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Startup question: AC generators


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Hi everybody!

 

When is the best moment to switch on the left and right generators? The actual startup procedures provided by DCS and you guys are different in that point.

 

DCS manual says before engine startup. But then the bleed air has to drive the generators as well. Doesn't it make more sense to switch on the generators after starting the engines? Or left generator after left engine start, right gen. after right engine start?

 

And when has the APU gen to be switched OFF? After one generator is running or both?

 

What do you think?

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Hi everybody!

 

When is the best moment to switch on the left and right generators? The actual startup procedures provided by DCS and you guys are different in that point.

 

DCS manual says before engine startup. But then the bleed air has to drive the generators as well. Doesn't it make more sense to switch on the generators after starting the engines? Or left generator after left engine start, right gen. after right engine start?

 

And when has the APU gen to be switched OFF? After one generator is running or both?

 

What do you think?

 

Bleed air does not run the AC gens, they are mechanically driven by the engine, the only reason the switches are there, if a generator fails you can disconnected. For the APU gen, if the APU is running, keep the gen on. It is not modeled in DCS, but without the APU gen on, APU would suffer damage since it drives the cooling fan.

 

For me, gen switches are always on, before engine start and after engine shutdown.

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Real world prior to the pilot stepping to the ACFT I switch on both AC generators and all four boost lump switches. When the pilot returns form his sortie he leaves the switches on and I switch them off.

 

And like stated above bleed air switch has nothing to do with the generators.

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I usually turn on the engine AC generators when turning on the inverter and battery at the start of the flight. Learned to do that after countless times of the plane going dark when shutting off the APU :doh:

 

(I wonder if real life pilots learn the plane by jumping in and flicking buttons until it works.. probably not)

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The beeping is a caution that the engine driven generator is supplying power and the APU generator is no longer needed.

 

If you have both generator switches in the on position before starting the engines, which I believe is the proper procedure, then the first one kicks in once the first engine started reaches 50%. The Master Caution comes on then, and switching the APU generator off clears the caution.

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The beeping is a caution that the engine driven generator is supplying power and the APU generator is no longer needed.

 

If you have both generator switches in the on position before starting the engines, which I believe is the proper procedure, then the first one kicks in once the first engine started reaches 50%. The Master Caution comes on then, and switching the APU generator off clears the caution.

 

For the APU gen, if the APU is running, keep the gen on. It is not modeled in DCS, but without the APU gen on, APU would suffer damage since it drives the cooling fan.

Just press the Master caution light, warning tone will go away

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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The beeping is a caution that the engine driven generator is supplying power and the APU generator is no longer needed.

 

but without the APU gen on, APU would suffer damage since it drives the cooling fan.

 

:huh:

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DCS manual says before engine startup. But then the bleed air has to drive the generators as well

 

It has to drive them either way, they can not be disconnected mechanically AFAIK, only electrically. A generator with open loop needs only little torque to be turned. The loop is only closed once the engine has reached the minimum speed for the generator to put out the correct frequency AC.


Edited by sobek

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It has to drive them either way, they can not be disconnected mechanically AFAIK, only electrically. A generator with open loop needs only little torque to be turned. The loop is only closed once the engine has reached the minimum speed for the generator to put out the correct frequency AC.

 

That sounds logical. Thank you. (otherwise there must be some kind of coupling - never heard that - will ask the mechanics)

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(otherwise there must be some kind of coupling - never heard that - will ask the mechanics)

 

Most large/civilian airplanes use a Constant Speed Drive (CSD) that is between the gearbox and the generator. The entire assembly is called an Integrated Drive Generator (IDG). In an emergency, the CSD can be mechanically disconnected, causing the generator to shut down.

 

You don't typically find this setup on small/fighter type, though...A-10 included.

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You don't typically find this setup on small/fighter type, though...A-10 included.

 

How do the small fighters generate constant frequency AC then? Do they have DC generators?

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How do the small fighters generate constant frequency AC then? Do they have DC generators?

 

Depends on the aircraft and generators. Some use frequency converters to regulate the output. Some have CSD. Some have DC generators also, I call them Batteries :D

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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How do the small fighters generate constant frequency AC then? Do they have DC generators?

 

Some don't generate a constant frequency at all. Watch a HUD tape of a T-38 landing (there are a few on Youtube), and you can hear the hum in the interphone changing as power is changed.

 

As the engine is decelerating, you can actually hear the generator shift to a lower gear in order to preserve the generator's rotational speed above the minimum.

 

A-10s have an IDG

 

:doh: You're right, of course.

"They've got us surrounded again - those poor bastards!" - Lt. Col. Creighton Abrams

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Some don't generate a constant frequency at all. Watch a HUD tape of a T-38 landing (there are a few on Youtube), and you can hear the hum in the interphone changing as power is changed.

 

As the engine is decelerating, you can actually hear the generator shift to a lower gear in order to preserve the generator's rotational speed above the minimum.

 

:doh: You're right, of course.

 

 

Actually, the generators cannot shift gears, they are composed of only a rotar and stator. That is the purpose of the CSD's. There are only a few, like per say the F-15's emergency generator, that can variate it's own speed and that is because it's fluid driven as opposed to mechanical. The PMG voltage is constant and directly proportional to the speed, but the main gen output is varied by alot of things, like excitation voltage for instance.

 

And like stated earlier, all US fighter aircraft that I can think of off of the top of my head have a CSD. F-15's, F-16's, A-10's all use some form of an IDG. And, none of them will produce a completely consistent frequency. There are limits imposed depending on the speeds. At an optimal 1:1 drive, the tolerance may only be +-1 but may vary +-5 or more depending on the drive speed. Gen loading will also effect this a little. The pitch tone in the headset can/will vary in lots of aircraft. I can make it change pitch in an F-15 just by turning on/off the exterior lighting.

 

The only aircraft I know to have a mechanical disconnect ability are heavies. That's just one more thing to fail in most cases. Most fighters with CSD's have an internal solder ring and shear pins that will disconnect during an overheat condition or mechanical lock-up.


Edited by Rainmaker
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How do the small fighters generate constant frequency AC then? Do they have DC generators?

 

See above post with regards to the CSD's.

 

And as far as DC power goes, most use Transformer Rectifier(TR) units. It will take AC power from the main gens and convert to DC. AC power is more efficient and takes less of a push to get it down the line than DC does.

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See above post with regards to the CSD's.

 

It was said that small fighters don't have CSD. :music_whistling:

 

 

And as far as DC power goes, most use Transformer Rectifier(TR) units. It will take AC power from the main gens and convert to DC. AC power is more efficient and takes less of a push to get it down the line than DC does.

 

I know what possibilities there are, lots of my curriculum is made up of electric engineering, i'm just curious what is used exactly and possibly for what reason. ;)

 

Actually concerning propagation of power, you are not correct. High voltage DC is far more efficient for power propagation (but the technology to convert high power AC to DC has only emerged a few years back, Germanies large north sea wind farms use DC underwater cables for power transmission from offshore to the main grid, e.g.). AC is easier to generate and transform, not easier to propagate.


Edited by sobek

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Actually, the generators cannot shift gears, they are composed of only a rotar and stator. That is the purpose of the CSD's.

 

It is entirely possible that the CSD is coupled with a shifted gearbox with 2 speeds to avoid undesireable input-output RPM differential ranges in some designs.

 

I know that many hydraulic torque converters use such devices because as RPM of the input and output becomes equal, they become very inefficient. Same if the RPM differential is too high.

Good, fast, cheap. Choose any two.

Come let's eat grandpa!

Use punctuation, save lives!

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