate under varying load conditions and sequential switching due to requirement of handling materials of varied weights (i.e. loads).

Many continuous duty motors 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 efficient and cost effective approach is to select the motor with optimum rating on the basis of the load duration curve for the particular application.

Selection made on this basis also provides equally effective and satisfactory 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 for a short time duration, rather than selecting the motor of high rating that would operate at full capacity for only a short period providing optimum efficiency only for that much duration. Only concern for motors operating at higher than its rating is the thermal capacity of motor, which determines the speed of degradation of the winding insulation.

Applications of various factors discussed in foregoing paragraphs combined with this suggestion would provide better result. However, accurate rating is very difficult to determine for crane duty applications.


Most vital and primary technical consideration for selection of the motor for any particular application is the torque required by the load during starting and acceleration periods, especially the relationship between the maximum torque or breakdown torque generated by the motor and the start-up torque or locked rotor torque for both periods.

The thermal capability of motor is determined based on the duty / load cycle. Additionally, where the totally enclosed fan cooled (TEFC) motor is operated with speed variations, the cooling may be insufficient when the motor is operated at speeds below its rated value.

If the ambient temperature is more than 40°C, derating factor would affect the selection of motor. Good estimate of the switching frequency can help in selecting the appropriate motor for the proposed duty cycle.

Reliability is of prime importance in all the services, however, it is general practice to grossly oversize the motor unnecessarily for better reliability, which results into sub-optimal energy performance. Better understanding of the power system and knowledge of operating parameters can certainly aid in reducing over sizing with no adverse effect on reliability.

  1. Efficient 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 Motor Manufacturers

About Author //


Ashok Parikh

Ashok Parikh - Working as Electrical Engineering Consultant located at Vadodara, India providing System Design & Engineering services to various industries, possessing 40 years of experience in diversified industries and consultancy.


  1. Catherine
    Jul 12, 2016

    Hi Ashok,

    Can you please suggest the KW, module x teeth and the ratio for an EOT crane long travel.
    crane details as below;
    10T EOT crane/6m lift height/span 24m/double girder/dual speed.
    I need the details of the long travel motor to be used in a 100metre.

    • Ashok R Parikh
      Jul 12, 2016

      Dear Catherine,

      The rating of the motor can be decided by the crane manufacturer, which I cannot decide as number of mechanical as well as electrical factors are required to be taken into account for determining the rating of crane duty motor. However, you can ensure that the crane vendor supplies right type and rating of motor which you can built-up in the specification based on the few technical features and requirements suggested in the subject article for the crane duty motors.

  2. ochesel
    Mar 30, 2016

    could you give an example of website or a brochure that includes crane duty motors, with three speed, and their features?

  3. Pooja R
    Jul 16, 2015

    Dear sir,
    Foe crane duty motors electrical rating of motors are chosen to be higher than required in order to sustain temporary heavy inrush, starting torques etc. Is that motors are designed mechanically for such higher rating than required or not ?

    • Ashok Parikh
      Jul 16, 2015

      Dear Pooja,

      It is not only question of withstanding high starting torque, but time for which heavy load is to be sustained. The crane motors are therefore specially designed mechanically as well as electrically based on duty requirements to be provided in detail by the purchaser.

  4. Pooja R
    Jul 16, 2015

    for crane duty motor ratings are decided to withstand high starting torques for small duration. Is motors are mechanically configured to sustain this large starting or overload torques .

    Feb 25, 2015

    What is the signifiance of CDF? what does it actually implies..

    • Ashok Parikh
      Feb 26, 2015

      Dear Mr. Keerti Singh,
      As already brought out in the article, standard motors should not be used for crane duty function, but the manufacturer should be given standard technical data for the motor as well as CDF as per requirement of crane duty function. The Purchaser has to decide the best possible CDF based on anticipated cyclic duty to be performed by the motor. The motor vendor will design tailor made motor for that specific duty and manufacture so that the Purchaser will get the desired performance from the crane/motor.
      I hope, this clarifies your requirement.
      Ashok R. Parikh

    Feb 25, 2015

    Dear Sir,
    This article is indeed very informative. But what bothers me is the actual what is the actual signifiance of CDF … how & why it is in term of Percentage?? what does 40% cdf or 25% cdf or 60% cdf implies ??
    I will be grateful if these queries can be heard upon..


  7. lehuan
    Sep 07, 2013

    Thanks for serie of articles.

  8. Ashok Parikh
    Jul 28, 2013

    Dear sir,
    First of all I wish top convey my sincere thanks for publishing my article titled “Selection of Crane Duty Motors” in 3 parts.
    While reviewing Part 3, I have observed that formula for deriving Cyclic Duration Factor is missing for S5 – Intermittent Periodic Duty. The list of notations is given below graph projected for S5 duty cycle, however the formula is not printed, though given in original article. Kindly do needful so that readers get that information.

    With regards,
    Ashok Parikh

    • Edvard
      Jul 30, 2013

      Hi Ashok,

      It will be fixed very soon! Excellent serie of articles, i enjoyed reading.

      • Ashok Parikh
        Aug 01, 2013

        Hi Edvard,

        Many many thanks for reply and appreciation, I would continue to participate in your excellent and highly appreciated portal.


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