Generator loadbank calculations 8

[howitz]

When using a purely resistive loadbank how do you calculate the load current. This is to load check lets say a 100KVA generator at 0.8 powerfactor. this is how i thought it was done

kva = kw/pf therefore if the powerfactor was at unity then 100kva = 100kw therfore the generator would be loaded at 100kw

however i have recently started a new job and the techies say to calculated the load use this formula

100kva * 0.8 = KW then times the answer by 1.39

100kva * 0.8 = 80kw

80kw *1.39 = 111.2 kw

the generator should be loaded at 111.2kw resistive load.

I don't see the relationship of this calculation on any generator i have come across or am i missing something.

I think the answer they gave me was due to the diesel engine fitted to the alternator.

Any help would be appriciated

 

[davidbeach]

Generally a 100kVA/0.8pf generator is paired with an engine that can produce 80kW, loading it to 111.2 kW will over current the generator windings (max current would happen at 100kW/1.0pf) and overload the engine.

 

[waross]

To re-phrase davidbeaches post;
If the load is in excess of 80 KW the motor will be overloaded, and may slow down.
If the load is in excess of rated current the windings will be overheated.
Use nameplate voltage and nameplate KVA to find the max current if it is not given on the nameplate.
I prefer to use current rather than KVA because of the number of times that I have seen generators operated at other than nameplate rated voltage.
A good instance was a customer who needed a 50 KVA set at 120/208 volts.
The supplier sold him a set rated at 50 KVA, 120/240 volts 4 wire delta. At 208 volts the kva rating de-rated to 43.3 KVA.
I have encountered other instances.
That said, There may be a good reason why your techies are using a 39% overload on their test. Why don't you ask them the reason?


David; Did you mean maximum RATED current?
(max current would happen at 100kW/1.0pf)
Respectfully

 

[davidbeach]

waross, correct, max rated current into a resistive load bank would happen at 100kW, since at unity power factor kW=kVA. But the point is that the kVA rating of the generator, and as you correctly point out at rated voltage, establishes the max current of the generator. Exceed max rated current at your own peril. 39% overload best be accompanied by a lot of extra cooling as the internal heating of the generator stator will be at 193% of rated.

 

[ScottyUK]

Some generators of that size range are dual rated for standby and prime service. The difference is usually about 10%, and effectively allows the unit to be overloaded for short running periods. Could this explain the odd multiplier?


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Sometimes I only open my mouth to swap feet...

 

[honeykuwait]

The important thing is if the Gen is 100kva ,rated at 0.8pf, you should load the Gen for testing with maximum of 80kw load only. So if you are using a pure resistive load of 1 pf load it to 80KVA=80KW. Other details such as prime/standby rating, percentage overload all to be considered only on this primary factor.

 

[rbulsara]

Is the unit rated 415V??

I beleive the 1.39 factor was used to calculate the current (load) and it relates to a 415V system. And there is some confusion on part of the OP between amps and kW.

Amperes=kVA*1000/(sqrt 3*V), for a 3 phase system

For V=415V, 1000/1.732*415 yeilds 1.39

So ampere load for a 100kVA,415V 3 phase gen will be 1.39*100kVA= 139A.

or for 80kW, it will be 80*1.39= 111.2A (not kW)

The basic formulas are:

kW=kVA*pf

0.8 is a typcical power factor for genetator sets.

For resistive loads: kW=kVA and pf is 1.0

Since the engine (primemover) is rated only for real power kW, the load should not exceed 80kW (100*.8). As a engine-generator set, one should not exceed either of its kW or kVA limits. With a resistive load, you reach kW limit, much sooner than the kVA limit.

 

[itsmoked]

Nicely sleuthed rbulsara.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

Thanks Keith. Hope my assumption is correct.

 

[waross]

That sounds as if you have found the solution, rbulsara.
I would use a different philosophy for testing, however.
I would deliberatly exceed the KW loading.
How else can you determine whether the dead-rack, or maximum fuel adjustment in the engine governor has been properly set?
Many (most) of these sets have an engine that is capable of overpowering the generator. The governor is set to limit the prime mover power at the rated voltage.
For example, if a set is rated at 240 volts and then is used at 208 volts, it will be overpowered unless the governor is adjusted.
A load bank is seldom available in the field and a compromise solution to voltage changes in the field is to pay close attention to the connected load and the over current protection.
You can often load a 100KVA set up to 100% current by reducing the voltage setting 20%. Of course you have to recalculate the resistance value at the new voltage level.
respectfully

 

[peebee]

Another red star for bulsara.

I'd love to hear back from howitz to see if he was right....

 

[rbulsara]

thanks, peebee. Enough red stars
Only if howitz got back to us.

 

[howitz]

To all who have given advice on the question posed, I have been load testing generators for over twenty years and had never heard of the 1.39 method. mainly because all the generators I have load tested are of the 200V 400 hz type ie for aircraft use. I would like to thank you all especially rbulsara for his input. When we load check a generator we test what impact a 10, 50, 65, 100 and 110% load has on the generating set. with these parameters the engine and the alternator should be able to withstand the stresses placed upon them, mostly the engine and alternator are compatable during manufacture, however we have been caught out a few times when the engine has been to small for the job. It would be interesting to know how the size of engine is chosen in relation to the alternator, with respect to its horse-power or cylinder size, maybe i will research it first before proposing the question. The main point i have learned, and this is the first time I have used this site, the answers are all out there and many brains get better results, or maybe i should have done a degree in engineering after completing my HNC. But ultimately check the output voltage of the generator before load testing, as the volage my not always be 415V, it could vary between 400v and lets say 440v. finally thanks to you all for your wisdom.

Howitz