ALT field switch... why, really?

[DaleB]

Assume we have an electrical system with an alternator, external voltage regulator, field supply via either a fuse or breaker, and an overvoltage crowbar.

In what situation would I use an alternator field switch, if I had one? In other words, if the master switch is on, why would I want the alternator field off? What failure mode, in flight or on the ground, could be helped by turning the alternator field off?

I see an ALT switch in some form on every panel I can remember. Old or new, Piper or Cessna or RV. I understand what it does, but I've never once done anything other than turn it on and off with the master switch, nor have I seen anything similar on anything other than an airplane. Just wondering why.

[rolivi]

I'll be interested in the responses to this thread.

FWIW--

I was trained that when there is a split or separate alt switch, it's there for a reason, so use it. That was never followed by an actual reason. Odd that I just accepted that since it's rather out of character for me.

When so installed, such as my RV, I leave the the ALT off while starting, then turn it on, and then when shutting down it's the last switch off BEFORE pulling the mixture, and when the prop stops I shut off both ignitions and finally the master.

[kamikaze]

Though I now forget the details, I do remember reading, and being convinced, that starting the engine was best done with ALT *off* ... I could try to find the exact technical explanation ...

[kamikaze]

Right, to prevent a voltage spike during start, that's what it was ... think I was convinced by a few stories of people actually suffering from this at one point of another ... I guess it's just extra safety in case the crowbar doesn't do its job? Or maybe the aircraft in question didn't have overvoltage protection?

[Jesse]

Or if the voltage regulator fails and the voltage increases above acceptable levels, you can kill the switch or the breaker and turn the alternator off.

I have recently been somewhat convinced that the main alternator breaker is not as necessary as we always thought. Plane Power told a customer that the only reason we ever still use them is because we always have.

[rbibb]

Best justification is to be able to sit off the alternator in case of a failure mode (regulator, etc) that would allow the voltage to run away. If you have a "crowbar" style over voltage protection circuit then that use case disappears in case of a voltage regulator failure and voltage spike the crowbar causes the field breaker to blow and does the job.

I can't think of a reason to have it if you have a crowbar protector. In fact, if damage from over voltage is your concern I would posit that by the time you determined you had such a failure and flipped the switch it would be too late.

A lot of things we do are rooted in historical practice that probable have no practical need in modern electronic environment. Why do I say that? Because I've yet to see an alternator switch in a modern automobile and they have more electronics in them that even the most tricked out panel I've ever seen documented here.

For me the Crowbar circuit is a worthwhile insurance policy of sorts. The alternator switch - probably a waste IMHO.

[Canadian_JOY]

Think of this scenario for a minute... You're flying along and your alternator belt breaks. You now have field current flowing through a useless alternator, draining the already-limited amount of electrons available in the aircraft's battery. The alt field switch allows that flow of electrons to be easily controlled. If it were instead an alt field fuse tucked up under the panel, it would be far more difficult to control the field current.

The same story goes for when you're doing some ground testing... do you really want juice to be flowing through your alternator? For that kind of activity, keeping only the "BATT" side of the switch on makes sense. Again, it's a matter of ease of being able to isolate the power generation from the power storage functions of the electrical system.

As for turning the alternator on after engine start, there are a lot of opinions on this. Some alternators do not fare well with their field voltage being turned on and off while they are being turned by the engine. This has led to premature failure of some alternators. We often see recommendations to turn on both sides of the master switch prior to engine start, and there was a good thread on here discussing why we would want the extra drag of the alternator during engine start. The bottom line remains that powering the alternator field should be done in accordance with manufacturer's recommendations.

You might also want to think about how this is done in your car (the source of many alternators installed in RV's). When you turn the ignition switch on in your car, the alternator field is powered and its output gets to ramp up as the engine comes up to speed. Consider it a "soft start" feature for your electrical system.

[humptybump]

My last panel had no "ALT" switch setting. It was "ON" or "OFF". I had to chamgevthect artery one and forgot to pull the field breaker. The alternator was not happy. When I re-did the panel, I talked thru the decision and was convinced to use a three position switch so I now have OFF-BATTERY-ALT. it's not a significant benefit given. Still have a CB for the alternator field. It's just a matter of having the correct procedures documented in the POH.

[Weasel]

Originally Posted by Canadian_JOY

You're flying along and your alternator belt breaks. You now have field current flowing through a useless alternator, draining the already-limited amount of electrons available in the aircraft's battery.
The same story goes for when you're doing some ground testing... do you really want juice to be flowing through your alternator?

I did NOT include the field control switch in my setup. The above quote is my only regrets.

A pull type breaker is all you need for the times when you need to disconnect the alt.

[WenEng]

You turn on the master to do EFIS work, or other electrical work, testing, etc, and you're draining 3.5 amps for no reason. I have VPX and use the EFIS VPX option to turn off the alternator field while working on my plane. I chose to not install a panel switch for the alternator field.