Alternator Field

[oktex88]

I'm flying a Piper Seminole and I'm trying to understand the electrical system better for my checkride.

There are 2 Circuit Breaks for each alternator. There is a 70A CB for the "Alternator" CB and there is 5A CB for the "Alternator Field".

What is a alternator field?

What is the difference between the two?

Does just pulling one of these take the alternator offline?

Or do both need to be pulled to take the alternator offline?

My instructor really couldn't explain it to me well. Only that the Alternator Field CB is what she pulls to simulate an alternator failure when we fly because it doesn't drain as much from the electrical system.

Also...
Why do we have a split bus system? For example why is there two sources with two 60A CBs going to the main bus from the tie bus? I know to take the main bus offline I have to pull BOTH the CBs.....is this second source just a backup?

 

[Dugie8]

The 5 amp is for the alternator field coil. Most (I won't say all) aviation type alternator or AC gens do not have a permanent magnet (bar magnet) they are basically electro magnets, meaning the magnet needs an electric current to generate a magnetic field.

You would have to do some digging, but I believe the alternator does not receive power from the batter to "electrify" the field coil until you turn on the alternator switch (it very well could come when you turn on the battery switch).

The 70 amp CB is the actual power output of the alternator. Pulling the smaller 5 amp CB is a good way of failing the alternator by taking the field coil off line.


You have a split bus system because you have 2 alternators. Either alternator can power the aircraft, ie cross gen starts. I don't remember the SEMI electrical system off the top of my head, but if you needed to isolate the main bus from the alternators or all electrical power, you would need to pull both CB to cut the power, but that would most likely cut power to all electrical systems as IIRC the main bus is what all other busses are fed from, minus the batter bus.

 

[oktex88]

I attached a picture of the Seminole Electrical Diagram.

The 70 amp CB is the actual power output of the alternator. Pulling the smaller 5 amp CB is a good way of failing the alternator by taking the field coil off line.

So if I understand correctly pulling the 70A Alt CB will just disconnect the alternator from the system but the alternator would actually still be running and if I just pull the 5A Alt Field CB then it would actually stop the alternator. However with JUST the 5A Alt field pulled the connection to the tie bus would still be there but the alternator wouldn't be producing any power.

You have a split bus system because you have 2 alternators. Either alternator can power the aircraft, ie cross gen starts. I don't remember the SEMI electrical system off the top of my head, but if you needed to isolate the main bus from the alternators or all electrical power, you would need to pull both CB to cut the power, but that would most likely cut power to all electrical systems as IIRC the main bus is what all other busses are fed from, minus the batter bus.

The two alternators and battery power the Tie bus which powers are the rest of the buses. So in the SEMI it would just take the Main bus off... the rest would still be powered.

You said it is split bus system because I have 2 alternators but from what I learned that refers to "Dual Fed" system.

Looking at the Seminole diagram attached you can see there is TWO connections with 60A CBs from the Tie bus to the Main Bus. Is this what they are referring to when they say it is a "Split Bus" System?

What are the benefits of a Split Bus System vs one that is not?

 

[oktex88]

Forgot to attach the picture. Here it is.

 

[Dugie8]

I attached a picture of the Seminole Electrical Diagram.



So if I understand correctly pulling the 70A Alt CB will just disconnect the alternator from the system but the alternator would actually still be running and if I just pull the 5A Alt Field CB then it would actually stop the alternator. However with JUST the 5A Alt field pulled the connection to the tie bus would still be there but the alternator wouldn't be producing any power.

Pulling the 70 amp will (disconnect) the alternator from the bus. However, n this configuration when you check the voltmeter you will still show 24 volts and your ammeter will show 0 or close to, since there would be no load. Pulling the 5 amp will take the alternator offline via taking away the alternators ability to produce electricity. In that configuration your voltmeter will show 0 as well as your ammeter.




The two alternators and battery power the Tie bus which powers are the rest of the buses. So in the SEMI it would just take the Main bus off... the rest would still be powered.

You said it is split bus system because I have 2 alternators but from what I learned that refers to "Dual Fed" system.

Looking at the Seminole diagram attached you can see there is TWO connections with 60A CBs from the Tie bus to the Main Bus. Is this what they are referring to when they say it is a "Split Bus" System?

What are the benefits of a Split Bus System vs one that is not?

Now I member!

The split bus refers to the two avionics busses. It basically gives you a little redundancy by putting separate systems on separate busses. The alternators feed the tie buss that inturn feed the two avionics busses and the main buss. Those two lines going to the top of the main buss are the lines that give the field coil power so the alternator can produce power.

This isn't a true "dual fed" "split bus" system. A true split bus puts one half of the airplane on one side and the other half on the other side. The only thing "split" here is some avionics.

 

[oktex88]

Now I member!

The split bus refers to the two avionics busses. It basically gives you a little redundancy by putting separate systems on separate busses. The alternators feed the tie buss that inturn feed the two avionics busses and the main buss. Those two lines going to the top of the main buss are the lines that give the field coil power so the alternator can produce power.

This isn't a true "dual fed" "split bus" system. A true split bus puts one half of the airplane on one side and the other half on the other side. The only thing "split" here is some avionics.

Ah I see so if something is wrong with the wiring from the Tie Bus to Avionics Bus 2 (or even Avionics Bus 1) then they could still power each other via the 25A Tie CB. Therefore it is a "split" bus system.

Thanks!

 

[Dugie8]

Ah I see so if something is wrong with the wiring from the Tie Bus to Avionics Bus 2 (or even Avionics Bus 1) then they could still power each other via the 25A Tie CB. Therefore it is a "split" bus system.

Thanks!

Don't think of it so much in terms of wiring. Think of this in terms of load shedding should you lose both alternators and now you need to minimize your total load.

Also, take a look at the electrical fire emergency checklist and run though that and follow the diagram as you isolate busses and items.

 

[ScoutFlyer]

Think of this in terms of load shedding should you lose both alternators and now you need to minimize your total load.

This is the benefit of a "split bus" system. The Seminole also has a non-essential bus which powers things such as lights, that are really not that important to me if I'm running low on juice.

Good luck on the checkride Oktex :rawk: