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Basic Parameters Of An Aircrafts Electrical System  
User currently offlinenovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Posted (1 year 6 months 4 weeks 1 day 6 hours ago) and read 3981 times:

From my notes it states that the basic parameters of an aircraft's electrical system are

1. No paralleling of the ac sources of power.

2. All generator bus sources have to be manually connected through the movement of a switch that also will disconnect any previously existing source.

Can anyone explain are elaborate on these two points further to help my understanding of them?

Thanks

10 replies: All unread, jump to last
 
User currently offlinefr8mech From United States of America, joined Sep 2005, 5599 posts, RR: 15
Reply 1, posted (1 year 6 months 4 weeks 1 day ago) and read 3929 times:

Quoting novice (Thread starter):
1. No paralleling of the ac sources of power.

Incorrect. Most of your older generation aircraft (B727, DC8, B747, etc) allowed their generators to parallel as normal operation. I know the MD11 flies around with its generators paralleled. The B757 and B767 run isolated. Not sure about the other more modern aircraft.

Now, it is true that you can not parallel an APU generator to the engine generators, nor can you parallel external power to any of the ship's generators.

Quoting novice (Thread starter):
2. All generator bus sources have to be manually connected through the movement of a switch that also will disconnect any previously existing source.

Again, incorrect. I know that the B757/B767/B744/A300 and MD11 all automatically bring their generators onto their respective busses (or the sync buss) when the generator comes online and the power is "good".

I'm not sure where you got your information.

The very basic requirements are spelled out in FAR 25.1351, 25.1353 and 25.1355

http://www.ecfr.gov/cgi-bin/text-idx...browse/Title14/14cfr25_main_02.tpl

[Edited 2013-04-30 14:27:00]

[Edited 2013-04-30 14:31:11]


When seconds count...the police are minutes away.
User currently offlineairbuster From Netherlands, joined Mar 2007, 451 posts, RR: 0
Reply 2, posted (1 year 6 months 4 weeks 7 hours ago) and read 3797 times:

Novice,

The text you wrote is an exact copy of what is written in the 737 FCOM. And is correct for that type. As stated above, some do parallel and on the MD11 this gives no power transient and it runs very smoothly.



FLY FOKKER JET LINE!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17110 posts, RR: 66
Reply 3, posted (1 year 6 months 4 weeks 7 hours ago) and read 3788 times:

Quoting airbuster (Reply 2):
The text you wrote is an exact copy of what is written in the 737 FCOM.

The 737 is used as the "standard" jet airliner in several JAA/EASA exams. Are those the notes that you have novice?

[Edited 2013-05-01 07:41:19]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinenovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Reply 4, posted (1 year 6 months 3 weeks 6 days 8 hours ago) and read 3643 times:

'Airbuster' what is the type that the information is correct for?

Quoting fr8mech (Reply 1):
Now, it is true that you can not parallel an APU generator to the engine generators, nor can you parallel external power to any of the ship's generators.

Is this because of different voltage requirements?

Source: 'ACE THE TECHNICAL PILOT INTERVIEW' by GARY V. BRISTOW - Page 181


User currently offlinefr8mech From United States of America, joined Sep 2005, 5599 posts, RR: 15
Reply 5, posted (1 year 6 months 3 weeks 6 days 2 hours ago) and read 3604 times:

Quoting novice (Reply 4):
Is this because of different voltage requirements?

No, it's because external power units can not be controlled by the aircraft and APU generator speed, and therefore frequency, is regulated by the APU speed itself. The APU is not nearly as troublesome as an external power unit.

In order to parallel power sources, the frequencies must match. Ground power units are notorious for being a little squirrely in frequency. Yes, newer units have much better controls, but the bottom line is that the aircraft would be unable to tweak the frequency of the external power unit.

