Alternators & Electrical Supply

[uwochris]

Hey guys,

It is my understanding that the battery provides electrical power for starting and use of the equipment when the engines are not running. If the engines are running, then the alternator will carry all the load. In other words, the battery acts as a back-up in case of an alternator problem (except for starting).

Now some people (including a cfi) have told me that it is the opposite... the battery supplies all electrical power (even during normal flight), while the alternator is there simply to keep it charged. This does not seem consistent with what I have read in certain books and POHs, and I hope someone can clarify this issue. Also, seeing how the voltage of the battery is always a little less than the alternator would seem to disprove this theory. (?)

Another question... what is considered "acceptable" for the position of the needle on the ammeter? The C152 POH says 2 needle bar widths on the positive side- is this a "cessna thing," or does this rule apply to all a/c, including turbine powered a/c.

If my initial understanding is correct (i.e. the alternator supplies all electrical load once the engines are running, not the battery), it should make sense that the needle on the ammeter should be 0 (neither positive nor negative, since the battery does not need to be kept charged as the alternator is supplying all electical power). However, why then is it considered acceptable to see 2 needle bar widths on the plus side? I am guessing that anything above 2 needle widths is excessive and could cause damage due to high rates of charging, like overheating.

Thanks in advance for any comments,

Chris.

 

[mmmdonut]

The Legbone Is Connnected To The Ankle Bone.... The Hambone Is Connected To My.... Trom Bone

Wheeeeeeeeeeeeeeeeeeee

 

[mtrv]

Hey guys,

It is my understanding that the battery provides electrical power for starting and use of the equipment when the engines are not running. If the engines are running, then the alternator will carry all the load. In other words, the battery acts as a back-up in case of an alternator problem (except for starting).

Now some people (including a cfi) have told me that it is the opposite... the battery supplies all electrical power (even during normal flight), while the alternator is there simply to keep it charged. This does not seem consistent with what I have read in certain books and POHs, and I hope someone can clarify this issue. Also, seeing how the voltage of the battery is always a little less than the alternator would seem to disprove this theory. (?)

Another question... what is considered "acceptable" for the position of the needle on the ammeter? The C152 POH says 2 needle bar widths on the positive side- is this a "cessna thing," or does this rule apply to all a/c, including turbine powered a/c.

If my initial understanding is correct (i.e. the alternator supplies all electrical load once the engines are running, not the battery), it should make sense that the needle on the ammeter should be 0 (neither positive nor negative, since the battery does not need to be kept charged as the alternator is supplying all electical power). However, why then is it considered acceptable to see 2 needle bar widths on the plus side? I am guessing that anything above 2 needle widths is excessive and could cause damage due to high rates of charging, like overheating.

Thanks in advance for any comments,

Chris.

Chris, the alternator does supply the normal power load while it's in operation. And as you've mentioned, at a slightly higher voltage than the battery. Excess current does have to go to the battery as storage, which is the reason that the battery switch must be turned on while the alternator is in operation.

Ammeters can be wired at least two different ways that I know of, but after a few "after work drinks", I'm much too relaxed to get into specifics.

But at least, one way doesn't fit all.

 

[IFlyGC]

Well, it depends on how technical you want to get.....

Initially, the battery is solely the supplier of electricity to power the starter to get the whole thing turning. Once the engine is running the battery supplies a small charge to the voltage regulator to keep the alternator functioning properly. (hence it doesn't work with a completely dead battery.....you need a little power to make a lot of power)

In the circuit, the alternator provides power to the battery to keep it charged while the battery continually discharges to supply the equipment on the aircraft. The ammeter usually provides more power than needed to just charge the battery however and the extra powers the equipment.

Think of it like this, pour a bucket of water onto a sponge...... the bucket is the alternator with a LOT of power, the sponge is the battery. The battery absorbs (aka charges) the water it needs to stay drenched while some drips from the bottom. When you turn the engine off, the bucket stops pouring and all that's left is the water in the sponge ready for next time......squeeze the sponge and away ya go!

