AC generator ratings

[Delbert]

I am an aircraft technician and I need some help from fellow ATr's understanding some electrical current values. While I am familiar with ohms law, I need to see if the rules change any when applying AC power to the equation.
A single generator on the aircraft type I primarily work on outputs 115 volts, 400 hertz, 3-phase AC power. It has a nominal output load rating of 30 KVA. According to the documentation I have, the crew is alerted (via amber text message that says GEN OVERLOAD on one of the CRT?s in the flight deck) when the load is greater than 35KVA. I have never seen the load get over 20 (except in the simulator) so I will take their word for it. The documentation I have also says the generator will be automatically shut down if the load exceeds 187 amps. This is where my question comes. I can?t fit the 187 amp value into the picture. The math doest work out. It should be higher than the 35 KVA trigger. Anyone have a guess?

 

[Geniere]

Click on this link.

http://www.traceengineering.com/technical/tech_notes/tn11.html

Regards

 

[Delbert]

Thanks Geniere this looks to be exactly what I was looking for. I couldn't help but notice this was your first post at Anandtech forums. Welcome. You currently have a perfect score in the contribution factor. Again, thanks.

 

[Evadman]

Originally posted by: Delbert
Thanks Geniere this looks to be exactly what I was looking for. I couldn't help but notice this was your first post at Anandtech forums. Welcome. You currently have a perfect score in the contribution factor. Again, thanks.

And I can not rate him in his profile because it is not turned on. Poopie.

 

[Mark R]

That article is not actually relevant to the question.

The concept of power factor does not apply here: like all generators, the rating here is given in Volt-Amps which correspond to apparent power - similarly RMS current is only related to apparent power.

The problem is the 3 phase nature of the system. This system is probably arranged as 3 phases, each of 66V [115 / sqrt (3) ] with resepect to neutral. This provides 3 seperate 115 V supplies - each supply formed by connecting to a pair of phases.

If you have 3 circuits each drawing 100 A from each 115 V supply - then the current on each phase will be 173 A [100 * sqrt (3) ] In total power terms this is 100 * 115 * 3 = 34.5 kVA

The maximum current of 187 A per phase the maximum rating of the winding on the gen - if the current on each phase is equal then this corresponds to a total apparent power of 37 kVA. But also note, that it owuld not be possible to draw 200 A from 1 phase and 0 from the others: even though the total power is less than the gens rating, the currents exceeds the maximum safe current (even if it is only one phase rather than 3).

 

[Geniere]

Well Delbert, I ?m happy to have a perfect score, however Mark?s response may be more appropriate.

I?ve browsed this forum for quite a while trying to learn and now, contribute if I can.

Anyhow, thanks to you and also Evademan for trying.

Regards