20th Apr 2012, 09:54
Perhaps someone can help me understand how current limiter works.
On CJ there is a 225amps current limiter in crossfeed bus. According to training manual on the ground generator limit is 250 apms. So here is my question: When on the ground and one engine is running while crossfeeding power to all buses my limit is 225 amps and when both engines are running my amps limit now is 250 amps. Is that correct ?
Im asking because when we start one engine after lets say one week of no flying often gen amps is as high as lets say 220 amps. In such case I should wait till gen amps decrease below 200 before swithing on avionics in order not to exceed 225 amps of current limiter, correct ?
20th Apr 2012, 12:37
Im asking because when we start one engine after lets say one week of no flying often gen amps is as high as lets say 220 amps.
Dariuszw, Age: 17 :E
20th Apr 2012, 14:49
Dariuszw, Age: 17
...with a mental age:6
20th Apr 2012, 16:25
Oh, come on, it's just a question.
I have not flown the CJ but it sounds as if it's just a fuse (same in KingAir).
Yes, start first eng then allow the batteries to recover before starting second. Otherwise, the starting current reqd for the second eng start is likely to blow the fuse (current limiter).
20th Apr 2012, 20:04
If he knew the correct starting procedure for the CJ he would not need to ask the question, I believe the answer is in the checklist!
21st Apr 2012, 04:55
Let me explain my problem in different way.
Current limiter in crossfeed bus is only 225 amps, yet allowable generator load in flight with single engine is 300 amps. How can 300 amps gen load be acceptable when crossfeed limiter is only 225 ?
Electrics was never my strong domains so having said that I assume that 300 amps load is being divided as it feeds three seperate busses and perhaps thats why 225 amps crossfeed limiter would not pop. Nevertheless I hope my older, wiser coligues can shed some light on it. :rolleyes:
21st Apr 2012, 15:59
Because the single engine, 300A allowable load is supplying both its bus and, through the bus-tie, the bus for the failed generator. The working generator has to have a greater allowable load than the bus-tie in order to supply the bus-tie's load *plus* its own bus' load.
21st Apr 2012, 16:09
The limiter is there so that if the reason for the first generator going US was because of a short taking it out with over 300amps pulled.
Then when the bus tie is engaged it doesn't weld/crew keep there finger on it, it doesn't take the other one with it as well.
22nd Apr 2012, 23:37
It would probably help to understand what current limiting actually is. You won't fully understand why it's 225 amps until you understand what a current limiter does and why they're in airplanes.
Current limiting - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Current_limiting)