Why don't jets have alternators?

There are lots of turbine-powered vehicles. In fact, a turbo-prop (aka prop-jet) aeroplane uses gear reduction from a turbine to drive a propellor.
 
Roger Roger said:
Ummm... A gen puts out DC without any rectification. And since no alternator I've ever dealt with doesn't have an internal rectifier, for all intents and purposes an alternator spits out DC. It's semantics.
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That defies the laws of physics. For the scope of our discussion all spinning power generation has to be AC. It can be changed to DC with a commutator or rectifier.
 
z987k said:
That defies the laws of physics. For the scope of our discussion all spinning power generation has to be AC. It can be changed to DC with a commutator or rectifier.
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Ok, but the rectifier or commutator as applicable are internal to the component, so for all intents and purposes, DC.
 
Cloud Surfer said:
But in all those cases of having an alternator, it seems it is meant to act as a backup. Perhaps in addition to having the advantage of being combined into a starter/gen unit, the high RPMs at which turboprop/jets operate makes generators more efficient as a primary source?

Also, I'm not too familiar with what airline turboprops/jets/fans possess, so I'm glossing over them it until some of our airline members would like to add something in regards, although I do recall that the CRJ uses generator units and APUs for electrical production and respectively starting.

BTW can a turbine starter motor be combined with an alternator?
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Starter/Generators don't turn at a high RPM typically. They are much heavier than alternators but also put out a lot more current. The S/G on the single engine turboprop I fly puts out up to 300 amps whereas the alternator only makes 40. Also, on most singles that I'm familiar with both alternator and S/G are used so the alternator isn't really a backup, more like a second engine. Typically you have two (or more) busses, two batteries, two power generating sources, etc for certification requirements.

To answer your question, no, an alternator cannot function as a starter, at least not if we are talking about your typically small, field rotating one that is not self excited.
 
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ppragman said:
He's technically correct - the best kind of correct.
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The way they are built determines the kind of current they make. It's really pretty simple. A DC generator will never make AC current, and vice-versa. Yes, you can change it after the fact, but the former applies. How they are built matters.

Unless I missed something about his post.
 
mshunter said:
The way they are built determines the kind of current they make. It's really pretty simple. A DC generator will never make AC current, and vice-versa. Yes, you can change it after the fact, but the former applies. How they are built matters.

Unless I missed something about his post.
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Uhhh......

:cool::D
 
mshunter said:
The way they are built determines the kind of current they make. It's really pretty simple. A DC generator will never make AC current, and vice-versa. Yes, you can change it after the fact, but the former applies. How they are built matters.

Unless I missed something about his post.
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Ya I think you missed the part where a DC generator is physically impossible to have. You HAVE to convert it after the fact. Or kind of during the fact with a commutator, but it's still AC more or less being mechanically rectified.
You cannot spin magnets or wires around the other and produce DC current. (within the confines of of useful things for aviation. We'll not get into a Faraday disc and the like.)
 
z987k said:
Ya I think you missed the part where a DC generator is physically impossible to have. You HAVE to convert it after the fact. Or kind of during the fact with a commutator, but it's still AC more or less being mechanically rectified.
You cannot spin magnets or wires around the other and produce DC current. (within the confines of of useful things for aviation. We'll not get into a Faraday disc and the like.)
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While this is true as inductance is only sustained in a AC system, when the nomenclature "generator," or "alternator," is used it is typically understood as DC vs AC, depending on the application.
 
Big jets tend to have alternators but they tend to call them generators anyway. They use constant speed drives to maintain the frequency.
 
Just to confuse the issue, large cabin Gulfstreams have AC Generators, and if it's a G-II it has DC Generators also. Some of the AC goes through Transformer/Rectifiers and comes out DC. Hawkers, some Lears and Citations have alternators for the sole purpose of heating the windshields. And anyone who thinks an alternator (AC Generator) is light is more than welcome to come help next time I have to change one, it's usually a two man job anyway. Don't even get me started on IDGs.:eek:
 
Maybe I'm wrong, but every jet I've been involved with so far, CRJ-700, EMB-135/145, Airbus, LM-2500s have alternators as well, the PMA kind of alternators.

It's just that the alternators don't supply power to the aircraft, they supply power for the engine FADECs so that the FADECs can still control the engine during a loss of all aircraft power.
 
z987k said:
Ya I think you missed the part where a DC generator is physically impossible to have. You HAVE to convert it after the fact. Or kind of during the fact with a commutator, but it's still AC more or less being mechanically rectified.
You cannot spin magnets or wires around the other and produce DC current. (within the confines of of useful things for aviation. We'll not get into a Faraday disc and the like.)
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This is true. If you take a DC generator and hook it up to an oscilloscope, you will see pulsations, not a straight line. Although, unlike an AC generator, the periods/pulsations will never dip into the negatives, always positive output. If you were to increase the frequency of the DC generator, the periods would become tighter and therefore, the peak to peak output of the generator would start to look like a straight line. A commutator will produce this kind of oscillation without any kind of back end controls like a rectifier.

Power plants have both AC and DC generators on the same rotor/turbine shaft. The AC generators provide power to the people, and the DC generator provide the excitation current for the AC generator.
 
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