 

[sky37d]

Hey guys,


Another question... what is considered "acceptable" for the position of the needle on the ammeter? The C152 POH says 2 needle bar widths on the positive side- is this a "cessna thing," or does this rule apply to all a/c, including turbine powered a/c.

Chris.

The position of the needle on the ammeter is definitely a Cessna thing. Some aircraft do not have ammeters. The ammeter will deflect based on load, and the amount of current the ammeter is designed to handle.

 

[Lead Sled]

...the battery acts as a back-up in case of an alternator problem (except for starting)...

...some people have told me that it is the opposite... the battery supplies all electrical power (even during normal flight), while the alternator is there simply to keep it charged...

Chris...
Is a 12oz glass containing 6 ozs of water 1/2 full or 1/2 empty? In most light aircraft it could probably be argued either way. I realize that in the case of a fixed gear, VFR training plane being operated in non-controlled airspace the loss of all electrical power is not a serious event, but merely an inconvenience. (I learned to fly in an Aeronca 7AC Champ that had no electrical system.)

In the jets that I fly, there are checklist procedures that deal with the loss of the batteries (we have two) as well as the loss of the generators. If I loose one or both batteries, I can continue on without the loss any of the major systems indefinately. (OK, I know - until the fuel runs out and the generators drop off line.) The loss of generating capability is another matter entirely - it is a very serious emergency scenario, with very definate elapsed time constraints. You basically have to get the airplane on the ground, like right now. That being said, from an operation point of view - the battery must be considered as the backup to the generators.

'Sled

 

[uwochris]

Thanks a lot for the responses.

I find it interesting what IFlyGC wrote... in his/her post, he/she mentionned that the alternator cannot work with a completely dead battery. If I am flying and I shut down the battery side of the master switch, everything still functions properly. Why is this the case? If your battery becomes U/S, can you still use any equipment if your alternator is functionning properly?

I just do not see why a battery is required to be functionning if the alternator is what is supplying the load. Perhaps I do not understand the role of the voltage regulator (it was mentionned this is required for the alternator to function, and it runs off battery juice).

IFLYGC also wrote: "In the circuit, the alternator provides power to the battery to keep it charged while the battery continually discharges to supply the equipment on the aircraft." This seems to contradict what was written by mtrv: "...the alternator does supply the normal power load while it's in operation..."

All comments are welcome!

Chris.

 

[mtrv]

Thanks a lot for the responses.

I find it interesting what IFlyGC wrote... in his/her post, he/she mentionned that the alternator cannot work with a completely dead battery. If I am flying and I shut down the battery side of the master switch, everything still functions properly. Why is this the case? If your battery becomes U/S, can you still use any equipment if your alternator is functionning properly?

I just do not see why a battery is required to be functionning if the alternator is what is supplying the load. Perhaps I do not understand the role of the voltage regulator (it was mentionned this is required for the alternator to function, and it runs off battery juice).

IFLYGC also wrote: "In the circuit, the alternator provides power to the battery to keep it charged while the battery continually discharges to supply the equipment on the aircraft." This seems to contradict what was written by mtrv: "...the alternator does supply the normal power load while it's in operation..."

All comments are welcome!

Chris.

The battery supplies voltage to the alternator's field winding. It's not actually the voltage regulator that we're concerned with here. And those double red Cessna type battery/alternator switches are actually interlocked. You can't turn off the battery without turning off the alternator field. But you can turn off the alternator without the battery side.

If the alternator is in operation & supplying current to the electrical busses, then the alternator field is also supplied by these circuits & will operate, should the battery fail. But you can't get the alternator work if there is no battery juice to start with. Problem with taking batteries off line, is that it can cause the alternator regulator to fail.

 

[IFlyGC]

That's actually the primary reason some aircraft have generators and not alternators. In a generator you have a coil of wire spinning around a magnet which creates a current. In an alternator you have 2 coils of wire and no magnets. One of those coils is stationary and powered by the battery and the other spins, creating the current.

The down side to a generator is that it is usually much larger and heavier then an alternator and therefore not much use in small piston powered aircraft.

In reality, all a generator is, is a motor....backwards. When you supply power to a motor, it turns the shaft. If you just turned the shaft by some other means (such as turning it with the engine) you get electricity out instead.

