Dual alternator setup and master contactor failure.

[Finley Atherton]

Kaa,

Do I have this correct? Your concern is that the master relay fails taking the battery out of the system. The resulting undamped alternator voltage may? go over 16 volts and activate the overvoltage protection and shut down the alternator. The backup alternator will not come on as there is now no voltage in the system to excite the field circuit (battery out and main alternator out).

I now have a B&C alternator with it’s external regulator but previously had an internally regulated alternator with built in OV protection which I inadvertently operated with the Master switch off. I can’t recall the voltage but the alternator kept working and the only noticeable difference was some background noise in the headset.

I do have an alternate feed to the buss. It comes directly off the battery through a 50 amp fuse (to protect the wire) to a CB toggle switch to the buss. The only apparent downside is that the fuse will blow if you accidentally try to start the engine through the feed circuit .

Fin
9A

[kaa]

Thanks everyone for comments!

Yes, this was my concern. Is there any downside to connecting the backup field alternator to the e-bus, and its output to the battery side of master contactor? Like this: https://drive.google.com/uc?export=v...o8PwK5-8GMPkJ1

[Finley Atherton]

Originally Posted by kaa

Thanks everyone for comments!

Yes, this was my concern. Is there any downside to connecting the backup field alternator to the e-bus, and its output to the battery side of master contactor? Like this: https://drive.google.com/uc?export=v...o8PwK5-8GMPkJ1

Could drain the battery through the B lead when the engine is not running if there is a bad diode in the alternator?

Fin 9A

[rv7charlie]

How close are you cutting it on your E-bus load, to alternator capacity? If you can tolerate adding the ~2A load of the master contactor, why not re-arrange things so the 'main' bus feeds your essentials, and the 'E' bus becomes the 'load shed' bus?

For a little history, the reason the E bus came to exist was to eliminate *everything* non-essential, including even the master contactor, to stretch battery-only operation as long as possible. When the 1st vacuum pad mounted alternator came out, it was the little 8A PM model, so electron management was still an issue requiring an E bus.

Now if you have 20A-30A available from the backup alt, you might be able to keep the 'main' bus alive with the contactor and just 'load shed' the non essential stuff that would be on the (for lack of a better term) 'non-E' bus. If your total load was close to the backup alt's capacity, you could eliminate the E bus completely.

My build has the luxury of 2 identical alts. I won't need to shed load (either alt can tote the full load), so I have no E bus. I do have an 'airframe bus' and an 'engine bus' for my electron dependent engine (fed from both the battery and from the airframe bus and airframe bus).

Charlie

[RV10Pilot]

Originally Posted by kaa

Thanks everyone for comments!

Yes, this was my concern. Is there any downside to connecting the backup field alternator to the e-bus, and its output to the battery side of master contactor? Like this: https://drive.google.com/uc?export=v...o8PwK5-8GMPkJ1

Why not connect both the alternator field and output to the e-bus? If the master relay fails, close the alternate feed switch and turn on the back-up alternator. But you will not have a way to power to the main bus with failed main relay.

[vic syracuse]

Yes, it is possible to have the master contractor fail. I?ve had it happen to one of my customers.
On my airplane I run a separate properly-rated switch directly from the battery through a 20 amp fuse to power the avionics, and i labeled it emergency power.


So far I?ve never needed to use it, but it is there for the very scenario you describe.

I do replace my aircraft master solenoid at 1000 hours. Probably overkill, but for $25 it is piece of mind for me on an aircraft I need to be reliable.

Vic

[Walt]

If you're interested in a better quality contactor I went with the GigaVac MX11, about 4x the price of the cole hearse unit though.

[N729LS]

I got rid of my master contactor and replaced it with a >full load rated marine battery switch. These things are effectively indestructible, and don't draw any load. I have a 2-battery/1 alternator set up and there are switches for that, too.
Andy

[Carl Froehlich]

I?ve used Blue Sea 9012 solenoids for the master relay function on the last two projects. Very good quality, draws 3.6 amps when changing state but only 0.13 amp for holding current. One of these on each battery. If you shop around you can find these for under $100.

I have four 30 amp relays that feed power from each battery to the panel, a primary on each battery to feed half the panel, and an alternate on each battery (as in left battery feeding the right side of the panel, etc.). The 30 amp relays draw around 100ma.

Carl

[rvbuilder2002]

Originally Posted by N729LS

I got rid of my master contactor and replaced it with a >full load rated marine battery switch. These things are effectively indestructible, and don't draw any load. I have a 2-battery/1 alternator set up and there are switches for that, too.
Andy

Is the switch you used, located very close to the battery?

If not, and the battery cable passes through any structure (such as the firewall) to get to your switch, you have a rather large current capable wire connected to your battery that you are not able to de-energize if that portion of the wire path became shorted to ground.

This is the reason master relays are typically used.... that the battery is often located quite a distance from the main power bus or other devices (starter motor) and having one gives the pilot the control to de-energize all heavy cable down stream from the battery with the exception of the short cable that links the battery to the relay.