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AC Generator set


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We have a Diesel Engine Generating Set which supplies a Port Container Crane using a DC Variable Speed Drive.


Specification of Engine-Generator :


Engine Output Power: 534Hp (398 KW), 1800 rpm

AC Generator: 844 KVA (675 KW), 3 ph, 480V, 1014A, 60 hz, 4 pole


Take note that Generator Rated Power is approximately 150% higher than Engine Capacity.


Estimated Maximum load (Hoist DC Motor, Trolley DC Motor, other Inductive load, resistive loads) is about 300 KW


We intend to purchase the same arrangement of Generating Set as a spare power pack for our Crane, however most of the suppliers are confused ( and so do I ) with the Engine-Generator arrangement …A Power Pack in which the Driven Generator Rating is so much higher than the Prime mover Rating-Engine.


Unfortunately, I cannot get in touch with the Crane Supplier to inquire the Technical aspect involves in choosing such design, but according to the same Crane Type user, this arrangement is the optimum design for the Crane Load which employs computerized control system.


Our Crane is considered modern, using technology drive- Variable Speed Drive which employs power electronic semiconductor devices, computerized control system… a form of nonlinear-load which introduces harmonic distortion onto the electrical supply system, however, do we need an over capacity generator to operate the load (non-linear) satisfactorily?


To summarize, I just want to inquire if we can de rated the capacity of generator to match nearly with the Engine kVA without unsatisfactory consequences to the load and its control system.




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Hello moises


A generator set comprises two major components, The engine and the alternator.

The engine is loaded by KW and the alternator is loaded by KVA (or Amps)


If you have an electrical load with a power factor of 1.0, you could match up the engine and alternator such that the engine KW = the alternator KVA(ignoring losses), however in most situations, we are dealing with a powerfactor of less than 1.0 and so we have a KVA that is higher than the engine KW. For example, if we design for a load power factor of 0.8, then we would select the alternator KVA to be 1.25 time the engine KW.


In your case, you will have high harmonics in the load, therefore the power factor will be less than 1, so you need to oversize the alternator.


Best regards,

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