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What is Alternator Field

Dbro172

Dbro172

Well Known Member
I've been researching, but I just don't understand what the "alt field" is. Do I need a circuit breaker for the Alt field AND for the alternator itself?
 
When you spin a coil of wire in a magnetic field (or spin the magnet) you generate electricity.
A generator has a permenant magnet.
An alternator has an electromagnet, which gets its power thru the "field" line.
Yes, the field line should have a fuse or CB, usually about 5 amps.
 
Alt field is usually a 5Amp circuit breaker located on the panel next to the Master Switch. The field uses battery power to magnetize the alternator field windings(the rotating part of the alternator). The alternator produces power in the stator (the stationary part of the alternator), and this power goes back to your battery and aircraft loads.

The Alt Field uses less than 5 amps continuously when the Master Switch is in the Batt/Alt position and the engine is running. The Alternator puts out amps based on the charge/voltage of the battery, up to it's rated current.

For example, my Plane Power 70A alternator uses 5 amps of battery power to produce up to 70 amps of power to return to the battery and power the airplane's various electrical loads.
 
Ahh huh.

I think I understand it now.... It's like this:

The engine belt is spinning the alternator, the alternator is doing nothing, until you throw the master switch to alt, which then sends a small amount of power from the battery (through the breaker) to the alternator to magnetize its field, which allows the alternator to do its thing, powering devices and charging the battery. No?
 
Dbro172 said:
I think I understand it now.... It's like this:

The engine belt is spinning the alternator, the alternator is doing nothing, until you throw the master switch to alt, which then sends a small amount of power from the battery (through the breaker) to the alternator to magnetize its field, which allows the alternator to do its thing, powering devices and charging the battery. No?
Click to expand...

Yes and no.

Initially it is as you say. After the alternator is producing power it switches to a self-exciting mode.

Sounds kinda kinky, yes? ;)
 
DanH said:
Yes and no.

Initially it is as you say. After the alternator is producing power it switches to a self-exciting mode.

Sounds kinda kinky, yes? ;)
Click to expand...

Are you sure about the "self-exciting" bit? I think the whole purpose of the Master Switch and the Alt Field circuit breaker is to deenergize the alternator if there's an electrical fault. If the Master Switch / Alt Ckt Bkr can't do that, then why have them at all?
 
IIRC, a self-exciting alternator only has one wire going to it, that being the main power wire. In that kind of alternator you would have a CB rated at the alternator current.

Otherwise, like in my -4, there is another wire that goes from the alternator switch and a 5 A CB to the field wire.

I have a 35 A CB labeled ALT and a 5 A CB labeled FLD. If I pull the FLD CB when the engine is running the alternator no longer produces voltage.
 
DakotaHawk said:
Are you sure about the "self-exciting" bit? I think the whole purpose of the Master Switch and the Alt Field circuit breaker is to deenergize the alternator if there's an electrical fault. If the Master Switch / Alt Ckt Bkr can't do that, then why have them at all?
Click to expand...
Self exciting means that once it is producing electrons, some of them are used to excite the field, the battery is no longer needed for that purpose. The field circuit goes through the Alt Field switch (and the VR) so that it can be controlled. Cars don't have that switch.
 
DakotaHawk said:
Are you sure about the "self-exciting" bit? I think the whole purpose of the Master Switch and the Alt Field circuit breaker is to deenergize the alternator if there's an electrical fault. If the Master Switch / Alt Ckt Bkr can't do that, then why have them at all?
Click to expand...

You're right....two beers and thinking cars.
 
I'm on my third beer and still wondering if the I need a second breaker. I will have the alt field breaker at 5a, will I need another 60a breaker for the alternator itself? (Vans standard firewall forward)
 
number 4

You are correct ... 5A Field CB, and 60A Alternator CB assuming your alternator feed (B+) wire is matched for that current. Remember, the CB equates to wire size (see AC 43).

But I think you have the idea. Low current through small wire provides excitation voltage to alternator field, big engine turns alternator and converts kinetic energy into electromagnetic energy and sends big current down fat wire.

Now, have that fourth beer.

Don
 
The big (60 amp or so) CB or ANL is so that if there's a fault in the alternator where there's a direct short to ground it won't kill the entire electrical system. It is NOT protecting the alternator, in this case it's the battery capable of sourcing multiple hundreds of amps into a short circuit and you're protecting the electrical system. If there's a fault in the alternator, the CB or ANL will blow and then you're running battery only. Leave out the CB or ANL and that fault would short the main bus, including the battery, and you'd lose everything and wherever the short was would see some serious welding!

Bob
 
In an internally regulated alternator the alt. field also feeds the regulator circuit and senses charge and d
ischarge state. Too much voltage drop in the alt. field circuit can cause "seeking" and an unstable alt. output.
 
I recommend Bob Nuckolls' book The AeroElectric Connection. It can be downloaded for free from here: http://www.aeroelectric.com/Catalog/pub/pub.html or you can order a CD containing the book and some extra material for $18. There is a chapter that has an excellent explanation about how alternators work and how they differ from generators.
 
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