Master/Alternator switch

[tonyjohnson]

I am trying to decide which switch to use for the Master/Alt function.

The AeroElectric Connection by Bob Nuckolls contains two different schemes. When discussing the function of switches he recommends a 2-3 switch. The 2-3 would turn on both the battery master and the alternator field at the same time.

The Z-11 diagram for the generic airplane shows a 2-10 switch for this function. The 2-10 would be progressive, in that it the first position up would turn on one of the functions and the next position up would add the second. The diagram seems to show that the first position up should turn on the battery and the second position up adds the alternator field. This would seem to turn off the alternator before the battery, and leaves open the possibility that leaving the switch in the first position up would have the battery on and the alternator field off. I have read that one should never turn off the alternator when the engine is running. Therefore I am a bit confused about the 2-10 switch recommendation.

If I use the 2-3 plan, it is pretty simple and would eliminate the possibility of having the battery master on while the alternator is off. If I wanted to shut down the alternator for some reason, I could pull the alt breaker.

Any electrical gurus out there who can help me understand this?

Thanks,

Tony

[Walt]

I use the 2-10 3 postion toggle on my master/alt. What alternators don't like is not having a battery hooked up to them when they are running or dumping a full load on them from a high rpm is not a recommended procedure. I have turned off the alternator in flight to check other systems like my back up gen. but then I reduce RPM before swithcing it back on to reduce the surge. So this setup is no problem and I like being able to turn of the alternator using the switch, just be mindful of load and RPM when turning it off and on during flight but you really cannot damage the alt using this setup.

[Kevin Horton]

Originally Posted by tonyjohnson

The Z-11 diagram for the generic airplant shows a 2-10 switch for this function. The 2-10 would be progressive, in that it the first position up would turn on one of the functions and the next position up would add the second. The diagram seems to show that the first position up should turn on the battery and the second position up adds the alternator field. This would seem to turn off the alternator before the battery, and leaves open the possibility that leaving the switch in the first position up would have the battery on and the alternator field off. I have read that one should never turn off the alternator when the engine is running. Therefore I am a bit confused about the 2-10 switch recommendation.

The issues with turning off the alternator with the engine running are only seen with some internally regulated alternators. There have been a number of cases with alternators sold by Van's where turning off the alternator with the engine running has destroyed the alternator's internal voltage regulator.

If you install an externally regulated alternator, as espoused by Bob Nuckolls, then there should be no concerns about using the 2-10 switch.

[tonyjohnson]

Walt and Kevin, Thanks for the input.

I will actually be using the plane power alternator which is internally regulated.

Walt, are you using an internall or externally regulated alt?

Tony

[L.Adamson]

Even though I used a lot of Bob's electrical ideas and diagrams, I still prefer the Cessna type alt/bat switch. Bob doesn't think it looks that great with other switches. In my panel, I think it looks excellent. It's already setup, so that the battery can not be switched off, when the alt side is on.

[PJSeipel]

Originally Posted by tonyjohnson

I will actually be using the plane power alternator which is internally regulated.

You might consider giving Plane Power a call and asking what they think. Personally I'm looking at using a 2-3 because I can't find a 2-10 that matches the rest of my switches.

PJ
RV-10 #40032

[gmcjetpilot]

Originally Posted by tonyjohnson

If I use the 2-3 plan, it is pretty simple and would eliminate the possibility of having the battery master on while the alternator is off. If I wanted to shut down the alternator for some reason, I could pull the alt breaker.

Tony that is what I would do. I don't have Bob's "Z" diagram in front of me but think I understand your quandary. My advice is make it simple and comply with standards set by the GA industry. Space shuttle wiring and switches are cool but can cause problems. One throw, two functions (bat/alt) is the standard.

Consider a simple single throw switch (on-off) and two polls, one for the master contactor and the other for the ALT. Why have to make two throws to get the electical system on line or off line? It really is extra work.

You want to have a way to turn the ALT off only for emergency. 99.9999% of the time the ALT comes ON with master before start and OFF after engine shut down with the master being turned off.

For the unlikely emergency shut down of the the ALT (while master remains on) using a pull-able circuit breaker (CB) is great. I go with the kiss method.

Internally regulated alternator (I-VR) shuts itself down automatically with a little bit of microprocessor control, so in theory you don't even need a pull-able CB. Personally I like a little control and installed a CB for the ALT just in case I want to pull it. Even when its on the signal wire drain is milliamps. Keep in mind for internally regulated alternators only need a 1 amp CB/fuse, not 5-7 amps like an E-VR needs.

The reason AeroElectric goes with the double throw switch is to work around having a fuse and externally regulated alternator as a base line.

The idea of multi-position switches for a master, down-mid-up, is more complicated, NON STANDARD. How many Cessna's, Pipers and Mooney's have multi throw master switches? It is just not common in general aviation. It may be fine for you the builder/pilot but may screw up another pilot, say if you sell the plane down the road.

You can make it simple or complicated. With a multi position switch you have a chance of pilot error (ergonomics - crew / machine interface error) with multi throw switches. Sure you will have a low volt light but agree with Kevin, with a stock internally regulated alternators, throwing the power (the wake-up/sleep power, not field) can cause problems with engine running.