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Alternator field switch required ?

L

Larry DeCamp

Well Known Member
Installing B&C system with regulator, OV function etc. The field is protected by a 5A circuit breaker and the output is protected by an ANL current limiter which I assume would protect the buss from an internal short in the alternator. So, my queustion is: Why would I need a FIELD switch ????
 
There needs to be some way of shutting off the alternator in case of smoke in cockpit. If the circuit breaker also functions as a switch, then a separate switch is not necessary.
Shutting off the master contactor will NOT shut off the alternator once it is running because the field will be powered by the alternator output, not by the battery. In fact, once the engine is running at high enough RPM, everything is powered by the alternator, not by the battery. The battery is still required after engine start to help stabilize the electrical system voltage.
 
Mich48041 said:
There needs to be some way of shutting off the alternator in case of smoke in cockpit. If the circuit breaker also functions as a switch, then a separate switch is not necessary.
Shutting off the master contactor will NOT shut off the alternator once it is running because the field will be powered by the alternator output, not by the battery. In fact, once the engine is running at high enough RPM, everything is powered by the alternator, not by the battery. The battery is still required after engine start to help stabilize the electrical system voltage.
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Mitch is spot on. I don't have an alternator field switch for my primary nor my backup B&C alternator. Both are protected instead by pullable breakers on the panel in case all electrons need to be manually shut off.

I have verified that turning off the master in flight, while disconnecting the electrical system from the battery, does not stop the alternator from producing power. Pulling the breaker stops it instantly.
 
From my understanding, the ANL will blow at a particular % over rated current but will still carry high voltage to your components, this high voltage will fry some electronics.
 
ANL

The ANL is like an extremely slow blow fuse. It is there to protect the B lead and the alternator from the battery. It won't protect anything from the alternator. Say you have a B&C 40 amp alternator and a 40A ANL. The alternator will happily put out much more than 40 amps for a while if the battery was run way down and the ANL won't blow. It will probably suffer twice its rating for quite a long while. If there is a dead short, it will blow immediately because the battery will try and shove a thousand amps through it. It also does not care what the voltage is. You need an overvoltage sensor and/or "crowbar" device to blow the field breaker if the alternator runs away. The crowbar acts in milliseconds and will probably save your avionics.

Ed Holyoke


bret said:
From my understanding, the ANL will blow at a particular % over rated current but will still carry high voltage to your components, this high voltage will fry some electronics.
Click to expand...
 
Thanks for the high quality response.

My conclusion from all of this is that the CB works as well as a switch in a runaway scenario, and' IF the OV relay fails the Avionics will fry before you can react. Thanks much ....Larry
 
It has always been a common practice to start the engine before turning on the alternator, field switch. It wouldn't matter how you did this, a switch or a breaker.
 
37 years of master and alternator on during start with no problems.

I do shut off the alternator if I am sitting in the RV and playing with the panel.
 
Bicyclops said:
The ANL is like an extremely slow blow fuse. It is there to protect the B lead and the alternator from the battery. It won't protect anything from the alternator. Say you have a B&C 40 amp alternator and a 40A ANL. The alternator will happily put out much more than 40 amps for a while if the battery was run way down and the ANL won't blow. It will probably suffer twice its rating for quite a long while. If there is a dead short, it will blow immediately because the battery will try and shove a thousand amps through it. It also does not care what the voltage is.

You need an overvoltage sensor and/or "crowbar" device to blow the field breaker if the alternator runs away. The crowbar acts in milliseconds and will probably save your avionics.

Ed Holyoke
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If you have an external Voltage Regulator this crowbar function is often built into it. Just like this Plane Power one -

http://www.chiefaircraft.com/pp-r1224b.html
 
Larry,
Is your original question based on limited panel space or minimalistic system philosophy?

If limited panel space, then please remember that the field switch can be incorporated into the "master" by using a progressive double pole double throw DPDT switch as described in the Aeroelectric documents.

