fr0g
Well-known member
- Joined
- Jun 20, 2003
- Posts
- 104
WOW!! A lot of electronic smoke flying..
My 2 cents worth, from the perspective of a light, GA aircraft
Battery:
The battery can be thought of a electrical "reservoir" that STORES electricity.
It never creates electricity - it's kind of like a bank account - if you take money out, you have to put it back in - or have a drained battery. When the battery is "online" - usually by the master - it is connected to the main electrical bus.
The battery is there to start the engine, when the generator/alternator has no output yet, and to provide emergency power (for a while) if the generator/alternator goes out. When the generator/alternator is online, the battery is essentially out of the loop - the generator/alternator provides the electricity for aircraft systems. The only current going to the battery then is to charge it from the electricity "borrowed" during start.
Generator:
A generator has a spinning armature inside field windings. The field windings create a magnetic field, which is varied by the voltage regulator to regulate generator output. The intensity of the field, and the speed of the generator rotation are directly related to the generator output. Why is the regulator attached to the field coil instead of the output of the generator instead? The current draw of a typical field winding is much less than the current output of the generator (say 3 amps instead of 35 or 50 amps at full power) and therefore much "easier" (less components, cheaper) to regulate.
The commutator is the part of the generator that the brushes ride on, and provide an electrical connection between the spinning armature and the electrical outputs of the generator. They also convert the alternating current output of the armature to direct current - mechanically.
Most generators are self exciting - they will provide electricity with no input from the battery - so your generator can go "online" and start producing current without an external power source, such as a battery.
Alternator:
Basically the same as a generator, except the "field" windings are in the center - the spinning rotor. You can think of the generator as coils spinning inside a magnetic field, whereas the alternator is a magnetic field spinning inside coils.
the field from the rotor is controlled much the same way as in a generator, by the voltage regulator. Once again, it is much easier to control the field current because it is much less than the output current. As to whether alternators are self exciting - that is can START producing electricity without external power, I do not know.
They should have no problem CONTINUING to provide power once they are "online", however, even with a disconnected or dead battery.
The alternator output current does not go through a commutator, and therefore must go through a diode (one way electrical valve, so to speak) bridge and get "rectified" from AC (alternating current) to DC (direct current). The advantage over the generator? The mechanical commutator introduces a lot of noise, and the diode rectifies it with no additional noise. Also, they can generally be smaller.
ammeter:
This device can be connected 2 ways, I believe:
1. From the output of the generator/alternator to the electrical bus, where it measures the output of the generator/alternator. This will not directly tell you the current to the battery, but will tell you if the generator is working. This should never read negative. However, battery current = generator current - electrical load
so, if you have nothing on, and it reads 20 amps - you can bet the battery is charging (as long as there are no electrical shorts)
2. From the electrical bus to the battery: This will measure the current either charging (+) or discharging (-) from the battery. Simply put - if the generator/alternator output is sufficient for all the aircraft systems, it will be 0. (centered). The reason it usually sits on the charge (+) side of 0 is because there is some current going to the battery to keep it charged, or recharge it from starting the engine. This usually is about 5-20 amps right after the engine start, and will trickle off to about 5 amps or less after about 20 minutes. After an hour of flight, it should be about 1-5 amps, unless the battery was super drained to begin with.
If the ammeter indicates discharge (-), then the output of the generator/alternator is insufficient to keep up with the electrical load of the aircraft systems, and therefore, they are "borrowing" electricity from the battery. This is normal during idle with a bunch of electrical items turned on - landing lights, nav lights, pitot hear etc. However, even with ALL items turned on, it should be positive by about 1800 RPM.
How can you tell which system you have? Simple:
with the engine shut down, turn on the landing lights. If the ammeter stays at 0, you have system 1. if it deflects to negative (about 10 amps) then you have system 2.
The generator/alternator output voltage will always be greater than battery voltage, to overcome the battery's internal resistance during charging.
Hope this helps!
My 2 cents worth, from the perspective of a light, GA aircraft
Battery:
The battery can be thought of a electrical "reservoir" that STORES electricity.
It never creates electricity - it's kind of like a bank account - if you take money out, you have to put it back in - or have a drained battery. When the battery is "online" - usually by the master - it is connected to the main electrical bus.
The battery is there to start the engine, when the generator/alternator has no output yet, and to provide emergency power (for a while) if the generator/alternator goes out. When the generator/alternator is online, the battery is essentially out of the loop - the generator/alternator provides the electricity for aircraft systems. The only current going to the battery then is to charge it from the electricity "borrowed" during start.
Generator:
A generator has a spinning armature inside field windings. The field windings create a magnetic field, which is varied by the voltage regulator to regulate generator output. The intensity of the field, and the speed of the generator rotation are directly related to the generator output. Why is the regulator attached to the field coil instead of the output of the generator instead? The current draw of a typical field winding is much less than the current output of the generator (say 3 amps instead of 35 or 50 amps at full power) and therefore much "easier" (less components, cheaper) to regulate.
The commutator is the part of the generator that the brushes ride on, and provide an electrical connection between the spinning armature and the electrical outputs of the generator. They also convert the alternating current output of the armature to direct current - mechanically.
Most generators are self exciting - they will provide electricity with no input from the battery - so your generator can go "online" and start producing current without an external power source, such as a battery.
Alternator:
Basically the same as a generator, except the "field" windings are in the center - the spinning rotor. You can think of the generator as coils spinning inside a magnetic field, whereas the alternator is a magnetic field spinning inside coils.
the field from the rotor is controlled much the same way as in a generator, by the voltage regulator. Once again, it is much easier to control the field current because it is much less than the output current. As to whether alternators are self exciting - that is can START producing electricity without external power, I do not know.
They should have no problem CONTINUING to provide power once they are "online", however, even with a disconnected or dead battery.
The alternator output current does not go through a commutator, and therefore must go through a diode (one way electrical valve, so to speak) bridge and get "rectified" from AC (alternating current) to DC (direct current). The advantage over the generator? The mechanical commutator introduces a lot of noise, and the diode rectifies it with no additional noise. Also, they can generally be smaller.
ammeter:
This device can be connected 2 ways, I believe:
1. From the output of the generator/alternator to the electrical bus, where it measures the output of the generator/alternator. This will not directly tell you the current to the battery, but will tell you if the generator is working. This should never read negative. However, battery current = generator current - electrical load
so, if you have nothing on, and it reads 20 amps - you can bet the battery is charging (as long as there are no electrical shorts)
2. From the electrical bus to the battery: This will measure the current either charging (+) or discharging (-) from the battery. Simply put - if the generator/alternator output is sufficient for all the aircraft systems, it will be 0. (centered). The reason it usually sits on the charge (+) side of 0 is because there is some current going to the battery to keep it charged, or recharge it from starting the engine. This usually is about 5-20 amps right after the engine start, and will trickle off to about 5 amps or less after about 20 minutes. After an hour of flight, it should be about 1-5 amps, unless the battery was super drained to begin with.
If the ammeter indicates discharge (-), then the output of the generator/alternator is insufficient to keep up with the electrical load of the aircraft systems, and therefore, they are "borrowing" electricity from the battery. This is normal during idle with a bunch of electrical items turned on - landing lights, nav lights, pitot hear etc. However, even with ALL items turned on, it should be positive by about 1800 RPM.
How can you tell which system you have? Simple:
with the engine shut down, turn on the landing lights. If the ammeter stays at 0, you have system 1. if it deflects to negative (about 10 amps) then you have system 2.
The generator/alternator output voltage will always be greater than battery voltage, to overcome the battery's internal resistance during charging.
Hope this helps!
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