The aircraft can control the speed of the APU, therefore controlling the frequency of the generator output, but the APU also supplies air to the aircraft and APU speed will tend to vary a little as demand shifts on the pneumatic side, before settling back down to the speed that provides 115V/400hz power. Maybe not enough to kick the generator offline, but why risk it? The APU generator is sufficient, in of itself, to power the whole aircraft (as are each of the engine generators).



When seconds count...the police are minutes away.
User currently offlinenovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Reply 6, posted (1 year 6 months 3 weeks 4 days 1 hour ago) and read 3440 times:

I don't understand why can't the external power units can not be controlled by the aircraft if the aircraft can control the speed of the APU which therefore controls the frequency of the generator output?



Quoting fr8mech (Reply 5):
but why risk it?

Risk using the APU to supply air to the aircraft?


User currently offlinefr8mech From United States of America, joined Sep 2005, 5599 posts, RR: 15
Reply 7, posted (1 year 6 months 3 weeks 4 days ago) and read 3434 times:

Quoting novice (Reply 6):
I don't understand why can't the external power units can not be controlled by the aircraft


Because the external power unit is external to the aircraft. The units vary in design and how they generate the power. They can be diesel engine powered, they can be powered through the terminal network. Heck, I've seen GPU/Airstart units that are powered by a small centrifugal jet engine. The only thing they have in common is that they produce 115V/400hz AC and have a standard plug.

Bottom line is that there is no provision on the aircraft to control the quality of power being supplied to the aircraft. It can only kick it off if it doesn't like the power, by opening the external power breaker.

Quoting novice (Reply 6):
Risk using the APU to supply air to the aircraft?


No. The risk, and it's minimal, of an APU generator's output fluctuating while it's paralleled to an aircraft generator, during pneumatic operation.

As far as I know, the only time an engine generator is paralleled to the APU is just before the engine generator takes the buss in order to execute what is called a "no break power transfer". Immediately after the generator is synched to the buss, the APU is forced off. This makes for a smooth transition that isn't accompanied by flickering screens and lights.



When seconds count...the police are minutes away.
User currently offlineFlyHossD From United States of America, joined Nov 2009, 945 posts, RR: 2
Reply 8, posted (1 year 6 months 3 weeks 3 days 19 hours ago) and read 3389 times:

Quoting novice (Reply 6):

I don't understand why can't the external power units can not be controlled by the aircraft if the aircraft can control the speed of the APU which therefore controls the frequency of the generator output?
Quoting fr8mech (Reply 7):
Because the external power unit is external to the aircraft. The units vary in design and how they generate the power. They can be diesel engine powered, they can be powered through the terminal network. Heck, I've seen GPU/Airstart units that are powered by a small centrifugal jet engine. The only thing they have in common is that they produce 115V/400hz AC and have a standard plug.

Bottom line is that there is no provision on the aircraft to control the quality of power being supplied to the aircraft. It can only kick it off if it doesn't like the power, by opening the external power breaker.

   Well stated.

Further, in my experience, the quality of maintenance of the ground power units also varies widely. The older they get, the more variable the output becomes.



My statements do not represent my former employer or my current employer and are my opinions only.
User currently offlinenovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Reply 9, posted (1 year 6 months 3 weeks 3 days 5 hours ago) and read 3342 times:

Thanks for clarifying that fr8mech    makes sense now.

User currently offlinecelestar345 From Hong Kong, joined May 2013, 19 posts, RR: 0
Reply 10, posted (1 year 6 months 2 weeks 6 days 6 hours ago) and read 3107 times:

Quoting fr8mech (Reply 7):
Bottom line is that there is no provision on the aircraft to control the quality of power being supplied to the aircraft. It can only kick it off if it doesn't like the power, by opening the external power breaker.

Second to that. At where I work there are a few diesel units that the output quality is so bad that the aircraft will just rejects and shuts down... Many occasions when the electrical hydraulic pump is being selected on and the sudden load caused the power cut - quite funny (with hindsight) watching his/her stunned expression and the ADIRU warning sound at the background....


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