If it is philosophy, the main question is how often do you plan to cycle (make or break) that field connection. If it is few, then pull breaker will be fine, if it is a lot, put in a switch. Breakers are not designed for high cycle usage.
 
Clarification on two points requested .

Mich48041-I read your comment to say that if the master is opened, the alternator will still apply 14.2 volts to the master buss and everything supplied from it until the engine is stopped. Is this scenario an unidentified electrical concern that wiring protection devices have not resolved, ie. smoke ?

For clarification, It is my understanding ( TJ @ Band C) the 5A field CB is to protect the field and supply wiring from excess CURRENT. The OV crowbar function is a RELAY in the regulator that OPENS the field for VOLTAGE protection.


My purpose for understanding the details is simplicity. Less switches is less labels and opportunity for mistakes. That said, it sounds like if you smell electric smoke, you will need to be prepared to pull the breaker or glide.
 
oops...

My earlier reply was thinking about the AEC Z figures using the old Ford style regulators with separate OV module.

The 2 stage switch shown in the Z figures for the B&C regulator, or a separate alternator field switch, or a pullable CB, would all accomplish the shutdown of the alternator. So your choice on how to implement.

The 2 stage switch should be the simplest from an operational standpoint; hit one switch and everything (except battery bus items) goes dark. When the 2 stage master is off (all the way down), it disconnects the master contactor *and* opens the circuit to the regulator, killing the power to the alternator field. One click up powers the master but leaves the regulator (alternator field) unpowered. Full up brings the regulator & alternator on line.

Charlie
 
Mich48041-I read your comment to say that if the master is opened, the alternator will still apply 14.2 volts to the master buss and everything supplied from it until the engine is stopped.
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Yes, that is correct, unless the master switch is double pole, one half for the battery contactor and one half for the alternator field.
Circuit breakers and fuses protect against short circuits and over current. However, it is possible to have smoke without blowing a fuse or tripping a circuit breaker. It happened to me years ago. Depending on the fear factor or toxicity of the smoke, the pilot might want to shut off all electricity. I recommend having a way for the pilot to open the alternator field circuit. The battery contactor will not accomplish that.
 
Larry DeCamp said:
My conclusion from all of this is that the CB works as well as a switch in a runaway scenario, and' IF the OV relay fails the Avionics will fry before you can react. Thanks much ....Larry
Click to expand...

That *could* be true, but if the battery stays in the circuit, it will typically take several--to many--seconds for the OV to climb high enough to damage most of the current avionics. That's assuming you're running a 12/14 volt system and the avionics are current generation stuff that works on either 14V or 28V systems. It can take a while for the alternator to pump enough electrons into the battery to drive voltage up over 35-40 volts, which is probably close to what it would take to kill avionics designed to handle 28V plus surges. (Battery management systems in lithium chemistry batteries might alter this equation, if they disconnect the battery at the 1st sign of over voltage.)

The question is, how quickly the pilot will notice it, after the OV event starts.
 
rv7charlie said:
My earlier reply was thinking about the AEC Z figures using the old Ford style regulators with separate OV module.

The 2 stage switch shown in the Z figures for the B&C regulator, or a separate alternator field switch, or a pullable CB, would all accomplish the shutdown of the alternator. So your choice on how to implement.

The 2 stage switch should be the simplest from an operational standpoint; hit one switch and everything (except battery bus items) goes dark. When the 2 stage master is off (all the way down), it disconnects the master contactor *and* opens the circuit to the regulator, killing the power to the alternator field. One click up powers the master but leaves the regulator (alternator field) unpowered. Full up brings the regulator & alternator on line.

Charlie
Click to expand...

Note for other readers.... the "two-stage switch" mentioned above is how the AeroElectric diagrams show/use a lever switch. It's action is it is the same as the standard Cessna/Piper split master switch.

The common Z-11 AeroElectric diagram -

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z11P.pdf

- only shows one breaker used (rest are fuses) and that is a pullable breaker for the Alternator Field as previously mentioned.

On my panel I added a second, a back-up pullable breaker for Autopilot disconnect.
 
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