Guru question: Alternator wiring

[flyeyes]

Hi all.

I feel I'm generally pretty good with electrical stuff, but there's always the problem that I "don't know what I don't know."

Standard practice is to wire the B lead of the alternator to the switched side of the battery contactor.

Why?

It seems to me that if the contactor is opened while the alternator is running, the alternator will become unstable without the battery until (hopefully) the overvoltage protection shuts it down.

Is there a failure mode that I'm not seeing that would be better wired this way? Why not wire the b-lead to the battery, or the battery side of the contactor?

I'm assuming there is a reason, I just am not seeing it.

Thanks in advance for what is likely a stupid question.

[L.Adamson]

One reason I can think of, is to keep the battery cable short as possible when the master is off. Otherwise the B-lead would remain "hot" through the cowl area --- in case of some kind of accident, in which you want all power off.

L.Adamson --- RV6A

[Bob'sRV6A]

There must be a good reason but being all thumbs with electrons, I can't see it. FAR 23.1351 c (3) (I know, we are not required to comply--but good to know what the spam cans must comply with): "a means must also be provided to disconnect each alternator from the battery." Without the battery contactor, how would the alternator be disconnected from the battery?

[flyeyes]

Ok, so I'll buy the hot B-lead, but it seems like you could mitigate that pretty easily.

I'm just trying to imagine why you'd want to be able to run the alternator without the battery--There seems to be little benefit, a risk of frying electronics, and I've never had a car that could do it. I've had alternators fail in cars, but never a failure that would have been hazardous in an airplane.

[DakotaHawk]

Usually the Battery Master Switch and the Alternator Field Switch are ganged such that the Battery CANNOT be turned off unless the Alt is turned off also. Also ganged such that the Alt CANNOT be turned ON unless the Battery is turned ON first.

This is traditionally done on the Cessnas with a dual switch (see pic below)

Alternatively, you can do this with a single switch by using Nuckoll's Aeroelectric wiring diagram (I think it's Diagram Z-11 - Single Batt/Single Alt VFR wiring diagram).

[flyeyes]

Originally Posted by DakotaHawk

Usually the Battery Master Switch and the Alternator Field Switch are ganged such that the Battery CANNOT be turned off unless the Alt is turned off also. Also ganged such that the Alt CANNOT be turned ON unless the Battery is turned ON first.

.

I'm familiar with the Cessna switches, having owned two 1978 model cessnas (a 182 and a 337). As I recall though, the sides of the switch could be operated independently in the 182, and the 337 had three switches (one battery and two alternators. We currently have a 1964 Cessna with a single master (but a generator) and a Cirrus sr22. The Cirrus has two batteries and two alternators, and all four switches are independent, allowing for human error and turning the battery off without turning the alternator off.

[L.Adamson]

Just thinking of another scenario. Say the alternator somehow shorts to ground. Or breaks a bracket & catches the B-terminal on a metal part. Now the B-lead is a direct ground to the battery's positive terminal with no way to shut it off. These heavy wires get hot fast, and the insulation quickly melts off. I know, because I did create a direct short.

In my 6A, I have about six inches of positive battery cable to the master contactor and that's it. I feel much better, knowing that all "hot" wires beyond that, can be turned off with a flick of a switch.

L.Adamson --- RV6A

[elippse]

Originally Posted by L.Adamson

Just thinking of another scenario. Say the alternator somehow shorts to ground. Or breaks a bracket & catches the B-terminal on a metal part. Now the B-lead is a direct ground to the battery's positive terminal with no way to shut it off. These heavy wires get hot fast, and the insulation quickly melts off. I know, because I did create a direct short.

In my 6A, I have about six inches of positive battery cable to the master contactor and that's it. I feel much better, knowing that all "hot" wires beyond that, can be turned off with a flick of a switch.

L.Adamson --- RV6A

...and also if the rectifier diodes short out, giving a low resistance path to ground which will take down the battery, and also not be kind to the wiring

[flyeyes]

Originally Posted by elippse

...and also if the rectifier diodes short out, giving a low resistance path to ground which will take down the battery, and also not be kind to the wiring

Ahh, ok. I hadn't considered that. Makes sense.

I would think a dead short would let the smoke out of the diode and result in it failing open, but a partial short would behave as you described.

[the4hens]

Use the cessna switch and a 50 or so amp breaker between the alternator and the battery. This way you can always isolate the alternator if necessary.