Dual Alternator question

[N787R]

I am looking at installing a back up alternator.
Can I connect the two alternator outputs to each other?

I am thinking of installing a toggle switch that turns on field to one alternator at a time.

Questiion : If one alterrnator field is turned off, and its output is connected to an alternator that has field active, does that cause any problem?


Thanks in advance.

Ben

[Yukon]

Best configuration would be 2 alternators charging 2 separate batteries. A paralleling relay could be activated in the event of an alternator failure. Not good to disconnect the field on a running alternator.

[f1rocket]

Check out the wiring diagrams on my website. I have two B&C alternators with alternator controllers for each. They both return current back to the same single battery. You don't need a switch as the field breaker effectively acts as a switch when you pull it. Normal operations is to keep both on at all times. I have pulled the field breaker on the primary alternator to test the backup and it worked just like it should. The indicator light from the auxiliary alternator controller blinks until I reduce electrical load to the point the auxiliary alternator can carry the load, then it goes out.

[N787R]

Randy

Thanks a lot. I am glad I got to see your site. Very usefull.

In the Power Distribution page (page 1.1) you are connecting both Alternator outputs (A & B) together. In effect shorting them.

That was the part that I thought it would be problem.
So you dont see that as so.
Maybe you can explain how that works when the two alternators are shorted.

Thanks

Ben

[Ted Johns]

Originally Posted by N787R

Maybe you can explain how that works when the two alternators are shorted.
Ben

First a minor nitpick. Let's call the alternators connected, not shorted. Shorted implies an unwanted connection.

So, how does that work when the alternators are directly connected together? Depends on your risk perspective. Normally, the output diodes will keep an offline alternator from consuming any of the active alternator's current. However, these diodes have been known to fail, and if they do the good alternator could be shorted to the stator windings of the bad unit, possibly resulting in smoke.

Regarding the use model of keeping both alternator's on:
This isn't a great idea from a purist EE perspective. Typical alternators don't current share, which means the unit with the higher output voltage will be doing the lion's share of the work. If this is the main alternator, great. If this happens to be a lower current standby alternator, yuck. The low current unit might be always running at near full capacity, possibly lowering it's lifespan. If I were to wire up a two alternator ship, I'd think about setting up some sort of fail-over control, rather than run both on at all times. Either that, or make sure the primary alternator voltage was set about .25volt higher.

T.J.

[Ted Johns]

I looked over Randy's schematics. Nice. The alternators are isolated from one another by fuses, which keeps the possibility of smoke away.

I suppose there is some small chance that the big alternator could fail shorted, and the little alternator could blow it's fuse before the battery takes out the big alternators fuse. Not keeping both alternators powered up would eliminate that concern.

I would still worry about load sharing between alternators. I think the B&C regulators are adjustable? If they are, I would trim up the big alternators voltage a bit. Not keeping both alternators powered up would eliminate this concern as well.

T.J.

[f1rocket]

I would need to dig deeper but don't have the time at the moment. When you buy the alternator controllers from B&C, they are different for the primary alternator than the auxiliarly alternator. I suspect your concern is already handled by the controllers but don't know for sure. Why else would they be different?