from the Forum department...
basic generator queries
 Posted by max on 6 July, 2012 - 3:22 pm
Max is an electrical engineer at a company. He is given the task to purchase a generator to supply power to the R&D Department. He calculated the total load to be X kilo watt. Now he went to the dealer to buy generator. He checked the ratings and found a power factor mentioned there that is (.8).

He start wondering what is the use of this rating mentioned here? because he has read in electrical engineering books that Power factor belongs to the load.
Please enlighten max. He is new in the company and is clueless.

 Posted by CSA on 16 July, 2012 - 11:20 am
Max should contact multiple generator vendors (manufacturers; manufacturer representatives) to get help with choosing the proper machine for the application. Max should also be cognizant of the basic terms, as you seem to be, but a good salesperson will be able use the information you provide to recommend a proper generator for the application.

There's no need to re-invent the wheel. By contacting multiple sources you will get a feel for who really understands your application and their machines by who can help you understand why their machine is best for your application.

The power factor is, indeed, a characteristic of the load. And, the generator chosen should be capable of driving the reactive component (VAr) of the load as well as the real (watt) component.

 Posted by pragatri on 27 July, 2012 - 8:32 am
Generator rating is in KVA/MVA which is unit for electrical energy. Same is case for transformer also. As both these machines generate & transform energy respectively, their unit is always in KVA.

While all electrical loads such as motors (fans, pumps, compressor etc) & lighting load all utilise electric power ( KW/MW ) from supplied KVA/MVA depending upon power factor. Power factor can be termed as efficiency factor of load for utilising KW from supplied KVA. Thus,
`      KW= KVA * P.F.  or, for a given KW, KVA will be,      KVA= KW/P.F.`

Thus, for constant KW for 2 motors with different p.f. KVA drawn from load will be different.

Generator manufacturer also provides generator capability curve. Its KW vs P.F. graph & is divided in 3 parts. From 0.1 lag to 0.8 lag P.F. load capability is determined by field coil temperature due to over excitation,From 0.8 lag to unity where load capability is determined by generator winding temperature & unity to 0.1 lead where load capability is determined by low excitation & yoke temperature.

In case 1, due to far lower lagging p.f. current generated in armature produce flux in such direction which tends to demagnetize magnetic flux produced by field winding resulting in field winding drawing more current to compensate for this cross magnetizing armature reaction. In case 3, reverse happens as armature current produces flux in same direction as produced by field which tends to magnetizing & subsequently lower excitation current in field. If this armature reaction component is more, the magnetizing component results in overheating of yoke & may result in permanent magnetizing in yoke.

In 2nd part, one can use use generator to its rated output. In first case generator can be loaded upto 50-75% of its rated load & in third case generator loading capability is just 20% of rated capability.

Thus for a 100KW connected load generator capability with p.f. 0.5 lag will be approx 400 kva (100/0.5= 200KVA & 200/0.5= 400 KVA ). For 100 KW connected load with p.f. 0.8 lag, 125 KVA generator is enough but with p.f. 0.8 lead, 625 KVA generator will be required

Your use of this site is subject to the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expressly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2013 Nerds in Control, LLC. All rights reserved.

Users of this site are benefiting from open source technologies, including PHP, MySQL and Apache. Be happy.

Fortune
On Monday mornings I am dedicated to the proposition that all men are
created jerks.
-- H. Allen Smith, "Let the Crabgrass Grow"
Help keep our servers running...