If you have a friendly mechanic out at your local FBO who doesn't mind teaching you a few things, ask him if he'll disconnect the battery and start the engine using ground power. The engine will start fine and continue to run after the engine is disconnected (hence why we have magneto's) but then turn on the radios.....zip, nil, nada.

Oh, and just for the record....it's a he!

 

[njcapt]

Jeez, you guys are making my head hurt. Apparently, I have forgot more about light aircraft and basic aviation theory than most people ever learn.

Chris, you are going to graduate posessing more knowledge than 95% of your peers. Keep asking intelligent questions. You want to know the three to one rule for jet descent planning, let me know. Other than that, let the intelligent folks here help you out.

You go, Boy.

 

[fr0g]

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!

 

[NuGuy]

Hi Chris,

On top of what the others said, the battery provides a deep reserve of cranking power. If you are flying a multi engine airplane, in most cases, the generator/alternator is incapable of providing all the cranking amps that an engine start requires. Even with the other engine operating and its electrical power source online, you still need the battery.

Generators put out DC power. Alternators put out AC power, which is then "rectified" internally (via diodes) into DC power.

Turbine aircraft that use electric starters usually have starter/generators. Once the start is complete, some relays reverse, and now the motor becomes a generator.

The battery can also act as a buffer to even out any variations and surges in the system.

Nu

 

[IFlyGC]

frOg, you're a little off on the ammeter explainiation....

An ammeter is wired in series and measures the amount of current flowing through the circuit. You cannot have a "negative" amount of current....either you have some, or you don't.

A Load Meter is the correct name for what you described above. It shows the load across the battery....is there enough power to charge it and supply the system sufficiently (positive needle indication....though only slightly positive after the battery is fully charged) or is the alternator not producing enough to fit the needs of the aircraft (negative needle indication).

It's been a while and I could be wrong, but I believe the cessna in question has a load meter and not an ammeter.

uwochris: If your trying to better your knowledge just for the heck of it then great! If you have a checkride coming up and your trying to study for it then what's in the POH/AFM is all you really need. Everyone's input above is WAYYYY too heavy for even an airline systems oral! The only things in addition to the POH you might get asked (though s/he wouldn't bust you for it) is what's the difference between a generator and an alternator and what is the difference between voltage and current.

Current - The amount of electricity you have
Voltage - The force that it's being pushed around the circuit

V=IR anyone?

 

[Dumbledore]

Not completely correct

In the circuit, the alternator provides power to the battery to keep it charged while the battery continually discharges to supply the equipment on the aircraft. The ammeter usually provides more power than needed to just charge the battery however and the extra powers the equipment.

As you point out in your latest post, the voltage is a measure of electrical "pressure." Thus, the voltage describes which way the current will flow. If you have 28 volts on the alternator and 24-26 volts on the battery electricity cannot flow out of the battery and into the electrical system.

Once the battery's recharged after engine start, the battery just sits there, fully charged until there is insufficient "pressure" to to keep its contents stored inside (i.e. alternator failure or shutdown.

 

[fr0g]

An ammeter is wired in series and measures the amount of current flowing through the circuit. You cannot have a "negative" amount of current....either you have some, or you don't.

A Load Meter is the correct name for what you described above. It shows the load across the battery....is there enough power to charge it and supply the system sufficiently (positive needle indication....though only slightly positive after the battery is fully charged) or is the alternator not producing enough to fit the needs of the aircraft (negative needle indication).

That's 100% correct - When I stated (-) on the ammeter, I mean of course POSITIVE current flow - but from the battery to the bus - and therefore a discharge, hence the (-) indication on the meter. I used the term ammeter because that is what it is commonly called in the aircraft.

 

[Gatorman]

Gosh, you pilots are smart.......and complicated.

I believe that all of the above post have correct answers pertaining to the aircraft they are in reference to.

I think the basic theory that contains the answer you are looking for is this:
You need the battery to start the engine that drives the alternator/generator that resupplies the battery after the cycle is complete.
i.e. once the engine is running, you do not need the battery unless the alternator/generator is not supplying the minimun voltage/amperage.