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.