Why have a switch on ALT Field?

[RVG8tor]

I am brain storming here while I work on my electrical system design (Z12). I wonder why there is a switch for the Alt Field. From my reading on the forums internally regulated alternators get their life shortened if one turns them on under load, (they should be on before start until after start to get the most life out of the alternator). The B&C alternator is externally regulated and can handle the switching under load. But operationally there is no reason to switch the alternator on and off, if you have an abnormal situation and need to turn the alternator off you can do that just as easily with a circuit breaker as you can with a switch. So why not leave the switch out of the loop and wire the alternator field directly to circuit breaker so it is always on, in case of alternator problem, pull circuit breaker. On less switch a few ounces and dollars saved. So, am I missing something obvious here?

[hecilopter]

I would do if doing it over

[gasman]

I have a PLANE POWER ALT. and that's how I did it. Been working great for 250 hours. And you don't need that split hard to install master switch. Drill another hole and install your master sw.............

[rbibb]

If regulator fails youi can then shut off the Alternator. Of course fuse and/or circuit breaker will allow same thing. If you have crowbar overvoltage protection it will happen automatically.

[N999BT]

Be sure that when the master switch is off, so is the field circuit. In other words, if you are using an essential bus, make sure you attach it to the main bus, not the essential bus. If you can energize it from the essential bus, there would be current being used by the alternator even if it is not generating electricity.

If you are following Aeroelectric connection drawings you can run the field circuit throught the master switch and it shuts down the master solenoid and field circuit at the same time on separate circuits.

[RVG8tor]

Originally Posted by N999BT

Be sure that when the master switch is off, so is the field circuit. In other words, if you are using an essential bus, make sure you attach it to the main bus, not the essential bus. If you can energize it from the essential bus, there would be current being used by the alternator even if it is not generating electricity.

If you are following Aeroelectric connection drawings you can run the field circuit through the master switch and it shuts down the master solenoid and field circuit at the same time on separate circuits.

I am not sure I understand, if I have my alternator field going to just a circuit breaker on the main bus and it is IN, and I shut off the battery master (de-energize the main bus). I then turn on my E-bus alternate feed to power the E-bus from the battery. Would this cause trouble? If I need the alternator field off, I can do this with a switch or without the switch pull the CB. What I am asking about is wiring according to the AEC Z12 but I would not have the alternator field go to a switch it would just go to the main bus CB. My master switch would really be a "battery switch".

[rocketbob]

I'm not using a ALT switch on my rocket. I have one in my RV-6, and in over 1100 hours of flying I've never found a need to shut the alternator off. Of course if you were flying on battery alone, you can switch the field off which would drop the 3A draw off the bus if you have the switch...but if your fuses or breakers are accessible then its just a simple matter of pulling the breaker to disconnect the field.

[MCA]

Another reason is that if you have a primary and a backup alternator, you need a switch to select which one to use.

[kiwipete]

Another reason, if your running the battery master but not the engine you don't have the drain of the alternator field.

Peter

[N184JG]

If you have an under or overvoltage system installed you can reset by turning off the field. It worked very well in my PA32 & 172 I have the split sw in my RV8.