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Field switch
- Thread starter Hiebert
- Start date
The Field current/voltage is what energizes the alternator and controls the voltage output based on the voltage regulator. The field must be energized for the alternator to charge your battery and run your avionics. 3 amps is a normal current draw, 7 amps is the common circuit breaker value. Under normal operations, the field switch must be on..
AZSteve053
Well Known Member
In generic terms, it's likely a switch in series with the "field" winding of your alternator. When turned on, it allows current flow to energize the magnetic field inside the alternator used to generate output from your alternator when it is spinning. Field windings are often driven from a regulator to control how much output current is to be generated to maintain a predetermined "set point" bus voltage. From your description, sounds like your EFIS current sensor is connected between your battery & the main bus, so hopefully your Dynon is showing a "discharge" when the engine is not turning.
Hope this helps.
Hope this helps.
So during a start with it off the batt would have more power for start but leaving it on would not cause any issues if all is in good shape. True or false
This is the first airplane I have flown since 1951that had this switch
Its current is minimal. What kind of alternator? Internal or external regulated?
For internal regulated alternators, it has been suggested that you turn on the alt field before start and turn off after shut down. This mimics the typical automotive operation of which many internal regulated alternators were derived. For external regulators follow the manufacturers recommended operating procedures.
AZSteve053
Well Known Member
Most Piper / Cessna / Beech / Grumman / etc.... in other words most Certified aircraft have a split Master switch standard, the one side energizes the master contactor while the other side is a "field" switch.
In all likelihood it is not.
As someone else already posted, any Aircraft with a split master switch (typically red), had the right hand portion controlling the alternator field
The purpose of this was to give the pilot the ability to shut down the alternator if needed, but still keep the battery connected to the Aircraft electrical system.
The Plane Power (a.k.a Hartzell Engine Technologies) guys said at a forum at Oshkosh a few years ago that they recommended having the alternator off during engine start.
To be more specific, when asked if the alternator should be on or off during engine start, they said something to the effect of "I don't know why you would want to subject your alternator to all that."
After hearing this, I turn on my alternator after engine start, and turn it off after the engine stops. Kind of makes sense - I don't see a huge downside to activating the alternator after the engine is running.
I don’t think you’ll find consensus on turning an internal regulated alternator on before or after start.
If you have a nippo or automotive alternator with internal regulation, your car doesn’t have an alternator field switch, so it is always on. Ever had an alternator failure in a car?
I went through a couple alternators before I started switching it on before start and never had another issue. Was turning it on after start part of the problem, who knows…..
Plane Power has suffered some reputation damage over the past few years so I’m not buying what they are selling. I don’t get what they mean by “why would you put your alternator through that”, through what? What a normal alternator sees every day in a car?
Anyway, to the OP, best to identify what alternator you have and follow the manufacturer’s recommendations, if they have any.
If you have a nippo or automotive alternator with internal regulation, your car doesn’t have an alternator field switch, so it is always on. Ever had an alternator failure in a car?
I went through a couple alternators before I started switching it on before start and never had another issue. Was turning it on after start part of the problem, who knows…..
Plane Power has suffered some reputation damage over the past few years so I’m not buying what they are selling. I don’t get what they mean by “why would you put your alternator through that”, through what? What a normal alternator sees every day in a car?
Anyway, to the OP, best to identify what alternator you have and follow the manufacturer’s recommendations, if they have any.
That places you at least 90.... Fly on....
An alternator field switch is not really necessary. I used Bob Knuckoll's electrical system architechture, which just specifies a pullable circuit breaker for the alternator field. The only time you expect to pull the breaker is to corral a runaway alternator. Otherwise you just leave it closed.
I wouldn't worry about leaving the alternator field on for engine start. It's not drawing any significant power until the alternator is turning anyway.
Are you sure the car's onboard electronics aren't sequencing the Alternator field so it only engages once the engine is running? It would be trivial to do, and would be a lot easier than explaining to the average driver what the field switch was on the dash.
Don’t know. Sounds like a good research project for you.
Alternators have been around for like 200 years? Pretty sure my ‘57 truck with its fancy foot starter doesn’t have any electronics in it. My ‘85 dump truck has a nippo in it, and don’t think there’s anything going on there either.
I haven’t had an alternator failure since I changed procedures but, again, who knows….. might just be luck….
have you never flown a Cessna?
Turning it off during a start can only help the field winding’s life. With the voltage diving, I’m sure there’s a pretty high amp spike. If anyone’s measured this, please share.
I also believe that ground bound analogies have little value here. If there’s a max cranking time/recommended cool down period for car/truck/tractor/etc I’ve never heard of it. Of course, I’ve never read the thousand page owner’s manuals.
I also believe that ground bound analogies have little value here. If there’s a max cranking time/recommended cool down period for car/truck/tractor/etc I’ve never heard of it. Of course, I’ve never read the thousand page owner’s manuals.
I always used to start the engine and then turn on the alternator after start.. then after having an alternator failure, I started subscribing to the theory that it may be more gentle on the alternator to start the engine with the field on, and allow the alternator to come up to speed with the engine.. nothing scientific, but it seems logical to me that cars seem to do well like that.
While this used to be true, it no longer is. In modern vehicles, the alternators voltage regulator and therefore field current, is controlled by the ecu. To reduce starting load, the ecu will not ramp up the alternator output until after the engine starts. In addition, the ecu is constantly adjusting the voltage level, so it acts more like a smart battery charger. They no longer constantly throw 14.4 volts into the system. Most now float around 13.6 after they have determined that the battery is fully recharged.
Fyi, i have never had an alternator fail in a plane (1@1400 hours and 1@400) and I switch the alternator on after start up. So, no conclusive data either way. I have replaced probably 30 alternators in cars. However, the NDs usually last about 150k miles to be fair. Need to be cautious with conclusions here as a lot of folks use pp alternators and many of them die an early death regardless of how they are used. I am confident that if you put a pp on a car, it would die just as quickly.
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