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Alternator Failure Q

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Slye said:
"Flapping" was used in an exaggerated layman sense and I could see how it could be misconstrued. No, I have no idea what an alternator is. And yes, I'd love some chocolate a$$cream...


LMAO...
 
Austin Powers, anyone? Anyway...I didn't mean to tangentize the conversation, I tend to wander sometimes.
 
An alternator that overcharges is not an alternator failure, it's a voltage regulator failure. The alternator always has the capacity to overcharge but the voltage regulator throttles it.

If the alternator itself isn't charging then leaving the alternator master on isn't going to make much of a difference. If it's overcharging then leaving it on could make your battery explode or catch fire... this will obviously ruin your day.

Of course there's no real way to tell in flight if it's the alternator itself or the voltage regulator that's failed aside from a couple failure modes so turning off the alternator master for any charging system failure is good advice.
 
EagleRJ said:
Don't have anything besides light bulbs running if you do this, because the raw 14V output from the alternator will be hard on avionics and other gear without the battery acting as a buffer.

Huh? I don't think this is correct. Assuming the alternator contains an internal voltage regulator, its output will be around 14V DC. Most 12V electrical systems are actually designed to draw from this slightly higher voltage, because such a voltage is necessary to keep the battery charging at a faster rate than the discharge rate associated with the operation of various electrical devises. Subsequently, you'll likely find that a fully charged battery, with the engine running, will actually measure well over 12V with a multimeter. In other words, the battery does not act as a "buffer" and it isn't even necessary because the electrical system is designed to handle 14 volts. Not sure what you mean by a "buffer". If certain avionics equipment requires a reduction in voltage, it will be accomplished through the use of integral, solid-states devises. The battery is simply a box utilizing a crude chemical reaction to store electrical energy. It is not a buffer, and is certainly not used to reduce voltages/amperages for the operation of avionics equipment.

Also what is the difference between "raw" voltage and "refined" voltage, assuming they're both DC?????????

Not nagging, just wondering where you came up with this....
 
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TrafficInSight said:
An alternator that overcharges is not an alternator failure, it's a voltage regulator failure.

Yes, but we're assuming here that quite a few alternators utilize internal voltage regulators. If the device fails, you may as well conclude that the alternator has failed. Keeping the alternator offline during the failure of a voltage regulator WILL prevent further damage to other electrical components.
 
A voltage regulator is a seperate unit completely. I don't know of any aircraft alternators that have integral voltage regulators.

On a 152/172 it's a little box mounted at the top of the firewall near the center. It has a bundle of wires and a plug on it and some solid state ones have an LED on them.

Notice that I said turning off the alternator master is a good idea even if it wont change anything?
 
UnAnswerd said:


Huh? I don't think this is correct. Assuming the alternator contains an internal voltage regulator, its output will be around 14V DC. Most 12V electrical systems are actually designed to draw from this slightly higher voltage, because such a voltage is necessary to keep the battery charging at a faster rate than the discharge rate associated with the operation of various electrical devises. Subsequently, you'll likely find that a fully charged battery, with the engine running, will actually measure well over 12V with a multimeter. In other words, the battery does not act as a "buffer" and it isn't even necessary because the electrical system is designed to handle 14 volts. Not sure what you mean by a "buffer". If certain avionics equipment requires a reduction in voltage, it will be accomplished through the use of integral, solid-states devises. The battery is simply a box utilizing a crude chemical reaction to store electrical energy. It is not a buffer, and is certainly not used to reduce voltages/amperages for the operation of avionics equipment.

Also what is the difference between "raw" voltage and "refined" voltage, assuming they're both DC?????????

Not nagging, just wondering where you came up with this....

You're right- a healthy alternator puts out 14V and normally, an aircraft's electrical system operates at 14V. I'm talking about the quality of the power.

If you were to look at the system voltage during normal operations on an oscilloscope, the line would be reletively stable and level. If you looked at the raw output from the alternator, it would be a rough, jagged line with a lot of fast variations in voltage. The internal power supplies in avionics aren't able to adjust that quickly to voltage variances, so they would experience static, flickering displays, and maybe even malfunctions.
The alternator doesn't put out a rock-stable 14V anyway. It will put out around 12V at a low idle (when the alternator warning light is flickering), and go up to around 14.5V at full throttle. That's normal, even with the voltage regulator working properly. Unregulated, the alternator would put out 20V or more at full throttle.

The power supplies in most avionics are able to handle input voltages from around 10V to 30V, to accomodate installations in both 14V and 28V aircraft. It's noisy, unstable power they don't like. One of the functions of the battery is to help absorb that fluctuation and stabilize the voltage for the aircraft's systems.
 
EagleRJ said:
One of the functions of the battery is to help absorb that fluctuation and stabilize the voltage for the aircraft's systems.

Which is why those red Cessna alt/battery switches interlock to keep the battery on line anytime the alternator field is on.
 
EagleRJ said:
The power supplies in most avionics are able to handle input voltages from around 10V to 30V, to accomodate installations in both 14V and 28V aircraft. It's noisy, unstable power they don't like. One of the functions of the battery is to help absorb that fluctuation and stabilize the voltage for the aircraft's systems.

Okay that does make sense. Perhaps the alternator output is not as stable as I had believed. You're correct about the battery providing consistent voltage to the electrical system, but nevertheless I was confused about what you meant by "buffer", as I thought the avionics equipment was capable of dealing with minor voltage fluctuations.Time to hit the books.....
 
TrafficInSight said:
A voltage regulator is a seperate unit completely.

Frankly, I'm surprised by that. The trend for 30 years in the automotive industry has been internal regulators. Yeah, we're dealing with airplanes, bad comparission I guess....

Speaking of that, anyone know what aircraft fuel-injection systems are like? One of the great benifits of aircraft engines has long been the ability to operate without any need for outside electrical power. Is this still possible w/ fuel injection???
 

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